DIRECTIONAL PROTECTION - lrf.fe.uni-lj.silrf.fe.uni-lj.si/e_rio/Seminarji1718/SmernaZascita.pdf · Protection equipment has the basic role of detecting an electrical fault ... whenever

UNIVERSITY OF LJUBLJANA

FACULTY OF ELECTRICAL ENGINEERING

DIRECTIONAL PROTECTION

Seminar work in the course Distribution and industrial networks

Mentor: Author:

Prof. Grega Bizjak Amar Zejnilović

Ljubljana, May, 2018.


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Abstract

Power system protection is extremely important in order to achieve satisfactory level of

reliability and security of the distribution system. One of the most used protection that is used

in distribution networks is overcurrent protection. In traditional, radial, distribution networks

this overcurrent protection was enough. But in modern networks with more than one power

source, or in networks that have multiple lines for conveying electrical power the directional

protection is essential in order to achieve selectivity. Directional overcurrent relaying refers

to relaying that can use the phase relationship of voltage and current to determine direction of

a fault. Generally, there are three types of directional protection: phase directional protection,

earth fault directional protection and active and reactive directional power protection and all

three groups will be explained in this seminar paper.


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Contents 1. Introduction .................................................................................................................................... 5

2. Distribution network with 2 or more power sources ..................................................................... 5

2.1 Principle of operation ............................................................................................................. 7

2.2. Characteristic angle .................................................................................................................... 11

2.3 Directional relay construction ............................................................................................... 14

3 Protection of parallel distribution lines ........................................................................................ 16

4 Protection of closed loop .............................................................................................................. 19

5 Earth fault directional protection ................................................................................................. 20

6 Power relays .................................................................................................................................. 22

7 Conclusion ..................................................................................................................................... 23

References: ........................................................................................................................................... 24

QUESTIONS ........................................................................................................................................... 24

EXAMPLE: .............................................................................................................................................. 25


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Figure 1: A radial distribution system with 2 power sources ................................................................. 6

Figure 2: short circuit protection on a network with a two sources ...................................................... 6

Figure 3. (b): current flows in forward direction .................................................................................... 7

Figure 4. (a): circuit diagram ................................................................................................................... 8

Figure 5: Selecting a characteristic angle to properly align the forward and reverse direction zones of

the directional overcurrent relay ............................................................................................................ 9

Figure 6: polarizing voltage of (a) phase "A “and (b) phase "C" for a current in phase "A" and phase

"C" respectively [5] ............................................................................................................................... 10

Figure 7: Directional protection tripping zones on (a) phase "A" and (b) phase "C" for a θ=45°[5] .... 10

Figure 8: Range of phase angle values of current IA expected for a phase-to-ground fault on phase A

.............................................................................................................................................................. 11

Figure 9: Ranges of phase angle values of current IA expected for phase-to-phase faluts between

phases A and B and between phases A and C ...................................................................................... 12

Figure 10: Range of phase angle values of current Ia expected for faults on phase A ......................... 12

Figure 11: setting the characteristic angle to 45° properly aligns the forward and reverse direction of

the directional overcurrent relay with the range of phase angle values expected for the fault currents

.............................................................................................................................................................. 13

Figure 12: setting the characteristic angle to 30° and 60° properly aligns the forward and reverse

direction of the directional overcurrent relay with the range of phase angle values expected for the

fault currents: ....................................................................................................................................... 13

Figure 13: Two lines in parallel to convey power to a substation ........................................................ 16

Figure 14: protection of two parallel line using just overcurrent relays [4] ......................................... 17

Figure 15: Directional overcurrent protection of two power lines connected in parallel .................... 18

Figure 16: protection of a closed loop using directional relays and time-based selectivity ................. 19

Figure 17: measuring the residual current using 3 CT’s (figure above), and using a ring (figure below)

.............................................................................................................................................................. 20

Figure 18: measuring the residual voltage using 3 VT with two secodary windings (a), and using

auxiliary VT’s (b) .................................................................................................................................... 21

Figure 19: Layout diagram for measuring power [1] ............................................................................ 22


