Pre-charge

A high-voltage system with a large capacitive load can be exposed to high electric current during initial turn-on.

In some applications, the occasion to activate the system is a rare occurrence, such as in commercial utility power distribution.

The capacitor charging starts with an inrush current and ends with an exponential decay down to the steady state condition.

The objective of a pre-charge function is to limit the magnitude of the inrush current into capacitive loads during power-up.

While pre-charging, the system voltage will rise slowly and controllably with power-up current never exceeding the maximum allowed value.

There are additional benefits: pre-charging reduces the electrical hazards which may occur when the system integrity is compromised due to hardware damage or failure.

Activating the high voltage DC system into a short circuit or a ground fault or into unsuspecting personnel and their equipment can have undesired effects.

Arc flash will be minimized if a pre-charge function slows down the activation time of a high voltage power-up.

A slow pre-charge will also reduce the voltage into a faulty circuit which builds up while the system diagnostics come on-line.

Without pre-charge the high voltage across the contactors and inrush current can cause a brief arc which will cause pitting of the contacts.

Pre-charging the controller input capacitors (typically to 90 to 95 percent of applied battery voltage) eliminates the pitting problem.