GENERATOR PROTECTION

INTRODUCTION A generator is the heart of an electrical power

system, as it converts mechanical energy into its electrical equivalent, which is further distributed at various voltages. It therefore requires a ‘prime mover’ to develop this mechanical power and this can take the form of steam, gas or water turbines or diesel engines.

TURBINESSteam turbines are used virtually exclusively bythe main power utilities, whereas in industry three main types of prime movers are in use:1. Steam turbines: Normally found where waste

steam is available and used for base load or standby.

2. Gas turbines: Generally used for peak-lopping or mobile applications.

3. Diesel engines: Most popular as standby plant.

It will be appreciated that a modern large generating unit is a complex system, comprising of number of components:

• Stator winding with associated main and unit transformers

• Rotor with its field winding and exciters• Turbine with its boiler, condenser, auxiliary fans

and pumps.

GENERATOR HAZARDS AND PROBLEMS ARE AS FOLLOWSInternal faults1. Primary and backup phase or ground faults in the stator

and associated Areas2. Ground faults in the rotor and loss-of-field excitation

B. System disturbances and operational hazards1. Loss of prime-mover; generator motoring 2. Over excitation: volts or hertz protection 3. Inadvertent energization : non synchronized connection 4. Unbalanced currents: negative sequence ; breaker pole

flashover5. Thermal overload

SYSTEM DISTURBANCES AND OPERATIONAL HAZARDS6. Off-frequency operation for large steam

turbines7. Un cleared system faults: backup distance ;

voltage controlled time over current (50V)8. Overvoltage 9. Loss of synchronism: out of step10. Sub synchronous oscillations11. Loss of voltage transformer signal to

relaying or voltage regulator12. Generator breaker failure

GENERATOR PROTECTION The following are the main protection schemes

adopted for generator.1. Generator Differential Protection 2. Generator & Transformer Differential Protection 3. Loss of Field or Loss of Excitation Protection 4. Negative Sequence or Current Unbalance Protection 5. Over Fluxing or Over Excitation Protection 6. Over Current Protection 7. Stator Earth Fault Protection 8.

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GENERATOR PROTECTION 8. Rotor Earth Fault Protection (64R)

9. Restricted Earth Fault Protection 10. Backup Impedance Protection 11. Low Forward Power Protection 12. Reverse Power Protection 13. Pole Slip Protection 14. Pole Discrepancy Protection 15. Local Breaker Back Protection 16. Bus Bar Protection 17. Over Frequency Protection 18. Under Frequency Protection 19. Over Voltage Protection

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MANY DIFFERENT FAULTS CAN OCCUR ON THIS SYSTEM, FOR WHICH DIVERSE PROTECTION MEANS ARE REQUIRED. THESE CAN BE GROUPED INTO TWO CATEGORIES:

STATOR INSULATION FAILURE The stator insulation failure can lead to earth fault

in the system. The neutral point of the generator stator winding

is normally earthed so that it can be protected, and impedance is generally used to limit earth fault current.

Earth fault protection can be applied by using a transformer and adopting a relay to measure the earthing transformer secondary current or by connecting a voltage-operated relay in parallel with the loading resistor

GROUND-FAULT PROTECTION FOR UNGROUNDED GENERATORS GROUNDED BY THE CONNECTED SYSTEM.

OVERLOAD PROTECTION Generators are very rarely troubled by overload, as

the amount of power they can deliver is a function of the prime mover, which is being continuously monitored by its governors and regulator. Where overload protection is provided, it usually takes the form of a thermocouple or thermistor embedded in the stator winding. The rotor winding is checked by measuring the resistance of the field winding.

OVER CURRENT PROTECTION It is normal practice to apply IDMTL relays for over

current protection, not for thermal protection of the machine but as a ‘back-up’ feature to operate only under fault conditions. In the case of a single machine feeding an isolated system, this relay could be connected to a single CT in the neutral end in order to cover a winding fault. With multiple generators in parallel, there is difficulty in arriving at a suitable setting so the relays are then connected to line side CTs.

OVERVOLTAGE PROTECTION Overvoltage can occur as either a high-speed

transient or a sustained condition at system frequency.

Power frequency over voltages are normally the result of:

1. Defective voltage regulator2. Manual control error (sudden variation of load)3. Sudden loss of load due to other circuit tripping.

OVERVOLTAGE PROTECTION Overvoltage protection is therefore only applied to

unattended automatic machines, at say a hydroelectric station. The normal setting adopted are quite high almost equal to 150%

but with instantaneous operation

UNBALANCED LOADING Any unbalanced condition can be broken down into

positive, negative and zero sequence components. The positive component behaves similar to the balanced load. The zero components produce no main armature reaction. However, the negative component creates a reaction field, which rotates counter to the DC field, and hence produces a flux, which cuts the rotor at twice the rotational velocity. This induces double frequency currents in the field system and rotor body.

GROUND (ZERO-SEQUENCE) DIFFERENTIAL PROTECTION FOR A GENERATOR USING A DIRECTIONAL GROUND–OVERCURRENT RELAY.

UNBALANCED LOADING The resulting eddy currents are very large, so severe

that excessive heating occurs, quickly heating the brass rotor slot wedges to the softening point where they are susceptible to being extruded under centrifugal force until they stand above the rotor surface, in danger of striking the stator iron

It is therefore very important that negative phase sequence protection be installed, to protect

against unbalanced loading and it consequences.

