Through a fault of series compensation equipment， the reasons of the action of maximum external fault current protection， capacitor unbalance protection， and trigger of spark gap are analyzed in the paper. The reliability of protection action is improved with improving shield of secondary cable of platform. The design defect of capacitor unbalance protection is solved through processing filter and increasing the current transformer on aggregate expenditure capacitor branch. Through increasing the monitoring measures for spark gap trigger circuit， the analysis of mal-trigger of the spark gap in future is facilitated.