Research On Key Techniques Of Shunt Active Power Based On Frequency Dividing Control

As the most widely used energy resources, the development and usage level of electric energy is the symbol of national development level and comprehensive national strength. The nonlinear equipments including power electronic products supply efficient and energy saving methods for electric energy utilization, however, they cause the harmonics interference to the power system at the same time. Active power filters (APFs) become hot topics and will play its important role for power quality since they are the efficient, safe, steady treatment method to the harmonic interference. Since their convenience in installation and maintenance, APFs become the mainstream. The important and difficult points of APF are studied based on frequency dividing control in this paper.An introduction to the cause and perniciousness of harmonic current is presented in this paper. The standards of limitation for the harmonic current at home and abroad are introduced. The ways to suppress harmonic current, the classification and the basic principle of APFs are presented.In order to improve the compensation accuracy, to reduce the time of dynamic response and to raise the flexibility of compensation, the proportion-integration (PI) control based on the multiple synchronous coordinates is improved, and the proportion-resonance (PR) selective harmonic current frequency dividing control based on the quiescence coordinate is proposed in this paper, the equivalence is proved by mathematical technique. The PI control strategy based on the multiple synchronous coordinates transforms the selective detected load harmonic current to the direct-current component and regulates the component by the PI controller, which can achieve the selective and zero steady-state error frequency dividing compensation of the load harmonic current. The regulation is carried out in the cycle of carrier wave, which guarantees the good dynamic performance and advantageous robustness when the load currents have severe change. The PR control strategy based on the quiescence coordinate has infinite gain to the selective frequency harmonic current, which implements the zero steady-state error frequency dividing compensations. The regulation in the cycle of carrier wave guarantees the dynamic performance too. The performance under the unbalanced load harmonic current is compared. To implement the application in low voltage high capacity based on the intrinsic technique, a parallel control strategy in capacity proportion frequency dividing mode based on the multiple synchronous coordinates for APFs is proposed in this paper. According to the harmonic component proportion in the total harmonic current, this strategy optimal the capacity distribution, the APFs with a larger capacity compensate more harmonic current and the ones with a smaller capacity compensate less harmonic current, which is very practical for accurate harmonic current compensation and stable grid operation in high power applications. In order to guarantee the safe operation of single APF in parallel system, this paper analyzes the common protection and brings forward betterment measure. In order to reduce the switching frequency harmonic current, this paper proposed application in the parallel APFs system of carrier phase-shifted technique.At the present time, the researches on APFs are centered in the hypothesis of the ideal system voltages. However, unbalanced and distorted system voltages are more and more common. According to the compensation objective, two main control strategies for APF are unity power factor (UPF) and perfect harmonic cancellation (PHC). The influence on the system common coupling point voltage of two reference extraction is presented in this paper. A new optimization control strategy considering the three-phase system as a whole according to IEEE-1459 is proposed. A set of voltages of each phase are extracted by recursive discrete Fourier Transform. The reference system currents are obtained by multiplying the set of voltages with the conductance factors, which is desired by the optimization process based on Lagrangian function. Both unbalanced and distorted load currents are not completely eliminated, compensated system current THD is limited within the specified value by the standards by reducing the magnitude of individual harmonic component, which ensures an optimal value of power factor of three-phase unbalanced and distorted system.Harmonic current amplification effect happens when the APFs compensate the voltage-source type nonlinear load. This paper regards the uncontrolled rectifier with the capacitors and resistors as the typical voltage-source type nonlinear load. Based on the research on the characteristic of input impedance, this paper focuses on the current amplification effect when the shunt-APFs compensate these loads and point out that the system equivalent impedance, equivalent impedances of load alternating current side and the input impedance characteristic of load are the basic reason of current amplification effect. The practicable scheme to limit the amplification is presented and the summary design method is given. Finally, based on all aforementioned researches, two APFs were realized. On the basis of these prototypes, this paper did a plentiful research on the aforementioned control strategies. The theories and strategies proposed in this paper are verified completely by simulation and experimental results, which will be something referential value and act as a stimulus to the industrial applications of APFs.