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UNIT II VI-SEM 2016ST. JOSEPH UNIVERSITY(TZ)

Protection Apparatus Schemes

Electrical Protection Schemes take actions only after sensing the occurrence of the fault to prevent the electrical systems from damage

Then why protections are required?– to limit the damage to the components which

are under fault.– to save the rest of the Power System.

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• Generators• Motors• Transformer • Buses• Lines (transmission and distribution)• Utilization equipment (domestic loads)Note: Protective system cost is 4-5% of the total

cost in industries per IEEE

Basic Components to be Protected

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Protection Zones

Unit Generator-Tx zone

Bus zone

Line zone

Bus zone

Transformer zoneTransformer zone

Bus zone

Generator

~

Reactor Bus Line Bus Reactor Bus Motor

Motor zone

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• In electrical system, the generator, transformer and motors are the most expensive equipments and hence it is desirable to employ a protective system

• The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency

• Protective components utilizes one or more of these quantities to detect abnormal conditions in a power system

Quantities Affected in Electrical System

MAIN EQUIPMENT FOR SWITCHGEAR OPERATION

Potential transformer Current transformer Relay Circuit breaker DC Power Source for operation of the Circuit

Breaker and Relays

Switchgear is a general term covering a wide range of equipments concerned with switching and protection.

i.e. Circuit breaker, Isolator, Earth switch etc

6G

E Consumer &

IndustrialMultilin

VVPP

VVSS

Relay

• Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay

• VT ratios are typically expressed as primary to secondary; 14400:120,

Voltage Transformers

VT

7G

E Consumer &

IndustrialMultilin

• Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc.

• CT ratios are expressed as primary to secondary; 2000:5,

Current Transformers

Fast operation Auto electric protective device Act at Abnormal condition. Energizing an Alarm Disconnect fault zone Use system supply to operate.

1. Electromagnetic/conventional Relays2. Static/solid state Relays

Simple electromechanical relay

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• Reliability• Selectivity• Speed • Simple switchgear

Desirable Protection Attributes

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Basic Functioning of Protection Relay

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• Getting Inputs from CT and/or PT, Relay determines whether there is any fault

• If it detects any fault then gives trip command to the circuit breaker

• Getting command, circuit breaker disconnects the faulty sections from rest of the power system.

Functioning of Protection Relay

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Our current discussion will be based on: Generator Protection

SEMINAR•Transformer Protection(2.5marks for Assignment1)

•Bus bar Protections Plus CT & PT App(2.5marks for Assignment1)

TYPES OF GENERATOR

AC Generator

DC Generator

Induction Generator

SYNCHRONOUS GENERATOR

Self Excited DC Generator

Separately Excited

AC GENERATOR Synchronous Generator

Synchronous Generator : in this type the rotor speed is just equal to the flux produce by the stator . And receiving field excitation from separate field supply

Asynchronous(Induction) Generator

Asynchronous or Induction : in this type Rotor speed is not equal the Flux produced by the stator

Induction generator takes reactive power from the power system for field excitation. If an induction generator is meant to supply a standalone load, a capacitor bank needs to be connected to supply reactive power.

Due to lack of a separated field excitation , these machines are rarely used as generator

AC GENERATOR Synchronous Generator

Asynchronous Generator

Field excitation

GENERATOR MAIN PARTS

Parts: By Mechanical

Stator (Field winding)Rotor (Armature winding)

Parts: By ElectricalArmature winding

The winding which carries only the load current.

Field windingThe winding which carries only the field current required to produce the magnetic flux.

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SCHEME OF GENERATOR PROTECTION

• CLASS A TRIPPING This is adopted for those electrical faults of Generator and

Generator transformer(TG) and unit auxiliary transformer(UAT) for which tripping can not be delayed.

      -  Generator HV side CB      -  Field Circuit Breaker      -  LV side incomer breakers of UAT      -  Auto changeover from unit to station for unit auxiliaries and

tripping of turbine

• CLASS B TRIPPINGThis is adopted for all turbine faults (Mechanical) and for some

Electrical faults of Generator, Generator transformer and unit Auxiliary transformer for which it is safe to trip the turbine after sometime   

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SCHEME OF GENERATOR PROTECTION

• CLASS C TRIPPINGThis is adopted for all faults beyond the Generator system

which can be cleared by tripping of Generator transformer HV side CB alone

In this case the TG set runs with High Power-Low Power bypass system in operation and the Generator continues to feed the unit auxiliary load through unit auxiliary transformers.

FAULT OCURRENCE & FAULT CLASSIFICATION

Insulation failure• Tends to increase with rising

temp• Insulation failure may cause LLL

or LLG.• Bring winding in to direct contact

with core plates.• Any failure to restrict earth fault

may result into core plate damage.

• Insulation of rotor winding is also important

Stator FaultRotor faultAbnormal Running Condition

INSULATION FAILURE FAULT

Insulation failure. Tends to deteriate with rising temp. Insulation failure may cause inter-turn fault, ph

to ph or earth fault. Bring winding in to direct contact with core

plates. Any failure to restrict earth fault may result into

core plate damage. Insulation of rotor winding is also important.

PROTECTION APPLIED TO GENERATOR Relays to detect faults outside generator Relays to detect faults in side generator Over speed protections. Temp measuring device for bearings, stator

winding, Oil temp.

