Abstract: Compound voltage over-current and zero-sequence over-current protection are usually used as backup protection for phase and ground faults in generator-transformer unit. Since the boundary of protective zone of these two types of protection is difficult to delimit, it frequently leads to miscoordination with protection in grid side. In addition, it is difficult for protection in grid side to coordinate with the protection in power plant side. Both cases can give arise to misoperation of protection at faults. To meet the requirements of speed, selectivity and sensitivity, phase and ground impedance protection are introduced into generator-transformer unit. The setting principle for impedance protection proposed is discussed in detail and a calculation case is given taking a 750 kV power plant as an example. The scheme has been successfully applied to the large generator-transformer unit and its adjacent grid in Northwest China, which greatly improve coordination of backup protection in power plant side and grid side.

Key words: generator-transformer unit, backup protection, impedance protection, setting coordination