基于模块化多电平换流器（Modular Multi-level Converter，MMC）的高压直流输电系统由于其具有自换相能力、可以实现有功无功解耦控制的特点，已经越来越多的在电网中得到应用。但是，MMC需要大量的电容，这一方面增加了MMC的成本，一方面在常规的基于交流端口电流的控制方法下使得MMC在直流端口上呈现出一个较大的等效电容，对基于MMC的多端直流系统的稳定性也有不利影响。本文针对MMC所需电容量大和直流端口等效电容影响直流系统的稳定性的问题，对MMC的桥臂瞬时电流直接控制方法、桥臂电容电压优化运行方式和基于直流内电势直接控制的MMC多端直流系统下垂控制策略进行了研究。为了对MMC实现更为灵活的控制，本文研究了MMC的桥臂瞬时电流直接控制方法，并给出了这一控制方法下实现对交直流端口内电势实现解耦控制和进行桥臂间的电容电压平衡的方法。通过这一控制方法可以实现桥臂电容电压与直流端口电压的解耦，并实现对直流端口内电势的快速直接控制，从而为桥臂电容电压优化运行方式和基于直流内电势直接控制的MMC多端直流系统下垂控制策略提供实现的基础。针对MMC所需电容量较大的问题，本文研究了MMC的桥臂电容电压优化运行方式。这一运行方式利用桥臂瞬时电流直接控制方法的实现桥臂电容电压与直流端口电压解耦的能力，在不影响交流和直流端口电压输出能力的前提下，将桥臂电容电压的直流分量按照当前的运行工况进行适当的下降。桥臂电容电压直流分量的下降可以在不提升子模块峰值电压的情况下提升允许的桥臂电容电压波动范围，进而可以减少所需的电容的量。针对MMC的直流端口等效电容影响直流系统稳定性的问题，本文研究了基于直流内电势直接控制的MMC多端直流系统下垂控制策略。这一控制策略利用桥臂瞬时电流直接控制方法的对直流端口内电势实现快速直接控制的特性，在电压控制节点换流站实现直流内电势-直流电流的下垂特性，在功率控制节点换流站实现对直流电流的直接控制，显著提升了基于MMC的多端直流输电系统的稳定性。

More and more modular multi-level converter (MMC) based HVDC are being used in power systems because of their ability of self-commutation and decoupled control of active power and reactive power. But MMC needs a large amount of capacitors. The capacitors not only increase the cost of converters, but also create a large equivalent capacitor on the dc-port of the converters, which negatively affected the stability of the MMC-based HVDC. In this thesis, direct control of arm instantaneous currents, optimized operation of arm capacitor voltages and droop control of MMC-based multi-terminal HVDC based on direct control of dc electromotive force have been researched for solving the mentioned problems caused by the large amount of capacitors of MMC.For achieving more control flexibility of MMC, direct control of arm instantaneous currents has been researched. Decoupled control of electromotive forces of ac and dc port and inter-arm capacitor voltages balance control under this control method have also been researched. Using this control method, the dc-link voltage of MMC can be decoupled from the arm capacitor voltages, and the direct control of dc electromotive force can be achieved. Optimized operation of arm capacitor voltages and droop control of MMC-based multi-terminal HVDC based on direct control of dc electromotive force can be realized based on the characteristics provided by this control method.For reducing the capacitor requirement of MMC, optimized operation of arm capacitor voltages has been researched. Making use of the ability of decoupling dc-link voltage from arm capacitor voltages of the arm instantaneous currents direct control method, the dc component of the arm capacitor voltages can be appropriate decreased without affecting the voltage generating range of the ac and dc port. The decreasing of the dc component of arm capacitor voltages leads to a larger voltage fluctuation range with the same peak voltage limit, so the capacitor requirement can be reduced.Aiming at the problem that the dc-port equivalent capacitor of MMC negatively affected the stability of the MMC-based HVDC, droop control of MMC-based multi-terminal HVDC based on direct control of dc electromotive force have been researched. Based on the ability of fast and direct control of the dc electromotive force of the arm instantaneous currents direct control method, this control strategy realized dc electromotive force - dc current droop characteristic at voltage control stations and realized direct control of dc current at power control stations. The stability of MMC-based multi-terminal HVDC can be significant improved with this control strategy.