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1. Introduction

Protection equipment has the basic role of detecting an electrical fault and disconnecting

that part of the network in which the fault occurs limiting the size of the disconnected section

as far as possible. In modern medium-voltage (MV) distribution lines and in almost all high

voltage transmission lines, a fault can be in two different directions from a relay and it is

highly desirable for a relay to respond differently for faults in the forward or reverse

direction. In fact, in almost all situations the relay should respond only when the fault is on

one side, while for failures on the other side it remains inactive. And because of this, the

usage of directional protection is important in order to avoid disconnection of unnecessary

circuits. As normal overcurrent relays cannot provide this function, a directional unit is added

to activate the relay when the fault current flow is in a predetermined direction.

Directional protection enables better discrimination of the faulty part of the network than

with overcurrent protection. It is necessary to use it in the following conditions: [1]

in a system with several sources

in closed loop or parallel-cables systems

in isolated neutral systems for the feedback of capacitive current

and to detect an abnormal direction of flow of active or reactive power

(generators)

Directional protection is used for all network components in which the direction of flow

of power could change, for example for the short circuit between phases or for an earthing

fault (single phase fault): [1]:

phase directional protection is installed to protect two connections operated in

parallel, a loop or a network component connected to two power sources

earth fault directional protection is sensitive to the direction of flow of the

current to earth. It is necessary to install this type of protection equipment

whenever the phase to earth fault current is divided between several earthing

systems.

active and reactive directional power protection equipment is used to detect

abnormal power flow other than the one due to a short circuit; e.g.: in the

event of the failure of the prime mover, a generator will continue to run as a

synchronous motor, drawing power form the system.

The IEEE device number used to signify a directional element is 67-directional

overcurrent, generally based on the phase relationship of V (voltage) and I (current), with no

distance to fault capability. [2]

In the following pages of this seminar paper, all above mentioned usages of directional

protection will be describe in more details and while describing the usages, the operating

principle of relay will be explained as well.

2. Distribution network with 2 or more power sources

In this chapter, the usage of directional protection will be described on a network with 2

power sources. The problem, as well as the solution, will be described using the figure 1

which describes already mentioned distribution network:


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Figure 1: A radial distribution system with 2 power sources

Figure 1 shows the system which is radial but it has two sources connected to it. A and B

are power sources, C,D and E are busbars, F1 and F2 are places where short circuit happens

and the green and red squares are overcurrent relays. The circled relays are also directional

overcurrent relays, but for better explanation of the directional relays, it will be assumed that

are these regular overcurrent relays. With this assumption, the fault F1 will be analysed.

When short circuit F1 happens, both power sources supply this fault with current. It is

obvious that red relays should open and on that way separate part of the network where is

short current from the rest of the network. But without directional relays, it is a high

probability that one of the green relays would open before circled red relay and on that way

selectivity is not fulfilled because the distribution line between busbars C and D is out of

function although the fault isn’t on that distribution line. To overcome this problem, the relay

element has to be provided with additional discrimination feature to distinguish between

faults that it should respond to, and others that it shouldn’t. So, instead of regular overcurrent

relays, the circled relays on the figure 1 should be directional relays and on that way green

circled relay would never open when the fault is F1, as well as the red circled relay would

never open when the fault is F2. There is still one problem in this network that should be

solved, and that is: how to avoid the unnecessary operation of the green relay that is not

circled when the fault is F1? The reason why it’s not possible to use directional relays here is

that if this relay is directional relay it would never open, even when it should react, for

example when the fault is F2. The answer on previous questions could be seen on a figure 2:

Figure 2: short circuit protection on a network with a two sources

As it can be seen, the solution for the previous problem is to use overcurrent relays with

time delay because of selectivity. The operational time for each individual relay is given just

above them. The pointer above directional relays determines the direction for what would

relay open. Now, when all mention problems are solved, the F1 short circuit will be explained

one more time on the figure 2. When the short circuit happens, both power sources supply it

with electrical current. The power source B supply F1 through overcurrent red relay but

because that relay has time delay it wouldn’t react as fast as possible, but only after 0.4s (this

number is given just for this example, in the real system the time delay depends on the

complexity of the network and on the numbers of relays). The power source A supply short

circuit though two green relays and red directional relay. The regular overcurrent green relay


