VOLTAGE SELECTION OF POWER CAPACITOR

**IMPORTANT TERMS FOR VOLTAGE SELECTION:-


When you deciding capacitor voltage, it is important to know what is the percentage impedance(%Z) of the transformer?
The percentage impedance voltage drop on full load due to the winding resistance and leakage reactance of the transformer. That's expressed as a percentage of the rated voltage. 

Like that, 

 The transformer secondary voltage is rated 433 volts and %Z is 4%, so voltage is available at the load end is 415 volt (during full load condition) only.
so, When load decreases, voltage drop also decreases.
so, voltage is increase at the load end (during no load condition), The voltage can reach maximum of 433 volts.

So, If Capacitor is select for 415 Volts, it should be over voltage during no-load conditions.
So that, Capacitor life is drastically.

Power Capacitor KVAR rating is directly proportional to square of the applied voltages.
  • * i.e.  KVAR = V² / Xc.


Like That, 

If  a 50 Kvar Capacitor rated voltage 480 volt and if applied voltage is 440 volt.
Then, What is the actual output KVAR value in this applied voltage?

Actual O/P KVAR: -

KVAR O/P = [Rated KVAR * {(Applied Voltage)²  / (Rated Voltage)² }]
                   = [50 * {(440)²  / (480)² }]
                   = 42 KVAR at 440 Volts.

Likewise, In the Same capacitor if we apply 500 volt, 
the KVAR O/P = 50 * (500²  / 480²)
                           = 54.25 KVAR at 500 Volts.


Hints:- If the capacitor should be rated for 440 volt, even through  load end voltage is 415 volts. In this system, it is better to select greater capacitor voltage than rated secondary voltage of the transformer and avoid prolonged over-voltage conditions.




As per IS Rule:- 
  • 10% over-voltage for 12 hours in every 24 Hrs.
  • 15% over-voltage for 30 minutes in every 24 Hrs.
  • 20% over-voltage for 5 minutes in every 24 Hrs.
  • 30% over-voltage for 1 minute in every 24 Hrs.