Why Stator Earth Fault Protection is required for a Generator?
Nowadays, the capacity of generators increases by more than 500 MW. The protection of the generator is crucial to reduce the outage time. Stator Earth Fault Protection is one of the major protections providing for generators.
Short circuits between the stator winding in the slots and the stator core are the most common electrical fault in Generators. Interturn faults, which normally are difficult to detect, will quickly develop into a ground fault and will be tripped by the stator Earth fault protection.
Why Grounding is needed?
- Grounding improves the safety of the equipment by allowing the detection of faults in it.
- The ungrounded systems the chances of occurring transient overvoltages is likely too high whereas grounding suppresses these transient voltages.
- The impact of fault is reduced by preventing the fault to convert into multi-phase to ground faults. This is possible by detecting the Single line to Ground faults in the earlier stages and helps to operate protection.
- By introducing the required impedance in grounding, the ground-fault current is limited and hence damage caused by faults.
- The same ground source can be used for other zone protections also.
Generator Earthing Methods:
It is important to know the method of Earthing used for the given Generator. Based on the Earthing method only we can provide the necessary protection. The following are the various Earthing methods and protections offered.
- Stator ground overcurrent (Low, Medium Impedance Ground, Neutral CT of flux summating CT) – 50/51N
- Stator ground overcurrent (High Impedance gnd) – 51GN, 51N
Low Impedance Earthing: (50/51N)
Low impedance Earthing consists of a low-value grounding resistance connected between generator neutral terminal and ground.
The lower resistance provides better grounding source but at the same time, lower value of resistance causes more damage to the generator windings on internal ground faults.
In this method of grounding ground fault current is high, sometimes up to 400A.
High Impedance Earthing: (51GN, 51N)
In most countries, it is a common practice to ground the generator neutral through a Grounding Transformer called Neutral Grounding Transformer NGT having a loading resistor across its secondary.
This method of Earthing is called High Impedance earthing where the Earth fault current is limited to 5-10 Amps.
Stator Earth fault protection:
Stator Earth fault protection of any Generator provided in the following ways.
95% Stator Protection:
- Neutral Overvoltage Scheme
- Overcurrent Scheme
100% Stator Protection:
- 3rd Harmonic Under-Voltage Scheme
- Neutral Injection Scheme
95% Stator Earth Fault Protection:
Neutral Overvotage (59N) or Neutral Displacement Relay (64N):
For generators with unit transformer (GT) and with high impedance grounding of the neutral, a neutral voltage relay with harmonic immunity and the independent time delay is used.
High impedance ground limits ground fault current to about 10A.
- Conventional Neutral overvoltage relay provides 90-95% stator winding coverage.
- The last 5 to 10% near neutral is not covered.
- Provided by a neutral voltage relay with harmonic restraint and adjustable time delay
- The relay is normally set to operate at 5% of maximum neutral voltage with a time delay of 0.3 – 0.5 Second. With this voltage setting, it protects approximately 95% of the stator winding.
- The relay is fed either from a neutral voltage transformer or from the broken delta winding of 3-phase voltage transformers on the generator line side.
Why Only 95% Protection?
It is due to the sensitivity of Neutral Displacement Relay [NDR] current setting. The relay has a minimum sensitivity of 5%.
As shown in figure below
- NDR setting = 5 V
- Volts across NGT(11KV/240V) Secondary for terminal earth fault = 240 /1.732 = 138V.
- Winding coverage = (138 – 5) / 138 = 0.95 (95%)
- Identical coverage by a sensitive current relay with a 5% setting.
From the above discussion, we came to know that Current Relay with a sensitive current setting (say 5%) can provide about 95% coverage for stator winding.
Overcurrent Protection (51N):
This method is used for low impedance grounded machines. The relay 51N element detects overcurrents in neutral circuit. It coordinates with system ground fault protection for security and selectivity.
In the case of parallel operation of machines 67N protection is also used for directional ground fault detection. This relay is operated in case the zero-sequence current or ground current flowing towards the generator.
Drawback of 95% Stator Protection:
Undetected grounds in this region bypass grounding transformer, solidly grounding the machine.
100% Stator Earth Fault Protection:
This protection also takes care of the fault occurring near to neutral. Such type of fault is critical as the fault near to neutral shunts the high resistance that saves the stator from large currents with an internal ground fault.
3rd Harmonic Undervoltage:
Ground faults caused by mechanical damage may occur close to the generator neutral. Today there is a distinct trend towards providing ground fault protection for the entire stator winding (100% Stator Earth Fault Protection).
The 100% stator earth fault scheme includes a 95% unit, which covers the stator winding from 5% of the neutral and a 3rd harmonic voltage measuring unit which protects the rest of the stator winding.
Workings of Third harmonic Neutral Undervoltage relay (27TN):
- Generator during normal running operation inherently generates third harmonic currents due to many reasons. Like imperfections in stator windings, the pitch of the stator windings and variation in the amount of field excitation, etc.
- The percentage of these third component voltages is usually fixed for a particular design of the generator. Now in the case of stator ground faults, there is a fall in the level of these third harmonic component currents. By measuring this level we can provide 100 percent stator earth fault protection.
- Generator winding and terminal capacitance provide a path for the third-harmonic stator current via grounding resistor.
- The voltages developed in the grounding components due to third harmonic currents are used in protection schemes for enhanced ground fault protection coverage.
- A fault near the neutral shunts the third harmonic near the neutral to ground. This results in third-harmonic under voltages are developed.
- The relay (27TN) detects the fall in voltage in case of the ground fault near to neutral and issues trip signal.
Third harmonic ratio voltage- 59D
The 59d relay compares third harmonic voltages at terminal and neutral ends. These voltages are nearly equal to each other. In case of a ground fault, one of these voltages goes very low at either end of the winding.
Drawback of Third Harmonic Protection:
Third harmonic protections may not work with all generators, and may not work at all times on a generator. The third harmonic values that are available to the protection vary with operational mode and power output (real and reactive).
Neutral Injection – 64S:
This is called the subharmonic Injection method. In this advanced method, we can provide 100% stator earth fault protection. The advantage is that this scheme has the ability to detect ground faults anywhere in the stator or in the unit-connected zone regardless of the generator operation and loading.
The subharmonic injection is used because the capacitive coupling induced current effect is reduced by using a lower injection frequency.20Hz signal is injected into the grounding transformer secondary circuit. If there is Rise in real component of injected current means resistive ground fault presents. The element is sensitive for only zero-sequence currents in the generator neutral.
Stator Earth Fault Protection Summary:
Stator Earth fault protection provided for both Low and High Impedance generators.
- For High Impedance Generators:
95% Stator ground fault protection provided by 59N. Additional 5% near neutral is provided by 27TN or 59D
64S alone provides 100% Stator Earth fault protection.
- For Low Impedance Generators:
90 to 95% Stator ground fault protection provided by 51N and/or 67N.