GENERATOR PROTECTION SYSTEM

TYPES OF FAULTS :-

The following problems required addressing & consideration from the point of view of applying protection:-

• Short circuit protection • Stator electrical faults • Loss of excitation • Over voltage • Overload protection • Unbalanced load protection • Under /over frequency protection • Mechanical Faults

Different Protections used for Generator

Generator Differential Protection. Generator Inter Turn Protection. Generator Negative Sequence Protection. Generator Loss of Excitation Protection. Generator Over Voltage Protection. Frequency Protection Generator Over Load/ Over Current Protection Generator Rotor Earth Fault Protection Generator Back Up Impedance protection Generator Low Forward power Interlock Generator reverse power protection. 95% stator Earth Fault Protection

Instrument Transformers

Current Transformers

• Turns ratio is inversely proportional to the ratio specified

• Secondary current is according to primary current only

• Should not be left open circuited

Potential Transformers

• Turns ratio is directly proportional to the ratio specified

• Secondary current is according to burden connected

• Should not be short circuited

Protection Classes Class A Protection• This master trip will operate when the fault

needs to isolate the generator immediately• This gives command to open the generator

breaker and field breaker. Class B Protection• This master trip will operate when the fault is not

serious in view of generator or it is not directly harmful to generator

• This gives the command to trip turbine or Prime Mover Tripped

Elements of protection system• Current relays• Voltage relays• Power relays• Impedance relays• Frequency relays• Special function relays• Auxiliary Relays• Timers

Specifications of Generator

Make: BHEL MVA: 247 Connection: Three

Phase Double Star Connected.

Voltage: 15750 Volts Current: 9050 Amp Frequency: 50 Hz Speed: 3000 RPM No. of Poles: 2

Synchronous Impedance: 222%

Sub Transient Impedance: 30.5%

Transient Impedance: 21.4%

Cooling: water/ Hydrogen Cooled

Neutral Grounding: High Impedance (Through NGT)

PROTECTION AGAINST 3 PHASE , SHORT CIRCUIT (51 V)

Voltage restrained over current relay is one of the most commonly used protection against generator external short circuit protection.

The reason of introducing a voltage signal into over current device is to provide a relay that can ride through momentary overload conditions such as motor starting and still provide proper short circuit protection.

The voltage is said to restrain the current element. Typical voltage setting of this relay is (80-90%) of nominal.

EARTH FAULT PROTECTION :-( 51 G )

A relay connected to a current transformer mounted on the Neutral- Earth conductor can provide back up protection for all earth relays at the generator voltage level, It provides protection against generator internal earth faults, however not all the stator winding can be protected against earth fault using this protection function.

The relay settings should be higher than the expected harmonic current following the neutral during normal load condition and not more than 33% of the maximum earth faults generator currents.

STATOR WINDING ELECTRICAL FAULTS Failure of the stator winding or

connection insulation can result in a severe damage to the winding and stator core.

The extent of the damage depends on the magnitude and duration of the fault current.

DIFFERENTIAL PROTECTION (87) To respond quickly to a phase fault with damaging heavy currents, high

speed differential protection is normally applied to generator equal or

higher than 500 KVA.

Differential relays operates only for faults within their protected zone

they are inherently Selective and can operate much faster. Accordingly

they will provide better protection and in large generating systems

maintaining stability by fast clearing faulted generators.

The zone of protection of differential protection can be extended to

cover the connecting cabling.

Typical setting of the differential relay is 5- 10% difference in CT’s current.

STATOR OVERHEATING PROTECTION (49Q)

Accidental over loading might occur through the combination of full active load current governed by the prime mover (KW) output and an abnormally high reactive current component KVAR output, governed by the level of rotor excitation.

Thermal relay (49 Q ) is normally connected to RTD’S resistance temperature detectors imbedded in the stator winding of generators .It Provides protection for the machine in the over load area but not fault conditions

Each phase of the winding shall be connected to 2 RTD’S at the

hot spot defined by supplier. The trip & alarm setting of this relay depends on the insulation

class & temperature rise of the generator. The generator

manufacture should be consulted during the setting of this relay.

PROTECTION AGAINST LOSS OF EXCITATION

Loss of excitation can occur when the generator is operating at only 20- 30% of rated power. In this case the machine runs as induction

generator execited from the others machines on the system. The generator

quickly over heats due to the system slip frequency currents induced in it , generator terminal voltage decreases and the stator currents starts to increase. Rapid automatic disconnection is then required in order not only to protect the generator but also to protect other machines from which

the faulted machine withdraw its excitation as an induction generator . The last might cause instability of the power system & over heating of

the other machines.

PROTECTION AGAINST LOSS OF EXCITATION

The last might cause instability of the power system & over heating of the other machines.

Relay (40) loss of excitation is an impedance

viewed from the generator terminals. This relay must be used with a proper time delay to stabilize the protection against maloperation in response to transient conditions.

PROTECTION AGAINST OVER VOLTAGES Over voltage may occur due to the transient surges on the

network and can also be caused from other reasons such as :

a) Defective operation of AVR when the machine is running in

isolated operation.

b) Sudden loss of load (Due to the tripping of outgoing feeders

leaving the machine isolated or feeding over small load).

c) Operation under manual central of the voltage regulator. A sudden change in load particularly reactive powered component will give rise a substantial change in voltage.

