Research On The Modeling, Simulation And Control Strategy Of CSCC-HVDC Systems

With the increasing amount of HVDC projects and larger capacity of dc transmission, superior technical requirements are needed for the security and stability of ac-dc interconnected grids. The conventional line commutated converter (LCC) used in HVDC projects relies on the ac bus voltage for the commutation process, which makes the converters susceptible to successive commutation failure in the event of ac faults of the inverter side. What is worse, it may lead to the HVDC pole blocking or some other chain reaction. For the weak sending end, ac faults of the rectifier side also have impacts on the security of the entire system. To improve the security and stability of ac-dc interconnected grids, relevant studies on new techniques of HVDC have become hot issues in the field of power transmission. Research results indicate that the capacitor commutated technique can effectively improve the overall operation stability of HVDC systems.Based on the capacitor commutated converter (CCC) technique, theoretical analysis, modeling and simulation of the controlled series capacitor converter (CSCC) are studied in this dissertation. Moreover, a corresponding control strategy of its thyristor controlled series capacitor (TCSC) is proposed, as well as two adaptive control methods for the real-time preferences of PI controllers. As for the weak ac system, i.e. the short circuit ratio (SCR) is lower than its critical value, a specific CSCC-HVDC scheme, in which fixed capacitor is allocated additionally, is presented. The effectiveness of the model and its control strategy mentioned above is verified via simulation models. The main contents are as follows:(1) A mathematical model of CSCC-HVDC systems is studied and developed here. A simulation model of the ac-dc interconnected grid which contains the CSCC is built in PSCAD/EMTDC, to provide effective approaches for the study of its operation characteristics and the control strategy. (2) According to the operation characteristics and control principles, a control strategy of the thyristor controlled series capacitor (TCSC) in CSCC-HVDC systems is proposed, of which the corresponding proportion, as well as the capacitance impedance, is determined based on the real-time detection of the converter bus voltage and the extinction angle y, together with the equivalent impedance, to improve the operation stability of the ac-dc interconnected grids.(3) In order to avoid inexact tracking of the impedance reference when the operation variation of TCSC is in a wide range, two adaptive control methods for the real-time preferences of PI controllers, i.e. the ideal curve method and the linearization method, are proposed here, for the sake of fast and accurate tracking of the reference.(4) As for the weak ac system, a specific CSCC-HVDC scheme is presented to increase the SCR and ensure the normal start-up of HVDC systems. Besides, it can reinforce the anti-disturbance ability under fault conditions, improve the electromechanical transient stability and enhance the dc infeed ability of weak systems.(5) Based on the simulation model, the operation characteristics including the start-up of dc systems and transient-state characteristics under ac system faults, of CSCC-HVDC systems and LCC-HVDC systems, are compared to verify the effectiveness of the model and its control strategy.At present, studies on the modeling and control strategies of the CSCC technique are merely at the early stage both at home and abroad. Therefore, further researches on the CSCC-HVDC models and control strategies can have important theoretical and practical significance in promoting the development of the CSCC technique.