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GeneratorStator protection:

Stator faults include the following-i. Phase-to-earth faultsii. Phase-to-phase faultsiii.Inter-turn faultsFrom these phase faults and inter turn faults are

lesscommon ,these usually develop into an earth

faults.This causes-• Arcing to core• Damage of conductor and insulation

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DIFFERENTIAL/MERZ-PRICE PROTECTION(Phase to Phase Fault)

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• Differential protection is a very reliable method of protecting generators from the effects of internal faults

• Under normal conditions or for a fault outside of the protected zone, current through R1,2&3 are equal

• Therefore the currents in the current transformers secondary are also equal, i.e. CT1 = CT2 and no current flows through the relay

• If a fault develops inside of the protected zone, currents CT1 and CT2 are no longer equal, therefore current through R1,2&3 are not equal and there is a current flowing through the relay.

GENERATOR DIFFERENTIAL PROTECTION

Modified differential protection

Phase to Earth Fault

Modified differential protection:

• If any fault occurs near the neutral point then the fault current is very small and relay does not operate.

• Modified differential protection scheme is used to over come this.

• Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.

Stator Earth Fault Relay

mmmmmm

mmmmmm

mmmmmm

mm

mm

mm

mm

mm

mm

Loading resistor Over voltage relay

With time delay

STATOR EARTHFAULT RELAY

INTER-TURN FAULT PROTECTION

87’s are relays

INTER-TURN FAULT RELAY OPERATION

The inter-turn fault is a short circuit between the turns of the same phase winding

The current transformers are connected in the two parallel paths of the each phase winding

The secondaries of the current transformers are cross connected. The current transformers work on circulating current principle

The relay is connected across the cross connected secondaries of the current transformers.

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GeneratorRotor protection

Rotor Earth Fault ProtectionUnbalanced Loading/Negative sequence relay ProtectionLoss of Excitation Protection

• The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac)

• But dc source is generally used as over-current relay in case of dc is more sensitive than ac

• A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay

Rotor Earth Fault Effect First(E/F) Protection

D.C. Injection Method Rotor Fault Protection

AC Injection methodRotor earth fault protection

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• Unbalance loading causing negative sequence currents which produce a reverse sequence rotating field in the machine

• This induces double frequency eddy currents in the rotor

leading to overheating and

• Unbalance loading gives rise to double frequency eddy currents induced in rotor which may cause excessive overheating

NEGATIVE PHASE SEQUENCE CURRENT

PROTECTION

Negative phase sequenceprotection:

Negative Sequence Vectors

1250 1160

1200

Negative phase sequenceprotection:

• Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current .

• This current in rotor causes rotor overheating and damage to the rotor.

• This can be protected by negative sequence current filter with over current relay.

Negative phase sequence protection:

Field(Excitation) failure protection

Field(Excitation) failure protection• This normally closed contact of sensitive magnetic coil relay

remains open as the relay coil is energized by shunted excitation current during normal operation of the excitation system.

• As soon as there is any failure of excitation system, the relay coil becomes de-energized and the normally closed contact closes the supply across the coil of timing relay T1.

• As the relay coil is energized, the normally open contact of this relay T 1 is closed.

• This contact closes the supply across another timing relay T 2 with an adjustable pickup time delay of 2 to 10 seconds which in turn operates tripping coil

FIELD(EXCITATION) CAUSES

Loss of generator field excitation under normal running conditions may arise due to any of the following condition.1. Failure of brush gear.2. unintentional opening of the field circuit breaker.3. Failure of AVR control

4. Field open circuit 5. Field short circuit 6. Accidental tripping of field Breaker 7. Loss of supply to main Exciter 8. Poor Brush contact in Exciter 9. Field Current Breaker Latch Failure 10. Slip ring Flash Over

Loss of excitation Effect

When the excitation of generator is lost it operate as aInduction generator. It derives excitation from thesystem and supply power at leading power factor. Which may cause- A fall in voltage & so loss of synchronism & system

instability. Over heating of rotor due to induction current on it.A protection having MHO characteristicis used to detect loss of field.

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• Senses when the generator’s excitation system has been lost.

• When generator loses excitation it will steal excitation from other gensets & quickly overheat the rotor due to induced slip-frequency currents

• Reverse VAR protection overcome this problem

Loss of Field Protection

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Loss of Field Protection

When prime-mover fails machine starts motoring and draws electrical power from the system and this is known as inverted operation .

The generator can be protected from inverted operation by using single-element directional power relay(reverse power relay) which senses the direction of power flow.

Failure of the prime mover of a generator set ,will keep the set running as asynchronous compensator

Reverse Power Protection

Reverse power relay scheme

Over voltage protection:

Over voltage may be caused due to- Transient over voltage in the transmission

line due to lightening. Defective operation of the voltage

regulator. Sudden loss of load due to line tripping.

The protection is provided with an over voltage relay.

Overcurrent protection:

• Overloading of the machine causes overheating in the stator winding.

• This can be prevented by using over-current relay with time delay adjustment.

• But overheating not only depends on over-current but also the failure of the cooling system in the generator.

• So temperature detector coils such as thermistors or thermocouples are used at various points in stator winding for indication of the temperature.

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• Resistance temperature detectors are used to sense winding temperatures.

Temperature Protection

EQUIPMENT GROUNDING

Prevents shock exposure of personnel Provides current carrying capability for the ground-fault

current Grounding includes design and construction of

substation ground mat and CT and VT safety grounding

GENERATOR PROTECTION SUMMARY

Name Input Protecting toDifferential protection

Differential Current Stator core and winding

Stator earth fault Voltage Stator core and windingOver current Current Stator core and windingOver voltage Voltage Stator core and windingInter-turn short circuit Current Stator core and windingRotor Earth Fault Current Rotor windingOver and under frequency

Frequency Turbine protection

Reverse power flow Voltage and current

Turbine protection

Loss of excitation Voltage and current

Power System Protection

Back up protection for lines

Voltage and current

Generator protection

Questions?