1.1 Protection of generators
An AC generator forms the electromechanical stage of an overall energy
conversion process that results in the production of electrical power. A
reciprocating engine, or one of many forms of turbine, acts as a prime mover
to provide the rotary mechanical input to the alternator.
There are many forms of generating plant that utilise a variety of sources of
energy available, eg. combustion of fossil fuels, hydro dams and nuclear
fission. Generation schemes may be provided for base-load production, peaklopping
or for providing standby power.
Electrical protection should quickly detect and initiate shutdown for major
electrical faults associated with the generating plant and, less urgently, to
detect abnormal operating conditions which may lead to plant damage.
Abnormal electrical conditions can arise as a result of a failure within the
generating plant itself, but can also be externally imposed on the generator.
Common categories of faults and abnormal conditions which can be detected
electrically 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
System related
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
Undervoltage
TECHNICAL GUIDE TG8614A
MiCOM P342, P343 Volume 1
GENERATOR PROTECTION RELAYS Chapter 2
Page 2 of 103
In addition various types of mechanical protection may be necessary, such as
vibration detection, lubricant and coolant monitoring, temperature detection
etc.
The action required following response of an 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. A non-urgent
shutdown may be initiated in the case of continued unbalanced loading. In this
case, it is desirable that an alarm should be given before shutdown becomes
necessary, in order to allow for 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 are self-reset, so that production of power can be re-started as soon as
possible.
1.2 MiCOM Generator protection relays
MiCOM relays are a new range of products from ALSTOM T& D
Protection & Control Ltd. Using the latest numerical technology the platform
includes devices designed for the application to a wide range of power system
plant such as motors, generators, feeders, overhead lines and cables.
Each relay is designed around a common hardware and software platform in
order to achieve a high degree of commonality between products. One such
product in the range is the P340 Generator protection relays. The relays have
been designed to cater for the protection of a wide range of generators from
small machines, providing standby power on industrial sites, to large machines
in power stations providing for the base load on the grid transmission network.
The relays also include a comprehensive range of non-protection features to aid
with power system diagnosis and fault analysis. All these features can be
accessed remotely from one of the relays remote serial communications options.
1.2.1 Protection features
The P340 relays contain a wide variety of protection functions for the protection
of generators. There are 2 separate models available to cover a wide range of
applications. The protection features of each model are summarised below:
Generator differential protection - Phase segregated differential protection
operating on a biased or high impedance principle. Provides high speed,
discriminative protection for all fault types. { P343 only}
Phase fault overcurrent protection - Two stage non-directional back-up
protection.
Voltage dependent overcurrent/ under impedance protection - Back-up
protection for generators with limited fault current capacity.
TECHNICAL GUIDE TG8614A
MiCOM P342, P343 Volume 1
GENERATOR PROTECTION RELAYS Chapter 2
Page 3 of 103
Earth fault overcurrent protection - Two stage non-directional back-up
protection.
Neutral voltage displacement protection - Provides protection against earth
faults on high impedance earthed systems.
Sensitive directional earth fault protection - Discriminative earth fault
protection for parallel connected generators.
100% Stator earth fault protection - Provides protection against earth faults
close to the generator star point. { P343 only}
Under/ overvoltage protection - Two stage undervoltage and two stage
overvoltage protection.
Under/ over frequency protection - Four stage under frequency and two stage
over frequency protection.
Reverse power - Protection against loss of prime mover.
Low forward power - Provides an interlock for non urgent tripping.
Over power - Back-up overload protection.
Field failure - Two stage element for protection against loss of excitation.
Negative phase sequence protection - Provides protection against
unbalanced loading which can cause overheating of the generator.
Overfluxing - Provides protection for the generator / transformer against
unusual voltage or frequency conditions.
Unintentional energisation at standstill ( dead machine) protection - Protection
against inadvertent closing of the generator circuit breaker when the
machine is not running. { P343 only}
Voltage transformer supervision - To prevent mal-operation of voltage
dependent protection elements upon loss of a VT input signal.
Thermal protection via RTD inputs - Thermal protection for the machine
provided by measuring the temperature of winding/ bearings etc. via
resistive thermal devices embedded within the machine. 10 RTD inputs can
be provided.
Programmable scheme logic - Allowing user defined protection and control
logic to suit particular customer applications.
1.2.2 Non-protection features
Below is a summary of the P340 relay non-protective features.
Measurements - Various measurements of value for display on the relay or
accessed from the serial communications, eg. Currents, voltages,
temperature etc.
Fault/ event/ disturbance records - Available from the serial communications
or on the relay display ( fault records only on relay display) .
Real time clock / time synchronisation - Time synchronisation possible from
relay IRIG-B input.
Four setting groups - Independent setting groups to cater for alternative
power system arrangements or customer specific applications.
TECHNICAL GUIDE TG8614A
MiCOM P342, P343 Volume 1
GENERATOR PROTECTION RELAYS Chapter 2
Page 4 of 103
Remote serial communications - To allow remote access to the relays. The
following communications protocols are supported; Courier, MODBUS,
WorldFIP and IEC870-5-103 ( VDEW) .
Continuous self monitoring - Power on diagnostics and self checking routines
to provide maximum relay reliability and availability.
Circuit breaker state monitoring - Provides indication of discrepancy between
circuit breaker auxiliary contacts.
Circuit breaker condition monitoring - Provides records / alarm outputs
regarding the number of CB operations, sum of the interrupted current and
the breaker operating time.
Commissioning test facilities. |
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