Study On MOA On-line Monitoring Equipment Used In Distribution Network

Arresters are used to limit over-voltage and protect the insulation of the electrical equipment. At present, zinc oxide arresters (MOA) are increasingly used in electrical power system. Due to the operational features of the MOA, in the long run under the operation voltage, the insulation of the MOA gradually aging. It manifested as the leakage current increases with the time increasing. Serious insulation aging can lead to insulation damage. Thus the electrical equipment losses protection, and the power network blackout. Detecting the leakage current on a regular basis is an important measure to ensure the safe operation of the MOA. But due to the large number of the distribution arresters, if taking the preventive test, it needs power off and consume a great amount of manpower and material resources. Therefore, a lot of operation department extend the inspection cycle or even cancel the preventive test, waiting for changing a new one after the MOA breakdown. The MOA usually use synthetic insulation coats. It is difficult to find the fault point, as the fault can not be shown on the outside. Therefore, on-line monitoring is the preferred method.Due to the low cost and the small impact on the distributed network, the monitor of MOA used in 10kV distribution network is always ignored. As users become increasingly demanding on power quality, and the intelligent development of the distribution network, the electrical department and the users pay more and more attention to the safe and reliable operation. But the existing monitoring methods are usually used in the high voltage level. Although those devices can be used in the distribution network, but they are too expensive for widely use.The paper takes simulation and experiment on MOA, and on this base the paper proposes a new method:indirectly obtaining the resistance leakage current by calculating the capacitive current. The paper adopts a new means:measuring total leakage current with "zero flux" current transformer. Also the paper designed a protection and anti-jamming circuit to ensure the safe operation of the monitoring systemAt present, those online monitoring methods usually measure the total leakage current, or the resistive leakage current, and they are mostly used in high voltage level. The grids under those voltage levels usually install voltage transformers. They facilitate the measurement of the reference voltage. In this paper, the equivalent capacitance is treated as a function of time. The capacitive current can be calculated by using the relationship between grid voltage and capacitance. And using the detected total leakage current signal, the device ultimately gets the discrete resistive current signal. The advantage of this method is that it only monitors the current signal. So it is very suitable for distribution network which does not install voltage transformer. The method does not need measure the reference voltage and does not need consider the phase angle error brought by the reference voltage.The key of online monitoring is the total leakage current measurement. The paper designs a current transformer based on the "zero flux" method to extract microampere-level current. It uses compensation circuit compensating the excitation current in the core to achieve "zero flux". By this, the current sensor can increase the precision. After the total current leakage is collected, it flows though a designed signal acquisition circuit to get discrete data. And the discrete data are sent to the MCU to process. The warning is issued or not depending on the processing result. Taking into account the surrounding electromagnetic fields, dirty, lightning and other factors, This paper also designs a set of protection and anti-jamming circuit to ensure the normal operation of the online monitoring equipment...