Differential instantaneous protection sets for fast fault clearing when serious fault happens inside a transformer and the general percentage differential protection may be no longer in force. Differential instantaneous protection maloperation is a big problem in actual engineering application. This maloperation is caused by low-pressure side current transformer saturation which happens in a load transformer such as the high-voltage house service operating transformer. For a case of external fault maloperation of differential instantaneous protection for high-voltage house service operating transformer

it is based on an analysis of the saturation characteristics of the current transformer to research the causes of differential relay maloperation. Countermeasures have been proposed to solve maloperation of differential instantaneous protection by using differential instantaneous protection made up of power frequency differential relays. Moreover

this maloperation is caused by low-pressure side current transformer saturation with high-voltage house service operating transformer and load transformer of direct distribution line.