Analysis Of AC/DC Harmonic Characteristics In LCL-HVDC System

Aiming at the problems of traditional LCC-HVDC(line commutated converter based HVDC),such as serious noise pollution of converter transformer,commutation failure of inverter side,harmonic control and reactive power balance,large harmonic loss of transformer and so on,the LCL-HVDC system is proposed to solve these problems.There are a series of theories and technologies that need to be studied for this new type of HVDC technology to be applied in engineering.This paper only analyzes the AC and DC harmonic characteristics,and focuses on the harmonic and load loss of converter transformer.Compared with the topology of LCC converter,the LCL converter has shunt capacitor C in the valve side winding of converter transformer,then series current limiting inductor L,and removes the filter and reactive power compensation capacitor in AC side.Among them,the current limiting inductance L can limit the di/dt value of the converter valve,and the shunt capacitor C can reduce the harmonic current of the valve side winding of the converter transformer,provide the commutation voltage required by the converter valve and provide reactive power compensation.The ideal capacitance value should meet the harmonic limitation of AC bus and low reactive power exchange with AC system.Firstly,this paper introduces the AC and DC harmonics in HVDC system,and analyzes the principle of AC and DC harmonics generated by 12 pulse converter in detail.When using 12 pulse connection,the characteristic harmonic number of Yy or Yd converter transformer is n=6k±1.The characteristic harmonic number of the AC system after the confluence of two converter transformers is n=12k ± 1.The characteristic harmonic number of DC line in DC system is n=12k.Secondly,according to the CIGRE model,the simulation models of LCC-HVDC and LCL-HVDC are established in MATLAB / Simulink.Taking the inverter side as an example,the values of parallel capacitance and current limiting inductance in LCL converter are introduced.The simulation results show that the LCL converter without AC filter can also achieve the purpose of AC side filtering.Thirdly,the harmonic of different equipment in AC system is analyzed.The harmonic and load loss of converter transformer are introduced.The harmonic voltage and current of valve side winding of converter transformer in LCL-HVDC system are simulated and compared with LCC-HVDC system.After the harmonic current of the converter transformer is obtained,the load loss of the converter transformer is further calculated.The rated voltage and installed capacity of shunt capacitor are calculated and compared with the reactive compensation and filter capacitor in LCC-HVDC system.At the same time,the rated current and installed capacity of current limiting reactor in LCL converter are calculated.Finally,the influence of DC harmonics on DC lines is analyzed.Firstly,the DC pole voltage and current fluctuation coefficients in LCC-HVDC and LCL-HVDC systems are calculated by simulation.According to the comparison of the fluctuation coefficients,the smoothing reactor in LCL-HVDC system is re selected.In addition,the design method of DC filter is introduced,and three kinds of double tuned DC filters are selected to judge the filtering effect of DC filter in LCC-HVDC and LCL-HVDC systems by calculating the equivalent interference current.