| In view of the serious noise pollution of the converter transformer,commutation failure on the inverter side,harmonic control and reactive power balance,and large harmonic loss of the transformer that exist in the traditional LCC-HVDC(Line Commutate Converter Based HVDC),the research team proposed a LCL-HVDC attempts to basically solve these problems.For this new type of direct current transmission technology to be applied in engineering,a series of theories and technologies need to be studied.This article only conducts research on the commutation characteristics of the converter.As one of the frequent failures of traditional HVDC(High Voltage Direct Current)systems,commutation failure will interrupt the transmission of DC power.In response to the above problems,this paper compares the topological structures and commutation methods of HVDC transmission.Based on the traditional LCC-HVDC system,a LCL-HVDC system structure is proposed and its basic theory is analyzed.Compared with the LCC type converter topology,the LCL type converter has a capacitor C connected in parallel to the valve side winding of the converter transformer,and then a current limiting inductor L is connected in series,and the AC filter and reactive power compensation capacitor are removed.Among them,the current-limiting inductor L can limit the di/dt value of the converter valve,and the function of the parallel capacitor C not only provides DC voltage and commutation voltage,but also undertakes the functions of reactive power compensation and harmonic suppression.The ideal capacitance value should meet AC bus harmonic limit and low reactive power exchange with AC system.Firstly,this paper analyzes the topology and basic wiring mode of LCC transmission system.On this basis,according to the working principle of the system in a cycle,the commutation process of rectifier side and inverter side of lcc-hvdc system is reasoned,and the basic concept,discrimination method,harm and prevention method of commutation failure are introduced.Secondly,taking CIGRE model as an example,this paper compares LCL system with LCC system,introduces the advantages of the new system,analyzes the control mode of rectifier side and inverter side of the new system and the commutation process of inverter side converter,and concludes that the converter of the new system has faster commutation speed compared with the traditional LCC system.The DC quasi steady state model of the converter is established,and the main parameters of the new system are calculated.Finally,the model of the LCC and LCL system is built in the PSCAD simulation software.The steady state of the LCL system is verified by analyzing the DC voltage,current,and the installation capacity of the parallel capacitor and the current-limiting reactor of the LCL-HVDC system under steady-state operation.The working characteristics are good at the time and the parallel capacitor can filter out the harmonics flowing into the AC grid well.By analyzing the voltage and current waveforms and harmonic content on the converter transformer grid side,valve side and converter valve of the two systems,and comparing the ability of the system to resist commutation failure under different fault conditions,it is verified that the parallel connection in the new system The capacitor has a good filtering effect,at the same time,it can reduce the commutation overlap angle,reduce the number of commutation failures,and ensure the safe and stable operation of the DC transmission system.It has a good engineering application prospect. |
