Research On Several Issues In Modular Multilevel Converter For HVDC Application

The application of power electronic technology constantly plays a significant role in modern power system to make it intelligent, clean, and efficient, including high voltage direct current transmission (HVDC) and flexible alternating current transmission system (FACTS). Modular multilevel converter (MMC) is considered as a next-generation technology of high voltage and high power conversion. MMC is considered as a representative application in flexible-HVDC technology, with the advantages like high capability, decoupled active and reactive power control, high level of power quality with no need of complicated ac filters, high reliability, and ability to weak ac networks or even dead networks. In this dissertation, several key issues in MMC for HVDC application are amply discussed following a literature review.In MMC circuit, there are a large number of semiconductor switches and dc capacitors. These components leads to a high-order, discrete and non-linear mathematical model which is hard to be deduced and analyzed. Based on the physical lumped parameter circuit model, a switching model is given and simplified to an average model through three steps of simplification and coordinate transformation. Finally, a low-order, continuous and linear model is given through small-signal linearization. The preconditions of the proposed simplification are defined to offer different degrees of simplification according different analysis requires. With dc voltage controlled variable, the proposed state-space model makes MMC mathematic model more complete in control theory, and provides a theoretic foundation for the n-controllable modulation for MMC.In MMC-HVDC application, nearest voltage level modulation (NLM) is widely reported and used for the valve group control. However, selective switching strategy based on submodule (SM) voltage and collision avoidance rules of two adjacent switching operations are still needed to be discussed in detail. In this dissertation, control points of the selective switching strategy are defined. An algorithm and collision avoidance rules for the control points are also given. The algorithm proposes different selecting methods corroding different type of control points, gives the rule of control points design. An n-controllable NLM (n is the number of SM per phase) is proposed to control the dc side of MMC as a controllable voltage source under certain conditions for dc current control, and to make a full use of redundancy SMs from the control point of view. Modified strategies of n-controllable NLM are proposed to expand the application of NLM for odd n or H-bridge SMs.With the n-controllable NLM, dc and ac sides of MMC are controlled as controllable voltage sources which provide a fundamental base for various system control strategies in a higher level. A direct dc current control strategy and its parameter design method are proposed for an inner-loop controller of MMC in this dissertation. As a coordinative outer-loop controller, an output control strategy, a system control strategy and their parameter design methods are proposed with power balance limitation.There are cases of unidirectional power transmission in power system especially for renewable energy source connection and offshore platform power supply. It is proposed to reduce system complexity and cost and to improve system reliability that a unidirectional low-cost MMC-HVDC (ULMMC-HVDC) system with a diode rectifier and a MMC inverter. Distinguished by energy storage elements, two types of energy storable ULMMC-HVDC is proposed and discussed. Parallel connected with electronic double layer capacitors (EDLCs) on dc side, the large number of SMs are fully used to achieve the advantages as good performance of SM voltage balancing control, distributed storage that is flexible and easy to maintain. The proposed system also offsets the self-discharge and the low voltage rating problems brought by EDLC. An energy management control strategy and its parameter design method are given to limit the energy stored in EDLC between upper and lower bound. Parallel connected with battery on dc side of SM, energy storable ULMMC-HVDC achieves higher voltage rating compared with conventional battery energy storage system, and makes damaged battery models easily to be maintained or replaced. Compared with supper capacitor energy storage system, it provides a storage scheme with higher capacity and efficiency for power system. A state of charge (SOC) control strategy and its parameter design method are also given for SMs and SM clusters.Electromagnetic transient simulation model for large-scale MMC has the problems of high-order and low-efficiency, because there is a large number of SM with semiconductor switches and dc capacitors in the circuit. As a completely equivalent model of the switching model of MMC, a proposed simulation model greatly improves the efficiency and provides a feasible way for the simulation through building fast-simulation SM model and optimized control algorithm. The algorithm includes double-sorting operations for selective switching and collision avoidance rules for over frequency of SM switching.One of the main problems in practical MMC is a large number of SMs requests high-throughput digital IOs for control and measurement. The control system of MMC is generally a multi-controller system. However, another problem is to realize an interface for high-speed real-time communication and large amount of non-real time data between the controllers. A MMC prototype, including a400V AC/±400V DC/3kVA/288-SM main circuit and a full-scale control system, is developed and tested. The hardware design and the software structure are given in this dissertation. A control and measurement signals multiplexer for SM is proposed to greatly reduce the cost and difficulty, by reducing the number of fiber channels, using only logical circuits without bringing in any communication burden. A double-bus structure is proposed to transmit real-time and non-real time data separately. The structure solves the problem of data interaction between the controllers. The experimental results verify the fundamental principle of MMC and the proposed control strategies. All performance indexes of the prototype are adherent to the designed specification upon examination.