SWITCH GEAR AND PROTECTION

BY

RAMU SRIKAKULAPU

ASSISTANT PROFESSOR

SHARDA UNIVERSITY

UNIT -1 INTRODUCTION TO POWER

SYSTEMS

Need for protection Nature and causes of faults Types of faults Fault current calculation using symmetrical

components Power system earthing Zones of protection Primary and back up protection Essential qualities of protection Typical protection schemes.

TEXT BOOKS Badri ram , D.N.Vishwakarma, ‘power system

protection & switchgear’ , Tata McGraw –hill publishing company ltd, New Delhi.

C.L Wadhwa , ‘ electrical power systems’ , New Age International (p) limited.

B. Ravindranath, and N. Chander, ‘Power System Protection & Switchgear’, Wiley Eastern Ltd., 1977.

Sunil S. Rao, ‘Switchgear and Protection’, Khanna publishers, New Delhi, 1986 .

Need for protective systems What is the power system ?

POWER FLOW

NEED FOR PROTECTION Heavy currents associated with short circuits is to

cause for damage If short circuit presents on a system for longer

time , it cause to damage the some of sections in system Fire exists System voltage reduces to low level and generators in

power station may lose synchronism By un cleared fault to cause for failure of the system

To protect the system from faults need to use automatic protective devices like Circuit breakers Protection relays ,isolate the faulty element

PROTECTION SCHEME OPERATION

C.B can disconnect the faulty element of the system Protective relay is to detect and locate a fault and

trip C.B Protective relay senses abnormal conditions in

system and gives alarm signal Those abnormal conditions are

short circuits Over speed of generators & motors Over voltage Under frequency Loss of excitation Over heating of stator& rotor of an alternator

CAUSES OF FAULTS Causes of fault on over head lines

Birds bodies touch the phases Direct lighting strokes Air crafts Snakes Ice Snow loading Earth quakes

Causes of faults in case of cables, transformers, generators Failure of solid insulation Flash over due to over voltages

DIRECT LIGHTING STROKES

Practical analysis for

causes of fault

AVERAGE 400 KV LINE FAULT FREQUENCY STATISTICS BY FAULT

CAUSE PER 100 KM PER YEAR

TRANSMISSION LINE FAULT CAUSES – 12,229 FAULTS FROM 132 KV TO 765

KV

TYPES OF FAULTS Unsymmetrical faults

Single phase to ground fault(L-G) Line to line fault(L-L) Double line to ground fault(L-L-G) Open circuited phases fault Winding faults

Symmetrical faults 3phases to ground fault(LLLG) 3phase fault

SYMMETRICAL FAULTS

All 3 phases are short circuited In case of symmetrical fault current is equally

shared by the 3phases In symmetrical to analysis fault by one

phase only Only +Ve sequence component exist in

symmetrical Remaining two components always zero.

OPEN CIRCUITED PHASES FAULT

Caused by a break in the conducting path Occurs when

one or more phase conductors break cable joint or a joint on the over head lines fails

Fault rises when C.Bs or isolators open but fail to close one or more phases

WINDING FAULTS

Occurs on the alternator, motor & transformer windings

SIMULTANEOUS FAULTS

PERCENTAGES OF FAULT IN POWER SYSTEM

L-G 70-80%

L-L 17-10%

LL-G 10-8%

3 PHASE 3-2%

EFFECTS OF FAULTS

Heavy short circuit current cause to damage to equipment

Arcs associated with short circuits may cause fire

Reduction in the supply voltage of the healthy feeders

Loss of industrial loads Heating rotating machines due to reduction

in the supply V&I Loss of stability

ZONES OF PROTECTION Separate protective schemes for each piece of

equipment or element of the power system. dependence on the equipment divided in to a

number of the zones Each zone covers one or a the most two

elements of the power systems The protective zone plan to cover the entire

power system. Adjacent protective zones must over lap each

other A relatively low extent of over lap reduces the

probability of faults in that region. Tripping of too many breakers does not occurs

frequently.

