The differential protection outlet is tripped in the charging of 750 kV shunt reactor during commissioning of certain EHV substation. Through the analysis of waveform

data

installation and test of primary equipment

the reason for the action is due to reverse connection of casing CT polarity at the end of reactor and type mistake of anti-pumping reply of phase a for 7521 circuit breaker. Moreover

the defects found are treated timely so to assure successful charging and smoothness of subsequence commissioning items.Through analysis of the fault it is important to measure correctly polarity of CT and prevent differential action and mal-operation due to wrong connection of polarity;and more strict requirement to the installation and testing of anti-pumping relay is proposed.