Study On The Design Of DC Filter In Multi-terminal DC Transmission System

With the power increasing of HVDC transmission system,it generally needs some rectifiers to provide the power and some inverters to consume it.MTDC transmission system can perfectly meet the above requirements.At present,a lot of HVDC projects,which are based on LCC topology,has been put into operation in our country.Connecting them to MTDC transmission systems will improve the flexibility and reliability of HVDC transmission system.Whether the design of DC filter is reasonable or not directly determine the operation characteristics of HVDC transmission system.However,there are few researches on the design of DC filter in MTDC transmission system.Therefore,this paper that talk about the design of DC filter in MTDC transmission system,which uses LCC topology,is very important.In this paper,the calculation method of main circuit parameters in MTDC system,which can be divieded into DC parameters calculaion and converter parameters calculation,is fisrtly introduced.It is the foundation of the design of DC filter and it will provide some basic datas for 3-pulse harmonic model to calculate.Then an advanced 3-pulse harmonic model,which can be applied to the asymmetrical and distorted AC system,is proposed.The 3-pulse harmonic voltage source and 3-pulse inductance in the advanced model can accurately reflect the influences of asymmetric factors,such as the deviation of commutation inductance and commutation angle,and backgroud harmonics.Further,the calculation method of harmonic current on DC side is derived based on the advanced 3-pulse harmonic model and node voltage method.Through the analysis of an example,the distribution law of equivalent interference current among converters is deduced and an ajustment principle of DC filter in MTDC system is propo sed.Finally,this paper describes the topology and impedance frequency characteristics of single-tuned,double-tuned and triple-tuned filters and proposes a new method to design the parameters of DC filter.In this new method,multi-tuned filter is equivalent to a corresponding number single-tuned filters in parallel.The parameters of multi-tuned filter is resolved by adjusting the parameters of single-tuned filter.The method can avoid the complicated calculation and improve the design efficiency of the filter parameter.