Generator Protections are broadly classified into three types.
CLASS – A :-
This covers all electrical protections for faults within the generating unit in which
generator field breaker, generator breaker and turbine should be tripped.
CLASS – B:- This covers all mechanical protections of the turbine in which turbine will be tripped first and following this generator will trip on reverse power / low forward power
protections.
CLASS – C:-
This covers electrical protection for faults in the system in which generator will be
unloaded by tripping of generator breaker only. The unit will come to house load
operation and the UAT will be in service.
Various protections of this class are:
1.220 KV (HV side of Generator Transformer) busbar protections.
2.Generator Transformer HV side breaker pole discrepancy.
3.Generator negative phase sequence protection.
4.Generator Transformer over current / Earth fault protection
5.Reverse power protection without turbine trip.
PROTECTION FUNTIONS:
I - For insulations failures
Differential
Inter-turn fault
Stator Earth Fault (95% & 100%)
Rotor Earth fault (2 Stage)
II – For abnormal running conditions
Loss of excitation (field failure)
Unbalanced loading (negative phase sequence)
Pole sleeping
Over frequency/Over speed
Over voltage
Reverse/Forward power
Impedance/Over current back-up protection, etc…
III – For Generator transformer protections
Differential protection
Bias test
2nd harmonics restrained.
REF protection
FOR INSULATION FAILURES PROTECTIONS:
1. GENERATOR DIFFERENTIAL PROTECTION (87 G): -
It is unit type protection, covering the stator winding for phase to phase faults due to breakdown of insulation between stator phase windings. This relay is not sensitive for single line to earth faults as the earth fault current is limited due to the high neutral earthing resistance. If CTs of identical ratios are used
on neutral and line side of generator, an operating current setting of 20% it can be adopted. It is instantaneous in operation and it trips the generator breaker (Class – A) to eliminate the system in – feed to the fault along with field breaker and turbines.
For all machines of ratings 10 MVA and above, this protection shall be provided.
Diagram:
c) STABILITY TEST:
Apply the full load current on both terminal and neutral side with an angle of 180 degree phase shift on any one side. (Angle will be vary depends on the vector group)
Dyn1 (-30° displacement between HV and LV)
Dyn11 (+30° displacement between HV and LV)
Dd0 (no phase displacement between HV and LV)
Dd6 (180° displacement between HV and LV)
2. INTER TURN FAULT PROTECTION OF THE STATOR WINDING (64GIT) :
Formerly, this type of protection was considered unnecessary because breakdown of insulation between points on the same phase winding, contained in the same slot, and between which a potential difference exists, will very rapidly change into an earth fault, and will be detective by either the differential protections or the stator earth fault protection. An exception is the generator designed to produce a relatively high voltage in comparison to it's output and which therefore contains a large number of conductors per slot. With the size and voltage output of
generators increasing, this form of protection is becoming essential for all generating units.
Diagram:
The recommended relay is the high impedance relay having a setting range of 10-40% of rated current.
Settings:
L-E Voltage of Faulted Phase Uph Min 110 V
L-E Voltage of Unfaulted Phase Uph Max 110 V
Uen> Earth Displacement Voltage 10 V
T-DELAY TRIP Uen/3U0 0.50 sec
It is an over voltage relay monitoring the voltage developed across the secondary of the neutral Grounding transformer in case of ground faults. It covers generator, LV winding of generator transformer and HV winding of UAT. A pickup voltage setting of 5% is adopted with a time delay setting of about 1.0 Sec. For
all machines of ratings 10 MVA and above this shall be provided. Relay application for this protection is mainly influenced by the method of stator earthing. Two methods are in common use.
Resistor earthing
Distribution transformer earthing
With resistor earthing, the fault current is limited to 200-300Amps while with distribution transformer earthing; it is limited to 5-10Amps. The latter method has the advantage of ensuring minimum damage to the
stator core, but it is only practicable when the stator winding is directly connected to the delta winding of the main transformer. The two schemes for stator earth fault protection (95%) are shown below:
100% STATOR EARTH FAULT PROTECTION:-
To provide 100% stator earth fault protection, an additional relay for covering 95-100% of the winding is provided..
This is a 3rd harmonic U/V relay. It protects 100% of stator winding. During the machine running condition there will be certain third harmonic voltage at neutral side of the generator. This 3rd harmonic voltage will come
down when a stator earth fault occurs causing this relay to operate. This shall have voltage check or current check unit, to prevent faulty operation of the relay at generator stand still or during the machine running down
period. The third harmonic relays setting is determined from the amount of generator neutral third harmonic neutral voltages. Calculations can be based on mission specifications and equipment capacitances or on field measurements.
To prevent 64G2 function from false tripping when there is no voltage, or low voltage, on the generator it's
supervised by the phase under-voltage relay 27.Set this relay at 90% of the rated voltage.
