Implementation of capacitor banks

Capacitor elements

Technology

The capacitors are dry-type units (i.e. are not impregnated by liquid dielectric) comprising metallized polypropylene self-healing film in the form of a two-film roll. They are protected by a high-quality system (overpressure disconnector used with a high breaking capacity fuse) which switches off the capacitor if an internal fault occurs.
The protection scheme operates as follows:

• A short-circuit through the dielectric will blow the fuse
• Current levels greater than normal, but insufficient to blow the fuse sometimes occur, e.g. due to a microscopic flow in the dielectric film. Such “faults” often re-seal due to local heating caused by the leakage current, i.e. the units are said to be “self-healing”
• If the leakage current persists, the defect may develop into a short-circuit, and the fuse will blow
• Gas produced by vaporizing of the metallisation at the faulty location will gradually build up a pressure within the plastic container, and will eventually operate a pressure-sensitive device to short-circuit the unit, thereby causing the fuse to blow

Capacitors are made of insulating material providing them with double insulation and avoiding the need for a ground connection (see Fig. L33).

---

b)

Electrical characteristics
Standard		IEC 60439-1, NFC 54-104, VDE 0560 CSA
Operating range	Rated voltage	400 V
	Rated frequency	50 Hz
Capacitance tolerance		- 5% to + 10%
Temperature range (up to 65 kvar)	Maximum temperature	55 °C
	Average temperature over 24 h	45 °C
	Average annual temperature	35 °C
	Minimum temperature	- 25 °C
Insulation level		50 Hz 1 min withstand voltage : 6 kV 1.2/50 μs impulse withstand voltage : 25 kV
Permissible current overload		Classic range(1)	Comfort range(1)
		30%	50%
Permissible voltage overload		10%	20%

Fig. L33:Capacitor element, (a) cross-section, (b) electrical characteristics