ANSI/IEEE Std. 18-1980 specifies that power factor correction capacitors must be applied within the following guidelines, no one of which may be exceeded:

1. RMS voltage not to exceed 110% of rated

2. Peak voltage not to exceed 120% of rated

3. RMS current not to exceed 180% of rated ^[1]

4. kVA not to exceed 135% of rated

The calculations are to be made as follows:

1. RMS voltage The RMS applied voltage is the square root sum of squares of all fundamental and harmonic voltages across the capacitor. The fundamental voltage should be taken as the maximum sustained line voltage and must include any voltage rise due to a tuning reactor. Harmonic voltages are calculated from the harmonic currents and the capacitor reactances at harmonic frequencies. The RMS voltage limit is associated with dielectric heating effects.

2. Peak voltage The peak voltage is calculated as the peak fundamental voltage plus the peak effect of all harmonic voltages. In principle, this requires an arithmetic addition of all harmonic voltages since they could, admittedly with a small probability, combine as an additive peak. Common sense, however, would suggest the arithmetic addition of the major harmonic voltages and an RMS combination of the remaining peak harmonic voltages. The peak voltage limit is associated primarily with the dielectric and corona stresses.

3. RMS current The RMS current is the result of fundamental current and all harmonic currents. The fundamental current must be adjusted for capacitance tolerance and terminal voltage. The current limit is primarily one associated with foil and terminal heating due...