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generator earth fault protection

2016/08/25
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Generator neutral is normally earthed in all systems. Normal Earth fault relay at the neutral of the generators will detect most of the earth faults occurring in the system. Hence the term 95% earth fault detection. However if an earth fault occurs very near to the earthed neutral of the stator and especially if the fault is just before the actual neutral earthing it is very difficult to detect that fault. Hence to detect such faults 100% stator earth fault relays are incorporated into the system. The earthing due to earth fault at that point normally does not affects the working of the system since the neutral point is already earthed. However if another fault occurs at another portion of the system this earth may carry heavy current at that time, damaging the neutral of the stator.

From symmetrical components we know that single phase to ground fault current is calculated by the formula I = 3 Ufn / (Z1+Z2+Z0+3Rn) where:
* Ufn is the phase to neutral pre-fault voltage at the point where fail occurs;

- Z1, Z2 and Z0 are the positive, negative and zero sequence Thevenin impedances (as seen from the point where the fault occurs)

* Rn is the neutral grounding resistance (or combination earthing transformer/resistor connected between generator neutral point and earth)

When we apply this formula for a SLG fault at the generator terminal, and considering that the generator is normally connected to the delta side of the step-up transformer, Z1, Z2 are the parallel combinations of generator and transformer impedances (Z1 = ZG1*ZT/(ZG1+ZT) and Z2 = ZG2*ZT/(ZG2+ZT)). Z0 and Rn are the generator zero seq. impedance and grounding resistance.

But when we apply the same formula for a ground fault internal to the generator stator winding, the voltage to be used in the formula will be k*Uph, where k is a factor which varies from 1 (terminal fault) to 0 (fault at neutral point). Now it will be clear that faults near the neutral point will produce very low fault current which cannot be detected by normal over current relays; so the rule is to detect them with residual over voltage relay measured by the earthing transformer mentioned above. This protection is good for faults covering up to 95% of the generator winding (k = 1 to 0.05). For faults in the portion of the winding between 95% and 100% (k = 0.05 to 0), ground fault current is negligible and must be actually injected in the winding from the external in order to detect such a fault.

Stator ground fault means 100% of the stator winding is protected against ground faults and that includes from the generator terminals all the way to the stator neutral. For 95% stator ground protection means only 95% of the stator winding is protected against ground faults and the remaining 5% which is the neutral part of the generator is not protected against ground faults. Detecting the ground faults at the remaining 5% of the stator winding which is the neutral can be very difficult when a ground fault should occur at that location.

There are schemes that are available to protect the remaining 5% to achieve a 100% ground fault protection to include the measurement of the third harmonic voltage 27THD against the threshold that every generator with active part winding should have, etc.
For 100% protection, low voltage at low frequencies(15Hz, 20hz,etc) are injected periodically into the stator winding and a measure low frequency current is done at the neutral and that is what predicts if there is a ground fault by measuring the harmonic voltages.

The principle is the following: a signal generator injects voltage pulses in the windings of the machine at a frequency different from the rate frequency of the system or any harmonic (typically 10 Hz to 25 Hz, 87 Hz...). These pulses are transmitted to ground through the winding insulation and back to the signal generator through the earthing resistor. See more on our blog.

Together with the signal generator, the relay also measures the current in the signal injected. In normal conditions, total impedance is high and low currents are measured. If there is a phase to earth fault in any false, impedance to ground decreases and current increases rapidly, indicating the fault.

This method works even with the machine out of service if the relay is connected (careful with this during maintenance). This system is normally implemented at the generator neutral transformer, but if it is not available (for example, because a resistance or reactance is used instead of a transformer, or the generator is delta connected), voltage transformers can be connected at the generator terminals with the secondary connected in broken delta, since this allows the injection of the same signal to all windings.

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