Eee-Viii-modern Power System Protection [06ee831]-Notes

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Modern Power System Protection 06EE831

CITSTUDENTS.IN Page 1

ELECTIVE-IV (GROUP D)

MODERN POWER SYSTEM PROTECTION

Subject Code : 06EE831 IA Marks : 25

No. of Lecture Hrs./

Week

: 04 Exam

Hours : 03

Total No. of Lecture

Hrs. : 52 Exam

Marks : 100

PART - A

UNIT - 1

STATIC RELAYS: Introduction, Basic construction, Classification, Basic Circuits,

Smoothing Circuits, Voltage regulation, square wave Generator, Time delay Circuits, Level

Detectors, Summation device, Sampling Circuits, Zero crossing detector, output devices.

8 Hours

UNIT - 2 & 3

COMPARATORS: Replica impedance, Mixing Transformers, General equation of phase

and Amplitude, Comparators, Realization of ohm, mho, Impedance and offset impedance

characteristics, Duality principle, Static amplifier comparator – Rectifier bridge circulations

current type, sampling comparator, static phase comparator coincidence circuits type Rectifier

phase comparator, Block split comparator, Zener diode phase comparator,

12 Hours

UNIT - 4

PRINCIPLES OF DIGITAL/ NUMERICAL RELAYS: Definition of Numerical

Protection System, Advantages of Numerical relays, Block diagram of Numerical Relays,

Processing Unit, non machines Interface, communication in protective relays, Information

handling with sub station monitoring system.

6 Hours





UNIT - 5

PART - B

STATIC OVER CURRENT, TIMER AND VOLTAGE RELAYS: Instantanense over

current Relay, Definite time lag relay, inverse time over current relay, static timer relay, Basic

relay circuits, monostable delay circuits Single phase Instantaneous over voltage and under

voltage relays, instantaneous over voltage relay using Op-amp.

UNIT - 6 & 7

10 Hours

DISTANCE RELAY: general Principle of operation, Zone discrimination, Fault area on

impedance diagram, Basic measuring elements, Different characteristics used in distance

relaying- Impedance, Reactance, Admittance. Ohm, Distance relay settings, Distance

measurement Problems.

UNIT - 8

10 Hours

DIGITAL RELAYS: Block Schematic approach of microprocessor based relays, over

current relay Protection, Transformer differential protection, Directional relay scheme,

Impedance relay scheme. 6 Hours





CONTENT

SL NO Topic Page No

1 UNIT - 1: STATIC RELAYS 4-28

Introduction, Basic construction, Classificat ion.

Basic Circuits, Smoothing Circuits.

Voltage regulation, square wave Generator.

Time delay Circuits, Level Detectors.

Summation device, Sampling Circuits.

Zero crossing detector, output devices.

2 UNIT – 2 & 3 COMPARATORS 29-44

Replica impedance, Mixing Transformers.

General equation of phase and Amplitude, Comparators.

Realization of ohm, mho, Impedance and offset impedance

characteristics.

Duality principle, Static amplifier comparator – Rectifier

bridge circulations current type, sampling comparator.

static phase comparator coincidence circuits type Rectifier

phase comparator, Block split comparator.

Zener diode phase comparator.

3 UNIT - 4 PRINCIPLES OF DIGITAL/ NUMERICAL

RELAYS

45-50

Definition of Numerical Protection System.

Advantages of Numerical relays, Block diagram of Numerical

Relays.

Processing Unit.

Non machines Interface.





Communication in protective relays.

Information handling with substation monitoring system.

4 UNIT – 5 STATIC OVER CURRENT, TIMER AND

VOLTAGE RELAYS

51-62

Instantaneous over current Relay.

Definite time lag relay.

Inverse time over current relay.

Static timer relay, Basic relay circuits.

Monostable delay circuits.

Single phase Instantaneous over /under voltage relays.

Instantaneous over voltage relay using Op-amp.

5 UNIT – 6 & 7 DISTANCE RELAY 63-75

General Principle of operation.

Zone discrimination.

Fault area on impedance diagram.

Basic measuring elements.

Different characteristics used in distance relaying- Impedance,

Reactance, Ohm, Distance relay settings.

Distance measurement Problems.

6 UNIT - 8 DIGITAL RELAYS 76-90

Block Schematic approach of microprocessor based relays.

Over current relay Protection.

Transformer differential protection.

Directional relay scheme.

Impedance relay scheme.





UNIT - 1

STATIC RELAYS: Introduction, Basic construction, Classification, Basic Circuits,

Smoothing Circuits, Voltage regulation, square wave Generator, Time delay Circuits, LevelDetectors, Summation device, Sampling Circuits, Zero crossing detector, output devices.

8 Hours

UNIT - ONE

General introduction to Static Relays

1.1. History of electronic Relays and their Relation to

Electromechanical counterparts

References to the design and application of electronic relays for power system protection can

be found in the literature from the year 1928 onwards. In that year Fitzerald (205) published a

scheme for pilot wire protection. Wideroe (31) in 1934 brought out a series of circuits for the

common types of protective relays while Loving in 1949 published refinements to these.

Macpherson, Warrington and McConnell (35) updated the developments upto 1948, and

these were extended in later years by Barnes, Kennedy, Honey, Reedman, Dlouhy, Cahen and

Chevallier. In all these schemes, either thyratrons or thermionic tubes have been employed.

None of these types has found general applicat ion for power system protection for reasons to

be discussed later. In the field of carrier current relaying, however, electronic protection with

thermionic tubes has been successfully employed. Even in this field, with the heavy power

supplies required for the electronic tubes, combined with the rapid development of

semiconductor components, the attention has been rapidly diverted to building carrier

equipment with solid state circuits. Appendix A. 1.1 to Chapter 1 discusses briefly he basic

electromechanical relay elements and their relation to static devices. The discussion therein is

brief and greater details can be found in later chapters. In appendices A. 1.2, A. 1.3 and A.

1.4 at the end of Chapter 1, the following details are given:

A. 1.2-Details of device numbers accepted internationally

A. 1.3-List of symbols for relays and protection

A. 1.4-Glossary of common relay terms





1.2. Application of Electronic Relays to Protection

The adequacy or otherwise of the present methods is always one of the most important

factors influencing technological research and development. Over 75% of the existing protective relay requirements are met without undue difficulty by electromagnetic relay

elements. The scheme in use depend on the characteristics of induction disc or cup, moving

coil or moving armature (hinged armature) elements. However, there are bigger issues and

considerations which have influenced the pace of development in static relays, such as the

following:

(a) Better performance and characteristics, e.g. higher speed with greater accuracy and

sensitivity in distance relays.

(b) Greater standardization in manufacture.

(c) Easier manufacture and reduction in maintenance time.

Edgeley and Hamilton (37) in 1952 claimed test and constructional advantages for

their relays employing transductors (magnetic amplifiers).

1.3. Reasons for the Short-lived Research into Relays Using Thermionic Tubes

As already mentioned,(the research effort up to the year 1956 was predominantly towards

circuits employing thermionic tubes, thyratrons etc., since till then solid state devices like

transistors, were just in their infancy. None of these circuits reached the commercial stage

and the research effort rapidly tapered off. However, there were certain disadvantages in

thermionic tube circuits in relaying and the main ones were the following:

(a) Provision of special power supplies for valve heaters-this imposes constant drain on

the power supplies.

(b) Provision of appreciable voltages for valve anodes and electrode bias.

(c) Incorrect operation under transient conditions.

In view of these disadvantages, attention has been diverted to the development of static relays

using solid state components. As there is at present no interest in relays using electronic

valves and as they are of historical importance only, we shall not further discuss such r elays

in this book. The reader is referred to the various papers appearing in the literature mentioned

under References.





1.4 Advantages of Static Relays

Static relays in general possess the following advantages:

(a) Low burden on current and voltage transformers, since the operating power is, inmany cases, from an auxiliary d.c. supply.

(b) Absence of mechanical inertia and bouncing contacts, high resistance to shock and

vibration.

(c) Very fast operation and long life.

(d) Low maintenance owing to the absence of moving parts and bearing friction.

(e) Quick reset action and absence of overshoot.

(f) Ease of providing amplification enables greater sensitivity.(g) Unconventional characteristics are possible-the basic building blocks of

semiconductor circuitry permit a greater degree of sophistication in the shaping of

operating characteristics, enabling the practical utilization of relays with operating

characteristics more closely approaching the ideal requirements.

(h) The low energy levels required in the measuring circuits permit miniaturization of the

relay modules.

Table 1.1 shows the comparative VA burdens of some static and electromechanical

relays, taken from certain manufacturers.

Table 1.1-Comparison of Burdens of Static and Electromechanical Relays.

Types of relay or protection

scheme

Electromechanical Static

Current Voltage Current Voltage

Definite time delay relay

Biased differential relay

Mho distance relays: 3-step

protection

-

Operating Coil 0.6 VA at 40%setting 3.7 VA

at 100% setting

Bias Coil 0.4

VA at 40%

setting0.4 to 8.5 VA

dependent onsetting

A.C. 110V25 VA

D.C. 110V

10W

20 VA

-

0.33 VAat 1.0 A1.00 VA

at 5.0 A

0.04 to 8.7

VAdependent on

A.C. 110V2.4 VA

D.C. 110V

4.46 W

8.7 to 11.2

VA





Switched distance schemefor phase and earth faults

Time-overcurrent relay

(IDMT)

Instantaneous overcurrent

relay

0.4 to 8.5 VAdependent on

setting

0.75 to 1.3 VA

dependent onsetting

0.7 to 10 VAdependent on

setting

1A, 2A, or

5A andsetting

20 VA 2 to 3 VA 8 to 9 VA

- 0.02 to 0.13

VA

dependent on

setting andcurrent rating

- 100 mVA

Overvoltage relay - 2.0 VA - 0.2 VA

Static relays with solid state components have certain limitat ions (as mentioned

below) but these can be overcome as indicated in each case.

(a) Temperature sensitivity-temperature compensation circuits have been developed (e.g.

use of thermistors).

(b) Ageing-this is eliminated by presoaking of components for several hours at a

relatively high temperature.

(c) Sensitivity to voltage spikes-this can be eliminated by filters and shielding.

(d) Damage due to overloading-this is eliminated by careful circuit design.

Static relays may be single, two or multi-input devices. Individual modules are now

being developed to provide critical measuring as well as non-critical switching functions to

be discussed in detail later.

Timing and counting requirements are much better satisfied by solid state circuit than

with electromagnetic elements. Availability of reed relays has made these more useful as they

have very high operating speed (1-2 ms). Thyristors are coming up rapidly as tripping

elements.

1.5 What Are Static Relays?

The term ‘static relay’ is generally referred to a relay incorporating solid state components

like transistors, diodes, resistors, capacitors, etc. In this type of relay, they functions of

comparison and measurement are performed by static circuits wherein there are no moving





parts. According to a recent decision of the Internat ional Electrotechnical Commission (IEC),

which has been adopted by the Indian Standards Institution, the following are the terms

relating to static relays.Static relay- A relay in which the designed response is developed by electronic, magnetic or

other components without mechanical motion.

Static relay with output contacts-A static relay having a contact in one or more of its output

circuit(s). The term previously applied to this type of relay was semistatic relay.

Static relay without output contacts-A static relay having no contact in its output circuit(s).

The term previously applied to this type of relay was fully static relay.

Most present-day static relays, incorporate a hinged armature or a polarized movingcoil relay in miniature size as the output device to provide at a low cost a number output

contacts capable of duly tripping. The alternatives are reed relays or thyristors (as indicated in

Section 4) which are being considered in recent designs but not yet adopted commercially.

