Study On Several Key Issues In Flexible HVDC Transmission Based On Multilevel Converters

The rapid development of modern power electronic technology lays a solid foundation for constructing an environmental, efficient and resilient smart grid, accelerating the development of Flexible Alternating Current Transmission System (FACTS) and High Voltage Direct Current (HVDC) transmission. Multilevel converters, represented by Modular Multilevel Converters (MMC), have a series of advantages including modular design, high quality of output waveforms, high transmission capacity, decoupled active and reactive power control, and ability to connect weak ac grid or even dead grid. They are considered the next generation of high voltage converter, and will be widely applied in flexible HVDC transmission. In this dissertation, several key issues in multilevel converters represented by MMC for HVDC application are studied, after summarizing previous study achievements.MMC has the characteristics of discretization and nonlinearity, because it contains large numbers of power electronic devices. By ignoring switching processes of power electronic devices, taking the topology of MMC as foundation, and applying Kirchhoff’s laws, the equivalent models of the Alternating Current (AC) and Direct Current (DC) are built. A 49 level, 380V MMC prototype is built, whose control system is designed as 35kV condition. It can achieve triggering, measurement, control and protection for 288 sub-modules.Modulation and voltage-balancing are the preconditions of MMC normal operation. For now, the most promising modulation method for flexible HVDC transmission is the Nearest Level Modulation (NLM), because it has characteristics of simple implementation, low switching frequency and low loss. As for few voltage levels and large harmonic contents, a modified NLM method which can generate 2N+1 levels is proposed. In the meantime, a zero error NLM method is proposed, which eliminates errors between the output and reference voltages. By the way, the 2nd harmonic circulating current is suppressed by the zero error NLM method.Different control strategies are applied to the MMC based flexible HVDC under different operation conditions. Station-level and system-level control strategies are studied under balanced AC system condition. Station-level controller uses two strategies:feed-forward cross decoupling control and feedback linearization decoupling control, while system-level controller determines the active and reactive power variables. The positive and negative sequence decomposition control and proportional resonant control are studied under AC asymmetric fault condition. The former suppresses the negative sequence current by the negative sequence controller, while the latter controls the fault current by the proportional resonant controller which is designed according to the internal model principle.Since high-voltage, large-power flexible HVDC needs hundreds of sub-modules, the processing speed and pin numbers of single processer maybe cannot meet requirements. To solve this issue, a two-layer controller including "system controller+arm controller" is proposed, while the arm controller is composed of "Central Control Unit (CCU)+Group Control Unit (GCU)". Each CCU controls several GCUs, while each GCU manages several sub-modules. The proposed control structure only needs small-scale processor, and the number of GCU changes according to the number of sub-modules.The modular structure of MMC makes it have the redundant protection function. However, different redundant protection schemes work out differently. The scheme that redundant sub-modules in spinning reserve is most convenient, after careful comparison. However, this scheme causes that sub-module numbers of the upper and lower arms in the fault phase are different, in other words the system operates under asymmetrical condition. This asymmetrical operation will cause ripples in the DC current. To solve this problem, an energy balancing control strategy is proposed, which raises the sub-module voltage in the fault arm to maintain energies in balance around six arms. This energy balancing control strategy eliminates the ripples in the DC current.Apart from MMC, many other multilevel converters are proposed and studied by scholars. An IGBT based Alternate Arm Multilevel Converter (IGBT-AAMC) has DC fault block ability. The working principle, modulation methods and DC fault ride-through schemes are studied. Meanwhile, a SCR based Alternate Arm Multilevel Converter (SCR-AAMC) is proposed. The alternating principle is studied, and device quantities of SCR-AAMC and MMC are compared. The result shows the SCR-AAMC has fewer power electronic devices.