1_FUNDAMENTLE Protection Principles

8/18/2019 1_FUNDAMENTLE Protection Principles

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Energy Sector Energy Automation

© Siemens AG 2009

Power System Structure

Generation Transmission / Sub transmission Distribution

Extra High Voltage 765 kV400 kV220 kV

High Voltage 132 kV110 kV66 kV

Medium Voltage 33 kV22 kV11 kV

Medium 24 kVVoltage 21 kV

15 kV13.8 kV

The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the

utilisation points with both reliability and economy.

The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the

utilisation points with both reliability and economy.

Many items of equipment are very expensive, and so the complete power systemrepresents a very large capital investment.

.

Many items of equipment are very expensive, and so the complete power systemrepresents a very large capital investment.

.


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Owner of high voltage networks

Public utility companies

Interconnected system, National network, Regional system, Urban network

Industrial companies

Steel, Cement, Chemistry, Automobile . . .

Power plantsRailways

Special systems

Airports, Hospitals, Testing stations, Ships . . .


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© Siemens AG 2009

Protective Relaying is the most important

feature of power system design aimed at

minimising the damage to equipment and

interruption to service in the event of faults. It

is therefore a co-factor among other factors

resorted to improve reliability of power system.

Protective Relaying

Role of Protection


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The Purpose of Protection

But it can:Limit the damage caused by shortcircuits

While:Protecting people and plant fromdamage

Selectively clearing faults inmiliseconds

Protecting plant from overload

conditions

The protection can not prevent system faults,

Power system must operate in a safe manner at all times.


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Causes and Probability of System Disturbances

Causes§ Operator Mistakes

§ Pollution/Condensation

§ Equipment failures, e.g. P.T.'s, Isolators

§ Transient Overvoltages

Probability

§ System faults (220/400 kV): 3p.a. and 100 km

§ 10-20 kV metal clad switchgear: 10-3 p.a. and feeder

§ GIS switchgear: 5-10-2 p.a. and bus

§ outdoor switchgear: 110/132 kV 7*10-2 p.a. and bus

220/275 kV 10-1

p.a. and bus400 kV 2*10-1 p.a. and bus


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© Siemens AG 2009

Since protective relaying comes into action at the time of

equipment distress, a certain safeguard is necessary in the

unlikely event of its failure to act at the hour of need.

Hence, two groups of protective schemes are generally

employed -

a) Primary Protection

b) Back-up Protection

Primary Protection is the first line of defense, whereas back-up

relaying takes over the protection of equipment, should the primary

protection fail.

Principles of Relaying


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The Primary Protection has following characteristic features -

1. It has always a defined zone of operation.

2. It should operate before any back-up protection

could operate, therefore, it should be faster in

operation.

3. It should be able to completely isolate the fault

from all the current feeding sources.

4. It should be stable for all operating conditions.

Primary Protection


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Primary protections failure could be due to any of

the following reasons -

1. Current or Potential Transformer failure

2. Loss of Auxiliary Control Voltage

3. Defective Primary Relays

4. Open Circuits in Control & Trip Coil

5. Failure of Breaker

It is therefore logical that back-up relays should not

utilise any of the above items as common with

primary relays.

Reasons of Primary Protection Failure


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Protection Concept

§ The system is only as strong as the weakest link!

DISTANCE RELAY

Circuit Breaker CT / VT

Protection Battery

Cabling


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System structure: meshed network


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System structure: radial network for public supply


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System structure: radial network in the industry

G G

M

M

M


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© Siemens AG 2009

Neutral earthing

Solid earthing Low-impedanceNeutral earthing

Isolated neutralEarth fault-compensation


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Main components of electrical networks

Bus coupler

Generator Three-windingtransformer

Cable

Shunt reactorOverhead

line

Consumer

MotorM

Filter circuit

Short-circuit currentlimiting reactor

Switch

Substation

Doublebusbar

Earth faultcompensation coil


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Protection target

Selective Fast


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Basic Protection Requirements

§ Reliability dependability (availability)

high dependability = low risk of failure to trip

§ Security stable for all operating conditions ,

high security = low risk of over-trip

§ Speed high speed minimizes damage

high speed reduces stability problems

§ Selectivity trip the minimum number of circuit breakers

§ Sensitivity notice smallest fault value


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Protected zone

Circuit-breaker

Current transformer


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Zones of Protection

n To limit the extent of the power system that is disconnected when a faultoccurs, protection is arranged in zones

n Zones of protection should overlap, so that no part of the power system isleft unprotected

n Location of the CT connection to the protection usually defines the zone

n Unit type protections have clear zones reach e.g Diff. Relay, REF relay

n Zone reach depends on measurement of the system quantities e.g OC ,EF, distance relays . The start will be defined but the extent (or ‘reach’) issubject to variation, owing to changes in system conditions andmeasurement errors.


