Research On Control Strategies Of Multilevel Modular Converter Based HVDC Power Transmission System

Compared with the traditional two-level or three-level voltage source converter, because modular multilevel converter (Modular Multilevel Converter, MMC) adopts modular design, there are no requirement of converter valves with directly series connected switching devices and problems of dynamic balancing voltage any more. At the same time, through adjusting the number of sub modules, MMC can change the voltage and power levels flexibly, extend to any level of output easily which can reduce the output voltage and current harmonic content. Modular multilevel converter based high voltage direct current (MMC-HVDC)transmission system has broaden the application prospects in the renewable energy integration, power supply for the passive power grid, asynchronous grid interconnection, distribution network and etc. Recently, MMC-HVDC has attracted more and more engineers’and scientists’ attention.MMC-HVDC control strategy plays an important role in ensuring system safe operation. Although the control strategy of two level voltage source converters based HVDC (Voltage Source Converter Based HVDC, VSC-HVDC) can be ported to MMC-HVDC, because their topologies and working mechanism are different with each other, the research of MMC and DC transmission system is still necessary and has important practical engineering significances. In this dissertation, MMC-HVDC topology and mathematical model, sub module topologies, circulating current, steady and control strategies are investigated by means of theoretical analysis and simulation validations.(1) Operation mechanism and steady mathematical model of MMC-HVDCFirstly, two general MMC-HVDC topologies (HVDC-Light and HVDC-Plus) and operation mechanism are analysed to investigate the control strategies. The mathematical models are built in the two different reference frames under balanced grid voltage condition. Secondly, based on the working mechanism of MMC, two modulating strategies are presented:nearest level modulation (NLM) and carrier phase shifting sinusoidal pulse width modulation strategy (CPS-SPWM). Lastly, improved capacitor voltage balancing algorithms are also designed with respected to these two different modulating strategies.(2) MMC circulating current and suppressing strategiesMMC structure and working mechanism have caused circulating current problem. Circulating current not only distorts arm currents but also increases the rated current of power devices, which may increases system cost and unnecessary power loses. Based on the arm current equations, firstly, the circulating current forming reasons are studied from different aspects according to the harmonic analysis theory, then the quantitative relationship between the ac current, arm current, dc link current and circulating current is derived. Secondly, through the deep analysis, the double base frequency characteristic of the circulating current is deduced. In order to reduce or suppress the circulating current, three measures, such as a modified voltage balancing algorithm, a vertical circulating current suppressing controller based on proportional resonant and an arm current filter are developed according to circulating current forming reasons and characteristics without changing the arm inductance.(3) Research on MMC sub module topologiesIn the event of the dc link failures, traditional half bridge sub module topologis of MMC can’t clear the dc link fault current after insulated gate bipolar transistors (IGBTs) are blocked because the forward diodes still conduct as rectifier bridges. In this case, the ac breaker has to be opened to cut off the fault current which may cause some adverse effects. This paper firstly summarizes the existing sub module topologic features and improved type of topologies. Secondly, the relationship between the arm current and capacitor’s charge or discharge states are analyzed after dc link fault, the traditional half bridge sub module topology is improved, series connected double sub modules topology (SDSM) and improved hybrid sub module type (IHSM), which can achieve dc link current blocking capability without affecting the control strategy, modulating and capacitor voltage balancing strategy, are designed. What is more, through investing the different current charge paths, the fault current blocking coefficient is defined as the ratio between arm equal DC voltage and ac voltage magnitude to indicate the blocking effect. The self startup methods are also designed according to the different capacitors rectified stages. Lastly, the different control strategies after dc link fault are designed and the simulation model is built to verify these topologies’and strategies effectiveness and feasibility.(4) Research on MMC-HVDC inner loop current controllers and strategies under unbalanced grid voltage conditionMMC-HVDC usually adopts double closed-loop vector control strategy under unbalanced grid voltage in positive and negative rotating reference frame, which has problems such as too many parameters needed to be adjusted and dynamic response being unsatisfied. To solve these problems, firstly, the mathematical model of MMC-HVDC in this case are analyzed in the two phase synchronous rotating and stationary a P reference frame, power characteristics and the corresponding relations between different power components are derived to calculate the current order for different control strategies. What is more, the corresponding voltage dependent current order limiter (VDCOL) is also designed to solve possible over current problem. Secondly, becaue there are ac and dc sequence currents, a hybrid current vector controller based on proportional and integral is used to control this hybrid current. In order to control the ac signal with no static error, the proportional resonant (PR) controller is introduced to. control positive and negative seence current uniformly in α β reference frame. This control strategy does not need the complex reference transformation, so the disadvantageous effect of coupled voltage which is easily influenced by circuit parameters is eliminated and the control algorithm becomes easy to be realized. At the same time, the circulating current characteristics under unbalanced grid voltage condition are also analyzed; the circulating current suppressor based on PR is designed. At last, the simulation platform of MMC-HVDC is built in the electromagnetic transient simulation software PSCAD/EMTDC to verify this controller’s effectiveness and feasibility.