MICOM P 342, 343 GENERATOR PROTECTION RELAYS

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.