Generator protection -- part 2 [Industrial Electrical Power Systems]

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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 grounded. If an ground 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 ground 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’s therefore important that rotor ground fault protection be installed. This can be done in a variety of ways.

Potentiometer method

The field winding is connected with a resistance having center tap. The tap point is connected to the ground through a sensitive relay R. An ground fault in the field winding produces a voltage across the relay.

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The maximum voltage occurs for faults at end of the windings. However, there are chances that the faults at the center of the winding may get undetected. Hence, one lower tap is provided in the resistance. Though normally, the center tap is connected, a pushbutton or a bypass switch is used to check for the faults at the center of winding. A proper operating procedure shall be established to ensure that this changeover is done at least once in a day.

Field winding --Potentiometer

++++ Potentiometer

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. An ground fault at any part of the winding gives rise to the field current, which is detected by the sensitive relay. Care should be taken to ensure that the bearings are insulated, since there is a constant current flowing to the ground through the capacitance.

Field winding AC injection AUX emf

++++ AC injection

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 ground. An ground fault causes the fault current to flow through the DC power unit causing the sensitive relay to operate under fault conditions.

Field winding E DC injection AUX emf

++++ DC injection

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 won’t have been designed to run continuously in such an asynchronous fashion. 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. If the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole pitch. If the disturbance has passed, by the time this pole slip occurs, then the machine may regain synchronism otherwise it must be isolated from the system.

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’s 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 won’t help in view of the large kinetic energy of the machine. The field cannot be destroyed immediately and the flux energy must be dissipated without causing excessive inductive voltage rise in the field circuit.

For small- to medium-sized machines this can be satisfactorily achieved using an automatic air circuit breaker with blow-out contacts. On larger sets above say 5 MVA a field discharge resistor is used.

Industrial generator protection

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. Note that the differential relay (though not discussed separately in this section) is normally necessary for generators in the range of megawatts.

++++ Typical protection scheme for industrial generator

Numerical relays

The above paragraphs described use of individual relays for different fault conditions.

However, the modern numerical relays combine most of the above functions in a single relay with programming features that make them useful for any capacity generator.

The numerical relays are manufactured by all the leading relay manufacturers. The various protections functions that are available in a typical numerical relay are as below.

  1. Inverse time overcurrent
  2. Voltage restrained phase overcurrent
  3. Negative sequence overcurrent
  4. •Ground overcurrent
  5. Phase differential
  6. Ground directional
  7. High-set phase overcurrent
  8. Undervoltage
  9. Overvoltage
  10. Volts/hertz
  11. Phase reversal
  12. Under frequency
  13. Over frequency
  14. Neutral overvoltage (fundamental)
  15. Neutral undervoltage (3rd harmonic)

++++ Generator protection relay by GE

  1. Loss of excitation
  2. Distance elements
  3. Low forward power
GE's DGP is a digital system which provides a wide range of protection, monitoring, control and recording functions for AC generators. It can be used on generators driven by steam, gas and hydraulic turbine. Any size of generator can be protected with the DGP. A high degree of dependability and security is achieved by extensive self diagnostic routines and an optional redundant power supply.


Stator differential
Current unbalance
Loss of excitation
Time overcurrent, voltage restraint
100% stator ground fault
Ground overcurrent
Over excitation
Over and undervoltage
Over and underfrequency
Voltage transformer fuse failure
Accidental energization
Eight configurable output relays
Up to three configurable inputs


Extensive self-test diagnostics
Trip circuit monitor
Metering: A V W vars Hz
Events - last 100
Fault reports - last 3

The relays can also be able to develop the thermal model for the generators being protected, based on the safe stall time, previous start performances, etc., which is used to prevent the restart attempt of the generator under abnormal conditions or after a few unsuccessful starts/trips.

In addition to the various protection functions, these numerical relays also record the generator output figures like voltage, current, active power, reactive power, power factor, temperature of stator/rotor windings, etc. on a continuous basis. Hence, the numerical relays are finding increasing applications in modern industries.

Parallel operation with grid

In modern industries and continuous process plants, it’s customary to have the plant generators (gas/steam turbine or diesel engine driven) to be operated in parallel with the grid to ensure uninterrupted power to essential loads. The basic protection employed in such systems are use of reverse power relays, which are used basically, to protect the grid from the faulty generators operating as motors.

It’s also quite common to see that these systems are provided with 'islanding' feature, which enables the unstable grid to be isolated from the stable generating sets due to transmission disturbances. The protection employed in such cases are under frequency and dv/df , which are basically the effects of grid disturbances.

It’s also common that power is exported to the grid from the industrial generators, when the power is generated in excess of the demand. The protective systems employed in all such cases shall be discussed with supply authorities to ensure that all protective functions as required per local regulations are met.

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Monday, January 21, 2013 0:34