Introduction: Multi-terminal Voltage Sourced Convertor (VSC) High Voltage Direct Current (HVDC) links are being considered by National Grid to provide additional capacity across transmission boundaries in the onshore transmission system and potentially to be used in the connection of offshore generation. Such a multi-terminal HVDC link might prove to be the most overall economic and efficient solution available when wider developments are taken into account.

National Grid has not previously implemented VSC HVDC converters on the transmission system and no multi-terminal VSC HVDC system has been implemented anywhere in the world. VSC HVDC and multi-terminal application therefore fall within the definition of new technology in accordance with PS(T)013 and their introduction onto the transmission system must be managed in a manner that takes due consideration of the risks. The tests proposed in this project form an essential part of the risk management strategy.

Cardiff University have done extensive research work in the area of multi- terminal VSC-HVDC for connecting off-shore wind-farms. They have built a 3- terminal test rig and tested the control strategies of the multi-terminal VSCHVDC lines. The test rig can be easily added with another terminal to meet National Grid required configuration.

Objectives: The key objective of the proposed work is to test and demonstrate the performance of control strategies for multi-terminal VSC HVDC systems. The tests will be carried out by using an analogue 4-terminal VSC-HVDC test rig. The rig can be configured to a grid source (3-terminals) and an off-shore wind-farm for NG required configuration.

The tests using the analogue 4-terminal VSC-HVDC test rig will complement the innovation work of the RTDS simulations to be studied by the University of Birmingham. As Cardiff University already have a 3-terminal test rig built, NG will be able to obtain some quick test results (in 6 months) to give us an early indication whether the control strategies proposed for a 4-terminal VSC-HVDC link is feasible, and to identify potential problems with application of the technology and inform specifications and risk register.