Application Research Of Multilevel Converters In STATCOM

With the rapid increase of China's electricity power consumption,voltage source converter based high voltage direct current(HVDC)technology has been widely adopted to realize energy conservation,thus,it accords with the concept of energy conservation and emission reduction proposed by the China Government.As one of the key units,the static synchronous compensator(STATCOM)has been a hot topic both in the industry and academy fields.In recent years,the multilevel converters have arisen as the leader and most competitive solution for voltage source converter applications operating in HVDC fields owing to the advantages such as overcoming the blocking voltage capability of the power switches,reduced harmonic distortion,low EMI and etc.The traditional multi-level inverter circuit structures are divided into three types,the diode-clamped multilevel inverter,the capacitor-clamped multilevel inverter or flying capacitor,and the cascade multilevel inverter.In addition,a number of variations of multi-level topologies have been proposed to overcome the disadvantages of the traditional ones,such as H-bridge mid-point clamped converter,the active clamp transformer and the modular multilevel converter(MMC)which will be addressed in this paper.In this paper,the mathematical models for both the MMC and STATCOM are firstly researched in detail,and based on them,the fundamental control strategies of them are fully studied.In addition,an improved and novel direct pulse-width modulation strategy is proposed here.Then both the simulation and experimental results are offered to illustrate its advantages.The main contents of this paper include the operating principle of MMC and its sub-module and the advantages of MMC over the traditional ones.The proposed novel direct pulse-width modulation strategy can achieve balancing capacitor voltages,low dv/dt and reduced switching losses with high quality of the output spectrum.The simulation established by Matlab/Simulink is adopted to illustrate the effectiveness of the proposed novel direct pulse-width modulation strategy with the double closed loop control of the STATCOM.At last,experimental tests of which hardware system includes a control circuit,a drive circuit for detecting conditioning circuit,protection circuits are established to show the correctness and feasibility of the system.The software codes are designed in the integrated development environment CCS 3.3.