The Research On Three-phase Phase Locked Loop Under Unbalance And Distorted Grid

Because of energy crisis,the development of new energy sources is aroused wide concern,such as wind power and solar energy,so more and more distributed generation systems will be implemented into grid.Inverters as the key part of distributed generation systems have enormous implications for whole system,and the ability for detecting grid amplitude,phase and frequency information restrict its efficiency.The common method is Phase-locked loop(PLL)that obtains accurately grid information.And the synchronous reference frame PLL(SRF-PLL)has been widely used due to its ease of operation and robust behavior under the clean and balanced grid voltage,however,its performance tends to worsen under unbalanced and distorted grid conditions.In this paper,a phase-locked loop based on reference current is proposed which strongly rejects these disturbances even if the fundamental frequency of the grid voltage varies.Firstly,the approaches of obtaining the fundamental positive sequence component of grid are discussed.Meanwhile,the paper makes deep-going discussion on the voltage vector relationship under unbalanced and distorted grid,and studies some method of positive and negative sequence voltage decoupling,then the paper analyze equivalence of those transfer function.Secondly,the positive and negative sequence voltage decoupling module with the classic PLLs are analyzed and summarized.Considering the strength of instantaneous power theory on handling harmonics distortion,the theory is introduced into the research on PLL.Then a reference current is derived from the instantaneous power theory,so a PLL based on reference current are proposed under unbalanced and distorted grid,and a variable parameter PI controller is applied for the better result.Meanwhile the advantages and disadvantages of each PLLs are analyzed and compared.Finally,the steady-state response and dynamic response of each PLL under unbalanced grid are analyzed thought simulation experiment on Matlab.Experimental result shows proposed PLL can detect voltage phase more accurately under unbalanced and disturbed grid.At last the superiority and inferiority of each PLL are listed in a figure.