Research On Power Control Strategies Of Voltage Source Converter Based Multi-terminal Dc Transmission System

Compared with traditional point-to-point high voltage dc(HVDC)transmission technology,the voltage source converter based multi-terminal DC(VSC-MTDC)transmission technology has the advantages of high reliability,redundancy and flexibility.It has application prospects in the fields of connecting offshore wind farms(WFs)and high-power long-distance transmission.At present,a number of VSCMTDC transmission projects have been built.However,researching on control strategies of VSC-MTDC transmission systems has drawn lots of attention.It has great significance to broaden the application of VSC-MTDC through further explore its control technologies.In order to improve the control performance of the VSCMTDC transmission control system,this thesis focuses on the following four key technologies:(1)Damp the oscillation in the dc side to improve the dc side stability of VSC-MTDC;(2)Optimize the power-sharing among converter stations;(3)Improve the design method of the traditional droop coefficient C by considering the influence of the dc line resistance on power-sharing;(4)Improve the accuracy of power-sharing by considering the parameters uncertainty.The specific contents of this paper are as follows:There is a risk of oscillations in the VSC-MTDC systems due to the existence of dc inducatance and dc capacitance.To address this problem,a virtual oscillation damping method is studied in this thesis.The equivalent mathematical model of VSCMTDC is derived by using the dc line equivalent model.The origins mechanism of oscillation in the VSC-MTDC system and their supression method are studied.This thesis analyzes the impact of the energy storage components in the dc system,for example the effect of inductors and capacitors on the amplitude and frequency of the dc voltage and power oscillation.Theoretical analysis shows that the energy storage components in the dc side is the main reason which causes the oscillations on the dc side.To solve this issue,this thesis proposes a virtual oscillation suppression method according to the structure of the VSC-MTDC system.By applying the principle of power balance between the ac side and the dc side,a dc damping signal is calculated.The dc damping signal is equivalent to the ac side and added to the current signal of the inner loop controller.This method is easy to implement,requires no additional sensors and does not cause additional losses.In the traditional droop control,the droop coefficient is fixed and the powersharing among the converters is not flexible.In order to address this weakness,this thesis proposes an adaptive droop control method by using power margin of VSC to participate in power regulation to improve the performance of flexible power-sharing.An adaptive droop coefficient adjustment method is designed according to the differential of the dc voltage and considers the real-time power margin of each converter.In addition,in order to avoid frequent changes of the droop coefficient,the hysteresis control is introduced to the dc voltage variable.Meanwhile,combining with the fixed dc voltage control,a general MTDC power coordinated control method is proposed.This method overcomes the shortcomings of the traditional droop control,optimizes the power distribution between different converters in the MTDC system,and avoids the overload of converters due to insufficient power margin.The proposed control method is simple to implement,requires no additional sensor and has a fast response speed.In order to compensate the impact of the dc line resistance on the power-sharing,this thesis proposes a droop coefficient design method to optimize the power-sharing accuracy.The impact of dc line resistance on power-sharing is analyzed based on the traditional droop control.The droop coefficient based on the voltage-power droop control is equivalent to the voltage-current droop control.Consider the impact of dc line resistance on the current-sharing among the converters and the voltage drop caused by the dc line resistance at the converter terminal voltage.The relationships between the power-sharing ratio,the droop coefficient and the dc line resistance are derived.A method to design the droop coefficient is presented with compensates for the effect of the dc line resistance on the dc current deviation and the terminal voltage drop.Therefore,the power-sharing among the converters can be accurately controlled.In a MTDC system,the values of parameters such as the dc line resistance,dc voltage and dc current measurement cannot be accurately obtained.Therefore,in order to reduce the inaccuracy of power-sharing caused by the uncertainty of these parameters,the impact of the dc line resistance variation caused by temperature and current,and the dc voltage and current measurement errors on the power-sharing among converters is considered.A power regulation method is proposed based on the proposed droop coefficient regulation method.The proposed power regulation method is simple to implement.The power-sharing error caused by dc line resistance variation,parameter measurement error and some other uncertain factors can be eliminated by the proposed power regulation method combined with the proposed droop coefficient design method.The proposed two methods collaborate to achieve an arbitrary power-sharing ratio among converters.