Research On Decentralized Control Of VSC-MTDC System

In recent years,as oil,coal and other conventional energy resources are increasingly scarce and the mounting pressure all kinds of environmental problems,wind,solar and other renewable energy have achieved a great improvement.At this time,the inherent defects of the traditional ac power grid are being gradually revealed.Offshore wind farm accesses to ac system,for example,is hardly can utilize ac transmision because wind power is randomness and volatility.With the breakthrough of full controlled devices technology,multi-terminal HVDC system based on voltage source converter(VSC-MTDC)gets a rapid development and pervasive application.Based on the above,a series of studies are carried out in this dissertation,focusing on decentralized control of VSC-MTDC and stability analysis.And main works are summarized as follows:VSC-MTDC not only has many advantages just like VSC-HVDC does,comparing to later,it is more flexible and economy,but its operation mode and control system is relatively more complicated.Master-slave control method is widely adopted in VSC-HVDC system,which highly depands on the real time communication between converters and the upper control system.However,VSC-MTDC always has many converters,which are geographically dispersed,the mutual coordination between the converters is more complex,and the control strategy based on real-time communication is difficult to meet the requirements of the system.Firstly,based on the autonomous decentralized control,this paper proposes a combined control strategy utilized in VSC-MTDC system.Combined control strategy combines with the advantages of two kinds of control strategies commonly used in VSC-MTDC:voltage droop control and voltage margin control,and makes up for the deficiencies of them.This strategy greatly improved the dynamic response of VSC-MTDC system.Secondly,the small signal model of VSC-MTDC based on the combined control strategy is deduced under the dq rotating coordinate system.This article adopts the modular modeling,the system can be divided into four parts:converter,control system,measuring system and dc lines.And the control system includes two layers:coordinated control between the converters(combined control strategy)and converter control(vector current control).At the same time,the corresponding detailed electromagnetic transient(EMT)model is also established based on PSCAD/EMTDC.Then two models are compared under the small disturbances in time domain in details.The simulation results show that the output waveform of the two models is almost completely overlap,which prove the small singal model is correct.Then,this paper analysis the the influence of control parameters on the stability of VSC-MTDC based on the small singal model by using the root locus method.This paper points out that slope parameters of combined control stragety have very important influence on system stability.And the influence of parameters of vector current control on the stability of system depends on the strength of ac system.When connecting to the weak system,PLL has very important influence on stability of VSC-MTDC and can not be ignored in the analysis.And with the increase of parameters of outer loop controller reactive component loop and the inner loop controller active component loop,the stability margin of VSC-MTDC will gradually increase;on the contrary,it may cause instability when the parameters are too small.And outer loop controller active component loop and the inner loop controller reactive component loop is not relative to the stability of the system.Under the condition of strong ac system,parameters of the inner loop controller active component loop may cause system instability when it is too small,and the rest will not affect the system stability.On the basis of guarantee the stability of the VSC-MTDC,in order to improve the dynamic response of the system,this paer proposes a parameter optimization method by means of invoking multiple PSCAD/EMTDC at the same time by the external optimization program.This mehod can make multiple cases parallel optimization,and by accumulating them with their own weighting,which can ensure that the results of parameter optimization is suitable for multiple running state and the efficiency of the optimization is greatly improved.Due to one of the most important application of VSC-MTDC is the renewable energy sources access,such as offshore wind farm.And wind farm is a typical weak system,therefore,in this paper,the stability of VSC-MTDC when connecting to the weak system are analyzed in detail.This paper indicates that phase-locked loop(PLL)becomes the key factor affecting the stability of the system,when connecting to the the weak ac system.And proposes a compensation method of PLL to improve the stability of VSC-MTDC which based on the dynamic relationship between the active power and voltage phase angle difference.And the proposed method is verified by the comparison between root locus curves when utilized traditional PLL and compensated PLL respectively...