Type & Correct Application Of Power Factor Correction Equipment
Like all other loads, Power Factor Capacitors are also susceptible to the negative effects of harmonic currents and voltages. Capacitors are a low impedance path to harmonic currents and will absorb a high percentage of the harmonic currents flowing in the distribution network. This leads to overheating of the capacitor elements and/or dielectric failure, causing greatly reduced product life. In extreme cases, the capacitors can interact with the distribution transformer to create a resonant circuit which can actually cause an amplification of the harmonic currents on the network.
In order to ensure longevity of the product and years of trouble-free operation, the following guidelines are presented. These guidelines are based upon the percentage of harmonic generating load on the same transformer as the capacitors. To calculate the percentage of harmonic generating load on the transformer, add up the kVA of the harmonic generating loads and divide these by the total connected load. For example, if you found you had 250 kW of motors controlled by variable speed drives, plus 300kW of high frequency high-bay lighting, by rough estimate then you have 550 kW of non-linear load. If then, your total load was 2000 kVA on a transformer which was sized at 2500 kVA, you would have 550/2000 = 27% non-linear load. Using this value, you would select Detuned PFC from the description below:
Less than 15% non-linear load:
Standard automatic power factor capacitors are suitable. Because harmonic currents can be imported into your facility from the utility network, always specify +460V dielectric capacitors. These capacitors can absorb some harmonic current without damage. Note: If the amount of kVAr exceeds 0.33 x kW, detuned capacitors are recommended to avoid resonance with any harmonics which may be present on the network.
15% to 50% non-linear load:
Detuned automatic power factor capacitors are suitable. Detuned capacitors employ reactors in series with each capacitor stage to limit the amount of harmonic current that is absorbed and to prevent resonance between the supply transformer and the newly installed capacitor bank. It is important however to remove any existing capacitors prior to installation of a detuned bank as their presence on the network can change the tuning characteristics of the detuned bank.
Over 50% non-linear load:
Filtered automatic power factor capacitors can be used but only after complete analysis of the network to ensure correct application. Filtered capacitors employ filter reactors in series with each capacitor step to produce a tuned resonant circuit which will offer a low impedance path to absorb a particular problematic harmonic (such as the 5th harmonic produced by variable speed drives). A filtered automatic bank will provide the necessary power factor correction and reduce the voltage distortion on the network. It is important however, to remove any existing capacitors prior to installation of a filtered capacitor bank as their presence on the network can change the tuning characteristics of the filtered bank.