Research On Several Issues In Modular Multilevel Converter Based HVDC

Modular multilevel converter (MMC) is an attractive multilevel topology of voltage-sourced converter based HVDC (VSC-HVDC). Since there is no requirement of converter valves with direct-series connection of IGBTs, the difficulty of valve manufacture technology is reduced. As a result, MMC is very suitable in high-voltage and high-power applications. Recently, MMC-HVDC has attracted many engineers’ and scientists’attention. A large number of references are published these years. However, MMC topology is still not well understood compared with2-level VSC topology. The dissertation makes researches on several important issues in basic control and operation of MMC-HVDC system.The dissertation is organized as follows:(1)An ac-dc decoupled model of MMC is presented based on the continues model. This model demonstrates that the outer power control loop and inner current control schemes developed for2-level VSCs are also suitable for MMCs. The range of inner EMF is analyzed, the nominal operation point, active and reactive power operation range are discussed.(2)Based on the comparison of different modulation schemes of MMC, an improved phase-shifted carrier PWM method is introduced. To reduce the switching losses of MMC, a reduced switching-frequency voltage balancing algorithm (RSFVBA) is investigated. The presented PSC-PWM and the nearest level control (NLC) method are compared from the aspects of harmonics and modulation errors.(3)The characteristic of the circulating current is investigated based on an analytical model. With the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a dedicated circuiting current suppressing controller (CCSC). The alternative current control is not affected at the same time. The CCSC can also reduce the capacitor voltage fluctuations.(4)To evaluate the power loss of MMC, the manufacture data of IGBT modules is used with the curve fitting method. The power loss is calculated with the real-time simulation voltages and currents. Based on the thermal circuit model, the variation of junction temperature is considered and a feedback method is implemented to calculate the power loss more accurately.(5)There are dc voltage ripples when MMC-HVDC operates under unbalanced grid conditions. To eliminate the second order harmonic voltage in the dc bus, a dc voltage ripple suppressing controller (DCVRSC) is introduced. The controller is incorporated with the negative-sequence current controller, which is widely used in2-level VSCs. Thus, the dc voltage ripple is eliminated as well as the ac current are balanced. To enhance the low voltage ride through (LVRT) capability of MMC-HVDC system, a dc voltage dependent current order limiter (VDCOL) is investigated. The VDCOL changes the active current reference according to the dc link voltage and keep the active power balanced in the dc system. Simulation results demonstrate the dc voltage is stable with the VDCOL, when the ac voltage of dc voltage control (DCVC) station is unbalanced.