Stability Analysis And Control Of DC Grid Of VSC-MTDC

The AC power system has shown some disadvantages with its development, high-voltage direct current transmission system (HVDC) has obvious advantages in adapting to energy crisis worldwide and the distribution of resources and load in China. As the operation and control technology have developed rapidly since 1990s and some practical projects have been put into commercial operation, this paper focuses on the operation and control of a VSC based multi-terminal HVDC system.In the first place, there are two kinds of mathematical models of VSC set up in two coordinate systems separately, containing abc coordinate and dqO coordinate. Then a double loop control system is proposed, which controls voltage in outer loop and current in inner loop. Secondly, this paper introduces three kinds of MTDC topologies, and chooses branch-paralleled VSC-MTDC as research topic. Some operating strategies of multi-terminal system are analyzed and studied in this paper. There are three operating means discussed:master-slave scheme, margin scheme and voltage droop control scheme, then this paper selects voltage droop scheme owing to its speedy response and independence from communication apparatus. Besides, this paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. An analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A four-terminal MTDC was built in PSCAD/EMTDC for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution. Finally, transient-state simulation studies are carried out to evaluate the effectiveness of the voltage droop control scheme.