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wouldn’t open as fastest as it could because of the time delay, the directional green relay

wouldn’t react at all because the direction of current is in the opposite of the direction of

reaction of this relay, and on the end, directional red relay would react because the direction

of the current is the same as the direction in which relay operate. With the reaction of this

relay, power source A stops supplying short current with the electricity, but power source B

still remains because regular red relay has time delay. But after 0.4sec after the fault happen

this relay would also react and the power sources wouldn’t supply short circuit anymore,

distribution line DE would be out of use, but line CD would continue with its normal

operation, which would not be possible without directional relays.

2.1 Principle of operation

After explaining one of the usages, the following question could be asked: how does

direction relay works? How does it recognize the direction of current flow, keeping in mind

that distribution network is network with AC and the direction of current is changing

constantly?

The solution for this is that in AC power networks, the direction of current flow is

determined from the phase shift between the voltage E and the current I at any given point of

the circuit. This will be explained with the help of figure 3: [4]

Figure 3. (a): circuit diagram

Figure 3. (b): current flows in forward direction Figure 3. (c): current flows in reverse direction


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When the current flows from left to right in the circuit, the absolute value of the phase

shift between the voltage and current is 90° or less (Figure 3b), the exact value of the phase

shift being dependent on the value of the circuit impedance Z. This direction is generally

considered as the forward direction. On the other hand, when the current flows from right to

left in the circuit, the absolute value of the phase shift between the voltage and current is 90°

or more (Figure 3c), the exact value of the phase shift being dependent on the value of the

circuit impedance Z. This direction is generally considered as the reverse direction.

In most power networks, the impedance has resistive-inductive character. In this case,

the expected range of phase angle values of the current is reduced to 90° for each direction of

current flow, as it is shown in Figure 4: [4]

Figure 4. (a): circuit diagram

Figure 4. (b): current flows in forward direction Figure 4. (c): current flows in reverse direction

The operation of the directional overcurrent relay along with its main parameters is

explained below: [4]

A directional overcurrent relay consists of overcurrent relay plus a directional

element that determines the direction of current flow. These two units operate

jointly for a predetermined current magnitude and direction. The relay is

activated only for current flow to a fault in one direction and when the current is

higher than the maximum current for which relay doesn’t operate.


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A directional overcurrent relay can monitor line current on two phases, on that

way measuring two currents and two voltages. With this kind of operation, any

phase-to-phase fault can be detected. In the case of monitoring line current on all

three phases, the relay measures three currents and three voltages. This allows

detection of any phase-to-phase fault as well as any phase-to-ground fault.

In electromechanical and static directional overcurrent relays, the reference

voltage for phase A is generally voltage EBC (VBC), and not phase voltage EA

(VA) as it could be expected. This is because voltage EBC isn’t affected when a

ground fault occurs on phase A, thereby providing a more stable reference

voltage. It is analogous for the phases B and C. This will be explained in more

details further in the paper. This logic could not be used for modern directional

overcurrent relays (digital and numerical units) because these relays generally

use phase voltage EA as the reference voltage for phase A and other techniques to

obtain a reference voltage that is even more stable (these techniques are beyond

the scope of this seminar paper, and because of that modern relays won’t be

further explained). However, there is still a reason to study electromechanical

and static directional relays because some of their principles and settings are used

in modern directional overcurrent relays even if a phase-to-phase voltage is used

as the reference voltage instead of a phase voltage.

The reference voltage may be rotated to properly align the forward and reverse

direction zones of the directional overcurrent relay. This rotation angle is referred

to as the characteristic angle. Figure 5 shows the forward and reverse direction

zones for two values of characteristic angle when line voltage EBC is used as the

reference voltage.