With the growth of power systems in both complexity and fault levels, the need for

more sensitive and faster, reliable protection schemes has arisen. The advent of the

semiconductors overcame the supply problems associated with thermionic valves. The use of

transistor amplifiers, etc. deriving their power from an external source, or in some cases self-

powered from the current and voltage transformers, made it possible to achieve greater

sensitivity and at the same time obtain excellent mechanical stability. The individual

components should be chosen carefully and should be used well within their designed rating.

It should be clearly understood that it is not usually economical to replace existing

electromechnical relays with their static counterparts just to reduce maintenance; source gain

in technical performance should also be obtained. At present protective relays are supplied

from iron-cored current transformer and hence excessive saturation should be avoided to

ensure high speed and discriminative operation. The use of static relays in general reduces the

burden on the current transferers.

One may be surprised to find that static relays have first been commercially produced

only in such cases as distance or differential schemes, while in the case of the much simper

overcurrent relay schemes, they have not been brought out. The reason behind this is the fact

that distance and differential schemes are more amenable to mathematical analysis while the

overcurrent characteristics like those given by the induction disc relays, are more empirical





and less mathematical. This a static relay cannot compete with the electromechanical

standard inverse time overcurrent relay as this characteristic is readily achieved by the

saturation characteristics of the electromagnet. On the other hand, the inverse time replayrequired for overload protection of rectifiers has a characteristic of the form I 8t=K which can

be achieved with static circuits, but would be extremely difficult to achieve with an induction

disc relay. In the field of distance protection, it is possible to derive many varied and complex

characteristics with the use of multi-input static comparators. It is possible to accommodate a

complete distance scheme in one or two conventional relay cases thus reducing panel space

and the cost of interconnecting wiring.

When static relays were first developed availability of components and their

reliability were poor. Now these components have attained a record of reliability much better

than conventional components. Many thousands of static relays have already been

manufactured and used-the field experience over the last decade has proved their superiority

in more than one sense.

As already mentioned, static relays using thermionic valves could not meet practical

requirements and never reached the commercial state. Rectifier relays, first used in Germany,

revolutionized the development of static relays. Transductors offered the advantage of d.c.

isolation between control and output circuits. They were widely employed in protective relay

applications, but have been discarded due to their slow speed of operation.

Hall effect relay permit instantaneous analogue multiplication of two measured

valued. However, because of their high cost they have not been sued in practical relays.

Gauss-effect relay employ semi-conductors whose resistance varies when a magnetic field is

applied. They are also quite expensive.

Measuring elements of static relays have been successfully and economically built up

from diodes, zener diodes, avalanche diodes, unijunction transistors, p-n-p and n-p-n

bijunction transistors, field effect transistors or their combinations. The main reason for the

rapid progress of transistors into static relays is the instantaneous readiness of operation due

to the absence of heaters and their substantially long working life Further, in recent years, the

manufacture of components has achieved considerable advances like encapsulation, planner

and epitoxial construction techniques, printed circuits, etc. Integrated circuits now occupy a





predominant place. These techniques have increased the reliability of the components and

reduced the sizes of the modules to small chips.

1.6. Basic Construction of Static Protective Relays

Basically, protective relays are analogue-binary signal converters with measuring functions.

The variables such as current, voltage, phase angle or frequency and derived values obtained

by differentiation, integration of other arithmetical operations, appear always as analogue

signals at the input of the measuring unit. The output will always have a binary signal, i.e.

either an open (or OFF) signal if the relay is not to trip or a close (or ON) signa l if the relay is

to trip. These output signals can therefore be easily evaluated by subsequent control elements

requiring very little technical effort. Each protective relay is built up of individual elements in

accordance with the basic block diagram show in Fig. 1.1. The signals, which occur in

analogue and therefore in the continuously variable

1. Measuring Circuit 2. Measuring Signals 3. Converter Element 4. Measuring Element 5.

Output element 6. Output signal 7. Controlled Element 8. Feed Element 9. Aux. Voltage

Source 10. Measuring Circuit Supply

Fig. 1.1 Basic block diagram of protective relays





form from the measuring circuit (C.T and/or V.T) are first fed to the converter unit in the

protective relay. This converts the measured signals so that they can be easily processed by

the measuring element which follows. This measuring element will be operated when theinput signal reaches a certain value-providing a close signal at its output. The output element

amplifies this binary but weak signal and transfers it to one or more controlled elements. The

controlled elements carry out the final switching functions as opening of circuit breakers, etc.

Power is supplied to the measuring or output element by a feed element. This power is

obtained either from an auxiliary voltage source or from the measuring circuit itself.

1.6.1 Converter Element

This element contains chiefly he matching transformers to obtain the required signal level.

The rest of the construction depends on whether one or two or more inputs are to be handled

by the relay.

Relays for one quantity are supplied with only one electrical quantity, e.g. current or

voltage. After suitable transformation by the matching transformers, this quantity is fed to

diode bridges at whose output it appears as a d.c. variable with ripple. Through setting

networks consisting of fixed and variable resistors, clipping diodes, etc. the measured value

of the quantity is fed into a harmonic filter (if the speed requirements are not too stringent)

since the subsequent measuring element deals only with d.c. variables. Sometimes smoothing

filters are used to eliminate ripples, but in high speed relays such filters cannot be used.

In relays for two quantities the converter element is fed at its input with two electrical

quantities. In each case the comparison can be either amplitude or phase comparison between

the two signals, the signals being converter to voltage or current signals (Fig. 1.2). There is

also a pulse type





VOLTAGE COMPARISON

1. Measuring Element

2. Feed-back Resistors

CURRENT COMPARISON

1. Measuring Element

2. Resistors

Fig. 1.2 Basic voltage and current comparators-two inputs

comparison in which one of the variables is converted into a pulse when passing through zero

and the second is converted into a rectangular block. When the pulse and the block coincide,

a close signal appears at the output of the comparator circuit. This gives extremely short

response time.

In relays for three or more quantities, the converter element is supplied with three or

more electrical quantities. In Fig. 1.3 it will be noticed that in two of the measuring bridgesthe feed-back resistors are combined into one.





Fig 1.3 Basic voltage comparison-three inputs

VOLTAGE COMPARISON

1. Measuring Element 2. Feed-back Resistor

1.6.2 Measuring Element

This is an analogue-binary signal converter with measuring functions. In the simplest form it

consists of the Schmitt trigger circuit shown in Fig. 1.4 as the basic circuit. The Schmitt

trigger circuit (which will be discussed in detail later in chapter 14) can be compared to an

extremely fast polarized d.c. relay and acts as a level detector. Transistors are used incommon emitter connection giving high input resistance and large current gain. The level

detector gives a step output when the input voltages exceed a specified level.

1.6.3 Output Element

This element amplifies the output signal from the measuring element, multiplies it, may

combine it with certain other signals and also introduce a delay if necessary. Since it has to

process only binary signals, this need not be a precision element. It may thus take the form of





auxiliary relays or contactors. These provide potential separation between controlling and

controlled circuits. It many also take the form of a bistable or monostable multivibrator

circuit and if required modulated by logic circuits like AND, OR, NOR or timing elements.Where large powers are involved, e.g. operating trip coils of circuit breakers, silicon

controlled rectifiers (SCR) are used after the logic element.

1.6.4. Feed Element

The function of this element is to supply the power necessary to operate the circuits and the

power is obtained either from a built-in auxiliary supply (nickel cadmium rechargeable cells)

or from station battery. In many cases it is derived from the measuring circuit itself. In all

case the feed element should supply a stabilized voltage to the static circuits, so that the

measuring accuracy is not impaired. In the initial stages of development, nickel cadmium

rechargeable cells. (commonly known as button cells) were used in the commercially

produced static relays, specially in the U.K., but experience has shown that their reliability is

poor. They are being given up at present and in their place station batteries with suitable taps

at the appropriate voltage are being preferred. In the case of several types of relays, the

supply is derived from the current and voltage transformers themselves, as mentioned above

with the refinement that the power supply to the relay is switched on only in case of a fault

being detected by a suitable fault detector.

1.7 Complete Electronic Protective Relays

We have seen how a static relay is basically designed. The detailed circuit in each case will

be discussed in the succeeding chapters of this book. We will also discuss briefly here (details

will be found in the respective chapters) the complete relay systems.

These relays can be built up in accordance with a unit construction system. This

system can be extended not only to individual relays but also to entire relay assemblies.

1.7.1 Overcurrent Relays

In a three phase system of relays, it is economical to build it on a single phase basis. Thus

there will be three converter elements, one for each phase, but thereafter the rest of the





circuits (e.g. measuring element, output element, etc.) will be common to all the three

circuits. This is shown in Fig. 1.5.

V Converter Element; OR OR Gate; M Measuring Element; t Timing Element; O Output

Element; S Feed Element.

Fig 1.5 Block diagram of an electronic three phase time-over current relay I R , I Y , I B=Three

inputs currents

1.7.2 Protective System for Generators and Transformers

Here again instead of three elements for the individual three phase relays, a single measuringelement and a single output unit are used. There are combined in Or-gates which in turn

modulate the high power control units. With this method it is also possible to combine

various measuring elements to save unit construction elements. This is shown in Figs. 1.6 and

1.7.

1.7.3. Busbar Protection

Figs. 1.8 and 1.9 show the static busbar protection scheme. This scheme offers (i) high

operating speed, (ii) use of current transformers of any desired characteristic, (iii)

independence of C.T. saturation, (iv) independence of the effect of load-in resistance, (v)

application for coupled or





I C.T. Connections, V V.T. Connections, CD Circuit Breaker, ES De-excitation switch Fig.

1.6 Electronic protection of generator-transformer unit NK =External voltage supply

subdivided busbars while maintaining the tripping selectivity of each section, (vi) avoidance

of C.T> change over, (vii) facilities for output signaling, and (viii) simple extensibility for

later extension of the busbar system. In case of (vi) there is thus no risk of opening of C.T.

secondary circuits since change over takes place only in the logic part of the circuit.

In the case of a short circuit fault, the protective system will measure the phase angle

of all outgoing currents, compare the phase angle of each busbar section and operate if a ll

these angles are within + 90 electrical degrees. The differential current is used as starting

quantity, while the reference value is a quantity which is an phase with the short circuit

current.





1.8. Complete System of Protection

Protective relays, being component parts of electrical switchgear, should be closelyassociated with the control section of such systems, e.g. indicating and signaling systems.

Thus the signaling and

PROTECTION

D Differential, MI Overcurrent, MV Overvotlage, RM Antimotoring, SE Stator E/F, RE

Rotor E/F

OTHER ELEMENTS





V Converter, M Measuring, t Timing, S Signalling, OR OR Gate, NOT NOT Gate, AS

Genral Signal, BS Fire Protection

indicating systems must also be designed on a static basis. The plug in type modular chassis

has been widely adopted already in electronic regulating and control equipment. These

contain a self contained plug in section which consists mainly of a printed circuit with the

appropriate circuit components as well as the front panel with setting scales, set ting

potentiometers and signaling elements. With the development of integrated circuits, these are

being used more and more. All functional units are mounted in a single rack and for more

comprehensive systems, several racks are combined into the hinged frame. The same

procedure is adopted for protective relays as well.

F Feeders.

B Reference Values

BS Busbar Protection

I Feeder Current

∆ I Difference Current

L Circuit Breaker

BUS Busbar

W Current Transformers

Fig 1.8 Busbars protected





The future circuit breaker may also follow these lines of static development and may

ultimately be a static device.

M Measuring Element

V Converter Element

NOR NOR-Gate

AND AND-Gate

Fig. 1.9 Block diagram of busbar protection

1.9 Characteristic Functions of Protective Relays

It is possible to obtain similar functions and characteristic from replay elements using

different operating principles. These principles and design criteria determine how well the

basic function is performed and how in pract ice it deviates from the ideal. Static relays can

achieve such a high performance that the departures from the ideal in practice are negligible.

