| The modular multilevel converter (MMC) is the core equipment in the new generation of voltage sourced converter based high voltage direct current transmission (VSC-HVDC), and has great prospects in the integration of renewable energy, dc interconnection of asynchronous ac system, power supply to big cities, island and off-shore drilling platform, etc. After ten years of development, research about MMC has made some progress, but there are still many problems to be solved, such as issues on designing the control system for large-scale MMC and handling the bipolar dc short-circuit, etc. Based on summarizing the previous research, this paper studies the half-bridge sub-module (SM) based MMC in-depth, and focuses on the following aspects:(1) The basic principles of MMC are studied. Based on ignoring the switching processes of SMs, the decoupled model between ac and dc side of MMC is built. According to this model, the MMC is divided into ac-loop and dc-loop equivalently, both of which are decoupled from each other. In dc-loop, the arm unbalanced voltage is decomposed into an inherent component and a modified component furtherly. The inherent component is caused by the exchanged power between MMC and ac system, whereas the modified component is generated by extra controller intentionally, for the sake of regulating the internal characteristics of MMC.(2) The basic components in arm current and SM voltage fluctuations of MMC under ideal conditions are analyzed. By using direct modulation, the interaction mechanism between the harmonic circulating currents and SM voltage fluctuations through the modulation function is studied. Based on this study, the mathematical model of harmonic circulating current, internal converter voltage and SM voltage fluctuation are derived. Particularly, the2nd harmonic component in circulating current is disscussed, followed by a discussion about the main hazards of harmonic circulating currents and their governance issues. For the possible asymmetrical operating conditions between the upper arm and lower arm, the influence imposed on MMC by asymmetrical arm impedance between the upper arm and lower arm is analyzed, as well as the influence caused by unequal number of SMs between them.(3) The controller composition of MMC-HVDC under symmetric ac system is studied. According to the decoupled model between ac and dc side, the station-level controller is divided into ac-loop controller, circulating current suppressing controller and damping controller. The ac-loop controller is responsible for regulating the exchanged power between MMC and ac system, the circulating current suppressing controller is responsible for cancelling the2nd harmonic circulating current, and the damping controller is responsible for regulating the dynamic response of dc current.A start/stop strategy of MMC is proposed. The start process is divided into uncontrollable start stage and controllable start stage. At the uncontrollable start stage, relationship between the limiting resistor and the maximum charging current is discussed, providing the basis for determining the limiting resistor. The stop process is divided into energy-feedback stage and capacitor-discharge stage, which improves the energy efficiency, reduces the shutdown time and lowers the discharging resistor’s requirements.(4) Under unbalanced ac system, due to dc voltage fluctuations with double line frequency at the dc side of MMC-HVDC, and due to the defect of singular matrix existing in the traditional dc voltage fluctuation suppressing algorithm, a new dc voltage fluctuation suppressing method suitable for MMC-HVDC is proposed. Using the storage characteristics of SMs, this method can suppress the negative-sequence ac current, and can eliminate the dc voltage fluctuation at the same time. The mathematical model of three-phase MMC under stationary frame is established, according to which the ac-loop controller, the circulating current suppressing controller and the dc voltage fluctuation suppressing controller based on internal model principle are designed.(5) Several single-sorting based straightforward voltage balancing strategies (SS-SVBS) are studied, including the traditional SVBS, the maximum voltage bias control based SVBS and the maintaining factor based SVBS. The defects about switching frequency and dead-time effect of traditional SVBS are analyzed. The control system architecture of MMC is discussed, and the control system based on main control unit and valve control units is proposed, which is suitable for large-scale MMCs. To solve the defects of SS-SVBS (for example, too much time is needed for the sorting process of SM voltages and great burden is imposed to the communication of control system), a dual-sorting based straightforward voltage balancing strategy (DS-SVBS) is proposed. The DS-SVBS divides the sorting process of SM voltages within one arm into two parts:the internal voltage balance and the external voltage balance, both of which can be executed in parallel. A three-phase41-level MMC prototype is developed. In addition, both the hardware and software design of the control system are introduced, providing ideas for control of large-scale MMC, such as the control system’s coordinated control, SMs’organizations and independent control, etc. Experimental results demonstrate that the maximum voltage bias control based SVBS can reduce the SMs’switching frequencies and deadtime effect effectively, the proposed control system is feasible, and the proposed DS-SVBS is working. |
PDF Full Text Request