Figure 5: Selecting a characteristic angle to properly align the forward and reverse direction zones of the directional overcurrent relay[4]


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As it is already mentioned, in order to detect the current direction, the phase

displacement between voltage and short circuit current must be determined. When is

cos(φ)≥0 (-

) then the active power seen by the directional relay is positive. The

active power seen by the directional relay is negative in the situation when is cos(φ)≤0

(

) .

The displacement φ can be determined by comparing the phase current with a polarizing

voltage as a reference. This explanation will be done for electromechanical and static relays,

which means that if the current is in the phase A, the polarizing voltage will be EBC (VBC),

which is perpendicular to the current Ia at φ=0 (Figure 6a). Analogously, the polarizing

voltage will be VAB if the current is in the phase C, and the voltage will be perpendicular to

the current at φ=0 (Figure 6b). The angle between phase current and chosen polarizing

voltage is called connection angle and has the value of 90° when φ=0. [5]

Figure 6: polarizing voltage of (a) phase "A “and (b) phase "C" for a current in phase "A" and phase "C" respectively [5]

The tripping zone is a half plane defined by characteristic angle θ. This angle is the angle

between polarizing vector and a line perpendicular to the boundary line (Figure 7). The usual

values for this angle are: 30°, 45° or 60°. Because of its importance, this angle will be more

briefly explained in the next chapter.

Figure 7: Directional protection tripping zones on (a) phase "A" and (b) phase "C" for a θ=45°[5]


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It can be seen that phase current Ia is in the tripping zone when θ-π/2<ψ1<θ+π/2 and for

the other half plane is in the nontripping zone. The similar conclusion could be made for

current Ic. This current is in the tripping zone when θ-π/2<ψ3<θ+π/2 and is in the nontripping

zone for all other angles. Angles ψ1 and ψ3 correspond to φ1 and φ2, respectively (φi= ψi+90°)

2.2. Characteristic angle

To determine the direction of the fault, the protection equipment measures the phase

displacement between the current and the polarisation variable. If the polarisation variable is

not in the axis of symmetry of the wished relay’s action, it’s necessary to re-phase it by

adjusting the characteristic angle. This angle must be determined in the process of designing

the protection coordination so any fault in the chosen direction causes a current that falls in

the tripping zone and that any current in the other direction causes a current falling outside of

this zone. The analyses will be done for typical values of characteristic angle θ=30°,θ=45°

and θ=60° and for three faults: phase-to-ground fault, a phase-to-phase fault between phases

A and B, and a phase-to-phase fault between phases A and C. These three faults will be

analyses separately. Also, the resistive-inductive character of the impedance will be assumed

(This assumption is almost always true).

When a phase-to-ground fault occurs on phase A, the range of phase angle values of

current IA that is expected is illustrated in figure 8: [4]

Figure 8: Range of phase angle values of current IA expected for a phase-to-ground fault on phase A

The ranges of phase angle values of current IA expected for phase-to-phase faults

between phases A and B and between phases A and C are shown in the figure 9: [4]


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Figure 9: Ranges of phase angle values of current IA expected for phase-to-phase faluts between phases A and B and between phases A and C

Superposition of the ranges of phase angle values of current IA from figures 8 and 9, is

shown on the figure 10. [4]

Figure 10: Range of phase angle values of current Ia expected for faults on phase A

As it can be seen on the figure 10, the total range covers 150°. The same analyses could

be done for phases B and C, and the results would be the same, i.e. the total range of phase

angle values of the current is also 150°.

As it is already said, the usual values for characteristic angle are 30°, 45° and 60°. The

principle of operation will be explained first for the angle of 45° and on this basis the

characteristic angles of 30° and 60° will be also explained.

Setting the characteristic angle to 45° properly aligns the forward and reverse direction

zones of the directional overcurrent relay with the vectors of fault current expected for phase

A, as it is illustrated in figure 11.


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Figure 11: setting the characteristic angle to 45° properly aligns the forward and reverse direction of the directional overcurrent relay with the range of phase angle values expected for the fault currents

From this figure it can be seen that forward direction zone encloses every side if the

expected range of phase A with a safety margin of 15° on each side of the expected range of

phase angle values of current. This ensures optimal operation of the directional overcurrent

relay. This 15° safety margin is for phase-to-phase faults, each side for one of these faults.

