What is the precharge design and what advantages does it have?

There is a capacitive load in the high-voltage circuit of the electric system. If there is no pre-charging design during power-on, the main positive relay is directly closed, and the instantaneous capacitive load is closed, which is equivalent to an instantaneous short circuit. This kind of high-voltage shock will cause damage to high-voltage electrical equipment and may bring danger. In order to avoid the transient impact damage to the high-voltage electrical equipment when the high-voltage is powered on, a pre-charging process should be designed for the high-voltage circuit system before the main positive relay of the high-voltage power-on is closed.

  1. Precharge principle and precharge model

1.1 The principle of precharging
As shown in Figure 1, the high-voltage circuit system precharge circuit.
In Figure 1, K1 and R1 are the pre-charging relay and pre-charging resistor respectively, K2 and K3 are the main positive relay and the main negative relay, respectively, R2 and C are the equivalent resistance and equivalent capacitance of the high-voltage system load, with Vb and Rb is the voltage and internal resistance of the power battery.

Figure 1 - Precharge Circuit
Figure 1 – Precharge Circuit

In the high-voltage circuit system, the high-voltage electrical equipment can be equivalently represented by a resistor and a capacitor, while the power battery can be simply represented by voltage and internal resistance. If there is no pre-charging design, the high-voltage main relay is directly closed when the high-voltage is powered on, which means that the high-voltage electrical equipment is directly connected through the capacitor, and the high-voltage components are also short-circuited. , it will generate a large transient current, and the transient current will cause transient impact on high-voltage electrical equipment, which will cause dangerous situations.

If the pre-charging process is introduced before the high-voltage power-on, that is, the pre-charging relay is closed before the main relay is closed, and the pre-charging resistor is connected, it will have a protective effect on the high-voltage electrical equipment. When the load high voltage reaches a certain precharge threshold, for example, the load high voltage is equal to or exceeds 90% of the total battery voltage during the precharge process as the conditional threshold for judging the success of the precharge. After the precharge is successful, the main relay is closed and the precharge is disconnected. Relay, the high voltage is powered on normally, and the high voltage electrical equipment works normally. Under normal circumstances, the high voltage on the main positive relay will be closed only when the pre-charging is successful.

  1. Precharge Model

The pre-charging circuit is simplified below, a pre-charging model is established, and then the equivalent load resistance and capacitance are calculated and deduced. In the precharge circuit, compared with the resistance value of the precharge resistor and the resistance value of the high voltage load, the internal resistance of the power battery is very small and can be ignored; The high-voltage load resistance is extremely large, and the high-voltage load resistance is negligible relative to the precharge resistance.

After simplification, the precharge model as shown in Figure 2 can be obtained.
In Figure 2, Vb is the power battery voltage, K1 and Rb are the pre-charging relay and pre-charging resistor, C is the high-voltage load equivalent capacitor, Vr is the voltage across the pre-charging resistor, Vc is the high-voltage load equivalent capacitor The voltage across the capacitor, i is the current of the high voltage loop. According to Kirchhoff’s law, we can get

Figure 2 - Precharge Model
Figure 2 – Precharge Model
  1. Selection of precharge resistors
    The precharge resistance is derived from the precharge model:

In the actual calculation, C is determined by the charging capacitor of the motor controller, and t is determined by the completion time of the pre-charging requirement in the power-on process. Usually, the bus voltage Vc outside the load reaches 90% of the total voltage Vb of the power battery as the condition for judging the successful pre-charging. . The selection of the pre-charge resistor is to select the appropriate pre-charge resistor according to the power-on time requirements of the pre-charge, the specification of the charging capacitor of the motor controller and the voltage requirements. The result obtained by deriving the load outside bus voltage:

The precharge process is shown in Figure 3.

Figure 3 - Precharge Process
Figure 3 – Precharge Process

For example, the charging capacitor at the motor controller end is 1100μF, and the total precharging time does not exceed 500ms. After removing the influence of factors such as the self-check time of the BMS and the action time of the relay, remove 200ms and leave the precharging process time no more than 300ms. The bus voltage reaches 90% of the total voltage of the power battery as the condition for judging the success of pre-charging, r=RC=55ms. Relay outside bus voltage:

The overload multiple of the precharge resistor is selected according to experience and cost, as long as the designed rated calorific value under the designed peak power is greater than the theoretically calculated calorific value.
If U=0.9Ubattery is used as the condition for judging successful precharge, then the precharge time t≈2.69, r=147.95ms; if U=0.8Ubattery is used as the condition for judging successful precharge, there is precharge time t≈1.98, r=108.9ms.
Considering the ability of the precharge resistor to pass the peak current, lmax=Ubattery/R=6A, and the peak power of the precharge resistor Pmax=I²maxR=1.8kW (the calorific value at the peak power is 9000J), so 50Ω100W (peak power delivery) is selected. Heat 15000J) metal precharge resistors, as shown in Figure 4.

Figure 4- Use 50Ω100W metal precharge resistor
Figure 4- Use 50Ω100W metal precharge resistor

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