1.Introduction

1.1Applicablescope

DC insulation monitoring deviceis very important in DC system. The operation practice proves that,the harm of DC system grounding may result in the maloperation andrefusal-operation of relay protection device, and even themaloperation and refusal-operation of primary e under DCcontrol, to seriously influence the safe and stable operation ofelectric power system. Therefore, it is necessary to have areal-time online monitoring of DC system grounding insulationconditions, and eliminate the faults as soon as possible in case ofgrounding.

In combinationwith the operation experience of WZJ-21 microcomputer-based DCinsulation monitoring device developed by our company in electricpower system for years, we have developed WZJ-31microcomputer-based DC insulation monitoring device (hereinafterreferred to as WZJ-31 device). It can realize the real-time onlinemonitoring of insulation conditions of DC bus and branch, timelyfind out and determine the grounding bus or branch in case of DCgrounding, and give a warning signal; moreover, it can detect outthe information of power frequency AC signal mixed in DC system aswell as the interconnection fault of parasitic branch and twobuses, so as to timely discover the hidden danger existing in thesystem. In addition, WZJ-31 device is provided with bus levelingfunction, conducting bus leveling when the system is grounded, toavoid maloperation. WZJ-31 device is made up of two parts, namelyhost and slave, in which, the host communicates with the displaythrough RS485, and the host communicates with the slave throughCAN. Besides, WZJ-31 device may communicate with upper computerthrough RS485. WZJ-31 device, adopting the principle of combinationbetween balanced bridge and unbalanced bridge, will detect the busgrounding insulation state, not inject AC to DC system, and noinfluenced by DC feeder line ground capacitance. This device isapplicable to many complicated DC systems used in power plants andtransformer substations, etc.