Power station generator zero-phase current is too high

The voltages will be minimum and the currents maximum, when the systems are 180 degrees, out of phase. Unbalanced faults and other system conditions can cause unbalanced three phase currents in the generator. The negative sequence components of these currents cause double frequency currents in the rotor that can lead to overheating and damage.

To respond quickly to a phase fault, sensitive and high-speed differential protection is normally applied to generators rated in excess of 1MVA. If CTs are connected as shown in Figure C8.3, it can be seen that current flowing through the zone of protection will cause current to circulate around the secondary wiring.

During the time when the generator is accelerating, very high currents are induced in the rotor and it may be damaged very quickly. Faults in the system can result in a system breakup into islands, which leaves an imbalance between available generation and the load. This results in an excess of power for the connected loads.

The zero-sequence impedance of a synchronous generator is typically about half of the positive-sequence subtransient reactance of the machine. This means that a single-phase-to-ground fault generates a per-phase fault current greater than the per-phase fault current generated by a three-phase fault .

A fault level that is too low can be a serious safety concern since it can prevent overcurrent protection from operating and lead to fire hazards, such as arcing faults. Occasionally, an ungrounded system is employed where the generator neutrals are not connected together. In this scenario, there will be no path for the circulating current to flow.

It could be useful to take one of the generators that exhibits the high B phase current and roll its connection to the system. If that generator winding continues to have high current the problem is likely with the generator, but if the problem remains with B phase it is a system issue.