The three-phase voltage source PWM rectifier（VSR） is key equipment in connecting the distributed generators to the small-current neutral grounding system. So

we have analyzed the frequency disturbance of the switching function as well as the change of positive-pole/negative-pole insulation for the DC side of VSR. The results demonstrate that the neutral-point voltage deviation（NPVD） of the three-phase VSR is a crucial factor which affects the response of insulation monitoring device（IMD）. Thus

a pair of methods that take the mean and spectra of the neutral-point voltage as the characteristics

are proposed to detect the NPVD

one is adopted to detect the mean value of the neutral-point voltage based on DC signal injection

and the other one is adopted to determine the frequency disturbance of the neutral-point voltage based on Duffing oscillator. So

the detection result for NPVD is obtained by the variety of the characteristics mentioned above

which is induced by the deviation of the neutral-point voltage. The test results demonstrate that the neutral point deviation caused by the change of DC-side insulation balancing is the primary factor which distorts the detection of insulation resistance; the relative error of detection results obtained by IMD is less than 4% under normal condition

and the relative error is no less than 41% while the neutral point deviation occurs. Further

the test results certify the feasibility of the proposed method

which can be used for improving the response precision of IMD.