High voltage direct current (HVDC) grids are seen as a key technology to integrate renewable power sources across long distances, while providing flexibility and redundancy. Among their challenges, power flow control becomes a concern since the converter stations cannot control the currents circulating inside the mesh, which depend on the resistance relation between conductors. The installation of new converter stations, modifications of the grid configuration, N-1 contingencies, etc. can modify the current distribution leading to overloads in some conductors, while others are underused. Consequently, power curtailments or the installation of new conductors may be necessary. An alternative solution is to install medium voltage converters inserting voltages in series with the DC conductors, known as power flow controllers (PFC) or current flow controllers (CFC). Those devices allow to control the current distribution in the HVDC grid and they can be understood as the equivalent of flexible AC transmission systems (FACTS) for HVDC grids. In the literature, several converter topologies are suggested, which are validated via simulations or experimental testing of scaled-down prototypes. However, less attention is being put on their integration into the HVDC grid: the type of busbar arrangement, the type of required switches, such as disconnectors, bypass switches, etc. Additionally, the busbar arrangement must allow the insertion, bypass and grounding of the PFC without interrupting the HVDC grid power transmission. This work discusses different DC busbar arrangements with PFC and selects a circuit providing good availability, moderate cost and allowing insertion, bypass and grounding of the PFC, while the HVDC grid is in operation. Then, the sequence of switches to insert, bypass and ground the PFC is presented and validated using simulations, showing that the PFC does not cause any major disturbance to the HVDC grid. The requirements of the external switches of the PFC are also assessed, identifying the opening voltage, current and time. This work outlines that the requirements of the PFC bypass switches are in between busbar transfer switches and line transfer switches, but much lower than DC breakers and also lower than other already implemented switches, such as the metallic return transfer breaker (MRTB). The other switches (except the necessary DC breakers) are expected to be in the range of DC disconnectors. Thus, it is shown that the realization of the busbar arrangement is technically feasible.
Florent Morel, Joan SAU BASSOLS, Sellé TOURE, Serge POULLAIN, Frank JACQUIER
Presented at CIGRE 2022