This high performance combined with the design flexibility makes it possible to satisfy a

wide variety of requirements with a limited number of functional units. The basic elements

needed to cover such a wide range of functions are discussed in the remaining sections of this

chapter.

1.9.1 Summation

The combination of a number of electrical quantities into a single quantity through static

equipment like summation transformers and sequence networks is already well known. These





fit into the concept of static relaying. Summing units can be reduced in size by using new

components and techniques, including active elements. Semiconductor circuits are well suited

to the use of summing junctions (sued in the analogue computation field) and can be appliedinto protective relaying.

1.9.2. Single Input Devices (Fig. 1.10)

These form the basis of many protective and control schemes as in the following:

(a) Non-critical repeat function (all-or-nothing relays)-This usually produces a switching

power gain of the order of 103, with a multiplicity of segregated outputs-these may be

in form of contacts. These devices are generally instantaneous with operating speedsof the order of 20 ms. Or less. They may also have an associated time-delay function.

Since these devices are either unenergized of energized very much in excess of

marginal conditions they ensure fast response and good contact pressure in the case of

electromechanical devices. Examples of this type are : attracted armature replays

(telephone type), reed relays (with high speeds of the order of 1-2 ms are now

available), semiconductors (specially thyristors). Repeat functions are usually initiated

by outputs from critical or measuring elements [Fig. 1.10 (a)].

(b) Critical or measuring function-This requires a response to an input when the latter

exceeds a prescribed critical level. Switching gain may be included but is not essential

since repeat devices can be provided. Practical requirements are fast response,

accuracy of setting and high reset ratio. This function is used in many protection

systems such as overcurrent, undervoltage, overvoltage, differential systems etc. The

elements used for these functions many be attracted armature, induction or moving

coil types. This function can also be derived through semiconductor circuits driving

electromagnetic relay like polarized moving coil or attracted armature types or reed

relays or thyristors [Fig. 1.10 (b)].

(c) Fixed time or definite time function-This necessitates a delay between input and

output (between the application of an input and the occurrence of an output or

between the removal of an input and the resetting of the output). The input is to be

only non-critical (either nothing or greatly in excess of the critical level) and

switching gains and multiple outputs are possible. The device can also provide a





repeat function. Measuring functions can be combined in the

inputs circuit. Practical requirements from these devices are accuracy of time setting

and repeatability under successive application [Fig. 1.10 (c)].

(d) Function time-dependent on input — The commonest form of such characteristic is

t=f(S n) where n is negative and real. Examples of this are over current and overload





protection. These may also contain in addition low set and high set instantaneous

critical function. [Figs. 1.10 (d) and (e)].

1.9.3 Two Input Devices (Fig. 1.11)

With two inputs, it is possible to obtain a wide range of characteristics using different

operating principles. The function is generally defined by the relationship between inputs,

which governs the boundary condition of operation. The two basic forms are as follows:

(i) Amplitude comparison-One input is a restraining quantity and the other is an

operating quantity so that an output is obtained when the ratio of these quantities is

less than a critical value. Ideally the comparison of the amplitudes of the two inputs isindependent of the level and phase relationship, of the inputs. The function is

represented by a circle in the complex plane with its centre at the origin-defining the

boundary of marginal operation. Examples of this are biased relays and impedance

type distance relays [Fig. 1.11 (a)].





(ii) Phase comparison-Output appears when the inputs have a phase relationship lying

within specified limits. Both inputs must exist for an output of occur-ideally the

output is independent of their magnitudes, but dependent only on their phase

relationship. The function as defined by the boundary of marginal operation is

represented by two straight lines from the origin of the complex plane. Examples of

these are directional relays, distance relays excluding the impedance type and other

phase comparison relays [Fig. 1.11 (b)].

These will be discussed in detail in the subsequent chapters. It will also be shown that

either comparator becomes equivalent to the other if it is fed through the appropriate mixing

units giving the sum and difference of the original inputs. Thus any relay characteristic can be

obtained by using the amplitude or the phase comparison principle, although practical

considerations might dictate preference for one scheme out of the two.





1.9.4 Multi-input Devices (Fig. 1.12)

With more than two inputs, the range of complex characteristics is extended. Usingamplitude comparison, the remaining conic section curves like ellipse, parabola, hyperbola,

etc, can be obtained.

Using phase comparison, characteristics are obtained which contain discontinuities as the

effective zone is the common area given by a number of straight lines and/or circular

characteristics.Equivalence of phase and amplitude comparators does not apply to multi-input case.

1.10 Semiconductors to Obtain Functional Requirements

The characteristics of modern transistors are specially suited to the functional requirements

mentioned above. These along with the other semiconductor and other components have

made possible commercial production of static relays. The properties particularly useful in

the realization of the functional requirements are amplification, switching characteristics,sensitivity, high speeds of response, flexibility of design and application, long life, compact

and rugged construction, and simplified power requirements. The development of

miniaturization resulting in the integrated circuit chips has accelerated the pace of

development and exploitat ion to commercial standards of the principles of static relaying.

1.11 Practical Non-critical Switching Circuits

The two main functions of such circuits in relation to protection are the following:





(i) Provision of a final signal for tripping to the circuit breaker; may also give

supplementary functions like inter-tripping, alarm and visual indication.

(ii) Acting as intermediate switching stages within the assembly of function elements.Both the above duties have been met in the past by using electromagnetic relays. For

(i) e.m. relays have proved to be economical and reliable, of course accounting nevertheless

for 10-30 ms of overall tripping time. For (ii) the operating time of e.m. relays may become

unduly long, while being reliable, but with unsuitable contact performance sometimes.

Conventional e.m. relays have been accepted in association with static relays for non-critical

functions if their operating times are tolerable. Progressively efforts have been made to

replace e/m/ relays by (a) reed relays and (b) static switching circuits using transistors or or

thyristors.

Reed relays have been found to be reliable with high operating speeds of the order of

1-2 ms even at small multiples of setting. They can therefore be sued for the intermediate

switching stages giving segregation between input and output. This gives added flexibility

and freedom of interconnection. Their operating power may, however, require a preceding

stage of transitor amplification with positive feed-back.

Transistor switching circuits are also suitable for the intermediate switching functions

and have been used extensively as such. The rating of trip circuits is generally unsuited to

transistor switching circuits and if a fully static trip circuit is required for various reasons,

then thyristors may be used subject to certain restrictions like interface problems.

1.12 Practical Critical Level Detectors

The inputs in such level detectors are generally at power frequency and may vary over a wide

range relative to the critical level. Basic requirements are accuracy, lo ng-term consistency,

fast operation and a controllable reset ratio of high magnitude. Obtaining these with e.m.

relays has always presented problems. When designing a static equivalent of an e.m. relay;

the circuit should be such that it retains the best features of the e.m. relay, but overcomes its

disadvantages. An example of such a design is shown in Fig. 1.13. Here the reset ratio is high

and snap action is retained. The circuit can be made relatively insensitive to the offset

transient conditions present in the fault currents. It may also be made self-energizing since it

permits associated units, for which auxiliary supplies may be essential, to be normally





unenergized. The circuit uses two basic elements, as shown in Fig. 1.13. (a), the critical level

detector and the pulse integrating circuit. The level detector compares an unsmoothed

rectified or an alternating signal against a d.c. datum. For peak inputs below the datum, theoutput is zero, but at the critical peak input there is a finite output pulse the width of which is

determined by





the reset ratio. The output pulse widens with increasing input, but at the critical level the

width substantially exceeds the marginal operating level of the second element. This circuit

retains snap action, even when the reset ratio inherently exceeds 1.0. With the exception ofthe datum signals in the level detector, the design parameters are non-critical.

A practical self-energized circuit using these principles is shown in Fig. 1.13 (b). A is

the measuring and switching circuit and B is the pulse integrating circuit. The rectified

unsmoothed signal feeds two alternative paths ( I 1 and I 2). Below the critical level, the

measuring and switching circuit is fully conducting so that current I 2 for all practical purposes

is zero. The datum V z is derived through a zener diode ZD and is substantially d.c. at the

critical level input. The loading on the input circuit up to the critical level input. The loading

on the input circuit up to the critical level is controlled so lely by R 3 which provides the

voltage for the measuring and switching circuit through R1 R2. The critical level occurs when

the voltage across R1 exceeds V z by ∆V required to operate the switching circuit, which then

switches to a high output impedance and diverts the current I 2 into the pulse integrating

circuit. The measuring accuracy depends only on Vz and resistors R1 , R2 and R3 if ∆V is small

compared to V z . Temperature compensation can also be incorporated in the circuit. At the

instant of switching, the input through R1 increases to a value dependent on the input

impedance of the pulse integrating circuit, relative to R3. This provides positive feed-back in

the measuring circuit, which controls the instantaneous, reset level. Surge and overload

protection are easily incorporated by using non-linear resistors on the input and an

electrostatic screen on the interposing transformer.

An alternative simple non-transistor arrangement based on reed relays is shown in

Fig. 1.14. The accuracy of the operating level of the first reed relay is important but the reset

ratio is not critical. Such a device can provide operating times of 5-10 ms at 2-3 times the

setting. The overall reser ratio can be about 0.95.





t.J +,.... R

.

...

E.-Ie_m_en_'---"" ,....

._ j _:t:...........

.:J.... · _

A.C. ReedElement

Fig. I.J4 Level deteclor using reed relays





UNIT - 2 & 3

COMPARATORS: Replica impedance, Mixing Transformers, General equation of phase and

Amplitude, Comparators, Realization of ohm, mho, Impedance and offset impedancecharacteristics, Duality principle, Static amplifier comparator – Rectifier bridge circulations

current type, sampling comparator, static phase comparator coincidence circuits type Rectifier

phase comparator, Block split comparator, Zener diode phase comparator.

12 Hours

UNIT TWO & THREE

2.1 Introduction

Comparators and Associated Elements

The basic block diagram of a static relay has been discussed in Chapter1. As per this diagram

the converter element is the heart of a static relay. This element contains the means for

converting system currents to voltages or vice versa and then comparing them in phase or

amplitude. Taking the case of the general two-input comparators, the inputs S 1 and S 2 can be

represented in the following form:

S 1=K 1 V L+Z R1 I L

S 2=K 2 V L+Z R2 I L

(1)

Where K 1 and K 2 are constants multiplying the system voltages and and are complex

impedances which convert the system current I L into suitable voltages. The comparator is

shown in Fig. 2.1, which shows the manner in which the two inputs are derived and applied

to the comparator.

In a static relay, it is therefore essential to convert the inputs to the comparator to

quantities of the same dimensions, e.g., voltages or currents. The values of K 1 and K 2 are





obtained through suitable potentio meters, while and are suitable transfer or replica

impedances.

2.2 Transfer or Replica Impedance

This could be either a self or a mutual-impedance as shown in Fig 2.2. Fig. 2.2 (a) shows a

self-inductance used as replica impedance. This is an iron core reactor with an air gap in the

core to give

linearity in the voltage/current relationship up to the highest value of current expected. We

have

And

where LS is the self-inductance of the reactor and r the resistance of the coil. This type of

impedance offers no isolation between I L and V S . In many relay circuit applications this

isolation becomes essential. Fig. 2.2 (b) shows a mutual inductance used as replicaimpedance. There are two windings- one is primary and the other is secondary, wound over

an air gapped iron core to give linearity of transformation over the required current range.

The secondary winding is loaded by a suitable resistance R2 (which includes the internal

resistance of the winding). The ratio of secondary voltage to the primary current can be

shown to be





Here R2 is the resistance connected on the secondary side, L2 is the self-inductance of

secondary, M is the mutual inductance between primary and secondary, and tan . This device

being a combination of a reactor and a tranfermer is known as Transactor, and offerscomplete isolation between the primary and secondary circuits, i.e., between I L and V S .

