Stator Earth Fault Protection

SUDHIR KUMAR SRIVASTAV

Additional General Manager-RAPDRP

NTPC LIMITED, NEW DELHI

sudhirksri1@gmail.com

Stator Earth Fault Protection

Stator winding faults: These types of faults occur due to the insulation breakdown of the

stator coils. Different types of stator windings faults are:

a) Phase to earth fault

b) Phase to phase fault

c) Inter turn fault

Phase to earth fault are limited by Grounding Transformer connected to the neutral of stator winding. There are fewer chances for the occurrence of the phase to phase and inter turn faults. The insulation between the two phases is at least twice as thick as the insulation between one coil and the iron core, so phase to phase fault is less likely to occur. Inter turn fault occurs due the incoming current surges with steep wave front. High impedance reduces the fault current and thus it is very difficult for differential protection system, to detect the high impedance faults. So the differential protection does not work for the high impedance grounding.

To overcome this limitation, an Inverse Definite Minimum Time over voltage relay (64G1) is connected to the secondary side of neutral grounding transformer & another Inverse Definite Minimum Time over voltage relay (64G2) is connected across an open delta of generator PT secondary winding.

Stator Earth Fault Main Protection (64G1): The secondary winding of neutral grounding transformer is shorted through loading resistance(R).  Inverse Definite Minimum Time over voltage relay (64G1) is connected across loading resistance(R). The separate relay to the ground neutral provides the sensitive protection. But ground relay can also detect the fault beyond the generator, so the time co-ordination is necessary to overcome this difficulty.

For an earth fault in stator winding, the E/F current flows in primary winding of neutral grounding transformer. As a result, a voltage is developed across the loading resistance (R) which activate earth fault sensing relay (64G1).

When the earth fault occurs at terminal end of stator winding, full voltage is developed at neutral of stator winding & maximum voltage is developed across neutral grounding transformer, resulting faster operation of inverse definite minimum time over voltage relay. But when the fault is near neutral, very less voltage is developed at neutral of stator winding & very less voltage is developed across neutral grounding transformer, resulting slower operation of inverse definite minimum time over voltage relay.  If the fault occur very near to the neutral i.e. less than 5% of stator winding, there is chance that relay may not operate.

So with this protection system only 95% of the stator winding is protected and 5% of the stator winding starting from neutral point remains unprotected because a fault in this portion will generate too low voltage for relay operation.

Stator Earth Fault Standby Protection (64G2): The Inverse Definite Minimum Time over voltage relay (64G2) is connected across an open delta of the generator VT secondary winding. When there is no E/F, the phasor sum of voltages of all three phase generator VT secondary will be zero, resulting non-operation of relay.

When there is E/F in one phase, the voltage of that phase will be less resulting unbalance phasor sum of voltages of all three phase generator VT secondary and a voltage will appear at relay, causing operation of relay & tripping the system.

Neutral third harmonic under voltage: There is the third harmonic present between the neutral and the ground , and protection schemes takes advantages of this and respond to the under voltage between the neutral and the ground.

100% protection scheme: This scheme provides complete protection of the stator winding by injecting the signal between the stator winding and monitors it for change. 95% scheme and third harmonics protection scheme provide protection only at rated speed and rated voltage but the 100% scheme also provide protection at standstill.

Overview of 100% Stator Ground Fault Protection

Third Harmonic Neutral Undervoltage Scheme. In the late 1970’s, a major European manufacturer introduced a third-harmonic neutral under voltage relay that in conjunction with the traditional 60Hz overvoltage protection, could provide stator ground fault protection over the entire stator winding. The third harmonic was measured across the generator neutral grounding resistor. The scheme’s basic concept is that when a generator stator ground fault occurs near the generator neutral, the third-harmonic voltage goes to zero. If the generator has enough third harmonic neutral voltage present during normal operation, such generators are candidates for 100% schemes using third-harmonic neutral detection.

Third-Harmonic Ratio Scheme. In the early 1980’s, a second third-harmonic scheme was developed by an American manufacturer. This scheme compared the third harmonic at the neutral and terminals of the generator. The major advantage of the scheme was that it was more secure than simply using third harmonic under voltage measured at the generator neutral. It required a broken-delta potential connection on the generator terminals to measure the terminal value of third-harmonic voltage. This required the installation of a VT-—the primary winding of which needed to be wye-grounded. Many generators, especially smaller units, required the addition of this VT since these generators used open delta-phase VT connections. This additional cost and wiring complexity reduced the number of people that used the scheme.  

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