Study On Active Power Filter In α-β Coordinate

Nowadays, the problem of power quality becomes more and more serious because of widely used nonlinear large-power load. As one of the most effective ways to restrain harmonics and improve power quality, the technique of the active power filter has been research focus.The existed topologies of the active power filters are mostly built in a-b-c coordinate. It is difficult to realize precise decoupled control because the dimension of control vector is less than that of output vector. Meanwhile, it’s not easy to overcome the impact from main circuit nonlinearity and un-modeled dynamic action on control strategy.Combining with control strategy, this thesis constructed a novel topology of the active power filter in a-P coordinate. The performance, detection algorithm and control method were deeply and comprehensively discussed for this novel topology and corresponding experiments were carried out. The whole basic theoretical framework for the active power filter of the novel topology were formed.The main research points are as follows:1. Research on topology of the active power filter in a-P coordinateAccording to equivalent transformation from a-b-c three-phase static coordinate to a-P two-phase static coordinate, the two-phase coordinate expression of three-phase current source model was derived, from which a novel topology of the active power filter in a-βtwo-phase static coordinate was constructed. The novel topology uses two groups of single-phase inverters as the main circuit. The calculation of control variables is directly conducted in a-βcoordinate and the results are directly used to two groups of single-phase inverters. It’s realized that two-phase current is injected into three-phase power system for compensation through specially linked transformer. Multilevel technique is introduced in this novel topology, which satisfies high-voltage and large-power application. Simulation results prove that this novel topology is feasible and has good compensation effect. In multilevel construction, even low frequency of switch can also gain quite good compensation effect.2. Research on performance of the active power filter in a-βcoordinateCommonly, dead-time effect which is of much stronger nonlinear characteristics is not reflected in control variable calculation. Taking dead-time effect as study object, the comparison between the novel topology of the active power filter in a-βcoordinate and the traditional topology in a-b-c coordinate was conducted. The results show the performance of the novel topology system is better and it helps solve the problem of maladjustment from coordinate transformation for control algorithm in nonlinear power electronics system.An iterative algorithm for dead-time effect which doesn’t depend on detection was proposed. This method originates from compressive map principle. Through mutual conversion of frequency domain and time domain, it iterates repeatedly using fundamental wave period every calculation until convergence. Then, errors introduced by dead-time can be gotten. Examples show that the algorithm has rapid calculation speed, good convergence effect and correct output results.3. Research on detection algorithm of the active power filter in a-βcoordinateA kind of sliding window recursive algorithm of electric quantity detection based on discrete fourier transformation was discussed. This kind algorithm avoids comprehensive coordinate transformation, which especially fits the active power filter construction in a-βcoordinate. Through analysis, it’s found that small frequency shift of signals has little influence on amplitude detection, but has serious impact on phase detection. So, it’s better to use phase-locked loop. It’s proposed that the detection algorithm essentially equals FIR filter. Through performance analysis of system function in frequency domain, the algorithm validity was proved theoretically. Calculation results further show that this kind of detection algorithm can fully satisfies the requirements of accuracy and real-time for the active power filter.4. Research on control strategy of the active power filter in a-βcoordinateThe control strategy of the active power filter directly aims at single-phase inverter in a-P coordinate, and most physical quantities in power system can be seen as periodic signals in steady state. So, an improved repetitive learning Boost converter control strategy was proposed. In this strategy, the internal model of the traditional repetitive control was modified and the control delay was shorten to half a period. Theory analysis and simulation results show that the improved control strategy has good robustness, rapid tracing property and small steady error.5. Experiments of the active power filter in a-βcoordinateThe experiment equipment was designed on the active power filter topology in a-βcoordinate. An approximate method was used aiming at the comprehensive transformation ratio of transformer, the error from which can be seen as a part of uncertain periodic interruption in system. Based on improved repetitive learning Boost converter control strategy, a control system using DSP and FPGA was built and the experiment research was carried out on it. The experiment results proved the correction of the above research in further.Power department and power consumers have high requirement for power quality, which makes the application of the active power filter as a trend. Mastering the key realization technique, the novel active power filter researched on in this thesis will affirmatively have wide application prospect.