| While measuring dielectric loss, the present traditional online insulation monitoring systems generally have the problem of installation complexity and low performance of anti-interference. Considering above shortcomings, we have designed a new type of online monitoring system that need not connect secondary-lines of PT (Potential Transformer) to every devices. It uses only one twisted-pair bus line to connect all the devices of the system.We lead a 50Hz AC power line to every measurement devices installed on capacitive high-voltage equipments and PT, which is used as a reference voltage signal and is also used as the power supply of all the devices. After the host computer sends the broadcast command to order measurement operation, all slave devices will start to sample the input signals synchronously. When the sampling operation has finished, devices installed on the capacitive equipments will calculate the phase angle difference (Φ　oi) between leakage current and the reference voltage signal ,and devices installed on PT will calculate the phase angle difference between secondary voltage and the reference voltage signal (Φou). When all the measurements operation have completed, the host will send a series of commands to get the calculated results. The program in the host computer can easily compute the value of dielectric loss (tan δ).Devices installed on high voltage equipments work under the control of single-chip computer of Intel 80C196KB.RS485 is chosen as physical standard of the communication system. It has excellent performance of anti-interference and a very low cost. To ensure the host computer can reliably communicate with all devices, we formulated a flexible communication protocol.The article describes the design principle of the system. We give schematic circuit diagram of hardware, communication protocol, software flow-chars and experiment data, and the measuring result data of low voltage experiments as well.
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