When the capacitive current of a generator is slightly large, a resistor must be added at the generator neutral point to avoid power-frequency overvoltage that could damage the motor insulation during a ground fault. The damping effect of this resistor reduces the overvoltage and limits the ground fault current. During a single-phase ground fault of the generator, the neutral-to-ground voltage equals the phase voltage, typically several kilovolts or even above 10 kV. Therefore, this resistor must have a very high resistance value, which is economically expensive.
In general, a large high-value resistor is not directly connected between the generator neutral point and ground. Instead, a combination of a small resistor and a grounding transformer is used. The primary winding of the grounding transformer is connected between the neutral point and ground, while a small resistor is connected to the secondary winding. According to the formula, the impedance reflected to the primary side equals the secondary-side resistance multiplied by the square of the transformer turns ratio. Thus, with a grounding transformer, a small resistor can function as a high-value resistor.
During a generator ground fault, the neutral-to-ground voltage (equal to the voltage applied across the primary winding of the grounding transformer) induces a corresponding voltage on the secondary winding, which can be used as a basis for ground fault protection—i.e., the grounding transformer can extract zero-sequence voltage.
The rated primary voltage of the transformer is 1.05 times the generator phase voltage, and the rated secondary voltage is 100 volts. It is easy to connect a resistor to the secondary winding, and a 100 V resistor is readily available. Although the reflected ground fault current on the primary side becomes large due to the transformer ratio, a generator ground fault should trigger immediate tripping and shutdown, so the current duration is very short, resulting in minimal thermal effects, which poses no problem.
Login