Phase-to-ground has “natural” margins with protecting the phase-to-phase faults so there is

no need for adjusting the characteristic angle just for this fault. In other words, if both

mentioned phase-to-phase faults are covered, the phase-to-ground fault is also secured.

Setting the characteristic angle to 30° and 60° the following figures are obtained:

Figure 12: setting the characteristic angle to 30° and 60° properly aligns the forward and reverse direction of the directional overcurrent relay with the range of phase angle values expected for the fault currents:


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As it can be seen from previous figure, all three faults are covered when the

characteristic angles is 30° or 60°. The only difference is in the margins; on what side are

they and what their value is. In these two cases, a safety margin is 30° but just on one side.

All the figures are done with the help of Autocad.

After explaining the operation of the directional overcurrent relays, the other usages of

this protection will be shown in the rest of this seminar paper.

2.3 Directional relay construction

As it is already mentioned, the overcurrent directional relay is consist of at least two

units: overcurrent unit and directional unit. The importance of directional unit is to detect the

direction of current flow, and based of that direction to trigger or not to trigger the relay. Of

course, the direction of the current isn’t the only condition that needs to be fulfilled; the

current value should also be high enough so that the relay would reacts. The logic of the

operation of this relay was already explained in the chapters 2.1 and 2.2 and it won’t be

repeat in this chapter. But in this chapter will be shown the construction of the relay and

physical operations of the directional relay when the fault happen, i.e. it will be shown what

physical changes inside the relay are going to happen when the current changes flow

direction.

On the figure 13 it can be seen operation of a simple inductive type of protection with

directional unit. [6]

Figure 13: Normal operation of directional protection


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A red colour is a winding where information is obtained about the reference value (voltage or

current), and a green colour indicates a winding with information about the current, which is

obtained through a current transformer of a protected device. There is also a cylinder with

two contacts, by which the relay is actuated or not. In normal operation (no fault), the

direction of reference and current value is shown with orange arrows. The cylinder is in the

position as it can be seen in Figure 13; the upper contact is closed and prevents further

rotation. On the figure 14 it can be seen what is happing inside the relay when the fault

happens and the current changes the flow direction. When the direction of current flow is

changed (blue arrows), the cylinder rotates in the other direction, as it is shown in Figure 14

with the purple arrow and contacts with the bottom contact and the relay is triggered. The

condition for the cylinder rotation is that the reference value is lower than the current value

that is measured via current transformer, but still enough to hold the switch on the upper

contact. In normal operation, the torque from referent value is helped with the torque from

current, in order to hold switch to upper position. When the fault happens, the current change

the flow direction and also the current torque change the direction, and because this torque is

higher than the torque from reference value, the cylinder would start to rotate.

Figure 14: the operation of directional protection [6]


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3 Protection of parallel distribution lines

The usage of multiple lines improves the availability of power, i.e. using multiple lines

in parallel allows more power to be conveyed to a given location. Two parallel connected

lines are the simplest and most frequently encountered example of a closed ring. The

protection system must be designed in such a way that when a fault is on one line, the other

distribution line should continue with its normal operation. For better explanation why the

directional protection is necessary in this case, the protection system will be firstly explained

just with regular overcurrent relays and it will be explained why it is not good solution and

why directional protection must be used if we want to have good selectivity of relay

protection.

As it is already mentioned, the simple system with two parallel lines will be observed.

Such a system is shown on a figure 15:

Figure 15: Two lines in parallel to convey power to a substation

One of the possible protection of this simple system is to use 2 overcurrent relays and a

circuit breaker at the source end of each line as it is shown on the figure 16. This

solution protects the line properly, but the selectivity is not achieved at all. In other


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words, wherever the fault occurs, both lines would be out of function.

