Abstract: Due to the service period was too long and the elements were aged seriously, the maloperation of MPR40C type motor comprehensive protection device were occurred repeatedly and influenced on the reliability of auxiliary power. It will need a lot of funds and longer construction period to replace all existing motor comprehensive devices. The maloperation reasons of motor comprehensive protection devices were investigated and analyzed, and relevant improvement methods were put forward. The measures, such as replacing few elements and adjustment the protection characteristics of the devices, were adopted, the problem of protection device maloperation was solved fundamentally and replacing the protection devices was not needed.

Key words: motor comprehensive protection device, maloperation, fault analysis, improvement scheme, reliability of auxiliary power