Displacement Vs Distortion

[kens]

Hi all, if we have a supply that is known to have significant current distortion (~ 25%), but also a large amount of inductive loads, is there any simple method of calculating the proportional effect on the total power factor. The reason for the calculation is that if the utility company is charging for a pf of <0.95 we need to know if we can improve the pf sufficiently using capacitors ( with blocking reactors) which is the less expensive option, or whether the correction requires the more expensive hamonic filtering. Generally the payback for standard capacitive correction is acceptable but hamonic filtering is not.

 

Again, all help appreciated.

 

Ken

[CJC_PE]

You need to first verify what the utility measures and how they use it in their rate structure. Historically, utilities have had only equipment to measure displacement pf but many are now paying attention to distortion in one way or another.

 

If you know the displacement power factor and the total harmonic current distortion, you can calculate the total power factor from Total PF = Displacement PF/sqrt(1+THDi^2).

 

[CJC_PE]

To calculate the total power factor for an entire plant, you must calculate or measure the plant’s total power and total RMS current (Irms) including the harmonic current content. Then you can calculate the total power factor from Total PF = Total Power/V x Irms x sqrt3 (assuming 3 phase).

 

For an accurate determination of Irms, you should determine the fundamental current (If) and each individual harmonic current for each individual load and then add the individual fundamental and harmonic currents to get totals for each, such as total fundamental (If), total 2nd harmonic (I2), total 3rd harmonic (I3) etc. The total RMS current for the plant is then Irms = sqrt(total If^2 + total I2^2 + total I3^2 + … total In^2).

 

If the loads have similar harmonic spectrums, I believe that you can just add together the total harmonic current (Ih) values for the individual loads to get a total Ih for the plant. For each distorted load, you can determine the total fundamental and harmonic currents from If = Irms/sqrt(1 + THD^2) and Ih = If x THD. To determine the total RMS current you add all the fundamental currents together and all the harmonic currents together and calculate the total RMS current from Irms = sqrt(If^2 + Ih^2). The currents of the loads without harmonic content are added as part of the total If.