A loss-of-excitation on a generator occurs when the field current is no longer supplied. This situation can be triggered by a variety of circumstances and the following situation will then develop:

1. When the field supply is removed, the generator real power will remain almost constant during the next seconds. Because of the drop in the excitation voltage, the generator output voltage drops gradually. To compensate for the drop in voltage, the current increases at about the same rate.

2. The generator then becomes underexcited and it will absorb increasingly negative reactive power.

3. Because the ratio of the generator voltage over the current becomes smaller and smaller with the phase current leading the phase voltage, the generator positive sequence impedance as measured at its terminals will enter the impedance plane in the second quadrant.

Experience has shown that the positive sequence impedance will settle to a value between Xd and Xq.

The most popular protection against a loss-of-excitation situation uses an offset-mho relay as shown in Fig. 9.8 (IEEE, 1989). The relay is supplied with generator terminals voltages and currents and is normally associated with a definite time delay.

Many modern digital relays will use the positive sequence voltage and current to evaluate the positive sequence impedance as seen at the generator terminal.

Figure 9.9 shows the digitally emulated positive sequence impedance trajectory of a 200 MVA generator connected to an infinite bus through an 8% impedance transformer when the field voltage was removed at 0 second time.

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