A controller protection circuit (114,214,314) for a voltage power optimiser (100,400,500), the circuit comprising: a first terminal (122,222,322) for connecting to a first end of a winding in the voltage power optimiser (100,400,500); a second terminal (124,224,324) for connecting to a second end of the winding in the voltage power optimiser (100,400,500); a thyristor (226,326,626,726,826) having an anode terminal, a cathode terminal and a gate terminal, wherein the anode terminal is connected to the first terminal (122,222,322), and the cathode terminal is connected to the second terminal (124,224,324), wherein the gate terminal is configured to receive a gate control signal; a thyristor gate control circuit, the thyristor gate control circuit configured to: set the gate control signal such that the thyristor (226,326,626,726,826) is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor (226,326,626,726,826); and set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller; wherein the thyristor gate control circuit comprises: a normally-on switch having a conduction channel and a control terminal, wherein the conduction channel is connected in series between the anode terminal and the gate terminal of the thyristor (226,326,626,726,826); and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller. a first terminal (122,222,322) for connecting to a first end of a winding in the voltage power optimiser (100,400,500); a second terminal (124,224,324) for connecting to a second end of the winding in the voltage power optimiser (100,400,500); a thyristor (226,326,626,726,826) having an anode terminal, a cathode terminal and a gate terminal, wherein the anode terminal is connected to the first terminal (122,222,322), and the cathode terminal is connected to the second terminal (124,224,324), wherein the gate terminal is configured to receive a gate control signal; a thyristor gate control circuit, the thyristor gate control circuit configured to: set the gate control signal such that the thyristor (226,326,626,726,826) is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor (226,326,626,726,826); and set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller; set the gate control signal such that the thyristor (226,326,626,726,826) is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor (226,326,626,726,826); and set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller; wherein the thyristor gate control circuit comprises: a normally-on switch having a conduction channel and a control terminal, wherein the conduction channel is connected in series between the anode terminal and the gate terminal of the thyristor (226,326,626,726,826); and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller. a normally-on switch having a conduction channel and a control terminal, wherein the conduction channel is connected in series between the anode terminal and the gate terminal of the thyristor (226,326,626,726,826); and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor (226,326,626,726,826) is configured not to conduct in response to a signal received from a voltage controller.