Study On Phase Locked Loop Under Unbalanced Grid

Modern grid standard requires that doubly-fed wind power system must have the ability of ride through faults.The fault ride through of doubly-fed wind power system under grid fault is an important factor to determine whether doubly-fed wind power system can be incorporated into grid.One of the important technologies of fault ride through--phase locked loop under unbalanced grid,provides the control datum for rotor-side and grid-side converter of doubly-fed wind power system,and plays an important role in the grid-connected operation of doubly-fed wind power system under faulty grid.In this paper,the technology of phase locked loop under unbalanced grid is studied deeply.Firstly,the dynamic and static performance of synchronous reference frame phase locked loop under the grid voltage amplitude unbalance,grid harmonic and DC bias voltage were studied which was based on the analysis of the structure and mathematical model of the synchronous reference frame phase locked loop.The error under the corresponding circumstances was analyzed.Secondly,the working principle of positive and negative sequence double dq rotating coordinate phase locked loop and dual second order generalized integrator frequency locked loop applied under unbalanced grid is introduced.In order to solve the problem that the error of the detection result is large when the DC bias voltage exists in the grid,the phase detector of stationary coordinate system phase locked loop is added to the structure of the the positive and negative sequence double dq rotating coordinate phase locked loop.In order to solve the problem that the frequency and phase of the grid can’t be detected accurately by the dual second order generalized integrator frequency lock loop when the DC bias voltage exists in the grid.The feedback loop is added to the structure of the phase-shift integrator,and the topology of the dual second order integrator frequency locked loop is modified.Finally,the simulation models of synchronous reference frame phase locked loop,double dq rotating coordinate phase locked loop,modified positive and negative sequence double dq rotating coordinate phase locked loop,dual second order generalized integrator frequency locked loop,modified dual second order generalized integrator frequency locked loop are built.And the simulation results are compared and analyzed.On this basis,the hardware platform of the PLL experimental system is designed and the experiment of phase locked loop under unbalanced power grid is carried out.Simulation and experimental results verify the correctness of the theoretical analysis.