Figure 16: protection of two parallel line using just overcurrent relays [4]

When a fault occurs on line 1, the fault is supplied with current form both lines, which

means that both overcurrent relays will react after 0.3 s (because of the time-delay), thereby

opening circuit breakers C and D. This opens both power lines and the result is: power in no

longer available at substation bus B

As it can be seen, using overcurrent protection without directional units, to protect power

lines connected in parallel works, but is not discriminative, in other words, even the healthy

line (line 2 in this case) is open and out of use. This leads to another problem, and that is

whenever a fault happen, on line 1 or line 2, the supply of power to the load is interrupted.

The results is that with this protection a chances to lose power to a load are doubled

compared to when a single line is used. This further leads to lower availability of the system.

To solve this problem, i.e. to archive discriminative protection of the two power lines

from figure 16, a directional overcurrent relays are required. The protection system with

directional units is shown on the following figure: [4]


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Figure 17: Directional overcurrent protection of two power lines connected in parallel

The analyse will be done when the fault is on the line 1 (as it can be seen on a figure 17).

In this case, fault current IF1 flows through line 1 and fault current IF2 flows also through line

1, but in the opposite direction. The protection equipment time delays are shown in the figure.

The current IF2 flows though relays D, F and E. Relay D detects the fault but because of time

delay it won’t react immediately, but after 0.4s. The relay F doesn’t detect the fault at all

because the direction of current is opposite to the direction set in the relay. Finally, relay E

would react because the direction of the current flow is the same as the direction set in the

relay and there is time-delay of just 0.1s (0.3s lower than for relay D). With the reaction of

the relay E, the overcurrent relay D resets before its time delay has elapsed. The situation

with current IF1 is much simpler for this fault because this current flows only through relay C.

At the moment when the circuit breaker at location E opens, the circuit breaker at location C

is still closed although this relay has reacted. The reason for this is that this relay has time-

delay of 0.4s. After this time, the circuit breaker on location C opens and the fault is isolated

while the healthy line is still in operation, providing electrical energy to the busbar B.

A time setting of 0.4 s is used on the overcurrent relay at location D to allow the

directional overcurrent relay at location E to trip when a fault occurs on line. This ensures

that line 2 is not disconnected when a fault occurs on line 1, thereby achieving proper fault

discrimination. For the same reason the relay on location C has the same time delay, just this

configuration has sense when the fault is on the line 2.


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4 Protection of closed loop

The principle of protecting the closed loop is the same as the principle of protecting the

system with two parallel lines. The only difference is that in the closed loop there are much

more lines, which means more relays and the time-delay of individual relays is much higher.

Depending on the application, longer fault clearing times may or may not be acceptable for

various reasons. On the next figure, one of the possible solutions for protecting the closed

loop system is shown: [1]

Figure 18: protection of a closed loop using directional relays and time-based selectivity

The direction of detection of each protection system is shown by an arrow. The archive

selectivity, every relay has time-delay unit. Analysing this simple system the problem with

time responding of some relays is obvious because some of the relays have time delay of 1.3s

and that could be too much. One of the possible solution for this problem is the usage of the

directional comparison protection. Directional comparison protection uses a pair of

directional overcurrent relays to protect each segment of power line, the relays being located

at the two ends of the protected line segment. Each directional overcurrent relay in a pair can

communicate with the relay at the other end of the protected line segment via a

communication link (optical fibre, microwave communication system, etc.). This key features

allows the implementation of fast relay tripping schemes providing short fault clearing times.

[4]


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5 Earth fault directional protection

There are two types of ground faults: single line ground faults (SLG fault) and line-to-

line ground fault (LLG fault). The ground fault is different from other types of short circuit

because it has a zero sequence of a current I0. Since in normal operations this current doesn’t

exist, the relays for detecting ground fault measure the component I0=(Ia+Ib+Ic)/3 and

declaring the fault of I0 exceeds a threshold.

However, in a system with multiple sources or parallel paths, it is required that earth

fault relays have directional unit. The reference phasor is sometimes called as polarizing

quantity or residual quantity. Also, both voltage and current polarizing signals are used with

ground fault relaying.

