| With the development of automation technology, examine and repair methods of high voltage devices used in the electric power system are developing. Condition-based maintenance, as an advanced technology, will be widely used in the near future. The data acquired by on-line monitoring system is the basis of condition-based maintenance. To realize condition-based maintenance for Metal-Oxide Arrestor (MOA), which is a kind of important device used tore strain over voltage in the electric power system, the research of on-line monitoring of MOA is important and necessary.According to simplify equivalent circuit of ZnO element, the characteristic of leakage currency of MOA and some disturb factors are discussed. Then the disquisition deeply analyzed the existing methods used in MOA monitoring. On account of the rich harmonic in power supply system, using basic resistance wave current as a monitoring element is more reliable than using third harmonic wave current. With digital waveform analysis method the basic resistance wave currency can easily be got. As for the structure of monitoring system, the distributed system is more suitable for MOA monitoring than centralized system. Based on the conclusion mentioned above, a distributed online-monitoring scheme using RS485 and MPU is designed. Choose AT89C52 as the system processor, the hardware platform of the local monitoring unit is designed carefully, especially the leakage current signal getting circuit and the primary-secondary isolation circuit. Also the printed circuit board is accomplished and debugged. Based on the hardware platform, the flow chart of the main program and module program are given in the paper. The application layer communication protocol between local units and center unit is also designed. The software of information management system was developed with Visual C++6.0. Data report can be formed in this system. The state of MOA can be inspected conveniently and accurately. Electric power system can run in the reliable state.
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