Study On DC Power Flow Control Technology Suitable For Multi-Terminal DC Systems

Multi-terminal DC system is the extension of traditional point-to-point high voltage DC (HVDC) system which can realize optimal allocation of energy and effective integration of renewable energy. It will be more difficult to control DC power flow of DC system when the structure of system becomes more complex. If DC power flow is out of control, then some transmission lines may become overloaded which will threaten the DC system normal operation. In order to sovle the problem, DC power flow control technology is proposed. DC power flow of transmission lines will be effectively regulated by inserting external DC power flow controller. This dissertation will study on the topology of DC power flow controller and influence of DC power flow controller on DC system.The main parts of this dissertation are as follows:1. The first part summarizes DC power flow controllers of existing literatures.They are categorized in two types:resistor controlled and voltage controlled based on control method. Topologies of those DC power flow controllers are discussed in detail and their advantages and disadvantages are compared. Finally, their application situations are pointed out.2. Based on the study on interline DC power flow control technology, the second part proposes a topology of interline DC power flow controller and analyzes its operation principle and characteristics in a three-terminal HVDC system. Simulations are carried out in PLECS through the model of a three-terminal HVDC system including the power flow controller. A three-terminal meshed dc system was built in the lab to validate the DC power flow controller. In order to realize a wide range of applications, the proposed topology is improved and simulations and experiments validate its effectiveness.3. In the third part a method based on sensitivity analysis to determine optimal allocation of DC power flow controller is proposed. It is found that the transmission loss can be reduced by introducing the DC power flow controller. The total transmission losses of DC system are calculated to identify the DC power flow controller location and its operational ratio. Finally, the theoretical analysis is validated by simulation of a typical three-terminal HVDC system.