Technical Articles
The increasing use of energy saving power electronics based loads (adjustable speed drives, switch mode power supplies, etc.) to improve system efficiency and controllability, is increasing the concern for harmonic distortion levels in end use facilities and on the overall power system. Active filter is the apt device for reducing harmonic levels in industrial and commercial facilities to meet IEEE 519 guidelines. The concept of active filter, what many try to explain is, it senses harmonics and generates 180Ëš phase shifted harmonics that cancels out the unwanted harmonics. This article, specifically describes the basic operation of active filters in a little more detailed way.
The active filter uses power electronic switching to generate harmonic currents that cancel the harmonic currents from a nonlinear load. The active filter configuration is based on a Pulse-Width Modulated (PWM) voltage source inverter that interfaces to the system through an Interface Filter as shown in Figure 1. In this configuration, the filter is connected in parallel with the load being compensated. Therefore, the active filter with this configuration is often referred to as shunt active filter. Figure 1 illustrates the concept of the harmonic current cancellation so that the current being supplied from the source is sinusoidal.
The voltage source inverter used in the active filter makes the harmonic control possible. This inverter uses DC capacitors as the supply and can switch at a high frequency to generate a signal which will cancel the harmonics from the nonlinear load. One leg of the inverter is shown in Figure 2 to illustrate the configuration.
The current waveform for cancelling harmonics is achieved with the voltage source inverter and an interfacing filter. The filter consists of a relatively large isolation inductance to convert the voltage signal created by the inverter to a current signal for cancelling harmonics. The rest of the filter provides smoothing and isolation for high frequency components. The desired current waveform is obtained by accurately controlling the switching of the IGBTs in the inverter. Control of the current wave shape is limited by the switching frequency of the inverter and by the available driving voltage across the interfacing inductance.
The active filter does not need to provide any real power to cancel harmonic currents from the load. The harmonic currents to be cancelled show up as reactive power.
For the stable operation of active harmonic filter and for the expected reduction of harmonics, the physical connection & location of power cables & CTs plays a critical role. Let us understand the connections through the below two cases.
In case any industry faces severe problems due to harmonics like over-heating of equipments, failure of power capacitors, frequent card failures, etc., it is advisable to arrest the harmonics right at the source. The connection shall be as below
Make sure that the APFC panels are detuned and are connected in the upstream of the AHF. Detuning of APFC panel helps in optimising the AHF rating, as detuning avoids the harmonic amplification. Connecting AHF below the APFC avoids the AHF being overloaded due to unwanted harmonic resonances. Also make sure that the CT of AHF shall be connected below the CT of APFC panel.
If almost all the main feeders from PCC generates high harmonics, ideally each feeder shall be compensated with individual AHF. But this will increase the installation cost and requires more space. Also managing all the AHF will be difficult. In such cases, a common AHF can be connected for harmonics compensation, as shown below:
Also make sure that the APFC panels are detuned and the AHF CT shall be below the CT of detuned APFC panels. This method is also simple enough, especially for industries where harmonics need to be reduced at the HT side, in order to meet utility regulations (like Tamil Nadu Electricity Board).
In some rare cases, there may be multiple sources, like two or more synchronised transformers. In such cases, individual CTs shall be connected to each sources and the common signal can be given to AHF through a summation CT.
The Power cables that are connected to AHF carries pure harmonic currents. Hence the cable shall be selected as per the below table for better life:
| AHF rating (A) | For 3 wire AHF (Cu single core)** | For 4 wire AHF (Cu single core)^^ |
|---|---|---|
| 60 | 4 x 25 Sq. mm | 6 x 25 Sq. mm |
| 75 | 4 x 35 Sq. mm | 6 x 35 Sq. mm |
| 100 | 4 x 50 Sq. mm | 6 x 50 Sq. mm |
| 150 | 4 x 70 Sq. mm | 6 x 70 Sq. mm |
| 200 | 4 x 70 Sq. mm | 6 x 70 Sq. mm |
| 300 | 4 x 120 Sq. mm | 6 x 120 Sq. mm |
| 400 | 4 x (2 x 70) Sq. mm | - |
| 600 | 4 x (2 x 120) Sq. mm | - |
| 800 | 4 x (2 x 140) Sq. mm | - |
Abhinav L Purkar,
Manager Product Management & Marketing (PMM)Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.
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