A controller protection circuit for a voltage power optimiser , the circuit comprising: a first terminal for connecting to a first end of a winding in the voltage power optimiser ; a second terminal for connecting to a second end of the winding in the voltage power optimiser ; a thyristor having an anode terminal , a cathode terminal and a gate terminal , wherein the anode terminal is connected to the first terminal , and the cathode terminal is connected to the second terminal , 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 is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor ; and set the gate control signal such that the thyristor 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 ; and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor is configured not to conduct in response to a signal received from a voltage controller. a first terminal for connecting to a first end of a winding in the voltage power optimiser ; a second terminal for connecting to a second end of the winding in the voltage power optimiser ; a thyristor having an anode terminal , a cathode terminal and a gate terminal , wherein the anode terminal is connected to the first terminal , and the cathode terminal is connected to the second terminal , 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 is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor ; and set the gate control signal such that the thyristor is configured not to conduct in response to a signal received from a voltage controller; set the gate control signal such that the thyristor is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor ; and set the gate control signal such that the thyristor 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 ; and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor 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 ; and a galvanically isolated signal transmission device configured to set the gate control signal such that the thyristor is configured not to conduct in response to a signal received from a voltage controller. <EOS>