ROTOR FAULTS The rotor has a DC supply fed onto its winding

which sets up a standing flux. When this flux is rotated by the prime mover, it cuts the stator winding to induce current and voltage therein. This DC supply from the exciter need not be earthed

ROTOR FAULTS If an earth fault occurs, no fault current will flow

and the machine can continue to run indefinitely, however, one would be unaware of this condition. Danger then arises if a second earth fault occurs at another point in the winding, thereby shorting out portion of the winding. This causes the field current to increase and be diverted, burning out conductors.

ROTOR FAULTS In addition, the fluxes become distorted resulting in

unbalanced mechanical forces on the rotor causing violent vibrations, which may damage the bearings and even displace the rotor by an amount, which would cause it to foul the stator. It is therefore important that rotor earth fault protection be installed. This can be done in a variety of ways.

ROTOR EARTH FAULT PROTECTION METHODS Potentiometer method AC injection method DC injection method

POTENTIOMETER METHOD

The field winding is connected with a resistance

having center tap. The tap point is connected to

the earth through a sensitive relay R. An earth fault in the field winding produces a voltage across the relay.

POTENTIOMETER METHOD

AC INJECTION METHOD

This method requires an auxiliary supply, which is injected to the field circuit through a coupling capacitance. The capacitor prevents the chances of higher DC current passing through the transformer.

AC INJECTION METHOD

DC INJECTION METHOD

This method avoids the capacitance currents by rectifying the injection voltage adopted in the previous method. The auxiliary voltage is used to bias the field voltage to be negative with respect to the earth. An earth fault causes the fault current to flow through the DC power unit causing the sensitive relay to operate under fault conditions

DC INJECTION METHOD

REVERSE POWER Reverse power protection is applicable when

generators run in parallel, and to protect against the failure of the prime mover. Should this fail then, the generator would motor by taking power from the system and could aggravate the failure of the mechanical drive.

LOSS OF EXCITATION If the rotor field system should fail for

whatever reason, the generator would then operate as an induction generator, continuing to generate power determined by the load setting of the turbine governor.

It would be operating at a slip frequency and although there is no immediate danger to the set, heating will occur, as the machine will not have been designed to run continuously in such an asynchronous fashion.

LOSS OF EXCITATION Some form of field failure detection is thus

required, and on the larger machines, this is augmented by a mho-type impedance relay to detect this condition on the primary side.

LOSS OF SYNCHRONIZATION A generator could lose synchronism with the

power system because of a severe system fault disturbance, or operation at a high load with a leading power factor. This shock may cause the rotor to oscillate, with consequent variations of current, voltage and power factor.

LOSS OF SYNCHRONIZATION Alternatively, trip the field switch to run the

machine as an asynchronous generator, reduce the field excitation and load, then reclose the field switch to resynchronize smoothly.

FIELD SUPPRESSION It is obvious that if a machine should develop a

fault, the field should be suppressed as quickly as possible, otherwise the generator will continue to feed its own fault and increase the damage. Removing the motive power will not help in view of the large kinetic energy of the machine.

L OSS OF P RIME -MOVER : GENERATOR MOTORING If the prime -mover supply is removed

while the generator is connected to the power system and the field excite d, the power system will drive the unit as a synchronous motor. This is particularly critical for steam and hydro units. For steam turbine s it causes over heating and potential damage to the turbine and turbine blades.

POWER DIRECTIONAL RELAY

The power directional relay is connected to operate when real power flows into the generator. Typical relay sensitivities with microprocessor relays are as low as 1 mA, which may be required when a generator can operate with partial prime-mover input. The operating time can be approximately 2 sec.

TYPICAL PROTECTION SCHEME FOR INDUSTRIAL GENERATOR

The various methods discussed above are normally applicable for an industrial generator protection. The following sketch shows the various protection schemes employed in an industrial environment.

Of course, not all protections are adopted for every generator since the cost of the installation decides the economics of protection required

• It is one of the important protections to protect generator winding against internal faults such as phase-to-phase and three phase-to-ground faults. This type of fault is very serious because very large current can flow and produce large amounts of damage to the winding if it is allowed to persist. One set current transformers of the generator on neutral and phase side, is exclusively used for this protection.

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Generator Differential Protection

GENERATOR DIFFERENTIAL PROTECTION

The differential protection can not detect turn-to-turn fault and phase to ground within one winding for high impedance neutral grounding generator such as ours. Upon the detection of a phase-to-phase fault in the winding, the unit is tripped with out time delay.

Typical differential (87) connections for the protection of wye- and delta connected generators: (a) wye-connected generator

Typical differential (87) connections for the protection of wye- and delta connected generators: delta-connected generator.

Ground (zero-sequence) differential protection for a generator using a directional ground–over current relay.

STATOR PHASE-FAULT PROTECTION FOR ALL SIZE GENERATORS

MULTI-CT DIFFERENTIAL PROTECTION (87) FOR ALL SIZE GENERATORS

CAPACITIVE COUPLING THROUGH TRANSFORMER BANKS FOR PRIMARY-SIDE GROUND FAULTS: (A) THREE-PHASE SYSTEM DIAGRAM