In the case of the self-inductance the angle and in the case of the transactor the angle

are designed to give the appropriate value depending on the particular relay applicat ion.

2.3 Mixing Transformers or Circuits

The inputs given in Eq. (1) are obtained through addit ion or subtraction of different voltages

or different currents. The addition or subtraction is done usually through appropriate

transformers. Some examples are shown in Fig. 2.3.

In Fig. 2.3 (a) voltage input V is derived by the addition or subtraction of two voltage,

one derived from the system voltage through a voltage transformer and the other from the

system current through a current transformer and a transactor.

Thus

In Fig. 2.3 (b) current input I is derived by the addition or subtraction of two currents,

on derived from the system current I L through a current transformer and the other derived

from the system voltage through a voltage transformer and a reactor.





Thus

These will be discussed in greater detail under the discussion of static circuit

applications to different relays.

Fig. 2.3 derivation of inputs: (a) ; and (b) current input.

2.4 Phase and Amplitude comparators

The most important component of the converter element is the comparator (where the inputs

are compared in phase or amplitude) which was introduced in Chapter 1. The discussion in

this chapter will be confined to two input devices, while multi-input devices will be

introduced later in Chapter 10.

2.4.1. Classification

Letting the two singnals be S 1 and S 2, the amplitude comparator gives positive (yes) output

only if , i.e. only if Ideally this

comparison should be purely scalar or in the other words independent of the phase relations

between the signals.

The output of a phase comparator is positive (yes) only if α=arg (S 1/S 2) satisfies the

relation

inputs.

. The comparison, ideally, should not depend on the amplitudes of the two

Both types of comparator can be arranged either for direct ( instantaneous) comparison, when

they would be known as direct (instantaneous) comparators or their output integrated over

each half-cycle, when they are called integrating comparators.





2.4.2 General Equation for comparators

To cover the complete range of conventional relay characteristics, let the two input signals S1and S2 be derived from the power system voltage and current as follows:

Fig. 2.3 has shown the manner in which these inputs are derived.

Putting these in the complex form,

Then

and cos

In the following discussion, K 1 and K 2 will be considered as real numbers, and V L taken as the

reference vector.

This

2.4.2.1 Phase Comparator (Cosine Type)

Criterion for operations of the comparator in general is In case of

symmetrical phase comparator λ 1 = λ 2 and all conventional characteristics are obtained when

λ 1 = λ 1 = π2. Thus the criterion for operation will be

i.e. cos α ≥ 0





Therefore the locus for zero torque (or the characteristic curve) for the comparator is obtained

when

Thus cos α = ac+bd = 0

Therefore (ac+bd)

Thus the criterion for operation becomes

Let the impedance seen by the comparator.Dividing Eq. (2) throughout by

(2)

From the general solution, the properties of a number of particular relay characteristics may

be predicted as follows.

Directional characteristic

Putting we have

i.e.

and





Fig. 2.4 Directional characteristic (phase comparison)

The characteristic obtained by this is shown in Fig. 2.4, where operation occurs to the right of

the straight lines shown shaded. The slope of the line may be controlled by varying θ2.

Ohm characteristic

Putting

we get

This is shown in Fig. 2.5(a). This is a general case and particular cases of this are

resistance and reactance characteristics.

Resistance characteristic

In the above general case, if θ = 0,

or

See Fig. 2.5 (b)

Reactance characteristic

In the above general case, if θ = 90o

or





See Fig. 2.5 (c)

In all the characteristics, positive operation taken place to the left of the line and

negative operations to the right of the line.

Offset impedance characteristic

Putting

This is the equation to a circle of radius and with centre at

. This is shown in Fig. 2.6(a). This I the general case and its particular

cases are the impedance and Mho characteristics.





Duality Between Phase and Amplitude Comparators:

Taking two input comparators, has shown that any relay characteristics could beobtained by either phase or amplitude comparison of the input quantities.

Let So and Sr be the operating and restraining inputs respectively into an amp litude

comparator. Thus operation is indicated when So > Sr. This can be written as Z <=1 where Z

= Sr/So

The characteristics of this equation is a circle on a polar graph as shown in fig.

Let S1 and S2 be two input quantities such as that

So = (S1 + S2) and Sr = (S1-S2)In which case for operation (s1+S2) > (S1-S2)

If now S1/S2= W where W is a complex quantity with an argument Y, then

W+1 >=W-1

This gives a straight line as shown in fig. along the vertical axis through the origin.

The shaded area (to the right of the line) indicates the operating region while the unshaded

area (to the left of the line) indicates the restraining region. This is the characteristic of the

ideal phase (cosine) comparator and the operating angular criterion can be expressed as de. Tr

2< 1° 2 This shows that S1 and SA satisfy phase comparison requirements while SI-f-S2=5;

and S1---St=S, satisfy amplitude camparison requirements. The converse of this can also be

shown to be true. In general, therefore, any relay characteristic w hich can be produced by one

comparator, can also be produced by the other comparator with the sum and difference of the

original inputs. The required relations are as follows:





So = Si +SA Sr Si — S,f Sr St 2 St- 2 The circuit arrangements necessary are shown in Fig.

2.10. It may be mentioned here that the phase comparator in this case must be a cosine

comparator, i.e. one in which zero torque is produced when the angle between the two inputquantities is ±n/2. Such cosine com-parators are possible only with static circuits using solid

state devices. In case of electromagnetic elements the phase comparison is of the sine type,

i.e. one in which zero torque is produced when the angle between

Though a given relay characteristic can be obtained using either of the two comparators,

considerations of the constants calculated for the required characteristic would indicate which





type of comparator is preferable. In general, an inherent comparator is better than die convert -

ed type, because if one quantity is very large compared with the other, a small error in the

large quantity may cause an incorrect comparison when their sum and difference are suppliedas inputs to the relay.

Static Types Static amplitude comparator may use two voltages, two currents, one current

and one voltage, or one voltage (or current) and one fixed reference as the two input signals.

Present static comparators are made from semiconductor elements. Use of therinionic valves

is not favoured. Transductors arc sometimes but not much emplo yed. These will be now

discussed in detail.

Rectifier Bridge Comparators Circulating current type The ccmparator uses current input

signals. As shown in Fig. 2.13, i, and i, are rectified full-wave and their difference Is

averaged. The full-wave bridge rectifiers are connected such that the two currents circulate

within the two bridges. The output device used should be an integrating device operating on

the average value of the difference between It, and ir.

Such device is a polarized moving coil relay or an integrating circuit followed by a level

detector to be discussed in detail later. The bridge rectifier consists of silicon or selenium

diodes. The operation is as follows. When io is zero, small values of i, flow in the polarized

relay in the blocking direction. This current raises the voltage drop across the relay, and acts

in the forward direction of the operating bridge. If is increased further, the voltage drop

across the relay rises further to V, the threshold voltage of the operating bridge (whic h is

twice the threshold voltage of each diode) when the operating bridge starts conduction. This





makes the current through the relay flat topped half waves. The action is similar when 1, 0

and lo is circulated. When io and i, flow simultaneously, the output device can respond to

small differences between the two currents. The current in the relay at all times is proportional to the difference between the two currents io and Ir. It will be found that due to

the non-linear nature of the rectifiers, the current through the relay does not exceed a

maximum value and the balance current flows through the rectifier bridge having the smaller

of the currents. This is shown in Fig. 2.I4(c). The voltage across the relay is limited to twice

the threshold voltage of each diode. Thus there is a limiting action making thc relay very

sensitive, and also protecting the bridge rectifier and the relay from damage due to high

currents and voltages.





and I,, but in

practice the wave shape is dependent on the phase angle. When the two sinusoidal input

currents io and are out of phase, (io — ir) has both positive and negative loops in- the wave

shape and, combined w:th the limiting action described above, the wave shape is of the formshown in Fig. 2.14 (b). When io > i, the positive loop becomes larger in time duration

compared to negative loop. When io < in the negative loop becomes larger in time duration

compared to the positive loop. When i,, the positive and negative loops arc equal, each loop

occupying one fourth of the time duration of one cycle of the input currents. Thus the output

waveform is a double frequency pulsation. The output device should therefore be, as stated

earlier, an integrating device responding to the average area over one cycle of the output

waveform. When lo and 1, are in phase, the output waveform is as shown in Fig. 2.I4(a).





Only in this case, a continuous unidirectional output is obtained. The maximum current that

can flow in the relay is dependent on the maxi-mum voltage drop attainable by the bridge

rectifier. It is also necessary in this case to have large values of source impedances for thecurrent sources io and 1, giving near ideal current sources. This also prevents mutual effects

between the two bridges. For the same reason, it is also necessary to have isolating

transformers between each source and the bridge rectifier as shown in Fig. 2.13. Further

discussion regarding its behaviour for relay schemes will be given in different chapters under

applications. Opposed voltage type This is shown in Fig. 2.15. The output relay has zero

torque when vo=1),. This type of comparator has no limiting action against high





Figg. 2.30(a) shows the coincidence type. This is a cosine type two input phase comparator

delivering to a pulse duration detector, a pulse during the positive coincidence period of two





sinusoidal inputs. The pulse duration detector can be a telephone type relay which operates if

the pulse duration exceeds 5 ms (90' duration) for a SO Hz system. 'Hie amplitude of the

output pulse is equal to only half tlac zener voltage due to the potential divider arrangement.Hence a very sensitive output relay is needed. The Thevenin equivalent circuit is shown in

Fig. 2.30 (b) for the positive coincidence period. In this case during the coincidence period

Zat is conducting with negligible voltage drop across it while ZD, is reverse biased with

zener voltage Vi across it. If the telephone relay has negligible resistance and an inductance

of L henries with N turns, the pick-up current will have to be Vz i --(1 — exp(-0.005R12L),1

R

Fig. 2.31. Zener dio.le non-coincidence comparator (a) non-coincidence comparator; and (t)

Thevenin equivalent circuit. Fig. 2.31 (a) shows the phase detector for non-coincidence

detection and Fig. 2.31(b) its Thevenin equivalent circuit. Voltage is developed across the

telephone relay only during the non-coincidence period — during the period ZD, is reverse

biased and ZD2 is forward biased. If the relay picks up at the same current as in the previous

case, the following values arc obtained. External resistance=R ohms Relay inductance =2L

henries Vz 1°=-R(I — exp( — 0.005RI2L)] The pick-up m.m.f. is as follows for the above

case.





UNIT - 4

PRINCIPLES OF DIGITAL/ NUMERICAL RELAYS: Definition of Numerical Protection

System, Advantages of Numerical relays, Block diagram of Numerical Relays, ProcessingUnit, non machines Interface, communication in protective relays, Information handling with

sub station monitoring system.

DEFINITION OF A NUMERICAL PROTECTION SYSTEM

6 Hours

According to ABB Review 1/93 (authors Majda liar and Gunnar Strannc of ABB Relays,

Switzerland and Sweden), the definition of a numerical protection system is as follows: (a) A

real time microprocessor system utilises sampled or pre-processed power system waveform

data. (b) Digital filtering and numerical calculations take place on the basis of a continuous

stream of data from the power system. (c) Programmes, algorithms and settings are stored in

the memory used by the microprocessor. (d) Extensive hardware and software monitoring

ensures high availability. (e) The protection system can communicate with the Station

Control System (SCS) and Station Monitoring System (SMS). (f) Event and disturbance

recording on printouts, with time tagging, is initiated by abnormal power system conditions,

being available on request afterwards. (g) The interface for the man-machine communication

(MMC) is either a personal computer, a mounted terminal or a remote terminal (with a

modem). Communication via the personal computer is menu-driven, highly structured and

provides full documentation of all the settings and recorded information. The implementation

of suitable algorithms allows an adaptive response by the protection functions to changing

power system conditions and changed system parameters.

