Design Of Phase Locked Loop Under Harmonic Pollution Condition

With the increasing application of power electronic devices more widely in human life, the harmonic pollution has become more and more serious. The active power filter is an effective solution to solve the problem. In order to ensure the normal work of APF, it is necessary to get the grid voltage fundamental frequency, amplitude and phase. When there is harmonic interference in the grid, the phase locked loop based on the synchronous rotating coordinate transformation cannot detect the information of the grid accurately. Therefore, this thesis makes a deep research on the technology of three phase locked loop under the condition of harmonic pollution.Firstly, this thesis studies the frequency tracking phase locked loop. It obtains the good performance of frequency tracking under the harmonic pollution conditions if the structure is improved. However, the simulation results point out that the output amplitude contains harmonic disturbances under the low order harmonic interference. Secondly, this thesis presents two phase locked loops to solve the problem of harmonic interference. In order to achieve different filter performance and function, it needs to adjust the control structure based on different coordinate system. It is also proved that the two is equivalence in eliminating the specific harmonics. However, due to the interaction between frequency feedback and phase feedback, the dynamic performance becomes worse. Finally, it puts forward the phase-frequency separation phase locked loop that combines with the function of frequency tracking and harmonic elimination and works better than them. In the end, the simulation and experiment results show the advantages of the phase-frequency separation phase locked loop. It proves that this phase locked loop has better dynamic performance and anti-jamming performance in theory and practice.