EXAMPLE

PRIMARY & BACK UP PROTECTION

Every zone have a suitable protection scheme

If fault occurs in a particular zone, it is duty of primary relays of the zone to isolate the faulty element.

If its fails, back up protection scheme is to clear the fault.

The protection schemes improves the system performance

BACK UP RELAY It operates after a time delay to give the

primary relay sufficient time to operate. When a back up relay operates a larger time

of the power system is disconnected from the power sources.

Types of back up relays are Remote back up Relay back up Breaker back up

REMOTE BACK UP It is cheapest and simplest

It is widely used for back up protection of transmission lines

Most desirable It will not fail due to the factors causing the

failure of the primary protection

RELAY BACK UP

Additional relay is provided for protection.

It trips the same CB if the primary relay fails and this operation takes place with out delay.

Costly

Used where back up is not possible.

BREAKER BACK UP Bus bar fault : When protective relay operates

in response to a fault but the C.B fails to trip, the fault is treated as a bus bar fault.

used for bus bar system , where a number of C.B’S are connected to it.

At time of bus bar fault , necessary that all other C.Bs on the bus bar should trip.

BACK UP PROTECTION

AC D

E

Breaker 5Fails

1 2 5 6 11 12

T

3 4 7 8 9 10

B F

QUALITIES OF PROTECTION

The basic requirements of a protective system are

Selectivity or discriminationReliabilitySensitivityStabilityFast operation

SELECTIVITY

The selectivity of protective system dependence on quality of a protective relay

It is able to discriminate between in the protected section & normal section.

It should be able to distinguish whether a fault lies with in its zones of protection or outside the zone.

The relay able to discriminate between a fault & transient conditions power surges inrush of a transformer’s magnetizing current.

INRUSH OF A TRANSFORMER’S MAGNETIZING CURRENT When a transformer is first energized, a transient

current up to 10 to 50 times larger than the rated transformer current can flow for several cycles

inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage (which for a pure inductance would be the current maximum in the AC cycle).

In the absence of any magnetic remanance from a preceding half cycle, the effective magnetizing force is doubled compared to the steady state condition

RELIABILITY The typical value of reliability of protective system

is 95% reliability dependence on

Robustness Simplicity In case of relay

Contact pressure Contact material of relay

To achieve a high degree of reliability Design Installation Maintenance Testing of the various element of the protective

system.

SENSITIVITY

It dependences on pick up current value Pick –up current:- a protective relay should operate

when the magnitude of the current exceeds the present value.

A relay should be sensitive to operate when the operating current just exceeds its pick up value.

STABILITY

Protective system should stable for Large current due to fault Internal fault External fault

FAST OPERATIONIt means Isolate the faulty element quickly. To minimize damage to the equipment . To maintain system stability To avoid the loss of synchronism

The operating time of the protective system should not exceed the critical clearing time.

Critical clearing time:- the minimum time taken by protective relay to clear the fault.

By fast operation of protection system to avoid Burning due to heavy fault current Interruption of supply to consumers Fall in system voltage it results Loss of industrial loads

Operating time is usually in one cycle or half cycle.

PROTECTIVE SCHEME CLASSIFICATIONS A protective scheme is used to protect an

equipment or a section of the line

The protective schemes areOver current protectionDistance protectionCarrier current protectionDifferential protection

OVER CURRENT PROTECTION This protection includes one or more over

current relays. over current relay operates when the current

exceeds its pick up value. It is used for the protection of

Distribution lines Large motors

DISTANCE PROTECTION It includes number of distance relays of same

or different. Distance relay measures the distance

between the relay location and the point of fault in terms of impedance, reactance

Relay Type Measurement1. Impedance relay impedance2. Reactance relay reactance3. Mho relay admittance

This protection scheme is used for the protection of transmission lines Sub- transmission lines

CARRIER CURRENT PROTECTION

In this used relays are distance type. Their tripping operation is controlled by the carrier

signal. A transmitter and receiver are installed at each

end of transmission line to be protected. Information of direction of fault current is

transmitted from one end of the line section to the other by carrier signal.

