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Multi-terminal VSC HVDC Modeling

Oak Ridge naional Laboratory

The goal of this project is to develop a multi-terminal VSC HVDC model and to implement this model into large power systems to enable integration of renewable energy sources like off-shore wind. The model is written as a user-defined model in DSA Tools. The proposed 4-terminal VSC HVDC (4TDC) system topology is built in PSAT of DSA tools, as shown in Fig. 1, with the right side connected to two independent off-shore wind farms and the left side connected to interconnected AC grids.

Multi-terminal VSC HVDC Modeling
Fig.1 4TDC static model in PSAT

The dynamic model of a 4TDC system is written in TSAT User Defined Mode (UDM) editor, with both a graphic interface and ASCII code. The main functions in the controllers are: DQ transfer, Phase Locked Loop (PLL), control functions such DC voltage control, active power control and reactive power control, modulation index and phase angle calculation. The normal control mode for a 4TDC system is one station controlling DC voltage while the others control active power. A simplified control structure of each converter is shown in Figure 2.

Multi-terminal VSC HVDC Modeling
Fig.2 Simplified control structure for different converters

Based on the proposed topology, a dynamic simulation is performed and a generation trip at terminal 3 is tested. The results show that the 4TDC system controlled both the DC voltage and the active power during the disturbance, and it can also provide reactive power to hold bus voltage.

Multi-terminal VSC HVDC Modeling

(a) DC voltage (b) Active power injection to AC grid (c)Reactive power injection to AC grid

Fig.3 Dynamic interactions of converters after a 10% generation trip of one AC grid terminal