Research On Control Strategies Of Modular Multilevel Converter For HVDC Application

The flexible direct current electric power transmission(HVDC)has obvious advantages in isolated island power supply,offshore wind power and so on.It solves the problems of large loss,high complexity and high cost of cable in traditional transmission lines.In practical engineering applications,the traditional direct current electric power transmission is thyristor,and the switching loss is large.In order to make the output waveform meet the requirements of harmonic,it is usually necessary to set a filter on the AC side.The MMC has a highly modular structure,and when applied to different voltage levels,the corresponding voltage levels can be achieved by changing the number of sub-modules.Due to the large number of levels,high sinusoidal output voltage,good harmonic characteristics,very suitable for flexible DC electric power transmission.Firstly,the topology of MMC is analyzed,and the working principle of the modular multilevel converter is described in detail through six working modes and three working states.The principle of three-phase modular multilevel converter is analyzed.Based on the characteristics of the direct electric power transmission,the basic principles of modulation strategies for modular multilevel converters are presented.Flexible direct electric power transmission require power transmission at both ends or even multiple ends.The mathematical models of abc three-phase coordinate system and dq two-phase synchronous rotating coordinate system are established,and the double closed-loop PI controller is designed.The principle of sub-module capacitor voltage equalization is analyzed,and a voltage equalization controller is designed.The effectiveness of the controller is verified by simulation.In the case of asymmetric fault in AC power system,two kinds of outer loop controllers are designed by modeling and Power Analysis.The inner loop adopts positive and negative sequence PI controller to track the reference signal produced by the outer loop,the corresponding control target is realized,and the effectiveness of the controller is verified by simulation.A model predictive control(MPC)inner loop controller for MMC-HVDC is designed to solve the problems of complex parameter tuning and controller complexity of traditional PI controller.When the power grid is symmetrical,the model predictive control has faster response speed than the traditional PI control.In the case of active and reactive power mutation,the decoupling control of active and reactive power can be realized without designing a special decoupling link.The model predictive controller is used to replace the original positive and negative sequence controller when the AC power network is not symmetrical.The comparison shows that the model predictive controller can respond quickly when the fault occurs and ends,and the structure of the controller is simplified.Finally,the effectiveness of the proposed controller is verified by a hardware-in-the-loop simulation platform.