Research And Analysis Of Control Strategy Based On HVDC Transmission System

In recent years,with the development of semiconductors and devices,high-voltage direct current(VSC-HVDC)based on voltage source converters has received more and more attention.Because VSC-HVDC transmission system has the characteristics of no power consumption,independent active power,reactive power control and power supply to passive systems,it is widely used in distribution network system construction,asynchronous grid trading and new energy generation.The application of VSC technology and Pulse Width Modulation(PWM)technology has many potential advantages,which will become an important part of future transmission and distribution systems.This paper studies the control of VSC-HVDC.The composition,working principle and control method of VSC-HVDC system are analyzed.Firstly,the transmission characteristics of VSC-HVDC system are analyzed.Since the high-frequency switching transient model of the conventional VSC-HVDC system converter is built in a three-phase stationary coordinate system,the presence of high-frequency components is not conducive to the design of the controller.On this basis,the dynamic model of the fundamental frequency in the α-β static coordinate system and the d-q rotating coordinate system is derived.At the same time,in order to achieve strong coupling,decoupling of multivariable and nonlinea r current inner loop control,a feedforward decoupler is introduced to prevent the control of the d-axis current and the q-axis current in the synchronous rotating coordinate system from being affected by each other.Aiming at the problem that the PI param eters in the inner and outer loop control system are difficult to adjust,the optimal PI adjustment method for inner loop current control and the symmetric optimal PI adjustment method for outer loop voltage control are proposed,which can improve the resp onse speed of inner loop current control and enhance the response speed.The anti-interference ability and stability of the ring voltage control.Finally,the mathematical model of the VSC-HVDC system was established.According to Kirchhoff’s law,a mathematical model of the VSC control system is established.A control system is obtained by combining the inner loop current controller of the system and the outer loop DC voltage controller.Then the simulation is carried out in PSCAD/EMTDC simulation software,and the vector control strategy is analyzed.The dynamic performance of the system under step change and system failure is studied.The simulation shows that the control system model can achieve two-way power transmission,fast response control and good steady-state performance.Controller parameters tailored to the analyzed tuning criteria provide better system performance.