Before explaining these two polarizing signals, the term residual variable should be

explained. For example, if the zero sequence variable Fh is defined as: [1]

1( 1 2 3)

3hF F F F (1)

The residual variable is:

1 2 3Fr F F F (2)

and it is three times greater than the zero sequence variable.

The residual current is either measured by three current transformers, one per phase,

or by a coil (ring CT) around three phases: [1]

o The usage of three current transformer, which is

shown on the figure 19 has certain advantages:

- CT’s are generally dependable

- it is possible to measure high currents

But it also has certain disadvantages:

- saturation of the CT’s in the instance of a short

circuit or when a transformer is switched on

produces a false residual current

- in practise, the threshold cannot be set to under

10% of the CT’s rated current.

o Measuring using a ring CT:

- has the advantage of being very sensitive

- has the disadvantage of the coil 9low voltage

insulated) being installed around a non-clad

cable to insulate it

Figure 19: measuring the residual current using 3 CT’s (figure above), and using a ring (figure below)


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Residual voltage is measured by three voltage transformers (VT), and usually these

transformers have two secondary windings: one is in the star connection and enables

both phase to neutral and phase to phase voltages to be measured and the other one is

in delta connection, enabling the residual voltage to be measured. Also, one of the

possible configurations is that main VT’s have one secondary winding that is in the

star connection, and grounded, while the set of auxiliary VT’s is used to measure the

residual voltage. These two ways of measuring the residual voltage are shown in the

figure bellow: [1]

figure 18: (a) figure 18: (b)

Figure 20: measuring the residual voltage using 3 VT with two secodary windings (a), and using auxiliary VT’s (b)

In theory both ways of polarising the protection equipment are equivalent. Also, the

residual voltage Vr, and the current of the neutral point, In, are related by the following

equation:

Vr=(Zh+3Zn) In (3)

where is:

Zh- transformer’s zero sequence impedance

Zn- the impedance of neutral point

In practice, the situation is different. Polarisation by the neutral point current is only used

in networks with an earthing fault current that is both large (several hundreds of ampers) and

at the same time much greater than the current due to parasite capacitance on the network. In

this situation, the measurement of current is more accurate that the measure of the residual


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voltage which has a small value. These situations are common in the substations that are near

to the neutral earthing connection.

The detection of the current flow direction is performed in the same way as it was

explained in the chapter 2.1.

6 Power relays

These relays won’t be explained as the previous situations, using the logic “what would

happen if this relay is just regular power relay, without directional unit”. The reason for this

is that power relays are naturally directional, i.e. every power relay has directional unit. This

type of protection equipment often uses a dual wattmeter method to measure the active and

reactive power.

As it can be seen from the figure 21, for

measuring the power, two current and two phase-

to-phase voltage transformers are required.

The only condition for correct measurements

is that there is no zero sequence current in the

circuit.

The active power is given by:

P=I1∙U31∙cos(I1,U31)+I2∙U32∙cos(I2,U32)

and the reactive power:

Q=I1∙U31∙sin(I1,U31)+I2∙U32∙sin(I2,U32)

The measured power is therefore an algebraic

variable and the direction of flow is indicated by

its sign (+/-). And this is the reason why every

power relay is naturally directional relay with no need for some additional directional unit.

These relays are used to protect alternators (generators). For example, it is possible that

generator, because of some problems with turbine, starts to receive and use active power from

network, i.e. generator became motor. In order to detect such operation, a directional active

power relay must be used. The threshold of this protection equipment is set to a low value

compared with the alternator’s rated apparent power, typically between 5 and 20%. [1]

Also these types of relays are used to detect the loss of excitation monitoring the

direction of the reactive power.

Figure 21: Layout diagram for measuring power [1]


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7 Conclusion

In modern distribution networks, the usage of directional protection is essential in order

to achieve satisfactory selectivity. Generally speaking, there are three types of directional

protection: phase directional protection, earth fault directional protection and active and

reactive directional power protection. But regardless of type, the operation principle of this

protection is the same. To determine the direction of the fault, the protection equipment

measures the phase displacement between the current and the polarization voltage and based

on this phase displacement the protection “decides” should it react or not. Also, one of the

most important parts of the direction protection is so-called characteristic angle. If the

polarisation variable is not in the axis of symmetry of the wished relay’s action, it’s necessary

to re-phase it by adjusting this angle. The typical values of this angle are 30°, 45° or 60°.

