Study Of Phase-locked Loop And Switching Function Modeling On HVDC

By 2015,there have been 26 HVDC projects put into operation in China.The Pearl River Delta region,and East China Power Grid have been multi-fed AC-DC hybrid power grids.The transmission power ratio through HVDC is increasing.The mutual impact between AC and DC systems becomes complicated,bringing new challenges to the safe and stable operation of the power grid.Modern HVDC transmission generally uses equidistant pulse control,requiring a phase-locked loop to provide reference phase synchronizing with AC voltage.The performance of phase-locked loop directly impacts on the control system regulation.Synchronous reference frame phase-locked loop(SRF-PLL)is widely used in HVDC transmission,but when the AC voltage is asymmetric,distorted,SRF-PLL cannot provide accurate synchronizing reference phase.This paper first analyzes the basic principle of SRF-PLL and the phase error when the system is faulty,and then introduces decoupled dual synchronous reference frame phase-locked loop(DSRF-PLL)and cascaded delayed signal cancellation phase-locked loop(CDSC-PLL).How DDSRF-PLL and CDSC-PLL exclude fundamental negative sequence voltage,and other harmonics is elaborated in detail.However,DDSRF-PLL and CDSC-PLL have their own defects,respectively.In this paper,the shortcomings of DDSRF-PLL and CDSC-PLL suffering from low-order harmonics are analyzed,and the analyzed results are verified by simulation in PSCAD software.Aiming at the shortcomings of DDSRF-PLL and CDSC-PLL,improved measures are put forward respectively.Due to variation of grid frequency is almost constant,this paper introduces decoupled multiple synchronous rotating frames,and proposes out-loop decoupled operation to extract the fundamental positive sequence voltage,realizing an improved phase-locked loop,DMSRF-PLL-Impr.As for the shortcoming of CDSC-PLL requiring 0.5 cycles to eliminate +2nd harmonic,the paper proposes to combine cascaded delayed signal cancellation method and doubled double synchronous rotation frames.The proposal can extract +1st,and +2nd voltages with fast dynamic response.A promoted phase-locked loop CDSC-PLL-Impr is implemented based on the proposal.In the following,the two improved PLLs are exemplified in PSCAD software.The improvement of phase-locked loop performance lays a foundation for precise control of HVDC transmission in the future.The harmonics generated by HVDC transmission are a kind of pollution to the quality of power supply,and it is one of the problems that must be solved in the operation of HVDC transmission.In this paper,using the advantage of improved phase-locked loop which it provides accurate and stable reference phase,considering actual commutating voltage zero-crossing points,commutation angles and effective firing angles,switching function model of HVDC converter is established.The proposed model does not need any modification under AC faulty conditions.The equations used to calculate actual commutating voltage zero-crossing points,commutation angles and effective firing angles are also presented.In the CIGRE Benchmark model,considering DC side impedance of converter,the proposed switching function model and the related calculation formulas are validated.The appropriate scope of application of the model is also illustrated by simulation.Finally,this paper further uses the established switching function model to analyze the relationship between AC positive and negative sequence components and DC side harmonics,and explains the source of DC side harmonics mathematically.When the system asymmetry fault is determined and only the fundamental voltage is considered,the mathematical relationship between three-phase firing angles and the 2nd harmonic of the DC side is analyzed.And according to the analysis,the method of suppressing the 2nd harmonic of DC side is proposed by a set of optimized three-phase firing angles.The final simulation results verify the correctness and feasibility of the method.At the end of this paper,the deficient aspects of the research are summarized,and the research of phase-locked loop and DC-side 2nd harmonic suppression method is prospected.