Based on the analysis and application investment of direct current system insulation supervisory device of power station and substation,two defects has been found which can t be resolved by itself.

Upon analysis of the parameter design and test results for four types of microcomputer-based insulation monitor,it is pointed out that the shortcomings of the traditional insulation monitor relay are solved with the extensive application of the microcomputer-based insulation monitor,but the voltage fluctuations of positive and negative poles are brought forward.