Saturday 23 January 2016

Harmonic Mitigation Techniques for Power Distribution Networks and Harmonic filtration

Harmonic Mitigation Techniques for Power Distribution Networks and Harmonic filtration





There has been much discussion and interest in recent years on the subject of Harmonics under ambit of network power quality.More and more frequently, industrial and commercial users are finding they have to deal with the problem of "polluted power.Many seminars and awareness programs have been conducted to effectively deal with the problems of harmonics. Lot has been done but still there are gaps in knowledge of harmonics in network and methods to effectively deal with them.

A growing number of harmonic mitigation techniques are now available including active and passive methods, and the selection of the best-suited technique for a particular case can be a complicated decision-making process. The performance of some of these techniques is largely dependent on system conditions, while others require extensive system analysis to prevent resonance problems and capacitor failure.

The nonlinear characteristics of many industrial and commercial loads such as power converters, fluorescent lamps, computers, light dimmers, and variable speed motor drives (VSDs) used in conjunction with industrial pumps, fans, and compressors and also in air-conditioning equipment have made the harmonic distortion a common occurrence in electrical power networks. Harmonic currents injected by some of these loads are usually too small to cause a significant distortion in distribution networks. However, when operating in large numbers, the cumulative effect has the capability of causing serious harmonic distortion levels. These do not usually upset the end-user electronic equipment as much as they overload neutral conductors and transformers and, in general, cause additional losses and reduced power factor . Large industrial converters and variable speed drives on the other hand are capable of generating significant levels of distortion at the point of common coupling (PCC), where other users are connected to the network.

Active Harmonic Mitigation Techniques

Active Harmonic filters use IGBT fired circuits to mitigate harmonic distortions in the load or the network of loads where they are installed. They can be separately installed on prominent individual loads generating harmonics or combined AHF unit on mains. The combination of both can also be used. It all depends on level of harmonics, type of operational issue for which it is being installed or level of compliance to standards as per utility / distribution company.



When using active harmonic reduction techniques, the improving in the power quality came from injecting equal-but-opposite current or voltage distortion into the network, thereby canceling the original distortion. Active harmonic filters (AHFs) utilize fast-switching insulated gate bipolar transistors (IGBTs) to produce an output current of the required shape such that when injected into the AC lines, it cancels the original load-generated harmonics. The heart of the AHF is the controller part. The control strategies applied to the AHF play a very important role on the improvement of the performance and stability of the filter. AHF is designed with two types of control scheme. The first performs fast Fourier transforms to calculate the amplitude and phase angle of each harmonic order. The power devices are directed to produce a current of equal amplitude but opposite phase angle for specific harmonic orders. The second method of control is often referred to as full spectrum cancellation in which the full current waveform is used by the controller of the filter, which removes the fundamental frequency component and directs the filter to inject the inverse of the remaining waveform.

 Essentially, the filter consists of a VSD with a special electronic controller which injects the harmonic current onto the system 180 out of phase to the system or drive harmonics. This results in harmonics cancellation. For example, if the VSD created 50 A of 5th harmonic current, and the AHF produced 40 A of 5th harmonic current, the amount of 5th harmonic current exported to the utility grid would be 10 A. The AHF may be classified as a single-phase or three-phase filters.

Hybrid Harmonic Mitigation Techniques

Hybrid Power conditioning system is a combination of Dynamic Activcomp Thyristor based Power Factor Correction System and IGBT fired Active Harmonic Filtration System offers most viable and economical solution for both Power Factor Compensation and Harmonic Mitigation. The designs offered by us are installed for compensation and Harmonic compliance of main transformer or at major load feeders. Though there are standard ratings but any design can be offered as per customer load characteristics and level of harmonic distortion.

Hybrid connections of AHF and PHF are also employed to reduce harmonics distortion levels in the network. The PHF with fixed compensation characteristics is ineffective to filter the current harmonics. AHF overcomes the drawbacks of the PHF by using the switching-mode power converter to perform the harmonic current elimination. However, the AHF construction cost in an industry is too high. The AHF power rating of power converter is very large. These bound the applications of AHF used in the power system. Hybrid harmonic filter (HHF) topologies have been developed to solve the problems of reactive power and harmonic currents effectively. Using low cost PHF in the HHF, the power rating of active converter is reduced compared with that of AHF. HHF retains the advantages of AHF and does not have the drawbacks of PHF and AHF. Figure shows a number of possible hybrid combinations. It is a combination of shunt AHF and shunts PHF. Using a combination of PHF will make a significant reduction in the rating of the AHF. As a result, no harmonic resonance occurs, and no harmonic current flows in the supply.In HHF the AHF can improve the filter performance and suppress the harmonic resonance of existing PHF.  shows a combination of AHF series with the supply and a shunt PHF. It is  found that this topology is not suitable for low-frequency interharmonic compensation because the AHF introduces a high compensation voltage which can interfere with downstream phase-controlled nonlinear loads.


 Conclusions


Electrical system reliability and normal operation of electrical equipment rely heavily upon a clean distortion free power supply. Designers and engineers wishing to reduce the level of harmonic pollution on a power distribution network where nonlinear harmonic generating loads are connected have several harmonic mitigation techniques available. Because of the number and variety of available methods, selection of the best-suited technique for a particular application is not always an easy or straightforward process. A broad categorization of different harmonic mitigation techniques (passive, active, and hybrid) has been carried out to give a general viewpoint on this wide-ranging and rapidly developing topic. PHF is traditionally used to absorb harmonic currents because of low cost and simple robust structure. However, they provide fixed compensation and create system resonance. AHF provides multiple functions such as harmonic reduction, isolation, damping and termination, load balancing, PF correction, and voltage regulation. The HHF is more attractive in harmonic filtering than the pure filters from both viability and economical points of view, particularly for high-power applications. It is hoped that the discussion and classification of harmonic mitigation techniques presented in this paper will provide some useful information to help make the selection of an appropriate harmonic reduction method for a given application on an easier task.





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