防止高压厂用变压器低压侧单相接地造成发电机单相接地保护误动

大型发电机应装设动作电压≥5V的基波零序电压（3U0)定子单相接地保护，但实际运行中，6kV厂用系统单相接地时，发电机侧基波零序电压高达13．2V和11．9V，造成发电机单相接地保护误动。误动原因是没有考虑高压厂用变压器耦合电容的传递零序电压，应该通过计算传递电压的大小，确定发电机3U0定子接地保护的动作电压值；或者为了获得动作死区≤5％的3U0定子接地保护，增设厂用系统零序制动电压。

Preventing the Generator Stator Ground Protection from Maloperation Caused by the Secondary Ground Fault of High Voltage Station Transformer

The large-sized generator ought to install the ground fault protection based on the zero-sequence fundamental voltage (3U 0)with the setting value not less than 5 V.But in the field operations,The 6 kV station service electrical system took place single-phase-to-ground faults reaulted in the zero-sequence fundamental voltages of generator reaching to 13.2 V and 11.9 V respectively,and the stator ground protection maloperated.Its reason lies in the zero-sequence transferring voltage caused by the coupling capacitance between primary and secondary windings of high voltage station transformer,which was not considered.The value of operating voltage 3U 0 should be set by calculation according to the transformer voltage,or the 3U 0 ground fault protection should extend the zero-sequence voltage of the station service electrical system as the restraint quantity for obtaining the operating dead-zone not more than 5%.