Protection of generators
Electrical protection is required to quickly detect and initiate shutdown for major electrical faults associated with the generating plant and, less urgently, to detect abnormal operating conditions which, if sustained, may lead to plant damage.
Abnormal electrical conditions can arise as a result of some failure with the generating plant itself, but can also be externally imposed on the generator. Common categories of faults and abnormal conditions to be electrically detected are listed as follows: (Not all conditions have to be detected for all applications.)
MAJOR ELECTRICAL FAULTS
* Insulation failure of stator windings or connections
SECONDARY ELECTRICAL FAULTS
· Insulation failure of excitation system
· Failure of excitation system
· Unsynchronised over voltage
ABNORMAL PRIME MOVER OR CONTROL CONDITIONS
· Failure of prime mover
· Over frequency
· Over fluxing
· Dead machine energisation
· Breaker Flashover
· Feeding an uncleared fault
· Prolonged or heavy unbalanced loading
· Prolonged or heavy overload
· Loss of synchronism
· Over frequency
· Under frequency
· Synchronised over voltage
· Over fluxing
· Under voltage
In addition to the range of electrical protection required for a generator, varying types and levels of mechanical protection are necessary, such as vibration detection, lubricant and coolant monitoring, etc.
The action required following response of an element of electrical or mechanical
protection is often categorised as follows:
· Urgent shutdown
· Non-urgent shutdown
· Alarm only
An urgent shutdown would be required, for example, if a phase to phase fault occurred within the generator electrical connections. A non-urgent shutdown might be sequential, where the prime mover may be shutdown prior to electrically unloading the generator, in order to avoid over speed in the case of a steam turbine. A nonurgent shutdown may be initiated in the case of continued unbalanced loading. In the event of unbalanced loading, it is desirable that an alarm should be given before shutdown becomes necessary, in order to allow for possible operator intervention to remedy the situation.
For urgent tripping, it may be desirable to electrically maintain the shutdown condition with latching protection output contacts, which would require manual resetting. For a non-urgent shutdown, it may be required that the output contacts reset without intervention, so that production of power can be recommenced as soon as possible.
LGPG111 integrated protection
The LGPG111 incorporates the commonly required protection functions for a wide variety of generating plant applications in a single integrated package, see Figure . Flexible scheme logic is also provided to allow the protection output contacts of the package to be configured to execute required categories of action. Optically-isolated logic inputs are provided to allow the status of external plant to be monitored and to exercise control over the protection functions. It is also possible for mechanical protection functions to initiate alarms or action via the LGPG111 scheme logic and to be monitored via the serial communications facility provided with the protection package.
With its frequency tracking system, the LGPG111 is able to maintain all protection functions in service over a wide range of operating frequency. It should not be necessary, for instance, to disable the negative phase sequence thermal protection when running a generator at low frequency; as might be necessary with existing discrete relay schemes. This LGPG111 capability will be of especial interest for pumped storage generation schemes, where synchronous machines would be operated from a variable frequency supply when in pumping mode. Additionally, in the case of combined cycle generating plant, it may be necessary to excite and synchronise a steam turbine generating set with a gas turbine set at low frequency, prior to running up to nominal frequency and synchronising with the power system.
When LGPG111 protection functions are required to operate accurately at low frequency, it will be necessary to use CT’s with larger cores. In effect the CT kneepoint voltage requirements will be multiplied by fn/f,where f is the minimum required operating frequency and fn is the nominal operating frequency.
In the case of a synchronous machine at a pumped storage plant being operated in the motoring mode, it would be necessary to disable reverse power protection in this mode. This could be accomplished by switching to the LGPG111’s alternative group of settings where the reverse power protection function would not be enabled.
LGPG111 protection functions
The following protection functions are provided within the LGPG111 package:
· Generator Differential protection (87G)
· Stator Earth Fault protection (51N)
· Neutral Voltage Displacement protection (59N)
· Sensitive Directional Earth Fault protection (67N)
· Voltage-Dependent Overcurrent protection (51V)
· Negative Phase Sequence Thermal protection (46)
· Field Failure protection (40)
· Reverse Power protection (32R)
· Low Forward Power protection (32L)
· Over Voltage protection (59)
· Under Voltage protection (27)
· Over Frequency protection (810)
· Under Frequency protection (81 U)
· Voltage Balance protection (60)
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