The following calculator computes the voltage decay on three-phase wye-connected capacitor banks after being disconnected from their power source. The calculation assumes that the system voltage is at 110% of nominal, and that the capacitor bank was disconnected at peak voltage.
Based on this data, the required discharge resistor rating is computed to meet industry standard discharge requirements for medium-voltage power capacitors of 50 Volts in 300 seconds (IEEE Standard 18-2012, Standard for Shunt Power Capacitors).
A custom resistor rating can also be investigated and is plotted on the discharge curve near the standard discharge curve.
Calculator-1
Known variables: Capacitor Bank Three Phase KVAR, System Voltage & Frequency
Formulas and Variables
Where:
| Vpeak = Peak Voltage on Capacitors before disconnecting (kV) |
| VLL = Line-Line Rating of Capacitor (kV) |
| C = Capacitance of Capacitor Stage per Phase (µF) |
| f = Frequency Rating of the Capacitor |
| KVAR = Three phase KVAR rating of the Capacitor Stage (kVAR) |
| R = Resistance of the Discharge Resistor (MΩ) |
| Vo = Initial Voltage of the Capacitor just after disconnecting (kV) |
| t = Time after the Capacitor has been disconnected (sec) |
| V(t) = Voltage at some time (t) after the capacitor has been disconnected (sec) |
Of Interest...
Capacitors when disconnected store energy and also trap voltage. This voltage can be a hazard if maintenance comes into contact with the terminals of the capacitor. As a result, NEC Article 460.28 (A), and most other national standards, states:
A means shall be provided to reduce the residual voltage of a capacitor to 50 volts or less within 5 minutes after the capacitor is disconnected from the source of supply.
Manufactures of medium voltage power capacitors, build discharge resistors into their capacitors to these standards. The above calculator computes the resistor rating, discharge or decay curve, resistor Watt rating, and capacitor discharge energy.
In some applications, a faster discharge in voltage is desired. A faster discharge is accomplished by decreasing the resistance of the resistor. In doing so, the Watt rating requirements of the resistor will increase. Typically Capacitors resistors can be sized to allow the capacitors to be discharged in no less than 2-minutes.
