During periods of northwards power flow on the HVDC link, the energy from the South Island is largely used in the Wellington region, and any surplus flows along five lines - four 220 kV lines via the Kapiti Coast and one 110 kV line via the Wairarapa, north to Bunnythorpe near Palmerston North. However, during periods of southwards HVDC flow, the 220 kV lines into Wellington must transmit electricity from the North Island grid for both Wellington and the HVDC link. The 110 kV line is normally unavailable for through transmission into Wellington due to a low circuit rating on the Bunnythorpe to Woodville section, necessitating the line to be split into two near Pahiatua to prevent the low-capacity section from overloading, and constraining all transmission into Wellington. Southwards HVDC power transfer therefore is limited by the capacity of the lower North Island 220 kV transmission circuits, and by the risk of voltage disturbances in the Wellington region in the event of a sudden disruption to HVDC transfer. The HVDC control and protection systems are also interlocked to prevent operators from operating the link with southbound flow in excess of the difference between the capacity of transmission lines into Haywards and Wellington's minimum regional load. Large southwards transfers on the HVDC link are not generally required except during period of prolonged low inflows to South Island hydro lakes, and the limited southbound capacity is not a major constraint.