What Is the DC Insulation Monitoring Device Working Principle?

In order to ensure the power supply safety of the DC power system, a DC insulation monitoring device is indispensable. This article will introduce the DC insulation monitoring device working principle in detail.

1. Basic DC insulation monitoring device working principle

The insulation monitoring of the DC power system involves the insulation monitoring of the DC bus as well as the insulation monitoring of each load branch. In practical applications, bus monitoring and branch monitoring will adopt different monitoring principles. The combination of them forms the insulation monitoring of the entire DC power system. The basic working principle of insulation monitoring device is in the following:

Under normal conditions, the busbar insulation monitoring is always active, and the busbar voltage deviation (the decrease in single-ended insulation resistance directly reflects the busbar-to-ground voltage offset) or the decrease in busbar-to-ground insulation resistance can initiate the detection of a branch circuit.

Branch monitoring typically operates in standby mode. Only when the insulation of the busbar is abnormal will the insulation examination of each branch be initiated in order to locate the appropriate insulation fault branch.

2. The insulation monitoring device working principle of the DC bus

As illustrated in the figure above, R+ and R- are the equivalent insulation resistances of the positive and negative busbars to the ground, while R1 and R2 are the balance resistances of the positive and negative busbars to the ground, respectively. These two resistors help to balance the voltage of the busbar to the ground under typical conditions, preventing the voltage of the busbar from becoming unstable due to the busbar’s excessive insulation resistance to the ground. The two groups of resistances switched by K1 ~ K2 are different, so the measured positive and negative bus voltages are different, so as to calculate the insulation resistance to the ground.

First, Under normal circumstances:

The positive and negative bus-to-ground voltages U1 and U2 are measured in order to derive the equations for R+ and R-. All switches (K1, K2) are disconnected. That is, under normal circumstances, the insulation resistance of positive and negative busbars to the ground.

Second, when a positive or negative bus voltage offset happens, you need:

(1) first close the switch K1, K2, and then measure the positive and negative bus-to-ground voltage U1, U2 to obtain the formulae for R+ and R-.

(2) After disconnecting K1, measure the positive and negative busbar voltages at U1 and U2, then use the results to derive the formulae for R+ and R-.

The grounding resistance of the positive and negative busbars can be properly identified by measuring the grounding resistance of the positive and negative busbars using the unbalanced bridge method.

3. The Insulation monitoring device working principle of the DC branch circuit

Leakage current detection is typically carried out on the branch output of the power system in order to precisely determine the fault point of the insulation drop of the DC power system. The corresponding magnitude of insulation resistance is then determined in accordance with the magnitude of the leakage current, and an alarm is given for the branch insulation fault.

The leakage current sensor is simultaneously passed through both the positive and negative connections. The positive and negative wires’ DC and DC currents are equal in size and flow in the opposite direction when there is no insulation drop, therefore the Hall sensor won’t produce an induction signal. The current flowing through the positive and negative poles will no longer be equal when one pole is grounded or when the insulation is decreased. The Hall sensor will now pick up the current difference signal.