ADVANTAGES OF NUMERICAL RELAYS

The introduction of numerical tahnology in protective relays los increased the performance

and flexibility of the equipment and has iricorporared new features such as self-supervision

and the possibility to store faults. These improvements have increased reliability and allowed

more efficient use of primary equipment. ABB introduced the nticroproctssor.based fault

locator called Rants in 1982. This was the first time that a truly numerical approach was used

to calculate the distance to the fault. resulting in significantly improved fault location

accuracy. The next big step was adding communication capabilities to the protection relays

during the mid-1980s. The first communicating sub-station was delivered in 1987. It was





equipped with the SPAC 500 series (of the SPAC()M family) feeder terminals with

protection and bay control. Full utilisation of the benefits of co mmunicating relays and fully-

automated sub-stations became possible when the devices were combined with monitoringand control software programmes, such as Micro SCADA. running on microprocessor

computers. This facilitated the fully integrated remote control of sub-stations.

They were also connected to. say. a Micro SCADA.based network control system thereby

allowingcomplete information exchange between the sub-station level and the network level.

Most numerical relays have a self-checking feature — they are capable of periodically

checking their hardware and software and in case of any problem. the relay gives an alarm fin

corrective action. It is easy to service a numerical relay as in most cases, it requires only

certain cards to be replaced. Many relays have built-in diagnostics so that the can identify

their own faults, thereby making repairs easy. The information can be accessed quickly and

effectively by means of the remote communication links resulting in improved post -fault

analysis of the faults. Thc relay hardware has to withstand sub-station environment including

temperature, humidity and dust. besides the electromagnetic interference generated within the

sub-station as well as the effects caused by lightning strokes. Hence tests are conducted as per

IR:. and ANSI standards. A microcomputer system is generally built with separate bus

structures for all internal connections between the processor and memory devices for Faster

access, while the input/output devices can be slower in operation while using filter circuits

and physkal separation to prevent els-Lulea] noise from corrupt ng the processor operation.

The internal power supplies for the processor are 5 V dc/12 V de for the circuits while

external supply voltages are the station battery voltages which car be 24,48.110 or 220 V dc.

Usually ddcic convenes, arc used. The twitching frequency is over 80 kHz in order to keep

the magnitudes small in size. The isolation of various circuits is necessary in the relay cases

as the electric circuits operate at a very low power. Hence sufficient screening has to be

providedbetween vatious circuits:Mc progressof numerical relay development from the 1970.

to the present is indicated in Fig. 6.1





PROCESSING UNIT

For high speed applications, digital signal processors are used. They provide high speed

computation on the input data winds enables the designer to implement complex algorithms

for a variety of protection functions and to perform real time calculation of impedances.

superimposed quantities, analysis of power system waveforms. etc. The requirement of

processing power for any relay increase, as the implementation of the protection functions

becomes more and more algorithmic. The newer versions of these relays have the capability

to include communication, fault location and disturbance recording. The core of the

numerical relay is mark up of a CPU an with 8-16 Nt processor depending upon the

application. The programme memory is used for holding the algorithm and code associated

with the man-machine interface. The data mernory consists of RAM for storing the sample

rabies of use in the algorithm and thedisturbance recorder funaions.The relay settings are

stored as a kuinp database, which is usually an EEPRON1. These values stored in rhe





EEPROM are used for computing the results. The input block consists of an ADC and its

associated circuits for processing the analogue signals.

MAN-MACHINE INTERFACE (MMI) This allows the user to make the entry of relay

settings and to read the relay internal information. The MMI usually consists of an

alphanumeric LCD or LED display unit. Knobs and switches are provided thus allowing the

user to enter the information using the Menu mode. Normally different levels of securities are

usually provided by means of a password so that only authorised personnel are allowed to

make changes in the settings to the relay. Local printers are also provided to print out the

settings and any stored information on the fault conditions and disturbances. Features like

remote communication links remove the need for the operating personnel to visit the sub-

station for collecting the information on the disturbance. The remote communication feature

can be used to simulate the display on the relay at the remote terminal. Thus the information

is available at a central station and a decision can be taken about the expertise needed for

conducting the mainte-nance work.

COMMUNICATION IN PROTECTION RELAYS

The most common transmission medium is the RS 232 link, which allows communication

from one relay to a monitor PC. It is also possible to make the system multi-terminal. wherein

up to 32 relays can be connected to a central computer by using suitable i 'ICC( face

equipment.This interface standard is RS 485. which use, only one pair of lines to connect up

to a distance of I km. RS 485 is a balanced interface and is less susceptible to common mode

interference. Another method of interconnection is through fibre optics. which are capable of

high data rates and very good noise immunity. However, they arc expensive and require

special purpose transmittertheceivers. Hence their use is restricted to transmission lines.

Optical fibres are very fine strands of glass or plastic with the ability to transmit light. They

are made up of a glass core inside a glass adding. The refractive index of the glass cadding is

lower than that of the core. When a ray of light is incident on the junction of the core and the

cadding. total internal reflection occurs if the angle of incidence exceeds the Okla angle. If

the diameter of the core is quite large with respect to the wavelength of the light, there will be

a multitude of light ray paths down which the light ray will pass. The input electrical signals





are converted into light signals by using suitable transmitters. Optical fibres arc usually of

core diameters of 50 microns with a cladding diameter of 125 microns.

INFORMATION HANDLING WITH SUB-STATION MONITORING SYSTEM (SMS)

Numerical protection and control terminals have a high functionality and contain huge

amounts of information.,' number of parameters need to be set for deriving benefit from the

flexibility of the terminals. The SMS is a tool for structured handling of information. It cats

be used for setting parameters, monitoring service values and self-supervision status and

event handling, and the collection and evaluation of disturbance information. The functions

can be implemented locally at the station or remotely in a central office via a public telephone

network or a standard "civil' network. With direct access to selected information, the data

stored in the numerical devices can be utilised in a much more efficient way. This is shown in

Fig. 6.2. The use of suitable software facilitates the clubbing of a number of protection

functions in one relay.





UNIT - 5

STATIC OVER CURRENT, TIMER AND VOLTAGE RELAYS: Instantaneous over current

Relay, Definite time lag relay, inverse time over current relay, static timer relay, Basic relaycircuits, monostable delay circuits Single phase Instantaneous over voltage and under voltage

relays, instantaneous over voltage relay using Op-amp.

Introduction

10 Hours

Overcurrent relays fall under the categories of instantaneous and time-overcurrent relays. It

will he realized that in the development of static relays, the first relays to be converted to the

static form were the high speed differential and distance relays, while the directional relay

came next and the time-overcurrent relay the last in the developmental series. The reasons forthe simplest relay to be taken up last are not far to seek. The distance, directional and

differential relays are based on mathemati-cal analysis and It is easy to build a mathematical

model or the circuit. On the other hand the time •overcurrent relays built through induction

disc elements gave characteristics which are not amenable to simple mathematical analysis. In

fact the inverse definite minimum Vine (IDMT) characteristic has been evolved because of

the induction disc element, the stopc of the curve being determined by the saturation pro -

perties of the magnetic core of the electromagnet. The four points specified by the Ind ian

Standards Institution (1.5.1 I and the British Standards Institution (13.51) have been entirely





due to the above curve being accepted in practice Moreover, from the systems authorities,

there was no demand for a static time overcurrent relay as the induction disc element

presented a rugged type or relay needing very little attention or adjustments. Similarly, thehinged armature unit offered a rugged type for the instantaneous overcurrent relay. Static

circuits are on the other hand more complicated especially in the case of time-overcurrert

relays giving the hitherto accepted types of characteristics (e g. 'MIT, very inverse, extremely

inverse) as the circuit elements have to be manipulated to produce these characteristics as will

be shown in this chapter. Althou-gh complicated in circuit construction, static overcurrent

relays offer the advantages of ,vw CT. burdens (about 1.10th of those for induction disc

relays; anti reduced space on the Nee atIdtt:On to the general It is, however, felt that for some

years it is unlikely that all electro-magnetic overcurrent relays will be replaced by static ones

till such time that a digital computer is universally employed for relay setting and introduction

of automatic test methods bv the manufacturers.

Instantaneous Overcurrent Relay

The circuit is indicated in Fig 6.1. The input from the main C.T. is fed to an auxiliary C.T.

(preferably a transactor to 'convert current to a voltage) with a tapped secondary winding. The

secondary current is fed to a full-wave bridge rect ifier which is protected from transient

overvoltage Spikes by means of the 125-C1 filter. The bridge output is then fed to the base of

transistor T. The output is developed across the resistor R,

Fir.6.I Instantaneous °vacuum War and is smoothed by the capacitor C,. The transistor T, (et-

p-n) and T, ( p-np) a-.r normally cut oft When the base voltage of T, exceeds the pick-up

value set by the potentiometer P., the transistor T, conducts leading to the switching on of

transistor T, and the trip relay TR. Thermistor Thin the collector T, is for temperature

compensation, and diode D is for the protection of the trip relay coil. The current pick-up

adjustment is made by means of the auxiliary transformer taps and the Potentiometer P,. In





the case of instantaneous relays there is a tendency for OVCISCASiii• vity under transient

fault currents with D.C. components. This can be prevented by making the auxiliary

transformer saturate just above the pick-up value. The transient filter described above wellalso reduce this tendency Fast reset is usually not necessary in overcurrent relays since the

setting value is much higher than the rated current, unlike voltage relays whose settings are

near to rated values and hence need fast reset features.

Improved Version of the Relay An improved commercial version of the static instantaneous

over current relay is given in block diagram form in Fig. 6.2(a) and in detailed circuit form in

Fig. 6.2(b).

voltage is then compared against a pre-set pick-up value and if it exceeds it a signal is given

to the output transistor through an amplifier. The output transistor is then driven to saturation

energizing the output relay (trip relay). The circuit design is such that no d.e. biasing voltage

is necessary separately for the circuit operation. The detailed circuit shows the current from

the main C.T. being given to the primary of the transactor which produces a proportional ac.

voltage. The ratio of primary current to secondary voltage of the transactor is made adjustable





by providing taps on its secondary so as to obtain different current settings. This voltage is

then rectified through a bridge rectifier and smoothed partially. Partial filtering only is used

to maintain high speed of operation. A zener diode ZD, acts as a limiter of the rectifiedvoltage to spfe values even when the input current is very high under fault conditions. A

fixed portion of the rectified voltage (through a potential divider) is compared against the

breakdown voltage of another zener diode ZDI. If the rectified voltage is greater than the

reference voltage, output transistor T2 is driven to conduction through the amplifier stage TI-

R2, whereupon the trip relay is energized. Reverse-biased DI protects the output transistor T2

from high reverse voltages induced when the inductive output circuit is opened.





application is the differential protection of transformers to present maloperation during

magnetization in rush currents. Also, this is used as back-up relays for differential and

distance schemes. Definite time feature when used with inverse characteristic is more useful

and gists inserse with definite time characteristic. An example IT protection of induction

motors against overloads using extremely inverse curve (n 2L In such cases it is desirable to

base an instantaneous or definite time operation at current levels corresponding to blocked

rotor operation. Definite time is pre-ferred to instantaneous to serve as A check against short

MC asymmetrical currents. (b) Very inverse Ore 1) This is generally employed in eases

where the source impedance is much smaller than the line impedance. Because of the steep

nature of the curve, they permit the use of the same time multiplier setting for several relays

in series. This reduces time errors and OveltraVel so that time margin for grading can te

reduced They are more suitable for earth fault protection as there is n greater variation of zero

sequence currents with distance than with phase faults. (c) Extremely inverse (nw, 2) Fuse

coordination and thermal protec-tion of transformers and induction motors require such





characte-ristic. They are useful, in conjunction with negative sequence filters to protect

against unbalanced operation of generators. The occurrence of an outage resulting in a

complete stsridsull of refrigerator Motors, factory equipments and lighting load andsubsequent restoration is another situation where this characteris-tic is useful -after an outage,

if power is switched on suddenly, there is a sudden inrush of current as the impedance during

hot conditions is 10 times the impedance during cold conditions. Extremely inverse relays

can distinguish between this condition and fault condition, since the former decays very fast

(d) Third power Inverse relay (a — 31 This can be used advantage-ously for coordination of

H V. fuses, In such eases, the slope 3 characteristic provides greater selectivity on load pick-

up than slope 3.

