According to a trip event of redundant pole control system in Baoan converter station, based on onsite description and observation and analysis for spoiled COL module, we found out that a capacitor breakdown led to the COL fault. The capacitor breakdown produced a nonlinear and low resistance short circuit. This translated into that COL was burned down. An improved scheme is given for COL installation and power supply circuit in pole control cubicle. The improved scheme can avoid trip caused by COL fault, and greatly improve the reliability and stability of pole control system.