Operation And Control Of VSC Based Multi-terminal HVDC System

VSC-HVDC(VSC based High Voltage Direct Current) is a new technology of electric power transmission, which is based on VSC(Voltage Sourced Converter) converters. The converter is consisted of high power full-controlled semi-conductor valves. The AC power system has shown some disadvantages in its development, such as the instability of long distance transmission, etc. In contrast to AC transmission, the traditional PCC(Phase Change Converter) based HVDC has unparalleled advantages in some areas, such as long distance power transmission or asynchronous interlink of large capacity system. The PCC-HVDC has great development since 1960s and continue to develop AC-DC interlinked network, which has promising future. The PCC-HVDC also has limits as fellows: unable to supply for the small capacity system and the load without revolving motors; produces large low frequency harmonics and absorbs large reactive power; expensive in investment of PCC converter station. Because of these limits, the traditional PCC-HVDC transmission technology is at disadvantage position in competing with AC transmission. The traditional PCC-HVDC mainly applied in long-distance large-capacity transmission with its voltage over 220KV, sea-ground cable transmission and asynchronous interlink between AC systems with same or different rating frequency. As the operation and control technology have developed rapidly since 1990s and some practical projects have been put into commercial operation, this dissertation focuses on the operation and control of an innovative VSC based multi-terminal HVDC system.VSC-MTDC is the multi-terminal HVDC system, which consists of VSC converters and features economical and flexible. However, when multi-terminal system enjoys more economical and flexible than two-terminal system, it also faces more complex situation in operation and control. Different from two-terminal system, MTDC needs master controller to offer every converter the optimized instruction in stable running state. The instruction follows change of load power flow. In abnormal state, such as faults occur in DC or AC side, the master controller will analyze the change of system structure and running parameters and coordinate the whole system to return to a new normal state.Firstly, a static model of VSC converter in dq0 coordinates is developed and a PI controller with a feed-forward sector is designed in this dissertation. As commuting loss included and AC-DC system decoupled, this model is very easy to apply in practical project. On the base of the model, a series of VSC controllers are studied and designed, which respectively adapt to active AC grid and passive AC grid. This control scheme has linear and decoupled characteristics.Secondly, some operating strategies of multi-terminal system are analyzed and studied in this dissertation. There are two operating strategies presented: voltage droop scheme and master/slaver control scheme. In order to merge the two strategies'advantages, there are also an improved multi-terminal DC voltage control strategy been developed, which improves DC voltage quality,increases commuting efficiency and makes master controller simple. One of the concerns is the operation and regulation scheme of multi-terminal VSC-HVDCsystem, which will solve the problem of coordination among VSC converters. Accordingly, the master controllers of different control schemes of VSC-MTDC are studied and designed.A main part of this dissertation is the AC-DC power flow calculation with VSC converters. The models suitable for power flow algorithm with VSC-HVDC converter are deduced and the equivalent power injection of VSC node is given. Then this dissertation introduced an alternative AC-DC power flow algorithm, which is available for multi-terminal DC system. The alternative AC-DC power flow algorithm has the advantage of perspicuous structure, conveniently programming, and wide application. Because this algorithm does not take the influence between AC and DC system into account, it also has some disadvantages, such as poor constringency and low calculation efficiency. Therefore, an improved united algorithm is proposed. This united algorithm is easy to programming and improved in constringency. Then the validity and efficiency of this algorithm are verified by practical examples. Nevertheless, the application of this improved united algorithm is limited because of the difficulty in analysis of DC system functions. In order to solve this problem, an improved alternative AC-DC power flow algorithm is brought forward, which not only has the advantages of both united algorithm and alternative algorithm, but also have wide applicability and high efficiency.Finally, the static stability and operations of inter-linked system are discussed in this dissertation. The essence of static stability of multi-terminal system is explained and the concept of mixed potential function is introduced. Then the concept is expanded to analyze the stability of multi-terminal VSC-HVDC system. The potential function belongs to Lyapunov function, so that the second criteria of stability determination is available in this power system. This manner contributes to theory research and practical engineering. In addition, this dissertation involves operation and protection of multi-terminal VSC-HVDC system in various situations and presents preliminary conclusions.