Settings:
100% Stator-Earth-Fault Protection ON
Pickup Value of Alarm Stage Rsef< 168 Ohm
Pickup Value of Tripping Stage Rsef<< 84 Ohm
Time Delay of Alarm Stage Rsef< 10.00 sec
Time Delay of Tripping Stage Rsef<< 1.00 sec
Pickup Value of I SEF>> Stage 0.75 A
Supervision Threshold of 20Hz Voltage 1 V
Supervision Threshold of 20Hz Current 10 mA
Test: (According to the theory the test method was given below. With the help of setting, the method was shorting the PT terminal of generator. For an example short GRP1 TB 18&20 then the relay will operate after time delay)
4. ROTOR EARTH FAULT (64F):
A single earth fault on the field winding or in the exciter circuit of a generator is not in itself a danger to the machine. Should a second earth fault develop, however, part of the field winding will become short circuited,
resulting in magnetic un-balance of the filed system with subsequent mechanical damage to the machine bearings. It is necessary to ensure that should a second rotor earth fault occur, the machine is disconnected. This is
achieved by the use of a second rotor earth fault relay which comprises adjustable resistors and a sensing element
Generator protection functions are essential to ensure the safe operation and reliability of generators, particularly in power systems. These functions are designed to detect faults or abnormal conditions that could damage the generator, reduce its efficiency, or cause instability in the power network. Here are some key generator protection functions and common test methods used to ensure they are operational and effective.
Key Generator Protection Functions
Overcurrent Protection:
- Protects the generator from excessive currents caused by short circuits or other issues. Overcurrent protection typically involves relays that trip the generator circuit breaker if currents exceed preset limits.
Overload Protection:
- Monitors current levels to protect against prolonged operation at high load, which can lead to overheating and insulation damage. Overload protection often involves thermal or electronic relays.
Differential Protection:
- Differential protection compares currents at the generator’s input and output to detect internal faults, such as phase-to-phase or phase-to-ground faults. This is one of the most sensitive and selective protection methods, as it responds only to internal faults.
Reverse Power Protection:
- Prevents damage caused by a reverse power flow, which can happen if the generator starts absorbing power instead of producing it (e.g., when an engine-driven generator motorizes due to an external source). Reverse power relays typically detect this condition and trip the generator.
Over/Under Voltage Protection:
- Protects the generator from voltage levels that are too high or too low, which can damage equipment or cause voltage instability. Over/under voltage relays detect these conditions and trigger an alarm or a trip.
Over/Under Frequency Protection:
- Prevents the generator from operating outside its frequency tolerance, which could destabilize the power grid or harm the generator. Over/under frequency relays will trip the generator if the frequency falls outside the acceptable range.
Loss of Excitation Protection:
- Protects against a loss of field current, which can cause the generator to operate in a leading power factor condition, potentially leading to severe voltage instability. Loss of excitation protection usually involves relays monitoring reactive power and field current.
Overtemperature Protection:
- Ensures that the generator does not exceed its safe operating temperature. This is particularly important to protect against insulation damage and potential fires.
Negative Sequence Current Protection:
- Protects the generator against unbalanced loading conditions, which can cause overheating and mechanical stress. Negative sequence current relays detect these unbalanced currents and protect the generator.
Testing Methods for Generator Protection Functions
Testing these protection functions is crucial to verify their reliability and accuracy. The main test methods include:
Primary Injection Testing:
- Primary injection involves applying a known current directly into the protection system’s current transformers (CTs) to simulate fault conditions. This method tests the entire protection scheme, including wiring and relays.
Secondary Injection Testing:
- This method injects test signals directly into the protection relay rather than the primary current circuits. Secondary injection is faster than primary injection and can isolate issues within the relay. It’s commonly used for relay setting verification.
Functional Testing:
- Simulates specific fault conditions (e.g., short circuits, loss of excitation) to check that each protection function responds appropriately. This method tests the end-to-end functionality of the protection system.
Simulation Testing:
- Uses digital fault recorders or simulation software to model and test different fault scenarios and their impact on generator protection. Simulation testing is valuable for complex protection systems.
Trip Testing:
- Verifies that the generator trips correctly when protection limits are exceeded. This includes testing the breaker’s response to ensure it disconnects the generator in case of a fault.
Calibration of Protection Relays:
- Periodic calibration ensures that the relay settings (such as current, voltage, and time delay) remain accurate. Calibration usually involves comparing the relay’s response to set standards.
Heat Run Testing:
- Tests the generator’s overtemperature protection by running it at or above its rated load to ensure the temperature protection relays function correctly.
Dynamic Testing:
- Evaluates the protection system’s response under dynamic loading and fault conditions. This method tests the relays’ performance under conditions that mimic real-world operation, such as load fluctuations and transient faults.
Each of these test methods is critical to maintaining the safety and reliability of generators in power systems. Regular testing is essential, as it ensures that protection systems will respond accurately to faults, thereby reducing the risk of damage to the generator and preventing outages in the power grid.
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