1.  Calculation of voltage rise on capacitors due to assembling series filter reactors:

 

Voltage drop on the reactors and at the same time voltage rise on the capacitors is:


                         ΔU =( 𝑓n∗𝑓n  /   fr∗fr−fn∗fn ) *100 in %

                          fr = √ (100 ∗ 𝑓n∗𝑓n)/P

   ΔU: Voltage rise on capacitors in %

fn: Nominal frequency

fr: Resonant frequency of the filter

p: Detuning factor in %.

The detuning factor in % is defined as the reactance of the reactor in the percentage of the reactance of the capacitor.

For example p=7% means IXLI = 7% of IXCI

Example:

Requested data:

Nominal power of Power Factor Correction Step: 25kvar and 50kvar

Nominal system voltage: 400V

Nominal frequency: 50Hz

Detuning factor: 7% means the filter frequency is 189Hz


ΔU = (50 * 50) /(189 * 189 - 50 *50)  *100 in % = 7.525%

 It means the minimum voltage on capacitors will be 400V*1.07525 = 430,1V

Due to this reason and considering voltage-high harmonics we should select a capacitor with a nominal voltage of at least 480V 50Hz or 525V 50Hz. 


2.    Calculations of capacitance depend on the power factor correction step:

Qc: Power of PFC step in kvar

Un: Nominal system voltage in V

fn = Nominal Frequency in Hz

ΔU = Voltage rise in %

C = Capacitance in µF

 

In case of PFC step is 25kvar at 400V 50Hz p=7%:

Qc = 25kvar

Un = 400V

fn = 50Hz

ΔU = 7.525%


  C  = Qc  / ( 2 * π * 50 * 400 * 400 ( 1 + 7.525 / 100 )*109

 If we calculate with previous data, C = 462.55µF or 3x154.18µF because of Delta connection capacitor windings into the capacitor.

 

The proper capacitor for such a case can be:

If we select capacitor with nominal voltage 440V 50Hz: KNK3053 28.1kvar (C=3x154µF) 440V 50Hz

If we select capacitor with nominal voltage 480V 50Hz: KNK3053 33.5kvar (C=3x154.3µF) 480V 50Hz

My suggestion is to use Heavy Duty capacitor from our catalog