Study On Harmonic Detecting Of Three-phase Four-line System

Active power filter is a new power electronic device which can effectively compensate harmonic and reactive current of network. It has many favorite work characteristic such as real-time and accuracy. Instantaneous reactive power theory has been widely used in active power filter, especially in three-phase three-line system. But there exits a lot of question when instantaneous reactive power theory is used in three-phase four-line system. This paper puts emphasis on three-phase four-line system. From the angle of power definition this paper brings forward several different power definitions based on instantaneous reactive power theory and their compensation algorithm. Beside this paper also discuss p-q-r theory and its application in harmonic detection in power system.Firstly, we discuss traditional instantaneous reactive power theory, Nabae instantaneous reactive power theory and their compensation methods. The conclusion is that when there isn't 0 sequence voltage the two algorithms above is exactly the same when there exists 0 sequence voltage the two compensation methods is different from each other and the traditional theory can compensation neutral-line current. While Nabae theory cannot compensate neutral-line current to zero.Secondly, we discuss p-q-r instantaneous power reactive power theory and its limitation in harmonic detection. We can conclude that when the voltage is not distorted, p-q-r theory can detect harmonic of three-phase system accurately. But when the voltage is distorted, the voltage of p axle contain high-order voltage, the dc currents of p axle and q axle also contains high-order current, so high-order harmonic of three-phase can not be detected accurately. We also prove that p-q-r harmonic detection is equivalent with tradition power theory no matter whether the voltage is distorted.Thirdly, we still apply resample theory and main filter to p-q-r theory. In p-q-r theory, the voltages and currents must be transferred through twice coordinate conversions and calculation amount is very great. Through theory analysis and simulation we can see that the currents of p axle and q axle only contain 3k order harmonic (k is a integer). If we resample the currents of p axle and q axle with the premise that the effective signal is not overlapped and at same time adopt suitable main filter, then the calculation amount and the delay time of filter are both greatly reduced. Simulation proves this method correct.Lastly, we put those algorithms to tests using an A/D card and the results prove their correction.