(e) Octal invent relay (n-8) One application of this is for the protection of mercury arc

rectifiers. A general expression for the operating time of a time-current relay is KM I^ -- I*,

where / is the multiple of tap current, l, is the multiple of tap current at which pick-up occurs,

K is design constant, and M is the time multiplier setting. If the relay picks up at tap value of

current, the above equation is simplified as follows:

KM — Fry As already mentioned, because of the nonlinear design factors ol induction disc

relays, the time characteristics that have been accepted in India and the U.K. are as follows in

approximate mathematical form (for conversion to static form). Type Approximate

mathematical form 0.14 (a) IDMT t aye-cry 13.3 (b) Very inverse I w (/ — I) 130 (4)

Extremely inverse (P-1) The definite-time characteristic needs no equation as the time of

operation becomes constant after pick-up. IDMT curses accepted in the USA. are somewhat

less inverse at the low current end than the above. Initially there will be a tendency for power

system authorities to have SIMI< overturrent relays with time-current characteristics to match

the existing induction disc relays. Howeser, in future, the characteristics will be of the form r

= giving a straight line on a log- r — log I graph. Since this curve will no longer be

asymptotic to the pick-up value of current, the pick-up will be controlled by a separate unit.

(In fact this is the practice even with static relays giving the conventional time-current

characteristics). The IDMT curve may be altogether eliminated and the definite time portion

will then be provided by a separate unit. Such simplified time-current curves will save

considerable time in the calcula-tion of relay settings and computers could be employed for

the purpose. This will also facilitate automatic gang testing of relays with a master relay.





=

(c) Octal inver.u relay (n=B) One application of this is for the

protection of mercury arc rectifiers.

A general expression for the operating time of a time-current relay is

KM I = /"-/•

• where I is the multiple of tap current, 1.is the multiple of tap current at which pick-up occurs. K is design constant, and M is the time multipfiu

setting. If the relay picks up at tap value of current. the above equation is simplified as follows:

KM t = J•-1

As already mentioned, because of the nonlinear design factors ofinduction disc relays, the time characteristics t bat have been accepted in

India and tbe U.K, are as foHows in approximate mathematical form (for

conversion to static form).

Type

(a) IDMT

(b) Very inverse

(c) Extremely inverse

.Approximate

mathematical form

t C::::: _ c0.1 _ 4. _

(]11•01. 1)

13.5 I = (/-1)

80 t ([2-1)

The definit·e-time characteris-tic needs no equation as the time ofoperation becomes constant after pick·up. IDMT curves accepted in the

USA.. are somewhat Jess inverse at the low current end than the

above.

Initially there wHI be a tendency for power system authorities to have

static overcurrent relay.s with time urrent characteristics to match the

existing induction disc relays. However, in future, the characteristics will

be of the form t = {. giving a straight line on a log- t

-

log I graph.





- Fia. 14.17 Monostable multivibrator

Rt n

c 0

z,

Zz

Vp

Fie. 14.18 lC monostable circuit





Overvoltage and Undervoltage Relays

Instantaneous relays of the overvoltage and undervoltage type can be

built ou t of the ba ic Schmitt trigger or level detector ci r cuit . These areshown in Fig. 4.JO(a) and {b).

ln the case of the overvoltage relay, the em i tter bjas voltage across R

keeps the ransistor .T1 off wh le T2 is condu ting. When the voltage i npu

a t the bndge r cct1fier V," mcreases the preset magnitude, producing a

voltage ncross resistor R1 , it overcomes the emitter bias voltage and makes

T conduct turni ng off T:!. When T!! is turned off, An output voltage V., 1

is produced ncross the co11ector ofTt·

In the case of the undervoltage relay, the input bias voltage across R, tries to keep the transistor T 1 conducti ng, while the voltage input V,.





In

both cases. capacitor C is used for smoothing the input voltage Yin after full-wave

rectification. The relays described are of the instantaneous type. In case delays are requiredthese could he incorporated by R-C delay circuits as discussed under timer relays. In the case

of instantaneous overvoltage/undervoltage relays, it is important that the circuit should hase a

high reset ratio (i.e. drop-out/ nick-up).

Over-and Under-Voltage Relay (a) Single phase — Two types of circuits are taken. In Fig

4.11(a) the relay ha :3 a polarity protection diode (for d.c. input voltages) against incorrect

polarity. The voltage supplied to the relay is fed to a voltage divider and compared with a

reference voltage. Depending on the difference voltage the level detector and the amplifier

(IC) are operated and the em. output relay picks up. Operating values are set by altering the

resistance of the voltage divider with a potentiometer. Since the reference voltage is obtained

from the supply voltage, no separate auxiliary voltage is necessary. Resetting ratios are : Over

voltage: > 97%; and undervoltage: < 1037„. In Fig. 4 .11( b) it is a combined over/under

voltage relay. There are voltage level sensing transistor circuits. The circuit uses the so called

7:- stabilized zener diode chains. Auxiliary 24V d.c. is necessary. Other things are as follows.

Setting range-under: 80-95%. over: 105-120; operating time 160 ms; resetting ratio — over :





98-997-0', under: 101-102%; accuracy + 1% for ambient temperature variation between 4- 5

C and -1- 50°C and auxiliary voltage between 20V to 30V. Power consumption — measuring

circuit: 0.2VA at 220V or 380V; and auxiliary voltage: 3.8W, 24V d.c. (h) Three-phase--InFig. 4.II(c) it is a three phase relay — operating both for symmetrical and asymmetrical

decreases or increases in the voltage and for phase open circuit. The three-phase voltages are

rectified and smoothed and they are applied to a diode circuit which acts as an OR circuit at

operation and as an AND circuit at reset; at operation of overvoltage relay it senses the

highest of the three voltages; while at reset all three voltages must be lower than the set value.

The undervoltage relay has the opposite function. The two voltages obtained after the diode

circuits are appled to level sensing circuits which actuate em. output relays. The values of

various parameters involved are as inlInws





(C)

Fia. 4.11 OveJ·and under voltagerelays (a} and ( b) si ngle phase: a nd (c) three phase.

Separate auxiliary : 24V d.c. is necessary

Setting ranges : Overvohage: 105-120%

Resetting ratio

Undervoltage:

: Overvoltage:

Undervoltage:

70-90%

95-97%

107-110%

Power consumption : Measuring circuit: 0.4 VA per phase at rated

voltage

Auxiliary circu it: 4.2W

Operating time Overvoltage c;; 70 ms

Undervoltage < 200 ms

Operating value deviates by·Jess than ± 1% when ambient temperature

varies between - 5 and + S5°C and auxiliary voltage varies between 20

and JIV.





UNIT - 6 & 7

DISTANCE RELAY: General Principle of operation, Zone discrimination, Fault area on

impedance diagram, Basic measuring elements, Different characteristics used in distancerelaying- Impedance, Reactance, Admittance. Ohm, Distance relay settings, Distance

measurement Problems.

General Principle of operation:

10 Hours

In electrical engineering, a protective relay is an electromechanical apparatus, often with more

than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit

breakers when a fault is detected. Unlike switching type relays with fixed and usually ill-

defined operating voltage thresholds and operating times, protective relays have well-established, selectable, time/current (or other operating parameter) operating characteristics.

Protection relays may use arrays of induction disks, shaded-pole magnets, operating and

restraint coils, solenoid-type operators, telephone-relay contacts, and phase-shifting networks.

Protection relays respond to such conditions as over-current, over-voltage,

reverse power f low, over- and under- frequency. Distance relays trip for faults up to a certain

distance away from a substation but not beyond that point. An important transmission line or

generator unit will have cubicles dedicated to protection, with many individual

electromechanical devices. The various protective functions available on a given relay are

denoted by standard ANSI Device Numbers. For example, a relay including function 51would be a timed overcurrent protective relay.

Electromechanical protective relays at ahydroelectric generating plant. The relays are in

round glass cases. The rectangular devices are test connection blocks, used for testing and

isolation of instrument transformer circuits.

The theory and application of these protective devices is an important part of the education of

an electrical engineer who specializes in power systems. In new installations, these devices

are nearly entirely replaced with microprocessor-based digital protective relays (numerical

relays) that emulate their electromechanical ancestors with great precision and convenience

http://en.wikipedia.org/wiki/Electrical_engineering

http://en.wikipedia.org/wiki/Relay

http://en.wikipedia.org/wiki/Electric_power

http://en.wikipedia.org/wiki/ANSI_Device_Numbers

http://en.wikipedia.org/wiki/Hydroelectric

http://en.wikipedia.org/wiki/Power_engineering

http://en.wikipedia.org/wiki/Digital_protective_relay

http://en.wikipedia.org/wiki/Digital_protective_relay

http://en.wikipedia.org/wiki/Power_engineering

http://en.wikipedia.org/wiki/Hydroelectric

http://en.wikipedia.org/wiki/ANSI_Device_Numbers

http://en.wikipedia.org/wiki/Electric_power

http://en.wikipedia.org/wiki/Relay

http://en.wikipedia.org/wiki/Electrical_engineering





in application. By combining several functions in one case, numerical relays also save capital

cost and maintenance cost over electromechanical relays. However, due to their very long life

span, tens of thousands of these "silent sentinels" are still protecting transmission lines and

electrical apparatus all over the world.

Operation principles

Electromechanical protective relays operate by either magnetic attraction, or magnetic

induction.

"Armature"-type relays have a pivoted lever supported on a hinge or knife-edge pivot, which

carries a moving contact. These relays may work on either alternating or direct current, but

for alternating current, a shading coil on the pole is used to maintain contact force throughout

the alternating current cycle. Because the air gap between the fixed coil and the moving

armature becomes much smaller when the relay has operated, the current required to maintainthe relay closed is much smaller than the current to first operate it. The "returning ratio" or

"differential" is the measure of how much the current must be reduced to reset the relay.

A variant application of the attraction principle is the plunger-type or solenoid operator.

A reed relay is another example of the attraction principle.

"Moving coil" meters use a loop of wire turns in a stationary magnet, similar to

a galvanometer but with a contact lever instead of a pointer. These can be made with very

high sensitivity. Another type of moving coil suspends the coil from two conductive

ligaments, allowing very long travel of the coil.

"Induction" disk meters work by inducing currents in a disk that is free to rotate; the rotarymotion of the disk operates a contact. Induction relays require alternating current; if two or

more coils are used, they must be at the same frequency otherwise no net operating force is

produced.

Protective relays can also be classified by the type of measurement they make. A protective

relay may respond to the magnitude of a quantity such as voltage or current. Induction types

of relay can respond to the product of two quantities in two field coils, which could for

example represent the power in a circuit. Although an electromechanical relay calculating the

ratio of two quantities is not practical, the same effect can be obtained by a balance between

two operating coils, which can be arranged to effectively give the same result.

Several operating coils can be used to provide "bias" to the relay, allowing the sensitivity ofresponse in one circuit to be controlled by another. Various combinations of "operate torque"

and "restraint torque" can be produced in the relay.