Relays placed at end It trips if the fault lies with in their protected section. Do not trip for external fault. It is used for the protection of EHV &UHV line (132kv

above)

DIFFERENTIAL PROTECTION C.T’S are placed on both side of each windings of a

machine. The outputs of their secondaries are applied to the relay

coils. The relay compare the current entering & leaving of a

machine winding This difference in the current actuates the relay. In case of internal fault , the current values are not equal. The relay inoperative under normal condition and external

fault. It is used for the protection of

Generators Transformers Motors of various size Bus zones

In bus zone protection, C.T’S are placed on the both sides of the bus bar.

FAULT CURRENT CALCULATION

SEQUENCE COMPONENTS

UNSYMMETRICAL VECTOR FROM SYMMETRICAL COMPONENTS

Relations of voltage components in matrix form

Relations of current components in matrix form

SYMMETRICAL VECTOR FROM

UNSYMMETRICAL VECTOR

SEQUENCE IMPEDANCE

SYMMETRICAL COMPONENTS OF GENERATOR

POSITIVE SEQUENCE IMPEDANCE

NEGATIVE SEQUENCE IMPEDANCE

ZERO SEQUENCE IMPEDANCE

SYMMETRICAL COMPONENTS OF

TRANSFORMER (ZERO SEQUENCE)

1

2

3

4

5

ZERO SEQUENCE IMPEDANCE OF 3Ф LOAD

No grounding Solid grounding Neutral impedance grounding delta

L-G FAULT

L-L FAULT

LL-G FAULT

NEUTRAL GROUNDING The performance of the system in terms of

the short circuits, protection etc. is greatly affected by the state of the neutral.

There are various method of grounding the neutral of the system. Solid grounding Resistance grounding Reactance grounding Voltage transformer grounding Zig-zag transformer grounding

UNGROUNDED SYSTEM

In case of L-G fault at phase Ccharging current = 3*(per phase

charging current)The voltages of the healthy phases

rises to 0.866VphThese voltages can be eliminated by

connecting an inductance of suitable value between the neutral and the ground or Arcing ground.

RESONANT GROUNDING

The value of the inductive reactance is such that the fault current balances exactly the charging current .

Resonant grounding is also called ground fault neutralizer or Peterson coil.

The resonant grounding will reduce the line interruption due to transient line to ground fault.

SOLID GROUNDING The neutral is connected directly to the

ground with out any intentional impendence between neutral and the ground.

For low voltages up to 600volts and above 33 kV.

RESISTANCE GROUNDING The value of the resistance commonly used is quite

high (in order to limit power loss in resistor during LG-fault) as compared with the system reactance.

It is normally used where the charging current is small i.e., for low voltage short length over head lines.

It reduces the ground hazards. It has helped in improving the stability of the

system during ground fault by replacing the power dropped as a result of low voltage, with an approximating equal power loss in the resistor.

To limit the stator short current . Generators are normally provide with resistance

grounding.

REACTANCE GROUNDING

Reactance grounding system (Xo/X1) >3

Solid grounding system (Xo/X1) < 3

Reactance grounding may be used for grounding the neutral of synchronous motors & synchronous capacitors and also for circuits having large charging current

For medium voltages 3.3kV & 33KV resistance or reactance grounding is used.

ZIG-ZAG TRANSFORMER GROUNDING

It is used , if neutral point is requied which other wise is not available (eg. Delta connection, bus bar points)

ADVANTAGES OF NEUTRAL GROUNDING

Voltages of the phases are limited to phase to ground voltages

The high voltages due to arcing grounding or transient line to ground faults are eliminated

The over voltages due to lighting are discharged to ground

ADVANTAGES OF ISOLATED NEUTRAL

It is possible o maintain the supply with a fault on one line

Interference with communication lines is reduced because of the absence of Zero sequence currents.