What is meant by pre-charge?

Pre-charge, in the context of a DC (direct current) system, refers to a process of gradually charging the capacitance or voltage level before the main power is switched on. The purpose of pre-charge is to reduce the initial current spike and thus avoid damage to the system and components.

When a DC system is started or reconnected after having been disconnected for a period of time, capacitances, such as for example capacitors, be fully discharged. Directly allowing the full voltage level to be applied to these capacitances can cause a sudden current spike that can be detrimental to components and the system as a whole. This current spike can be particularly problematic in power systems where high currents can cause disturbances or damage.

To avoid this, the pre-charge method is used. It involves gradually increasing the voltage level across the capacitances by using a limited current or a resistance-based power source. By slowly increasing the voltage across the capacitances, you can smooth out the current spike and protect the system. Once the voltage has increased to the desired level, the main power can be switched on and the system can operate normally.

Pre-charge is often used in areas such as power transmission, electric power systems and similar DC systems where capacitances may be present and current spikes should be avoided to protect equipment and ensure proper operation. In such a circuit, a pre-charge contactor is used.