| With rapid economic growth in China, the contradictions between economic development and energy consumption is becoming more and more prominent, which can be solved by two methods mostly, one is industrial upgrading, another one is energy conservation and emission reduction. In order to get this aim, it is need to save energy, reduce energy consumption and emission. In power system, the reactive transmission should be decreased to reduce loss, and harmonic current should be suppressed to promote use efficiency of equipment, by which the transmission efficiency of electric power can be guaranteed. At the same time, consumers need high quality and reliable power supply to keep their business being carried out without a hitch, which can promote production efficiency and reduce emission. Therefore, it has became an important things to keep good power quality and power supply for guaranteeing continuous growth of economic.Unified Power Quality Conditioner (UPQC) is a comprehensive compensator for power quality, by which current and voltage quality problem can be restrained at the same time. If UPQC can be applied properly, all kinds of power quality demand from consumers in one distribution area can be meet by it, coordination control problems of compensating equipments and overlapping investment for power quality can be avoided. However, because of restrictions of semiconductor device, the existing strcuture of UPQC just can be installed in low voltage distribution system. With the development of power electronics, benefied from the techniques of Modular Multilevel Converter (MMC), UPQC based on MMC (MMC-UPQC) can be applied in medium or high voltage distribution system, which can get a better balance point between function maximizing and investment minimizing, makes it have better cost performance than installed in low voltage system, and can be applied more widely.In this paper, focus on the possible problem of MMC-UPQC applicated in medium and high voltage distribution system, study work that had be carried out as below:(1) According to analyzing reslut on relationship between MMC level and total harmonic distortion of output voltage, which was modulated by Nearest Level Modulation (NLM) and Carrier Phase-Shifted Pulse-Width Modulation (CPS-PWM), a three-phase UPQC that can be installed in high voltage system based on MMC was designed. Then, the output formula on shunt and series converter in MMC-UPQC in dqO frame was listed based on simplified equivalent circuit of MMC-UPQC, and its control strategy was also discussed.(2) With idea of outputting fundamental component voltage by NLM and rest voltage by CPS-PWM, a hybrid modulation optimized for accuracy of output voltage in limited levels with NLM and CPS-PWM was presented. This method can be applied on a special MMC structure, in which there is a high switch frequence SubModular (HSF-SM) in every leg. It can take a obvious improvement in accuracy of output voltage with a little increasing of loss. Then, a improved hybrid modulation method was proposed, which can be applied in normal MMC structure. Finally, a low voltage experiment was done to verified this mothed.(3) Based on the slope of output current decided by voltage under connection reactor, the relationship between error of output current and output voltage was analysed, and a MMC hysteresis current control method based on current slope for minimized output current ripples was presented. Finally, Its feasibility and validity was testified by a closed-loop real time simulation system composed of a actual controller and a real time digital simulator (RTDS).(4) The pre-charging current of MMC-UPQC in three steps is analyzed in detail, which are uncontrollable rectification step, Sub-Modular voltage in series converter promotion step and controllable rectification step. In uncontrollable rectification step, the largest impact current would flow through the anti-parallel diode which is shunt with dc capacitor in SubModulars(SMs). In second step, current-limiting resistance plays the key role to limit the impact current which is less than the current in uncontrollable rectification step. Then, a pre-charging control strategies for less impact current is proposed, its validity and feasibility is verified by digital simulation and experimental prototype in low voltage.(5) With the control system that would be applied in a MMC-UPQC low volatge experimental prototype and extended cards of RTDS, a hardware platform for closed-loop digital simulation system was built up. The mathematical model of whole test system, including in MMC-UPQC, was set up in RSCAD. In this closed-loop real-time digital simulation system, the control performance of MMC-UPQC control system for shunt and series MMC was verify. |
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