How to Calculate Harmonics Filtersby 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.
Measure, using the harmonic analyzer on the circuit at 30% load, the load LD in kilowatts and the power factor PF.
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)
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))
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
- Power line detail image by Tasha from Fotolia.com