Research On Control And Protection Strategies Of Modular Multilevel Converter Based HVDC Power Transmission System

Modular Multilevel Converter (MMC) is a new type of voltage source converter(VSC) topology, which has great potential in high voltage and large power applications. Compared to the traditional two-level or three-level VSC topologies, the MMC-based HVDC system with low requirements for power electronic devices and good scalability can be easily extended to high voltage and power levels, and has better steady-state and transient performances. So MMC has become the trend of the VSC-HVDC projects for its advantages of less switching losses and independent technology of devices in series.The control and protection strategies of MMC-HVDC directly affect its practical operations. Thus to research on the control and protection strategies is of evident application value and practical significances. In this dissertation, the control and protection strategies of MMC-HVDC and its related technologies under steady state and ac system fault conditions are investigated, by means of theoretical analysis and the simulation validations. The main contents of this dissertation are as follows:(1) The mathematical model of MMC-HVDC systemFor the purpose of investigating the control and protection strategies of MMC-HVDC, the mathematical models of MMC-HVDC under ideal conditions and unbalanced ac voltage conditions are studied and developed respectively. Under the ideal conditions, the high frequency mathematical models and the low frequency dynamic models of the MMC in the three-phase static coordinates are developed. Based on these studies, the low frequency dynamic model of MMC in d-q coordinates is established. Using the concept of the instantaneous symmetrical components, the low frequency dynamic mathematical models of MMC under unbalanced ac voltage conditions are developed in d-q synchronous reference frame. The active power balancing relation between the ac and dc sides of MMC are analyzed under both ideal and unbalanced conditions. Simulation results show that the double-frequency fluctuation will appear in the dc voltage and the active power under ac system asymmetrical fault conditions. Analysis of the mathematical model and power balancing are the basis for the design of MMC-HVDC control and protection system.(2) Control and protection system framework of MMC-HVDC systemIn order to design of MMC-HVDC control and protection system from top to bottom, the control and protection system framework of MMC-HVDC system is built. Based on the topological structure of MMC-HVDC, the design principles and main functions of its control and protection system are illustrated. Since the control and protection system is of great importance, the necessity of its redundant configuration and multi-layer design are pointed out. Its overall structure is given and a four-layer framework is proposed:dc system control layer, polar control and protection layer, valve control layer and sub-module control and protection unit. The control functions of each control layer and relationship among these layers are further introduced. The research has guiding significance for the design of MMC-HVDC control and protection.(3) Research on the control strategies of valve control layer of MMC-HVDC systemThree main important issues of valve control layer of MMC-HVDC are studied which includes modulation strategy, capacitor voltage balancing algorithm and circulating current suppression controller (CCSC). According to the inner mathematical model of MMC through a double fundamental frequency negative rotational frame to decompose the circulating currents to two DC components, a CCSC based on the inner discrete mathematical model of MMC is designed to eliminate the circulating currents among the three phase-units, and the designed CCSC can minimize the peak value and the distortion degree of the arm current significantly. The modulation principle of nearest level modulation (NLM) and carrier phase-shifted pulse width modulation (CPS-SPWM) are investigated. For the purpose of balancing the sub-module capacitor voltages, an improved capacitor voltage values based raking method was proposed to reduce the averaged switching frequency and the losses. Simulations in PSCAD/EMTDC validated its correctness and effectiveness.(4) Research on the control strategies of polar control layer of MMC-HVDC systemThe control strategies of polar control layer of MMC-HVDC under ideal conditions are studied. The control system of the MMC based on the traditional series PI regulator is analyzed, and the theory of the passive control is applied to design the controller of MMC-HVDC. Then based on the Euler-Lagrange (EL) mathematical model in synchronous d-q coordinates and its passivity, the passive control with expected state balance points, states and state errors is proposed by damp injection. The proposed control algorithm can realize the decoupled control of real and reactive power, and maintain very good static and dynamic performances. Using the theory of computer discrete control, a discrete inner-loop current controller based on Smith predictor is also designed and the decoupled control of active and reactive currents is realized rapidly. So the MMC controllers are greatly simplified and the control performances are improved. The passive controller of MMC-HVDC supplying passive networks are designed and validated under multi-conditions such as variable reactive power, voltage regulating and passive load disturbance, etc. Finally21-level MMC-HVDC model in PSCAD/EMTDC validated the effectiveness and correctness of all the proposed control methods.(5) Research on the control and protection strategies of MMC-HVDC system under ac system faultsThe control and protection strategies of MMC-HVDC are studied under ac system unbalanced conditions. First, the decoupled positive-negative sequence passive controller based on Euler-Lagrange mathematical model is proposed to suppress the negative sequence current. The controller with favorable control performance and current limitation can restrain the negative sequence current caused by unbalanced faults, and then the control performance of positive-negative sequence passive controller is compared to the traditional vector controller. Second, the influences of different ac side faults of MMC-HVDC on the dc voltage control of the MMC are analyzed. To restrain dc voltage fluctuation caused by ac system faults, an active power controller integrating dc voltage control is developed, leading to improve the sustained operation capability. Moreover the protection strategy under fault condition is proposed to ensure MMC-HVDC meet the requirements of its safety operation. The results show that the dc voltage is controlled and current limitation of MMC is realized. Finally, the control and protection strategies of MMC-HVDC supplying passive network under fault conditions are proposed. The simulation results in PSCAD/EMTDC show that the proposed controller and protection strategies have very good dynamic performances and are valuable to the practical system.