Study On Harmonic Current Real-time Detection Method For Active Power Filter Based On Fourier Series

Active power filter (APF) is an effective power electronics-related equipment which has wide application foreground in suppressing harmonics and compensating reactive power, and harmonic current detection is a key technique for APF. For three-phase APF, the harmonic current detection methods based on the instantaneous reactive power theory of three-phase circuit are considered as recognized mature methods, but they also have some problems requiring solving. As for single-phase APF, there is not a mature method. Nowadays, many harmonic current detection methods have been put forward. Although each method has its own strong points, there are some problems which are difficult to overcome such as large computation quantity, bad real-time performance, not high detection precision, bad stability, low reliability etc. In this thesis, aiming at existent problems of harmonic current detection methods for APF, and considering Fourier series' expression of the nonlinear load current as theoretical gist, a series of innovative research findings are obtained, and a few the most main of them are as follows.(1) Using "When the load current is a periodic current, integral in a cycle for absolute value of subtracting the fundamental active current from the load current is the least." and "When the load current is a periodic current, integral in a cycle for absolute value of subtracting the fundamental reactive current from the load current is the least." as detection principles, direct computation method, which is a harmonic current real-time computation method based on direct computation, is proposed. This method has the traits of using formula to calculate directly, very small computation quantity, good real-time performance, nice stability, high reliability, etc.(2) Using "When the load current is a periodic current, integral in a cycle for absolute value of subtracting the fundamental active current from the load current is the least." and "When the load current is a periodic current, integral in a cycle for absolute value of subtracting the fundamental reactive current from the load current is the least." as detection principles, simple iterative algorithm, which is a harmonic current real-time computation method for APF based on simple iterative algorithm, is proposed. On the basis of simple iterative algorithm, optimal iterative algorithm, which is a harmonic current real-time computation method for APF based on optimal iterative algorithm, is proposed. The simulation waveforms of simple iterative algorithm are very similar to those of direct computation method, and optimal iterative algorithm is direct computation method which can judge the state of the load current. Both simple iterative algorithm and optimal iterative algorithm are not as good as direct computation method, but they provide an idea about judging the state of the load current, on the basis of this idea, two algorithms of judging the state of the load current are proposed based on dynamic iterative length.(3) According to Fourier series' expression of the nonlinear load current, a few basic concepts including the load current in stable state, the load current in mutative state, the true amplitude of fundamental active current when the load current is placed in stable state or in mutative state, the true amplitude of fundamental reactive current when the load current is placed in stable state or in mutative state are clearly defined.(4) On the basis of clear definitions for a few basic concepts, double linear construction ideas are proposed, which can calculate the amplitude of fundamental active current and that of fundamental reactive current. On the basis of double linear construction ideas, double linear construction algorithm, which is a harmonic current real-time computation method for APF based on double linear construction algorithm, is proposed. This algorithm can accurately calculate the amplitudes of fundamental active current and fundamental reactive current, accordingly it can accurately calculate the harmonic current needing compensating.(5) On the basis of the comparisons and analyses for simulation waveforms of direct computation method of harmonic and reactive current, double linear construction algorithm of harmonic and reactive current, and harmonic and reactive current detection method of single-phase circuit based on the instantaneous reactive power theory of three-phase circuit (method 2 having the shortest time delay), a question is proposed, which is "Is it correct that, the higher the detection precision of a harmonic and reactive current detection method for APF is, the better the method is, when the load current is placed in mutative state and following period of time?". Considering APF's work principle, APF's essential function and the Fourier series' expression of the nonlinear load current as theoretical gist, a new idea of evaluating harmonic and reactive current detection method for APF as follows is proposed. When the load current is placed in stable state, the higher the detection precision is, the better the method is. When the load current is placed in mutative state and following period of time, it is incorrect that the higher the detection precision is, the better the method is; the amplitude of fundamental active current detected out by the method should smoothly and slowly track the true amplitude of fundamental active current; the method should guarantee that "the fundamental active current" is an approximate sine-current and at the same time give attention to its detection precision, the both should be roundly considered.