【Capacity Measurement and Formula】

 

Fundamental Capacitor Knowledge

 

1. Application of Capacitors

①Capacitor compensation

  • No remarkable harmonic source in the power consumption system (large power of single unit or a certain number of small power non-linear load);
  • Total harmonic distortion (THDi) ≤5%；
  • If total harmonic distortion is 5%≤THDi≤15%, the rated voltage level of the compensating capacitor needs to be increased;

② Detuned filter (inhibiting harmonic amplification) compensation

  • A few of harmonic sources in the power consumption system;
  • Total harmonic distortion is 5%≤THDi≤25%;
  • If 5th or higher harmonic dominates in the harmonic current, then additionally install series reactors with reactance ratio of 5-8%;
  • If 3rd or higher harmonic dominates in the harmonic current, then additionally install series reactors with reactance ratio of 12-14.8%.

③ Tuned filter (removal of partial harmonics) compensation

  • Total harmonic distortion is 25%≤THDi≤50%;
  • Single tuned compensation and filter branches are available for main harmonic orders (generally F3, F5, F7 and F11, etc.);
  • Setting principle of single tuned branch is based on compensation of reactive power.

 

Important factors affecting capacitor service life:

  • Harmonics in the power grid (large harmonic quantity directly leads to damage of capacitor);
  • Long-term operation under over-voltage condition (life to be reduced by half per voltage rising by 10%);
  • Excessively high ambient temperature (life to be reduced by half per temperature rise by 8℃~10℃);

 

2. Commonly Used Calculation Formulas

(1) Reactive power compensation

 

Q=P（）
 

Where: Q-Reactive power compensation capacity (kvar)

          P-Active power of load (kW)

         COSФ1 — Power factor of load before compensation         

         COSФ2 — Required power factor of load after compensation

(2) Capacitor voltage after series reactors:

 

Where: U-Capacitor voltage (VAC)

            Uo- Fundamental voltage of the system

            λ-Reactance ratio (%)

(3) Rated capacitance of capacitor:



Where: CN-Rated capacitance of the capacitor（uF）
            QN- Rated capacity of the capacitor (kvar)

            UN-Rated voltage of the capacitor (VAC)

            FN- Frequency (Hz)

(4) Reactive power conversion among different voltage levels of capacitor compensation:


 

Where: Q1-Capacitor capacity at voltage U1 (kvar)

            U1-Capacitor voltage (VAC)

            Q2- Capacitor capacity at voltage U2 (kvar)

           U2-Capacitor voltage (VAC)

e.g. Reactive power of product (480VAC, 20kvar) under a voltage of 440VAC:

Q=440*440/(480*480)*30=25.2 (kVAr)

(5) Capacitor mounting capacity after series reactors:



 

Where: Q1- Capacitor rated capacity after series reactors (kvar)

            U1-Rated voltage of capacitor after series reactors

           Q0-system output capacity (kvar)

           U0-System voltage (VAC)

           λ- Reactance ratio (%)

e.g. Under 400VAC system voltage, 30kvar output capacity, 7.0% reactance ratio and 480VAC rated voltage of the capacitor, the rated capacity of the capacitor is figured out as below:

       Q= 480*480/（400*400）*30*（1-7%）= 40.2（kVAr）