PROTECTION AGAINST OVER VOLTAGES

A typical setting for over voltage relay (59) is 107 % of rated stator voltage with a time delay of 10 sec. to allow for transients due to load switch off / rejection , over voltages resulting from recovery from fault or motor starting .

UNDER VOLTAGE PROTECTION :-( 27) (OVER LOAD PROTECTION )

Under voltage protection is used for

generator feeding an isolated system. It protects generators against prolonged over load or failure of AVR.

Setting must be chosen to avoid

maloperation during voltage dips during power system fault clearance or associated with motor starting.

PROTECTION AGAINST UNBALANCED CONDITIONS RELAY FUNCTION (46)

Phase balanced load produces a reaction field that is constant and

rotates synchronously with the rotor field system. Any phase unbalance

can be resolved into positive and negative sequence component.

The positive sequence component is similar to the normal balanced

load. The zero sequence produces no main armature reaction.

The negative sequence component field reaction rotates in the opposite

direction to the DC field system, thereby producing double frequency

currents in the field system and in rotor body. The resulting eddy

currents are very large and cause severe heating of the rotor.

This protection is not required when the generator has large negative

phase sequence capacity.

UNDER / OVER FREQUENCY PROTECTION 81% SEQUENCE CAPACITY :-

The governor fitted to the prime mover normally provides protection against over frequency, however over frequency may occur in case sudden removal loads. Under frequency as a result of over load of generators operating in isolated systems. Prime mover & generator load must be protected against excessively low Frequency by tripping of the generators concerned.

Typical settings of under frequency is 49 HZ for 20 seconds

& 48 HZ

for 0.5 seconds.

Over frequency setting 51 HZ for 10 seconds and 52 HZ for

1 second

REVERSE POWER PROTECTION :(32)

Reverse power protection in its simple form means motoring of the prim mover , if the prime mover has lost power for any reason and start to be motored by the system. Diesel engine could catch fire or suffer from damages to its gear box or shafts. Gas turbine could suffers from gear box damage. The manufacture value for motoring percentage of rated power should be consulted. The reverse power protection should be provided with time delay, on operation to prevent operation of the relay with transient power swings that may arise following synchronization

Typical protection settings is 50% of motoring power .This setting must be checked during commissioning.

GENERATOR BEARING PROTECTION: -(38) Failure of the generator lubrication system may

cause over heating of generator bearing. Consequently causing

mechanical damage. Resistance temperature detectors (RTD’s) embedded

near the driving & non driving end bearing to trip the

generator breaker in case of over heating of the bearing. The generator manufacture should be consulted to

provide this setting.

Specifications of Generator

Make: BHEL MVA: 247 Connection: Three

Phase Double Star Connected.

Voltage: 15750 Volts Current: 9050 Amp Frequency: 50 Hz Speed: 3000 RPM No. of Poles: 2

Synchronous Impedance: 222%

Sub Transient Impedance: 30.5%

Transient Impedance: 21.4%

Cooling: water/ Hydrogen Cooled

Neutral Grounding: High Impedance (Through NGT)

Generator Inter Turn Protection

Difference of sharing between split phases

Reasons of diff. In currents • If there is any inter-turn

short with in the winging• If gen is supplying through

fault and small circulating currents gets proportionately increased

• Tripping provided at 5% of rated current

Generator Negative Sequence Protection.

Balanced and unbalanced systems Sequence components• Positive sequence component

I1 = 1/3 (IR +a2 *IY + a * IB)• Negative sequence component

I2 = 1/3 (IR +a *IY + a2 * IB)• Zero sequence component

I0 = 1/3 (IR +IY + IB) Effect of negative sequence component on

synchronous machine. Generator Capability for I2• Continuous I2 capability 5% of Im• I22 * t = 8

Balanced Phasers

Generator Loss of Excitation Protection. Study of Generator capability curve

Setting criteriaXa1 = 0.5*Xd*CTR/PTRXd = 30.5% = 0.305 pu ……sub-transient reactance of generator Xd in ohms = Xd (pu)*kv2/MVA

= 0.305*15.752/247 =0.306311993

Xa1 = 2.139 ohm Xb1 = Xd * CTR/PTRXd (ohmic) = Xd(pu)*kv2/MVAXb1 = 2.22(15.752/247)(10000/5)(110/15750) = 31.14 ohm

Generator capability curve

Generator Over Voltage Protection.

Generator high voltage is harm full to the insulation of generator, generator transformer, excitation transformer, UAT, etc

Generator Over Voltage Capability – 10% Setting Adopted• Stage – 1 = 10%, 5 Sec delay• Stage – 2 = 15%, 500msec delay

Frequency Protection

Over frequency protection• It indicates excess generation than

demand hence tripping should graded among the units

Under frequency protectionUnder frequency is harmful to turbine Adopted settings

Adopted Frequency settingsUNIT NO.

CAPACITY UNDER FREQ. TRIP SETTING

OVER FREQUENCY TRIP SETTING

1 30 MW 47 HZ, 0.5 SEC DELAY

54.5 HZ, 21.15 SEC DELAY

2 30 MW

3 210 MW 52.5 HZ, 1 SEC DELAY

4 210 MW 52.0 HZ, 2 SEC DELAY

5 210 MW 52.0 HZ, 2.0 SEC DELAY

GENERATOR PROTECTION SETTINGS EXAMPLES