A wind turbine cooling system comprising a cooling circuit arranged to convey a cooling fluid to and from a heat source, a cooling device arranged to cool the cooling fluid, a 5 pump arranged to circulate the cooling fluid in the cooling circuit, and a cooling fluid tank arranged in fluid connection with the cooling circuit and having a first fluid port adjacent a top of the tank and a second fluid port adjacent a bottom of the tank, wherein the second fluid port is arranged to communicate with the cooling circuit, wherein the pump has an outlet arranged in fluid connection with the cooling circuit and in fluid 10 connection with the tank, and wherein a fluid path between the pump outlet and the tank includes a flow restriction device.