Fault-tolerant Control Of Modular Multilevel Converter For Sub-module Fault

Modular multilevel converter(MMC)has become an effective solution to the transmission and efficient utilization of new energy,due to its high efficiency,low harmonic distortion,separate control of active and reactive control and small footprint.MMCs contain a lot of sub-modules which may have fault and do not work.Thus,it is necessary to take measures to maintain smooth operation of MMCs.This thesis is about the methods to keep the operation and safety of MMCs when MMCs with or without redundant sub-modules are facing sub-module fault.First,the thesis analyzes mathematical model of MMCs and its AC and DC equivalent circuit to build the foundation of following analysis.And then,two common-used modulation technologies and their applications are analyzed.Also,In the common-used control method,abc-dq coordinate transformation can help realize the active and reactive power.Finally,causes and solutions of capacitor voltage fluctuation and circulating harmonics are calculated.Based on the theories above,a 51-level MMC is built in MATLAB/Simulink to verify the correctness of modulation and control of MMCs.Second,if sub-modules of MMCs with redundant sub-modules breakdown,the most common-used methods are cold redundancy fault-tolerant strategy and hot redundant fault-tolerant strategy.However,each method has its own disadvantages.Cold redundant sub-modules do not generate power losses and extra heat,but its processes of putting into use is time-consuming and causes large circulating.Although hot redundant sub-modules can put into use smoothly,its conduction losses and heat are considerable.In this thesis,an improved method is proposed.This method combines the advantages of cold and hot redundant fault-tolerant strategy and use a control strategy to guarantee the smooth processes of putting in the redundant sub-modules,which avoids problems such as large circulating,over-voltage and over-current of power devices,over-voltage of capacitors and high loss.And then,the simulation of MMC verifies the validity of this method.Finally,if sub-modules of MMCs without redundant sub-modules breakdown,a set of methods are proposed for fault-tolerance to avoid negative impact of fault on MMCs as much as possible.The following three methods’ negative effects are increasing,so the first method taken into consideration is the sub-module fault control strategy based on operating redundancy,which relies on the modulation of operating and has no negative effects on MMCs.The second method to consider is the strategy based on injecting 3rd harmonics to modulation waveform to decrease the maximum of arm voltages,which has little negative impact on MMCs.The last strategy to consider is the one based on unbalancing operation of arms,which increase the capacitor voltage to support the output.However,the circulating current of fault phase is large.To solve this problem,a base frequency circulating current suppressor is designed to effectively decrease the base frequency of circulating current.Finally,the feasibility of this set of schemes are verified through the 51-level simulation model of MMC.