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中文摘要：
分析基于电压源型换流器的多端直流输电系统（voltage sourced converter multi-terminal DC,VSC-MTDC）现有控制策略（电压裕度控制与电压倾斜控制）的特点与不足,提出一种新的基于自律分散思想的控制方法。该方法结合了上述二种控制方法的优点,并相互弥补了各自的缺点。正常运行时,主导站定直流电压控制,辅助站定功率控制。一旦主导站因故障丢失,辅助站能够快速并稳定地切换到定电压控制。由于倾斜特性的引入,各换流站的传输功率始终随直流电压的变化而变化,且保持固定的比例关系。因此直流电压不会因电压裕度的选取而产生不同程度的过调量。此后辅助站将作为多端系统的平衡节点,吸收/发出其余端子的功率差额,使系统重新找到新的运行点（预先设定的最优运行点）。在整个暂态过程中,其余端子能够始终不受扰动的影响而保持正常运行,使扰动对直流系统的影响最小化。文中基于PSCAD/EMTDC分别设计这3种控制器,并对其暂稳态特性进行仿真分析与详细比对。仿真结果表明：组合控制改善了系统的暂态特性,并能够保证系统在扰动前后都始终运行在最优点。
英文摘要：
The features and defects of the common control methods （voltage margin control, voltage droop control） utilized in voltage source converter based multi-terminal direct current transmission system （VSC-MTDC） are analyzed, and a novel control strategy ＇combined control method＇ based on the idea of autonomous decentralization is proposed. This strategy combines the advantages of both above methods, and also makes up their defects. In normal operation, the leading converter controls DC voltage and others control active power. If the leading converter is disconnected from the system due to fault, the auxiliary converter will alter to DC voltage control fast and smoothly. Because of introduction of droop control, the over-correction of DC voltage caused by the voltage margin will be eliminated. Afterwards, the auxiliary converter will work as a slack bus of DC system and absorb/send out the difference of active power of other converters, allowing the system back to the normal operation which has been set as optimal operating point previously. In the whole transient process, the other converters remain stable and safe from any influence. All three kinds of control were designed based on PSCAD/EMTDC, and the transient characteristics were simulated and compared. The results show that the combined method makes a great improvement of the transient characteristic and ensures DC system working in optimal point in the whole process.
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