Analysis And Improvement Of Phase-locked Loop On The Compensation Characteristics Of SAPF

With the widespread use of power electronic devices,harmonic pollution in power systems is increasingly serious,which greatly affects the power quality of the power grid.The Shunt Active Power Filter(SAPF)has a wide range of applications,which detects harmonic currents and reactive power and outputs compensation currents to the grid to achieve harmonic suppression and reactive power compensation.However,due to the unbalanced voltage and harmonic interference on the grid side,or the problem in the parameter design of the phase-locked loop,it may cause errors in the phase-locked loop.Because the control of SAPF need to use the phase angle information,the phase locked error may affect the compensation performance of SAPF.The understanding of the impact of PLL can provide a theoretical basis for the design and performance of SAPF phase-locked loop,which has some research value.Firstly,this paper introduces the generation,harm and related standards of harmonic problems.Then the common active power filters are classified and compared.For the three-phase three-wire shunt active power filter studied in this paper,the mathematical models in three-phase stationary coordinate system and synchronous rotating coordinate system are given.And the PLL in it is analyzed.Then this paper analyzes and compares the advantages and disadvantages of different current command extraction schemes,and gives the system structure diagram of SAPF for harmonic suppression and reactive power compensation.The effects of phase-locked errors on the harmonic suppression and reactive power compensation of SAPF are studied in terms of the two possible forms of dynamic and static errors,which may occur in phase locking.The theoretical analysis results show that within a certain error range,the two error forms basically do not affect the harmonic suppression effect.The lead static error will cause the current ahead of the voltage after the reactive compensation,and the effect of the lag static error will be opposite.dynamic phase angle error will cause SAPF to output harmonic current and affect the power quality of the grid.The impact of phase locking error on the reactive power compensation of SAPF can be reduced by reducing the load active current component after the detection point.Then the paper analyzes the voltage unbalanced conditions,the reasons for the phase-locked error,and the effect of voltage unbalance on the compensation performance of SAPF.The positive-sequence extraction link was designed by using the symmetrical component method to improve the phase-locked loop so that it can lock phase correctly under unbalanced conditions.In addition,the method for extracting reactive current command is optimized by using the positive-sequence component extraction method,so that SAPF does not output additional harmonic current when performing reactive power compensation.Finally,the simulation model is built in Simulink and the experiment is carried out by the 66 kVA active power filter.The correctness of the theoretical analysis is verified by simulation and experiment results.