By use of a permanent magnet in the magnetic circuit, a relay can be made to respond

differently to current in one direction than in another. Such polarized relays are used on

direct-current circuits to detect, for example, reverse current into a generator. These relays

can be made bistable, maintaining a contact closed with no coil current and requiring reverse

current to reset. For AC circuits, the principle is extended with a polarizing winding

connected to a reference voltage source.

http://en.wikipedia.org/wiki/Magnetic_attraction

http://en.wikipedia.org/wiki/Magnetic_induction


http://en.wikipedia.org/wiki/Reed_relay

http://en.wikipedia.org/wiki/Galvanometer

http://en.wikipedia.org/wiki/Galvanometer

http://en.wikipedia.org/wiki/Reed_relay




http://en.wikipedia.org/wiki/Magnetic_attraction





Light weight contacts make for sensitive relays that operate quickly, but small contacts can't

carry or break heavy currents. Often auxiliary telephone-type armature relays are triggered by

the measuring relay.

In a large installation of electromechanical relays, it would be difficult to determine which

device originated the signal that tripped the circuit. This information is useful to operating

personnel to determine the likely cause of the fault and to prevent its re-occurrence. Relays

may be fitted with a "target" or "flag" unit, which is released when the relay operates, to

display a distinctive colored signal when the relay has tripped.





Lfllt trap flldacttlllet • l()()v.H : N'•pfl111cqtJ£ i tor • 10()() pF

Band

2

J

.. s

0

7

I

9

10

Mldba11d f"qt urry Btlltdwld t hkHr kHz

75., 7 11

17.S I0-9S

99.5 90-100

112 JOG-125

125 110.1<10

138 12 H8

HI 130·175

179 15 214

22S 11 280

)SJ 2S SOO





The fourth closing side of the characteristic is obtained by the zone differentiating circuit as

shown in Fig. 10.17. For faults within zone I reach, zone I — AND gate opens at the arrival of

the fart pulse and goes to the output stage via the OR gate. For faults within zone 2 and zone 3

reach, the respective gates open after required time delays. The block diagrams of the

complete relay circuits arc shown in Fig. 10.18 (a) and (b). Two alternatives have been given

for zone 2 and zone 3 discrimination.





Wramt I Fig. 10.18(01 A -• Pulse circuit B - Buffer amplifier CWel swing detector I (to be

described later) ••• Power swing blocking E - Phase comparator F VT G - Phase splatter and

rectifier H'- RC terser I - Threshold detector timer ••, PAT - Pulse amplifier and thyristor sou

Lit 2 If tg. 10.13 (b)I Significance of various blocks are the 411Tte as in whew I except the

following: •- Monostable timer I. — Backward monmlable circuit Some of the detailed

circuits are shown in Fig. 10.19. In Fig.10.19(a) the input circuits are shown. The auxiliary

VT has six secondary wind. rigs. One of them has two taps K. and K, for zone land zone 3

measure -roans. Phase splitting network with diodes is shown in Fig. 10.19 (b) for obtaining

direct voltages with loss ripple content. Pulse shaping and phase detector circuits arc shown in





Fig. 10.19 (c). Under normal no fault conditions T, and T, are conducting and the pulse

arriving at the collector cf T, is shorted. When a fault occurs inside tripping,' area, input

voltages V. VI. -5C' and (h R VI) ha-Come positive simultane-ously when the sampling pulseis present. Transistors T, and T. stop conduction, allowing the pulse logo through. Power

swing Mocking-Here either rate of rive of current or blinders can be used for detection of

power swings. In the method using blinders, the sampling tuts: hta is taken through another

gate with control Input positive 'and' of (Vt + leR.) and (leR, - Vt) and the NAND of a the

phase detector. The output from this gate feeds the monostable multisibrator which blocks the

phase detector (transistor T,) for a preset time. Now, R, is taken larger than the maximum

setting of R (for the phase detector). Steady lamp ind•c+- lion or continuous alarm for

uncleared faults in the non-tripping dire-ction of the relay or when a power swing is in the

buffer area without receding or passing through the trip area of the relay, can be ptosided as

shown This is shown in Fig. 10.19(d). In the rate of rise current detection, normally the phase

detector AND gate is closed by the nrgato es bias provided by the flip-lop circuit. In the tvert

of a fault, the high value of dIrMr produces • pulse of height proportional to Wadi thioughing

the flip-flop into another state of stability and brings the relay into operation This is shown in

Fig. 10.19(e).





Zone differentiator — Zone I AND gate is an instantaneous amplitude comparator — the outputfrom this triggers a thyristor sia an OR gate and a pulse amplifier.

Zone 2 and Zone 3 time delays are provided by means of (i) RC circuit or (ii) by moncstable

circuit. In the RC timing method, a level detector is used to measure the voltage across the

capacitor, while in the monos-table circuit method, delay is obtained by it s sett ing. A

backward mono-stable circuit is used with an AND gate to prevent the output of the

monostable circuit from unwanted triggering of the thyristor, in case the fault is cleared by the

earlier zone units. TYPE 2: In this type of multi-input comparator two phase detectors are

used one for zone I and the other for zone 2 and zone 3. Zone





Zone I operation : Angle 8, of Z., is made equal to fine angle 8, Ye is converted to a negative

pulse at its zero point. The sampling pulse will appear at the output only if all the inputs S„

.S'„ and S, are simul-taneously positive when the pulse appears. The angular limit of the

phase comparison p -- 8,). Operating criterion is sin (I, + • — 4) Ze < and sin,

tripping limits are 8,<4<8, The trip area is controlled by ft and', For the case of 90° shift, the

operating criterion is Ze<Zat, cos (8, Oland el<4<e, Thus for the same value of 5, the trip area

as compared with that for r 90'. can be increased or decreased by adjusting r< or ....90° ; the

maximum value being (180' — 8,), respectively. Zone 2 and :one 3 operation The inputs are

•=ii fare, (pulse) S. (hR — Ye) S, — S. — — 90* This comparator is exactly similar to the one

in type I previously discussed and hence needs no further discussion. Zone differentiation is

provided as before. Complete Relay Circuit--The complete relay circuit in block form is

shown in Fig. 10.21. The AND gates are normally blocked by the nega-tive bias provided by

the flip-flop circuit. The sudden collapse of Ye under a fault triggers the flip-flop thereby

removing the blocking — this prevents false operation when voltage is first applied or when it

recovers after the clearing of a nearby fault. For Zone I fault, AND gates of comparator D and

M, open at the arrival of sampling pulse Vt. This finally triggers the thyristor. SCR can be

reset by the auxiliary contact on the CB. A chain of flip-flop circuits (F) arranged for negative





triggering is used to provide time delay T, for zone 2 — the time delay can be counted in

terms of the number of cycles since the sampling pulse is produced once every cycle. The

delay provided by a chain of three flipiflop circuits is 8 cycles. For zone 3 time delay, amonostablc circuit is used. A backward monostablc circuit is used as in type I for preventing

unwanted triggering nf SCR In Fir,. 10-71. the vannut hIneka are as. (Allows :

The relay makes measurement Once every cycle at the appearance of the sampling pulse, i.e.

when Pc passes from positive to negative. The time required for operation depends only on

the angle after 180° at which the fault occurs. Faults which occur just before 180° have nearly

zero operating t ime while fault occurring just after 180° have a one cycle operating t ime. Theoperating time can be reduced to half-cycle by duplicating the circuitry for the other polarity

so that phase angle measurement is made on both half cycles. With 100% d.c. transient, the

range of the relay could reach a value of about 50 and the transient overreach less than 10%.

(iii) 4 AloOed Technique for Quadrilateral Distance Relay The principle of this method is

illustrated in Fig. I0.22(a) and (b). If a point P, is defined by the phasor Zr, and the

quadrilateral ch•racte.istic is defined by the extremsties of the phasors 4, Z., and Z,, then the

relaying inputs are





S, — Zi. a la, — kit — S. et Z. - Zt liZ, — Vt. 5, a Zt. hZ0 — S, — — where Vi. hZt It can be

stated that whenever the point P, is in the restraining region, all the relaying inputs are

confined to 180° phase angle margin, while if the same point is in the trip region [as Pt in Fig.10.22 (b)) all the relaying

inputs are conned to more than ISO' phase angle margin as shown in the figure. There will be

finite coincidence period in the restraining region governed by two extreme phasors, while in

the tripping region there is no coincidence period. The above is realized by circuits shown in

Fig. 10.23. Fig. 10.23(a) shows the block diagram. The relay input* are derived by the help of

replica impedance Z., Z., 4 and an auxiliary V.T. as shown in Fig. 10.23 (b). These inputs are

fcd to a coincidence gate, which delivers output pulses periodically, every 20 ms based on a

power frequency of SO Ile for 7,e in the restraining region. The pulse stretcher, which is a 20

ms monostable triggering on the leading edge of the input pulses, delivers continuous output

to energize is slave relay having normally closed con-tacts. The contacts are thus kept open

for the restraining region. When Ze falls in the operating region, the coincidence gate and the

pulse stretcher deliver no output resulting in the drop-out of the slave relay — closing its

contacts. Fig. 10.23(c) shows the transistor circuitry consis. ring of four clippers, and NAND

gate. a monostable gate as a 20 ms pulse stretcher and the slave reed relay. It is claimed that

this technique require: only 4 inputs, has an accuracy of I0Ø up to a range of 30 with a

maximum transient overreach of 7%. The VA burdens have been claimed to be very low e.g0.4 VA and 0.5 VA on the V.T. and CT.. respectively. It may be noticed that the circuits

stated above are more complicated. In Western Europe, where the quadrilateral re/ay has

gained popularity, the normal practice is to separate the direction and distance measure-ments

by having two restricted directional characteristics looking in opposite directions (these are

AND compounded). This permits the use of sound phase voltages for directional

measurements resulting in the following advantages (i) Unlimited directional sensitivity in the

esent of as)numerical faults.





(i) Unlimited di rect i onal ensitivity in the event of asymmetr ical

faults.

PhOM Sl!lflinv

8

5-limf!\otion

(ill

Fig. 10.2J Cireu it1 (or quadrilater a l diMance rday: (a) blOC>)( drarrom·

C b) dc:rivnt ion of relay in puts; and fc) del:lied circuli. '





Here Io Zr produces a negative pulse at its zero point and is applied to a phase detector AND

circuit. This pulse will appear at the output if Vo L90' is positive at the same time — this gate

operates a directional element. The pulse is then applied to a zone differentiation circuit —

these gates arc controlled by rectified voltage I I KV:. I — I (IL + 3Kqe. VA for amplitude

comparison. The characteristic obtained is also shown in the same figure.





UNIT - 8

DIGITAL RELAYS: Block Schematic approach of microprocessor based relays, over current

relay Protection, Transformer differential protection, Directional relay scheme, Impedancerelay scheme.

6

Hours

Over Current Relays: An over current relay is the simplest form of protective relay which

operates when the current in any circuit exceeds a certain predetermined value, ie the pickup

value. It is extensively used for the protection of distribution lines, industrial motors and

equipment. Using a multiplexer, the micro processor can sense the fault currents of a number

of circuits. If the fault current in any circuit exceeds the pickup value, the microprocessorsends a tripping signal to the circuit breaker of the faulty circuits. As the microprocessor

accepts signals in voltage form, the current signal derived from the current transformer is

converted into a proportional voltage signal using a current to voltage converter. The ac

voltage proportional to the load current is converted into dc using a precision rectifier. Thus

the microprocessor accepts dc voltage proportional to the load current.

The bloack schematic diagram of the relay is shown in fig. The output of the rectifier is fed to

the multiplexer. The microcomputer sends a command to switch on the desired channel of the

multiplexer to obtain the rectifier voltage proportional to the current in a particular circuit.

