The working principle of insulation monitor



The DC power supply of power plants and substations is a very large multi-branch power supply network as the security power supply and control signal power supply for the main electrical equipment. In general, one point grounding does not affect the operation of the DC system, but if the ground fault point cannot be quickly found and repaired, and another point grounding fault occurs, it may cause a major fault.

The existing detection methods mainly include the bridge balance principle and the low frequency detection principle. The insulation monitoring device based on the principle of bridge balance is widely used, but it cannot detect the situation when the positive and negative insulation of the DC system are equal to each other. Even if the insulation monitoring device gives an alarm, it cannot directly obtain the insulation resistance of the system to the ground. Using low-frequency detection principle to detect grounding fault is a new method used in recent years, but the grounding resistance it can detect is restricted by the distributed capacitance of the DC system to the ground, and the low-frequency AC signal is easily interfered by the outside world, and the low-frequency AC signal injected increases the voltage ripple coefficient of the DC system. It can be seen that both the bridge balance principle and the low-frequency detection principle have some insurmountable defects. In this paper, a new detection method is proposed, that is, the main circuit uses an unbalanced bridge to detect the total insulation resistance, and the branch uses a DC transformer to detect which way the insulation is reduced. At the same time, the single chip microcomputer is used to realize this detection method.

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