Technical Articles
Modern industries are defined by widespread use of non-linear loads such as DC drives, VFDs etc. which introduce large content of harmonics into the system and its subsequent effects. There arises the need for intelligent solutions for harmonic mitigation. Active harmonic filters have developed as the most ideal solution for harmonic filtering in both industrial and commercial facilities. Added advantage is that they also offer solutions for phase balancing and power factor improvement. This article briefs about its basic principle of operation, need for active compensation and sizing.
An active harmonic filter is based on the following principle:
It detects the difference between the ideal current sine wave (IMAIN) and the actual current which has been deformed by harmonics (ILOAD). It, then, injects this difference (IFILTER), which is the negative of the harmonic currents present in the load current, into the system on a real-time basis. This cancels out the high frequency harmonics and results in almost pure sine wave.
The active filter monitors the line current in real time and converts the data to digital signals. The compensating current, as already explained, is generated by an IGBT bridge using Pulse Width Modulation (PWM) technology. Source of IGBT Bridge is a DC link capacitor which is charged simultaneously with generation of compensating current to the network. The generated output is injected into the network via a reactor or filter circuit.
Active Harmonic Filters provide many advantages over the various solutions for harmonic mitigation as under:
Unlike detuned passive filters, active filters provide actual reduction in THD as recommended by IEEE. The IEEE 519-2014 guidelines on limits for current and voltage harmonics at Point of Common Coupling (PCC) are as under:
| Maximum Harmonic Current Distortion in % IL | ||||||
| Individual Harmonic Order (Odd Harmonics) | ||||||
| Isc / IL | 3 ≤ h < 11 | 11 ≤ h < 17 | 17 ≤ h < 23 | 23 ≤ h < 35 | 35 ≤ h < 50 | TDD |
| <20 | 4.0% | 2.0% | 1.5% | 0.6% | 0.3% | 5.0% |
| 20 - 50 | 7.0% | 3.5% | 2.5% | 1.0% | 0.5% | 8.0% |
| 50 - 100 | 10.0% | 4.5% | 4.0% | 1.5% | 0.7% | 12.0% |
| 100 - 1000 | 12.0% | 5.5% | 5.0% | 2.0% | 1.0% | 15.0% |
| >1000 | 15.0% | 7.0% | 6.0% | 2.5% | 1.4% | 20.0% |
where
Isc = maximum short-circuit current at PCC [Can be calculated as MVA / (%Z x V)
IL = maximum demand load current (fundamental frequency component) at PCC
| Voltage Distortion Limits | ||
| Bus Voltage at PCC | Individual Voltage Distortion | VTHD |
| V ≤ 1.0 kV | 5.00% | 8.00% |
| 1 kV < V ≤ 69 kV | 3.00% | 5.00% |
| 69 kV < V ≤ 161 kV | 1.50% | 2.50% |
| > 161 kV | 1.00% | 1.50% |
The use of active harmonic filters helps in reducing harmonics as can been seen under:
Some of the areas where active harmonic filters can be used are:
Active harmonic filter is rated in Amperes. The current rating is decided on the basis of harmonic content (THD) in the system which can be obtained from harmonic study. The required nominal current can be obtained by multiplying the initial current of harmonics measured in the load by a safety factor (SFh) of 20%. In other words,
Where,
Ifilter (AHF) : Nominal Current of Active Filter (A)
Iload : Maximum Load Current (A)
% THD(I) : Load Current Harmonic Distortion (%)
Majority of the energy saving devices are non-linear in nature. Consequently, the problem of harmonics has become inevitable. Advanced devices like active harmonic filter provide an ideal solution to this problem. These filters help in maintaining a stable and healthy power system thereby increasing productivity and efficiency.
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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