The Channel Tunnel 25 kV railway system will be supplied from the UK and French 400/225/132 kV grid systems. The paper describes the preliminary unbalance and harmonic studies carried out for the tunnel traction system. The studies explore the interaction between the tunnel traction loads and the grid supply systems. The studies were carried out for the most severe loading conditions, determined from previous studies of rail traffic patterns, imposed on the grid systems in their electrically weakest state. Specific harmonic and unbalance limits were used against which the results of the studies were assessed. These studies form the first stage in the design process that will lead to the definitive design of the Channel Tunnel traction systems.