Generator Protection

ENERATOR

ENERATOR

PROTECTION

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A generator is the heart of an electrical power system, as it A generator is the heart of an electrical power system, as it converts mechanical energy into its electrical equivalent, converts mechanical energy into its electrical equivalent, which is further distributed at various voltages. It therefore which is further distributed at various voltages. It therefore requires a

requires a ‘prime‘prime mover’mover’ to develop this mechanical powerto develop this mechanical power and this can take the form

and this can take the form of steam, gas or water turbines orof steam, gas or water turbines or diesel engines.

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A generator is the heart of an electrical power system, as it A generator is the heart of an electrical power system, as it converts mechanical energy into its electrical equivalent, converts mechanical energy into its electrical equivalent, which is further distributed at various voltages. It therefore which is further distributed at various voltages. It therefore requires a

requires a ‘prime‘prime mover’mover’ to develop this mechanical powerto develop this mechanical power and this can take the form

and this can take the form of steam, gas or water turbines orof steam, gas or water turbines or diesel engines.

TURBINES

TURBINES

Steam turbines are used virtually exclusively Steam turbines are used virtually exclusively byby the main power utilities, whereas in industry

the main power utilities, whereas in industry three main types of prime movers are in use: three main types of prime movers are in use: 1.

1. Steam turbines: Normally found where waste steam is Steam turbines: Normally found where waste steam is  available and used for 

available and used for base load or standby.base load or standby. 2.

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

applications. 3.

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

a complex system, comprising of number of components:

•• Stator windingStator winding with associated main and unit transformerswith associated main and unit transformers

•• RotorRotor with its field winding and exciterswith its field winding and exciters

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FOLLOWS

FOLLOWS

Internal faults Internal faults

1. Primary and backup phase or ground faults in the

1. Primary and backup phase or ground faults in the stator andstator and associated Areas

associated Areas

2. Ground faults in the rotor and loss

2. Ground faults in the rotor and loss-of-field excitation-of-field excitation B. System disturbances and operational hazards

B. System disturbances and operational hazards 1. Loss of prime-mover; generator motoring

1. Loss of prime-mover; generator motoring 2. Over excitation: volts or hertz protection 2. Over excitation: volts or hertz protection

3. Inadvertent energization : non synchronized connection 3. Inadvertent energization : non synchronized connection 4. Unbalanced currents: negative sequence ; breaker pole 4. Unbalanced currents: negative sequence ; breaker pole

flashover flashover

5. Thermal overload 5. Thermal overload

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6. Off-frequency operation for large steam turbines 7. Un cleared system faults: backup distance ;

voltage controlled time over current (50V) 8. Overvoltage

9. Loss of synchronism: out of step 10. Sub synchronous oscillations

11. Loss of voltage transformer signal to relaying or voltage regulator

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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. Rotor Earth Fault Protection (64R)

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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

MANY DIFFERENT FAULTS CAN OCCUR ON THIS SYSTEM, FOR WHICH DIVERSE PROTECTION MEANS ARE REQUIRED.

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 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

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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.

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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.

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 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 regulator

2. Manual control error (sudden variation of load) 3. Sudden loss of load due to other circuit tripping.

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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%

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 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

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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

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 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

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 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.

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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

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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

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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.

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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

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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.

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 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.

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 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.

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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.

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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.

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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.

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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

 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

STATOR PHASE-FAULT PROTECTION FOR

ALL SIZE GENERATORS

MULTI-CT DIFFERENTIAL PROTECTION (87)

FOR ALL SIZE GENERATORS