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Failure analysis of pre-charged wiring harness circuit

release time:2021-07-08publisher:Howdy

   1) The addition speed of voltage Uc is slower than expected, and the addition speed of Ip is greater than expected

   Defect 1: Insulation defect. The load has a short circuit or a small resistive load due to defects, such as a capacitor breakdown, etc., which will cause Uc to consistently fail to rise during the pre-charging process. The current is too large at the moment. The increase in the amount of heat generated by the pre-charging resistor will burn the resistor and cause the failure of the pre-charging process. The main relay cannot be turned on and the vehicle cannot operate normally under high voltage. At this moment, the precharge loop current may flow into the low-voltage electrical network of the vehicle, and there is a risk of direct contact with the occupants, so the defect has been admitted. The pre-charging circuit should be actively disconnected.

  Defect 2: RC becomes larger. In the planning or equipment process, misunderstanding constitutes improper matching; in use, due to time, environment and other factors, the electrode corrosion of the capacitor, the electrical aging and thermal aging of the dielectric, and the self-healing effect, etc., will affect the parameter change of C; The increase of the wire and the contact resistance between the wire and the electrode will constitute a change in the R value.

  2) Voltage Uc is added faster than expected, and Ip is added faster than expected

   Defect 1: Open circuit, open circuit. Open load leads to false precharge completion. The possible reasons are that the load is not connected or the capacitor is disconnected due to defects, such as the lead wire and the electrode contact surface oxidation, poor contact, constitute a low-level open circuit; the liquid electrolyte is dry or frozen, etc. At this moment, the Uc detected by the BMS through the output El is not the actual voltage on the load capacitor, but the open circuit voltage (OCV) of the battery pack. The false Uc=Ub information is immediately obtained, which may lead to the completion of the precharge, but because the output is open, There is no risk. However, suppose that the load is suddenly added at this moment, because the pre-charging has been completed, there is no pre-charging circuit resistance R to bind the current. It will generate a large current and damage the circuit or relay. Therefore, in the general pre-charging strategy, the one that is completed as soon as the power is turned on can be judged as a defect, and the subsequent stop is performed. In this case, the problem can be admitted by querying the conduction status of the pre-charging circuit.

  Defect 2: RC becomes smaller. In the process of planning or equipment. Misunderstanding that the composition is not properly matched: the capacitor is used in the process of moving with the silver ions, the molecular structure of the dielectric is changed, and the value of the parameter of C becomes smaller due to the arcing between the electrodes in the environment of high humidity or low pressure. Affect the pre-charge function.

In order to prevent the occurrence of pre-charging failure, when selecting each component, car-grade products should be selected first, and industrial-grade products are not applicable; some parts that are prone to insulation failure and easy to touch should be added with rated protection under the premise of existing insulation protection. Ways to reduce wear and damage to the circuit caused by human causes, and reduce the defect generation rate.