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无功补偿:Reactive power compensation

http://www.leonardo-energy.org/drupal/node/1825

http://citeseer.ist.psu.edu/old/393182.html

http://www.areva-td.fi/finland_home/liblocal/docs/EN-TH04-11_2004-Harmonics_and_Reactive_Power_Compensation_in_Practice.pdf

Reactive power compensation and harmonic filtering
By Ved Prakash Sinha / Published on Tue, 2007-03-13 15:58 Further readingApplication Note: Passive Filters (4209)
There is enough evidence available about the reactive power compensation and its benefits for loads having low power factor. Though such loads, which impose only low power factor are increasingly replaced by loads which generate significant harmonics as well. As a result, most of the modern compensators are designed to carry out a dual task for compensation as well as harmonic filtering. And then, there is also a dimension of reaction time which is quite important for certain loads. For example, a welding load which generates high harmonics, has low power factor, can never be properly compensated by a classical passive compensator. It needs a fast acting device like a thyristor switched capacitor bank to match the speed of the load variation.

Several solutions for power factor compensation and harmonic filtering are available in the market like conventional automatic banks with contactor switching or fast acting real time banks with thyristor switching and active filters. There is a variety of capacitor banks available which are used to compensate loads which need support in terms of power factor as well as harmonic correction. In a network polluted by harmonics, capacitor banks often use inductors in series with the capacitor steps for safeguarding the capacitors from harmonic overloading and inrush current. Such banks are called “detuned banks”.

Specially designed capacitors and inductors make a filter circuit (passive filters) which serves a dual purpose of providing reactive power and filtering of harmonics. Most commonly used passive filters are of fixed type, sometimes automatic switched banks are also provided with steps tuned as passive filers. This however imposes a limitation on the design of such banks. Principally, if there are several steps available, they are required to be tuned at different frequencies. For example, a 200 kvar rated capacitor bank can have 4 steps, each of 50 kvar, tuned to 5th, 7th, 11th and 13th harmonic filter. The immediate requirement of such bank will be to keep the 5th harmonic bank in service first, followed by 7th, 11th and 13th order. This means, any fluctuation on reactive power demand around 200 kvar will be met by switching on/off the last steps (11th & 13th harmonic filter) only. The switch controlling these steps will be overstressed compared to other steps (tuned to 5th & 7th order). It will not be possible to operate these 4 steps in “circular switching” mode or first in first out (FIFO) mode as it will result in dangerous amplification of higher order harmonics. This also means, if there is any problem in the 1st step, tuned to 5th harmonic, none other steps could be switched ON, leading to non-availability of the complete system. Other limitations come from the change in load (like additional of one more drive to the same bus) or network parameter changes (like replacement of feeding line or transformer) which always affect the performance of a passive filter.

An automatic bank detuned with a reactor does not impose such limits. For example, a detuned bank using 7% reactor (which results in a tuning point below 4th order harmonic) provides a robust solution to such application. The bank with several steps can all have the same detuning reactor and the bank can operate in first in first out (FIFO) mode ensuring all switches are used evenly. Even if any of these steps are not in service, remaining healthy steps can continue to perform without any danger of amplification.

Above concept works well with classical solution (contactor switched banks) or fast acting real time banks. The thyristor switched banks are more apt for loads which changes very fast. Examples are welding machines, drives, compressors and similar applications. Cranes used at harbor, which mainly employ DC drives, need fast reactive compensation as well as harmonic filtering. thyristor switched real time banks with detuning reactors are used for these loads with best result as they provide much needed reactive power and reasonable harmonic filtering with better performance than classical automatic banks with contactor switches. If harmonic pollution is severe (high voltage distortion), the passive devices need special design considerations. Under certain situation, a combination of dedicated power factor compensation device (like a thyristor switched bank) and harmonic filtering device (like an active filter) may produce the best result.