Research On Modeling And Control Strategy For Modular Multilevel Converter

The high voltage direct current system based on voltage source converter is widely used in these situations:renewable energy connecting to the grid, supplying power to the passive network and city grid, interconnection between asynchronous AC grid. As a new topology of converters, modular multilevel converter has many advantages, such as independent control of the power, no additional filtering device, small footprint and modular structure. There are some actual project put into operation at home and abroad currently.Modeling for modular multilevel converter is the basic to study its dynamic characteristics and the control strategy, but with the shortages of heavy workload in modeling, big occupancy resource and slow computing speed in simulation. This paper establishes the converter’s equivalent model through replacing the arm of converter with Thevenin circuit, which can achieve the simulation of high level modular multilevel converter system and is of the ability to emulate the internal fault in converter. The error analysis and applicability range for equivalent model are also included in this paper.An improved capacitor voltages balancing control is presented in this paper, which avoids the sort of all sub-module capacitor voltages. The detailed approach of the improved control is establishing a capacitor voltage container with several layers, and put all the capacitor voltages in corresponding layer according to its value, then choose the sub-modules optimally to put into operation from the container layers according to the value of arm current and the number of sub-modules needed to input. Finally, a criterion is introduced to determin whether to reset the container layers, which can reduce the time complexity of the improved control.A redundancy proteciton based on carrier phase shift modulation is also presented in this paper, it can reduce the converter switching loss compared with the system based on nearest level modulation. It consists of three parts:carrier dynamic allocation, additional carrier control and capacitor voltage optimization control. Carrier dynamic allocation allocates carriers for all the sub-modules dynamically, additional carrier control make the redundant sub-module of priority to be charged through changing the carriers when a fault occurs, capacitor voltage optimization control makes sure a small fluctuation in all capacitor voltages.