But in this case the protective relay must be slightly time-delayed to avoid spurious trips caused by circulation of a false zero sequence current following a brief period of saturation in the current transformers (magnetizing current, switching surge or down stream faulty). This limitation leads to too high a setting compared to the maximum available earth fault current, and consequently to a loss of sensitivity.
This difficulty can be largely overcome by using a ring current transformer in neutral point of power transformer in order to achieve zero-sequence current measurement. In this case the earth fault relay can simultaneously provide both rapid and sensitive earth fault protection.
This technical recommendation is applied to a new island electrical system with power auto transformer by a design group. They used the earth fault relay and related NCT in neutral point of auto transformer. But they had not suitable response from earth fault protection against the occurrence of ground faults.
How can you explain the reason of protection system inability in this case?