How to Calculate Harmonics Filters

by Jonah Quant

In power supply systems based on alternating current (AC) -- such as the main power distribution network from electric utilities -- non-linear loads can feed some amount of power back into the wiring. This feedback typically occurs in the form of harmonics: multiples of the frequency of the original AC wave. Harmonics need to be eliminated from a power circuit by a harmonic filter to prevent them from causing voltage distortions and excessive currents in grounding connections. A harmonic filter consists of a power capacitor connected in series with a tuning reactor, with both of them placed between the power line and ground. The parameters for a harmonic filter depend on the electrical circuit in which harmonic elimination needs to happen.

Step 1

Measure, using the harmonic analyzer on the circuit at 30% load, the load LD in kilowatts and the power factor PF.

Step 2

Calculate the phase angles for both the actual and desired power factors (a typically desirable power factor is 0.97) by evaluating: PAActual = arccos(PF) PADesired = arccos(0.97)

Step 3

Calculate KVAR, the kilo-volt-amperes required to raise the power factor from PF to, for example, 0.97 by evaluating: KVAR = LD x (tan(PAActual) - tan(PADesired))

Step 4

Calculate the capacitance required for the capacitor in the harmonic filter by evaluating: C = KVAR / ((KV)^2 x 2 x Pi x F x 0.001) Replace KV by the power line's voltage in kilovolts, and F by the power line's frequency in Hertz.

Calculate the reactance required for the tuning reactor in the harmonic filter by evaluating: X = 1 / (2 x Pi x F x C)

Items you will need

  • Harmonic analyzer

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