To achieve selectivity, this protection must be used in the following situations: in a

system with several sources, in closed-loop system or system with parallel cables, in isolated

neutral systems for the feedback of capacitive current and to detect an abnormal direction of

flow of active or reactive power. Also, the relays that are used for this protection almost

always have a time-delay unit, especially in distribution meshed grid. And this necessity of

time-delay unit in every directional relay is the biggest problem of directional and generally

of the overcurrent protection at all, because in meshed grids time relays could have

unacceptably large time-delay and in that situation some other protection must be used. But

beside this disadvantage, the directional protection is used more and more in modern

distribution system, especially because the number of power sources that submit electrical

energy directly in distribution system is increasing.


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References:

[1] Pierre BERTRAND, “Directional protection equipment”, march 1998

[2] John Horak, “Directional Overcurrent Relaying (67) Concepts”

[3] Module 5: Directional Overcurrent Protection; lecture 18: Directional Overcurrent

Relaying

[4] Electricity and New Energy, Directional Protection, Courseware Sample

[5] Abdelhay A. Sallam, OM P. Malik, “Electric Distribution System”, chapter 5.5

[6] Gašper Lenassi, „Smerna zaščita“-seminarska naloga

QUESTIONS

1. In what conditions is it necessary to use directional protection?

In a system with several sources, in closed loop or parallel-cables system, in isolated

neutral systems for the feedback of capacitive current and to detect an abnormal direction

of flow of active or reactive power.

2. How does directional protection determine the direction of current, keeping in

mind that distribution network is network with alternative current?

To determine the direction of the fault, the protection equipment measures the phase

displacement between the current and the polarization voltage and based on this phase

displacement the protection “decides” should it react or not.

3. What is characteristic angle, and what are usual values for this angle?

Characteristic angle is the angle between polarizing vector and a line perpendicular to the

boundary line. If the polarisation variable is not in the axis of symmetry of the wished

relay’s action, it’s necessary to re-phase it by adjusting the characteristic angle. Typical

values for this angle are: θ=30°, θ=45° and θ=60°.

4. What is the biggest disadvantage of directional protection?

The relays that are used for directional protection usually have time-delay unit. In big,

meshed grids, time-delay for certain relays could be higher than the value that is allowed,

and because of this some other protection should be used for this kind of networks.


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EXAMPLE:

For the distribution system given on the picture bellow, answer on following question.

a) If all relays are regular overcurrent relays (without directional unit) and all of

them have same time-delay, what relays would react if the fault is F1? And what

about fault F2?

If the fault is F1, the relays R1 and R2 would react, which means that both distribution

lines L1 and L2 are out of order and the consumers aren’t supply with electricity

anymore. If the fault is F2 the situation is the same.

b) Hatch the relays that should be directional, draw the direction of current flow

for which they react and write what time-delay (approximately) should have all

the relays in the system in order to achieve selectivity?


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c) In a system from b) explain what relay/relays would open if the fault is F2?

If the fault is F2, the current would flow through distribution lines L1, L2 and L3. First of

all, relays R3 and R4 wouldn’t react at all because the direction of the current is opposite

from the direction set in relay. Relays R1 and R2 detect the fault but because of time

delay they won’t react immediately, but only after 0.4s. Relay R5 also detects the faults

and it will react after 0.1s (for 0.3s faster than relays R1 and R2) which means that this

relay would open first and the power source doesn’t supply fault F2 with electrical current

anymore. The overcurrent relays R1 and R2 resets before the time-delay elapsed.


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DIRECTIONAL PROTECTION - lrf.fe.uni-lj.silrf.fe.uni-lj.si/e_rio/Seminarji1718/SmernaZascita.pdf · Protection equipment has the basic role of detecting an electrical fault ... whenever