Research On Phase-locked Loop Algorithm And Application In APF Under Unbalanced Grid Condition

In the three-phase four-wire low-voltage power supply system,more and more non-linear loads such as power electronic devices has been put into use and power users don't use electricity at the same time.These lead to increasingly prominent problems of grid harmonic and unbalanced issue.Active power filter(APF),as one of the effective measures to solve the problem of harmonic,has received extensive attention.Phase-locked loop is the key technology of harmonic current detection in APF system.On the basis of traditional PLL algorithm in unbalanced three-phase grid voltage,this paper researched PLL algorithm which was suitable for the APF in the case of including dc offset component and unbalanced power grid.The harmonic current detection of APF requires that the grid synchronization signal is tracked quickly and accurately.This paper established the mathematical model and analyzed the dynamic performance of traditional SRF-PLL system.Under the condition of balanced and unbalanced grid voltage,this paper analyzed the phase lock effect and designed the parameters of PI controller in the system.Under the balanced grid voltage,SRF-PLL can lock the phase accurately.However,SRF-PLL can't lock the phase accurately because of the second harmonic distortion created by the negative sequence component under the unbalanced grid voltage.The conclusion is verified by simulation,and it is consistent with theoretical analysis.The APF system presented higher demands for PLL under the unbalanced grid voltage.Based on the separation of positive and negative sequence voltage,this paper researched the PLLs based on the double second-order generalized integrator and cross decoupling complex filter deeply.It also designed the parameters of the orthogonal signal generator and the positive and negative sequence filter.On the basis of theoretical research,the two kinds of PLL were verified by simulation.The results of simulation and experiment showed that the dynamic response of the PLL based on cross decoupling complex filter is faster than the DSOGI-PLL.In order to improve the dynamic response performance of PLL system,this paper researched the independent frequency-locked loop based on second-order generalized integrator deeply.According to the principle of SOGI-FLL,this paper designed the FLL based on the positive and negative sequence filter under static reference frame.The dynamic and static performance of this FLL depend on debugging parameters ?cand ?.The ? is needed to be linearized and the structure is complex relatively.To avoid the effects on the performance of FLL,this paper improved the FLL based on the positive and negative sequence filter.The improved FLL structure is simple and it's performance will not be affected by the debugging parameter.The simulation and experimental results verify that the overshoot of the improved PLL based on positive and negative sequence filter is smaller and the dynamic response is faster than before.In addition to consider the condition of unbalanced grid voltage,this paper also researched the PLL in the condition of containing dc component in grid voltage.The ac and dc voltage signal can be separated by the sine amplitude integrator.The dc signal vq that is extracted by SAI as the input of PI controller,the grid signal synchronization can be realized.The results of simulation showed that the PLL with SAI could detect the grid phase,frequency and amplitude information accurately.