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Criteria indicating fault condition

Current I I > > I >> I I

Voltage U U< U>

Impedance Z Z<

Phase angle

Power S S (t)

Frequency f f

r r

P Q

U

I

ddt

I r

I

I I (t) = sin t + e-

t

T×


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Overcurrent-time protection

Definite-time

overcurrent-protection

Inverse-time

overcurrent-protection

t

I I N I > I >>

t2

t1

t

I I N

t

I I N


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Differential protection

Load condition

Fault condition

Load condition

Fault condition

I A I B

I C

I A I B

I C

Line

Busbar

Istart - Iend = 0 è DI = 0

Istart - Iend¹ 0 è DI ¹ 0

IA + IB + IC ¹ 0 è SI ¹ 0

IA + IB + IC = 0 è SI = 0


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Overvoltage - Undervoltage

U >

UN

U <

Overvoltage

Undervoltage


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Back-up protection II

Z<

I > I > I >

t = 400 ms

t = 100 ms

t = 1000 ms

Z< Z<

t = 700 ms

t = 300 ms

t = 0 ms

I

I

I


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Survey Equipment - Type of protection

Line

TransformerHigh voltage - Medium voltage

Busbar

TransformerMedium voltage - Low voltage

Motor

Time-graded protectionDifferential protection

Differential protection

Time-graded protection

Reverse interlockDifferential protection

FuseTime-graded protection

Time-graded protectionOverload protection


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EquipmentSignal

conversion

Signal

tailoring

Processing(calculation)

Signalanalysis

Trippingsignal

Trippingcoil

Circuitbreaker

Protection device

Auxiliary supply Settings Annunciation

Equipment : Lines, cables, transformers, machines Processing : Digital Filters,Numerical Methods,

Measuring AlgorithmsSignal Conversion : CTs and VTs Signal Analysis : Comparisonwith

Settings, gradingSignal Tailoring : Signal matching, Anti-Aliasing Filters, A/ D Conversion

Binary Inputs

General Structure of a Numerical Protection Device


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PCPC--InterfaceInterfaceSystemSystem--InterfaceInterface

Hardware for Digital RelaysHardware for Digital Relays

100V/ 1A, 5A100V/ 1A, 5Aanalogueanalogue

IInput/nput/OutputOutputPortsPorts

32/16 Bit32/16 Bitprocessorprocessor--SystemSystem

Memory :Memory :RAMRAMEEPROMEEPROM

EPROMEPROM

RS232/RS232/485/FO485/FOSerialSerial

InterfacesInterfaces

0001000101010101

00110011

AmplifierAmplifier

A/DA/D--ConverterConverter

FilterFilter

Measur.inputMeasur.inputss(max. 11)(max. 11)

max. 7max. 7VoltageVoltage--inputsinputs

(140 V cont.)(140 V cont.)

max. 11max. 11CurrentCurrent--InputsInputs(100/N. 1s)(100/N. 1s)

10V10Vanalogueanalogue

digitaldigital InputInput--/Output/Outputcontactscontacts

Input/OutputInput/Output--

unitunit

2....112....11binarybinaryInputsInputs

5....115....11AlarmAlarmRelaysRelays

2....52....5TripTripRelaysRelays

8....168....16LEDLED

IndicatorsIndicators


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Analog to Digital ( A / D ) Conversion of Measuring Signals

IL1

Filter

•

•

•

•

•

•

1 kHz

S & H

MUX

ME

IL2 ; IL3 ; IE

UL1 ; UL2 ; UL3

UE•

•

•

•

•

•

1 kHz

ME Measuring Input S & H Sample u. Hold

MUX Multiplexer

PGA

1 1 10 00A

D

PGA Programmable Gain Ampliflier


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Analog to Digital ( A / D ) Conversion of Measuring Signals

Example of an A/D Conversion of a Sinusoidal Signal.

A voltage signal 10 sinω

t is sampled at a rate of 1 kHz (Sampling time Δ

T = 1 ms) .ω = 2πf, with f being the power frequency = 50 Hz..

How does the output of a 12 bit (11 bits + sign) ADC look like ?

Note : 1 ms for a 50 Hz system corresponds to 18 electrical degrees

t t

Input of ADC Output of ADC

10 sin ω t


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Sampling Rates used in Siemens Numerical Protection

S.No Relay Designation Sampling Rate

1. 7UT612 12 Samples / Cycle

600 Hz for 50 Hz system





4. 7SJ61-64 16 Samples / Cycle


5. 7SA... 20 Samples / Cycle


6. 7SD... 20 Samples / Cycle


7. 7SS... 20 Samples / Cycle


8. 7UM Depends on network frequency

Documents

1_FUNDAMENTLE Protection Principles