There is disclosed a controller (112) for a power electronic network element (110) of a power transmission network (100), wherein the controller (112) is configured to vary a control parameter of the network element (110) which at least partly determines a ripple profile in a transmission line of the network. The controller (112) is configured to vary the control parameter between at least a first value and second value to cause the ripple profile in the transmission line to change; and the controller is configured to vary the control parameter periodically or in response to a signal indicating a threshold temperature at a hotspot location along the transmission line. A method of optimising control parameters for a power transmission network (100) is also disclosed.