Research On Control Strategy And Fault Detection Of Modular Multilevel Converter

Modular Multilevel Converter(MMC) combines multiple submodules with low voltage level in series, which can make it suitable for high-voltage applications. MMC can achieve excellent output voltage waveform with lower switching frequency, and it is convenient to extend to higher voltage level and power volume because of the highly modular construction, so it has been widely adopted in the fields of motor drives, HVDC transmission, power compensation and so on. This paper emphases on the following study of MMC, including the balance control of the capacitors voltage, control methods without the feedback of capacitors voltage, fault detection:Firstly, the basic principle of MMC will be analyzed in detail, then its equivalent model can be deduced by mathematics. Balance of the arm capacitors voltage is vital for the normal operation of the system. The common modulation schemes and voltage balance methods will be discussed thoughly. Though different modulation schemes combined with various kinds of voltage balance methods can both achieve excellent performance, they differ from each other in certain aspects, such as the harmonic of the output voltage, the suppression of the circulating current, dynamic response. This paper takes the selection mechanism based on nearest level modulation and the closed-loop energy control based on carrier phase shift modulation for example, then analyzes their difference through simulation.Given that the number of submodules is large enough, traditional control methods will increase the cost of hardware, the transmission delay of multiple signals may lead to the instability of MMC system. In order to solve the potential problems, three novel strategies without the feedback of arm capacitors voltage are introduced in this paper. Firstly, the small signal model, the steady-state model, discrete model of MMC can be obtained through theoretical deduction, hereby the control method based on voltage observer, capacitor voltage estimation, capacitor voltage prediction will be introduced. Then stability analysis and the research on how the deviation of the model parameter influences the steady-state of system will be discussed comprehensively. Finally in virtue of simulation, the comparison of three different control method is given and the validity of them is verified by experiment.Failure of submodule may occur during normal operation, if the fault submodule cannot be detected immediately, serious impact will happen in the MMC, it is necessary to carry out the fault detection for the system. This paper firstly adopts the detection method based on the Kalman filtering theory to determine the failure occurs in which phase, then analyzes the features when a submodule is in fault condition, which can rapidly locate the position of the fault submodule. At last, simulation will be established to prove that the proposed method can detect the fault module effectively and in time.