Research On Control Method Of Flexible DC Transmission Based On Voltage Source Converter

With the rapid development of semiconductors,the Flexible DC Transmission Technology based on voltage source converters and PWM modulation technology is being paid more and more attention by academics and electricity workers.As the VSC-HVDC transmission system has the characteristics of not consuming reactive power,independent control of active and reactive power and supporting the passive system,the technology has a wide range of applications in the construction of urban distribution network system,asynchronous grid transactions,the new energy power generation and other fields.In this paper,the operation principle and the control strategy of the VSC-HVDC transmission system are studied.The power transmission characteristics and basic working principles of VSC-HVDC system are analyzed firstly.Then,the switching binary logic function is introduced to establish the transient model of the high frequency switching function of the VSC system in the three-phase stationary coordinate.Though the physical meaning of the model is clear and the precision of the model is high,the high frequency component is not conducive to the design of the controllers.To simplify the mathematic model of the converter system,the fundamental frequency dynamic model of the commutation system is deduced in the two-phase stationary coordinate system and the two-phase synchronous rotating coordinate system respectively.In order to decouple the strong coupling,multivariable and nonlinear current inner loop control,the feedforward decoupler is introduced and analyzed in detail in this thesis.The system frequency of the power grid can sometimes fluctuate in the actual operation,which could have a bad influence on the synchronization phase detection of the phase-locked loop.With the concept of the soft phase-locked loop,the traditional phase-locked loop can be improved,so that the new system with the improved PLL can detect the synchronization phase more accurately when the system frequency fluctuates in a small rage.Aiming at the problem that the PI parameters are difficult to adjust in the control systems,this paper proposes the modulus optimum PI tuning method for the inner current control loop and the symmetrical optimum PI tuning method for the outer power control loop,which can make the response speed of the inner loop faster and the stability of the outer loop stronger.Finally,the model is set up on the simulation software PSCAD to verify the validity of the control strategy.For the case of asymmetric power grid failure,this paper analyzed the adverse effects of negative sequence components on the synchronous phase detection and system operation.By using instantaneous symmetric component theory,the dynamic models of positive and negative sequence components in two-phase coordinate system were established.By suppressing the negative sequence current to compensate the negative sequence voltage,the negative sequence control part can make the system not affected by the negative sequence components.At last,the whole control system was simulated on PSCAD to test its validity.