SVC for the Channel Tunnel rail link: Providing flexibility and power quality in rail traction

When the high-speed electrified railway line between London and the Channel Tunnel to France is finished, it will be possible to travel between London and Paris in just over two hours. The railway power system is designed for loads which are high (power ratings in the range of 10 MW) and which fluctuate due to rapid acceleration and retardation. The traction feeding system that was chosen is a modern 50 Hz, 2 × 25 kV supply, incorporating an autotransformer scheme to keep the voltage drop along the traction lines low. Power step-down from the grid is direct, via transformers connected between two phases. A major feature of this power system is the Static Var Compensator (SVC) support, the primary purpose of which is to balance the unsymmetrical load and to support the railway voltage in the case of a feeder station trip when two sections have to be fed from one station. The second purpose of the SVCs is to maintain unity power factor during normal operation. This ensures a low tariff for the active power. Thirdly, the SVCs mitigate harmonic pollution by filtering out the harmonics from the traction load. This is important, as strict limits apply to the traction system's contribution to the harmonic level at the supergrad connection points.