Harmonics :
In order to understand harmonic filters first we have to study what are harmonics. Well, normal alternating current power supply follows ideal sine wave. For example linear electrical load such as a heater draws a current that follow sine wave.
But in case of non linear load, the load changes its impedance with instantaneous applied voltage. The changing impedance means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non sinusoidal currents contains harmonic currents that interact with impedance of power distribution system to create voltage distortion. This distortion then can affect both the distribution system equipment and the connected load itself.
In past these non linear were found in heavy industrial applications such as VFD, heavy rectifiers for refining etc. But nowadays these harmonics are very found not only in industrial applications but also in commercial buildings . For example computers, photocopiers, telecom devices etc which are very common also can produces these harmonics.
Increasing use of power electronic device also demands for voltage stability and power quality.
But in case of non linear load, the load changes its impedance with instantaneous applied voltage. The changing impedance means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non sinusoidal currents contains harmonic currents that interact with impedance of power distribution system to create voltage distortion. This distortion then can affect both the distribution system equipment and the connected load itself.
In past these non linear were found in heavy industrial applications such as VFD, heavy rectifiers for refining etc. But nowadays these harmonics are very found not only in industrial applications but also in commercial buildings . For example computers, photocopiers, telecom devices etc which are very common also can produces these harmonics.
Increasing use of power electronic device also demands for voltage stability and power quality.
Harmonic filters:
Harmonic filters reduces the distortion or harmonics in a sine-wave to a level recommended . In cases where the existing preventive action is insufficient, then we have to go for a filtering system that can minimize the distortion.
There are three types of filters:
- Passive filters
- Active filters
- Hybrid filters
Passive filters:
Passive harmonic filters are used where
- Industrial installations with a set of non-linear loads representing more than 500kVA such as VFD, UPSs, rectifiers, etc.
- Installations requiring power-factor correction
- Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
- Installations where current distortion must be reduced to avoid overloads
How it works:
An LC circuit as shown in picture below, the passive filter as installed in parallel with non linear load absorbs the harmonics , thus avoiding their flow in the distribution network.
Generally speaking, the passive filter is tuned to a harmonic order close to the order to be eliminated. Several parallel-connected branches of filters can be used if a significant reduction in the distortion of a number of harmonic orders is required.
Active filters :
Active harmonic filters are used where
- Commercial installations with a set of non-linear loads representing less than 500kVA such as variable-speed drives, UPSs, office equipment, etc.
- Installations where current distortion must be reduced to avoid overloads.
How it works:
These systems, comprising power electronics and installed in series or parallel with the non-linear load, compensate the harmonic current or voltage drawn by the load.
As shown in diagram below, parallel-connected active harmonic conditioner (AHC) compensating the harmonic current (Ihar = -Iact).
The AHC injects in opposite phase the harmonics drawn by the non-linear load, such that the line current Is remains sinusoidal.
Hybrid filters
Hybrid filters are used where
Industrial installations with a set of non-linear loads representing more than 500kVA such as variable-speed drives, UPSs, rectifiers, etc.
- Installations requiring power-factor correction
- Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
- Installations where current distortion must be reduced to avoid overloads
- Installations where strict limits on harmonic emissions must be met
How it works:
As shown in diagram below passive and active filters are combined in a single system to constitute a hybrid filter. This new filtering solution offers the advantages of both types of filters and covers a wide range of power and performance levels.
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