There are four types of protection for "All-film" H.V. capacitors:

• Without internal fuses and external protection with unbalance monitoring

• With internal fuses and external protection with unbalance monitoring

• Without pressure monitoring device and external protection with HRC fuses

• With pressure monitoring device and external protection with HRC fuses

The choice between these four possibilities depends on the following criteria :

• electrical characteristics of capacitors (power, voltage, connection)

• customer's requirements concerning protection sensitivity

The following table gives, according to the above criteria, the possible type of protection for the capacitor and its advantages.

Internal view of a "All-film" H.V. capacitor with internal fuses
1 discharge resistor
2 internal fuse
3 elementary capacitance

Protection with internal fuses

Due to their advantages, whenever possible, internal fuses are mostly used to protect "All-film" H.V. capacitors.
In this technology, each elementary capacitance forming the capacitor is protected by its own internal fuse.
In the event of an elementary capacitance fault, the internal fuse eliminates the corresponding capacitance and guarantees continuous capacitor operation.
Given the high number of elementary capacitances forming the device, the power loss resulting from the first fault is negligible (less than 2 %).
The external unbalance protection is only activated when the number of "blown out" elementary capacitances in the same capacitor is significant and liable to cause an excessive balance.

An internal fuse is activated:

• When the capacitor voltage reaches its maximum value and, therefore, when the current reaches its minimum value. The potential difference at the terminals of the "faulty" elementary capacitance causes to the corresponding fuse to blow out.

• When the current reaches its maximum value and, therefore, when the voltage reaches its minimum value, the outflow in the "faulty" capacitance of the energy stored in the operational capacitances in parallel causes the corresponding fuse to blow.

Protection with pressure monitoring device

Protection with a pressure monitoring device is of interest whenever the capacitor cannot be protected correctly (due to electrical characteristic or cost problems) with internal fuses or by unbalance monitoring.

This protection is individual for each capacitor. It is formed of a pressure switch sealed on the capacitor case. This pressure switch is composed of a "membrane" sensitive to pressure rises generated in the case due to a elementary capacitance blow-outs and an "O.C." contact used to trigger the bank control device (contactor - switch, etc.).