Some of the causes of energy loss in a power capacitor are:
1. Dielectric loss
2. Loss in series resistance in joints
3. Loss in discharge resistors
1. The dielectric loss is a complex phenomenon involving molecular physics. It varies with the dielectric material, and can be considered as a loss in a series resistance. In polypropylene film, it is 0.5W/kVAr, in polyester 5.0 W/kVAr, while in polycarbonate it is 1.5 W/kVAr. This is the reason that PP film is preferred for capacitors. Please note that for impregnated paper, it is 2.5W/kVAr. Impregnated paper was used 20 years back. Now almost all low voltage ac capacitors are made from PP. This loss can be reduced if break through takes place and better dielectric materials are developed.
2. Other loss takes place in series resistance of connection of wires to metallised ends of film winding through metal spray. This loss is usually nominal and well controlled by the manufacturer. A high loss capacitor will not last long.
3. A source of energy loss in a power capacitor is the loss in the discharge resistors, which are permanently connected across each pair of terminals. IEC 831 specifies that discharge resistors are to be provided across each pair of terminals, such that within 3 minutes of switch off, the capacitor terminal voltage should drop to a value below 75V. For a 50 kVAr, 415V, 50 Hz, 3 phase capacitor, minimum value of the discharge resistors is 200 kOhms, and the power consumed by each resistor is app. 1 watt. This accounts for 3 watts being consumed permanently for each 50 kVAr capacitor. This loss may add up to a lot world-wide, but provision of the resistors is essential from safety point of view. Please note that prior to IEC 831(1988), IEC 70 & 70a were in use, requiring resistance values such that the capacitor terminal voltage be reduced to below 50V within 1 minute of switch off. This needed resistors of 60 kohms minimum causing a power loss of 9 watts