1. Standards and principles of DC insulation test
In the Gb/T18384.1-2015 on-board rechargeable energy storage system, it is stipulated that bMS shall conduct insulation tests on the integrated state of all components of the power lithium-ion battery system, and use the insulation resistance value to calculate the insulation state. Insulation resistance can be divided into total positive to ground and total negative to ground.
The existing bMS test of the energy storage system usually directly borrows the on-board system and its standards, and it is important to use the bridge method to measure, combined with the PCS (energy storage converter) system, the insulation test principle of the entire energy storage system is shown in Figure 1-1. :
Figure 1-1 Principle of insulation test for energy storage system
According to the above figure, Rx is bAT+ resistance to ground, Ry is bAT- resistance to ground, R1 and R2 are standard resistances with known resistance values for measurement. The measurement method is as follows:
Step 1: Close R1, disconnect R2, collect the voltage of U1 to the ground as U1, and collect the total voltage of the battery as U
Step 2: Close R2, disconnect R1, collect the voltage of U2 to the ground as U2, and collect the total voltage of the battery as U
Switching R1 and R2 in time-sharing, according to steps 1 and 2, and the above two equations, the values of Rx and Ry can be solved; Rx and Ry are the total positive and total negative insulation resistance values of the battery to ground, respectively.
2. Problems and decomposition of insulation test of energy storage system
After applying the above test method to a large-capacity energy storage system, there was a false alarm that the insulation resistance was too low, and there was actually no problem of insulation abnormality. The reason for the false alarm will be broken down below.
1. The problem of the electric bridge method
In the process of measuring the insulation resistance by the bridge method, when the KM2 is closed, the battery is connected to the DC side of the PCS system. Because there is a Y capacitor to the ground inside the PCS, the insulation test circuit of the battery is shown in Figure 2-1 according to the test principle. When the channel is switched, Rx, Ry, R1, R2 and other resistors are connected to the positive, negative and PE of the battery. When the positive and negative are switched respectively, the positive and negative levels of the relevant battery will be bounced at the PE point, which will pass through the PE, The Y capacitor and battery cable form a loop to charge and discharge the Y capacitor inside the PCS
Figure 2-1 Insulation test circuit to PCS side Y capacitor charging and discharging circuit
During the switching process of the battery positive and negative impedance test circuit to the ground, the Y capacitor on the DC side of the PCS will be charged and discharged to the ground. During the AC coupling process, the impedance of the capacitor to ground is very small, and the insulation impedance test result will also be It is very small, and then an insulation test failure is reported.
2. Non-isolated system
Regarding the non-isolated PCS system, the AC side of the PCS is connected to the mains. Since the mains may be connected to various loads, the impedance of the AC incoming phase of the mains is relatively small. In the non-isolated system, except for the DC side test In the charging and discharging circuit, once the PCS closes the AC contactor or relay, compared to before the PCS starts to work (closes the AC side contactor or relay), the bridge method is also used to measure, the insulation resistance test on the DC side will be lower, even Approaching 0.
Figure 2-2 Schematic diagram of the mains side circuit of the non-isolated system insulation test
3. Multi-machine non-isolated parallel connection
Regarding the use of a multi-module non-isolated parallel system, non-isolated PCS can be simply understood as a certain impedance from the battery end to the AC end, so that the bMS internal resistance is connected to the AC end in parallel through the PCS internal resistance, thereby presenting the effect of impedance paralleling, and large-scale paralleling Later, when using the bridge method to measure the insulation resistance, the test loop of the bMS will also be connected in parallel through the non-isolated system to reduce the insulation resistance test value.
Therefore, for the multi-machine parallel test, a certain bMS timing control is required, and the internal resistance of the bMS test is for mutual influence.
Through the above decomposition and the use of the entire energy storage system, the two most important factors that affect the bMS insulation test of the energy storage system are the bridge method and the internal resistance of the bMS test;
Three, processing methods
The most straightforward and effective method is not to use the bridge method to test the insulation resistance. Instead, use the active injection test method, etc., which will not be described in detail here; you can directly test the non-isolated, multi-machine parallel AC and DC insulation resistance in real time.
If the bridge method is still used, based on the internal resistance of the bMS insulation test and the capacitance of the PCS port Y, bMS extends the channel switching to insulation resistance sampling and reading time to ensure the stability of the charging and discharging circuit during the bMS test; at the same time, it is related to the multi-module parallel system. Method, the insulation test adopts the polling method, which can deal with the problem of low insulation resistance test value introduced by the bridge method.
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