The rated current (IN) of a capacitor is the current flowing through the capacitor when the rated voltage (UN) is applied at its terminals, supposing a purely sinusoidal voltage and the exact value of reactive power (KVAR) generated. Capacitor units shall be suitable for continuous operation at an r.m.s. current of (1.3 x IN).
The characteristics of a capacitor, reported on its nameplate, are: According to IEC 60831-1 standard, the rated voltage (UN) of a capacitor is defined as the continuously admissible operating voltage. Capacitors can be selected with their rated voltage corresponding to the network voltage.
Capacitors can be selected with their rated voltage corresponding to the network voltage. In order to accept system voltage fluctuations, capacitors are designed to sustain over-voltages equal to 1.1 times UN, 8h per 24h. This design margin allows operation on networks including voltage fluctuations and common disturbances.
Q – rated power of the capacitor at rated mains voltage. Not only capacitors should be protected against short circuit, but the whole capacitor bank as well. Usually, in the switchgear from which the CB is supplied, there is an additional circuit breaker for the capacitor bank. Its value should be selected as:
For each step power rating (physical or electrical) to be provided in the capacitor bank, calculate the resonance harmonic orders: where S is the short-circuit power at the capacitor bank connection point, and Q is the power rating for the step concerned.
Considering power capacitor with rated power of 20 kvar and rated voltage of 440V supplied by mains at Un=400V. This type of calculation is true, if there is no reactor connected in series with capacitor. Once we know the total reactive power of the capacitors, we can choose series of capacitors for PF correction.