The output of the mux is fed to the A/D converter to obtain the signal in digital for.





microcomputer reads the end of conversion signal to examine whether the conversion is over

or not. As soon as the conversion is over, the microcomputer reads the current signal in digital

form and then compares it with the pick-up value. In the case of a definite time ovcrcurrentrelay, the microcomputer sends the tripping signal to the circuit breaker after a predctumined

time delay if the fault current exceeds the pick-up value. In case of instantaneous overcurrent

relay there is no intentional time delay. In order to obtain inverse-time charac-teristics, the

operating times for different values of currents are noted for a particular characteristic. These

values are stored in the memory in tabular form. The microcomputer first dettsmints the

magnitude of the fault current and then selects the corresponding time of operation from the

look-up table. A delay subroutine is started and the trip signal is sent after the desired delay.

Using the same program, any characteristic such as IDMT, very inverse or extremely inverse

can be realised by simply changing the data of the look-up delay subroutine is started and the

trip signal is sent after the desired delay. Using the same program, any characteristic such as

IDMT, very inverse or extremely inverse can be realised by simply changing the data of the

look-up table according to the desired characteristic to be realised. The microcomputer

continuously measures the current and moves in a loop and if the measured current exceeds

the pick-up value, it compares the measured value of the current with the digital values of

current given in the look-up table in order to select the corresponding count for a time delay.

Then it goes in delay subrou-tine and sends a trip signal to the circuit breaker after the

predetermined time delay. The program flowchart is shown in Fig. 8.1(b) and the program is

as follows.









Memory Machine Address Oxles

201E 21,00,22

Label Mnemonics Operands

LXJ H,2200

Comments

2021 46 MOV B,

M Count for lookup tablein reg. B.

2022 23 SEEK lNX

2023 BE CMP 2024 D2-.2E,20 JNC

2027 OS OCR 2028 C2A20 JNZ 202-B CA,04,20 JZ

202E 24 AHFAD [NR

202F 46 MOV 2030 OE,FF BBHlNO MVl

2032 16.FF CHAIN MVl

2034 15 MOVE DCR 2035 C2,34,20 JNZ

2038 OD OCR 2039 C2,32,20 JNZ 203C 05 OCR 2030 C2,30,20 JNZ

2040 3E,Ol MVl 2042 DJ,Ol OUT

2044 76 HLT

Look-up Table

H M AHEAD

B

SEEK MESH H B,M Count for delay in B.

C.FFD,FFD

MOVE c CHAIN B

BEHIND

A.Ol 0 1 Send tri p signal.

Memory

Address

Digital Values

of Current

Memory

Address

Count for Delay

in Register B

Delay Time

ins 2200 OC(a:>UNl)

2201 7F' 2301 3 0.19 2202 7A 2302 5 0.32

2203 60 230.3 9 0.51 2204 66 2304 OB 0.70

2205 60 2305 OD 0.&3

2206 SA 2306 tO 1.00

2207 53 2307 16 .1.40

2208 40 2308 lE 1.90

2209 46 2309 2D 2.90 220A 4(1 230A 50 5.10

2208 3A 2308 7D 8.00

220C 33 230C DB 14.00

In order to llvoid false tripping of an overcurrcnt relay due to transicniS the

program can he modincd slightly.When the fault current exceeds the pick-up

value., the fault current i.s measured once again by the microprocessor to

confirm wbetber it is a rault current or tran.'iient. ln case of any ttan,.!cai of

sbort dumtion, the mea.orurcd currcntabovcpick wup valuewillool appear in the

secund measwement. But if lhete is an actual fault, it wiII again appear in thesecond measurement also, and then the microproc.e&'>Or will issue a tripping

signal to disconnect the faulty pan of lhe system.1be program flowchart is

shown 1n F ig. 8.2 and the program is given below.





Transformer Differential Protection:

Generally Differential protection is provided in the electrical power transformer r ated

more than 5MVA. The Differential Protection of Transformer has many advantagesover other schemes of protection.

1) The faults occur in the transformer inside the insulating oil can be detected by

Buchholz relay. But if any fault occurs in the transformer but not in oil then it can not

be detected by Buchholz relay. Any flash over at the bushings are not adequatelycovered by Buchholz relay. Differential relays can detect such type of faults.

Moreover Buchholz relay is provided in transformer for detecting any internal fault inthe transformer but Differential Protection scheme detects the same in more faster

way.

2) The differential relays normally response to those faults which occur in side the

differential protection zone of transformer.

http://www.electrical4u.com/electrical-transformer/index.php

http://www.electrical4u.com/electrical-transformer/index.php





Differential Protection Scheme in a Power Transformer

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique. The

differential relay actually compares between primary current and secondary current of

power transformer, if any unbalance found in between primary and secondary currents

the relay will actuate and inter trip both the primary and secondary circuit breaker of

the transformer.

Suppose you have one transformer which has primary rated current I p and secondary

current Is. If you install CT of ratio I p/1A at primary side and similarly, CT of ratio

Is/1A at secondary side of the transformer. The secondaries of these both CTs are

connected together in such a manner that secondary currents of both CTs will oppose

each other. In other words, the secondaries of both CTs should be connected to samecurrent coil of differential relay in such a opposite manner that there will be noresultant current in that coil in normal working condition of the transformer. But if anymajor fault occurs inside the transformer due to which the normal ratio of the

transformer disturbed then the secondary current of both transformer will not remainthe same and one resultant current will flow through the current coil of the differentialrelay, which will actuate the relay and inter trip both the primary and secondary circuit

breakers. To correct phase shift of current because of star - delta connection oftransformer winding in case of three phase transformer, the current transformer

secondaries should be connected in delta and star as shownhere.

At maximum through fault current, the spill output produced by the small percentage

unbalance may be substantial. Therefore, differential protection of transformer should

be provided with a proportional bias of an amount which exceeds in effect the

maximum ratio deviation.

Directional Over Current Relay:





The classic electromechanical and solid state relay, as well as some common numeric

relays, determines the direction to fault by comparing the phase angle relationship of

phase currents to phase voltages. If only per phase watt flow (32 element) is to be

considered, the basic concept would be that if IPh is in phase with VPh-N (0°, ±90°),then power flow on that phase is indicated as forward (or reverse, depending on one’s

perspective). However, for a phase to ground fault, the VPh-Nmay collapse to 0, and I

may be highly lagging, so that VPh-N x IPh may be mostly VAR flow, and thus prevent the relay from making a correct directional decision. To resolve the low

voltage issue, quadrature voltages (i.e., VBC vs. IA) are commonly used. To resolve

the issue that fault current is typically highly lagging, the relay current vs. voltage

detection algorithm is skewed so that the relay is optimized to detect lagging current

conditions rather then 1.0 power factor conditions. One approach, seen in Fig. 1, is to

phase shift the voltage signal so that the relay’s internal voltage signal (VPolarity,abbreviated as VPol) is in phase with current when current lags the 1.0 power factor

condition by some setting, typically between 300 and 900.

The angle setting is commonly referred to as the maximum torque angle, MTA. In

some designs of this concept, the current signal is skewed rather than the voltage

signal. In some designs, other phase voltages are used. For instance, IA could be

compared to VAB, VCA, VBN, or VCN, and the detection algorithm would work,

though the quadrature voltage VBC gives the most independence of the voltage signal





from the effects of an A-N, A-B, or A-C fault.

OVERCURRENT AND DIRECTIONAL ELEMENT NAMES AND CONTROL

One needs to understand which directional decision controls which overcurrent

element. There is no standard way to name all of the overcurrent elements that are

involved. Assume for the discussion that there are 67/51P (phase), 67/51G (ground)and 67/51Q (negative sequence) elements and similar 67/50 elements, and that each

has a forward or reverse looking mode with different settings for each direction. Thereare three directional elements called the 67POS (positive sequence), 67NEG (negative

sequence), and 67ZERO (zero sequence) that control the 67/51 and 67/50 elements.The protective elements and their directional controls are:

A given relay may have more than one copy, or no copy, of the indicated element, anda given relay may or may not give the user direct access to 67POS, 67NEG, and

67ZERO.

IMPEDANCE RELAY SCHEME





Impedance relays are used whenever overcurrent relays do not provide adequate protection.

They function even if the short circuit current is relatively low. The speed of operation is

independent of current magnitude.

Impedance relays monitor the impedance between the relay locat ion and the fault. If the

impedance falls within the relay setting, the relay will operate. The basic construction forimpedance relays on which the principle of operation is easily explained is the balanced

beam.

Figure: Principle of Impedance Relay

The relay consists of a balanced beam. At each end of the balanced beam is a coil that exerts

a force on the beam at that end. One coil is connected to a current from a current transformer,the other coil is connected to a potential transformer. The voltage coil functions as a

restraining coil, the current coil functions as an operating coil. Under normal conditions, the

contact of the relay is kept open. During a fault, the voltage drops, and the current rises. The

torque due to the current coil overpowers the torque due to the voltage coil, and the relay

closes its contact.

The torque caused by the current through the current coil is

where K i is a constant of proportionality that depends on the relay construction

The torque caused by the voltage coil is

where K v is a constant of proportionality that depends on the relay construction

When the torques are balanced,

The ratio of voltage and current is the impedance the relay detects at the point of its

connection.





To close its contacts,

The contacts will close anytime the impedance the relay sees is less than a preset value given

by

This can be represented on an impedance graph X vs R

Figure: Operating Diagram of an Impedance Relay

This type of impedance relay is not directional. It will detect a fault in any direction. If it is

used, it is used together with a directional relay that eliminates half of its characteristic.

Figure: Operating Diagram of an Impedance Relay with a Directional Unit

OFFSET IMPEDANCE RELAY





Offset impedance relay is also known under names ADMITTANCE RELAY or MHORELAY

Phasor Diagram

The torque of the watt element is

construction

where K w is a constant of proportionality that depends on the relay

The torque caused by the voltage coil is

where K v is a constant of proportionality that depends on the relay construction

Relay operates if





The phase angle q depends on impedance Z s . Zs can be chosen so that q = 0. Also, both

vo ltage coils are connected to the same vo ltage V. ( )

It follows that

Similarly as for the impedance relay, this impedance can be represented by an X vs R graph.

This time the circle is offset from the center.

Figure: Operating Diagram for an Offset Impedance Relay with Characteristic Angle equal to

0

If impedance Zs is chosen so that q ¹ 0, the circle shifts:

Figure: Operating Diagram for an Offset Impedance Relay with Characteristic Angle

Different from 0

For impedance relays detecting short circuits on transmission lines, impedance Z s is chosen

so that q is the same as the impedance angle of the line. This relay will detect a fault in only

one direction.





RESISTANCE AND REACTANCE RELAYS

The torque of the watt element is

where K w is a constant of proportionality that depends on the relay construction

The torque caused by the current coil is

where K i is a constant of proportionality that depends on the relay construct ion

Relay operates if

The phase angle q depends on impedance Z s . Zs can be chosen so that q = 0. Also, both

current coils are connected to the same current I.

It follows that

The operating characteristic of this relay is a straight line





Figure: Operating Diagram for a Resistance Relay

Figure: Operating Diagram for a Reactance Relay

All distance relays are connected to the power system through instrument transformers. The

relay monitors the impedance in secondary ohms. Secondary ohms are related to the primary

ohms by the equation

where CTR is the current transformer ratio

PTR is the potential transformer ratio

Zones of Protection

In general, distance protection includes three steps of protection, with each step reaching a

fixed preset distance and operating in a preset time.




Zone 1 reaches 80 - 90% of the protected line. The tripping is instantaneous.

Zone 2 extends beyond the protected line up to about 50% of the adjacent line. The tripping

has a time delay, usually set to a value between 0.3 s to 0.5 s.

Zone 3 covers the protected line, the adjacent line, and up to 25% of the line next to theadjacent line. Tripping is delayed between 0.6 s to 1.0 s.

Documents

Eee-Viii-modern Power System Protection [06ee831]-Notes