Technical Articles
Detuned Filters are a combination of series inductors and power factor correction capacitors that are meant to:
Typically a detuned filter has a series connected capacitor and reactor. The capacitor terminal voltage varies with respect to the tuning factor (%p) of the reactor. Tuning factor (%p) is the ratio of inductive impedance to the capacitive impedance (XL/XC). Common tuning factors of detuned filters are 7% and 14%.
Every series LC combination behaves capacitive below its tuning frequency [fres = 1/ (√2πLC)] and inductive above. The inductive element of the detuned filter is selected such that the tuning frequency of the filter is significantly lower than the lowest order harmonic frequency present in the system. The filter is thus ‘detuned'. The ratio of inductive reactance (XL) and capacitive reactance (XC) is known as the tuning factor.
A tuning factor of 7% implies XL / XC = 0.07.
The tuning frequency using tuning factor can be calculated as:
Where,
fs = Supply Frequency = 50 Hz
For tuning factor of 7%,
ft = 189 Hz.
As can be seen from the above graph, for all frequencies above the tuning frequency (ft), the combination will provide increasing impedance. The combination will not provide a low impedance path for harmonics that the capacitor did earlier, thus preventing harmonic amplification. Further as the tuning frequency of the combination is lower than the lowest order harmonic in the system, there is no question of resonance. At 50 Hz the combination behaves capacitive and power factor correction is achieved.
Can i add de-tuned filters in my existing panel?
The voltage that appears across the terminals of a capacitor increases the moment you connect an inductor in series with it. This can be illustrated by the below phasor:
VS :System Voltage; VC :Voltage across the capacitor; VL :Voltage across the inductor; I :current.
As can be seen VC > VS by an amount VL. Thus if reactors are to be added to an existing APFC panel, the capacitors will V have to be replaced with those capable of withstanding higher voltages. More over, the output of the capacitors will have to compensate for the reactive power that will be consumed by the reactor.
As can be seen from the above graph, for all frequencies above the tuning frequency (ft), the combination will provide increasing impedance. The combination will not provide a low impedance path for harmonics that the capacitor did earlier, thus preventing harmonic amplification. Further as the tuning frequency of the combination is lower than the lowest order harmonic in the system, there is no question of resonance. At 50 Hz the combination behaves capacitive and power factor correction is achieved.
Reactors are a major source of heat and existing panel may not have sufficient space or cooling arrangement to handle the heat generated by the newly installed reactors.
For these reasons, it is not advisable to add de-tuned reactors to existing APFC panels.
| Reactor Tuning Factor | Tuning Frequency | Application (Harmonic Orders) | Typical Loads |
|---|---|---|---|
| 7% | 189 Hz | 5th harmonic (250 Hz) and above | 6 pulse drives (AC / DC), 3 phase UPS, frequency converters |
| 14% | 133 Hz | 3rd harmonic (150 Hz) and above | Single phase UPS, CFL lamps, SMPS, dimmers |
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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