Protecting an operation control unit connected to an electric machine via a long cable

1. A system for operating an electric machine comprising:

an operation control unit controlling operation of the machine,

an alternating current cable connecting the machine with the operation control unit, which alternating current cable is more than 10 kilometers long, and

an electric short-circuiting arrangement adapted to protect the operation control unit, said arrangement connected to terminals of the operation control unit facing the cable, said arrangement being configured to short circuit said terminals based on an overvoltage detected at said terminals.

2. The system according to claim 1 , wherein the short-circuiting arrangement comprises a diode bridge connected to said terminals of the operation control unit.

3. The system according to claim 2 , wherein the short-circuit arrangement comprises a switch connected for short-circuiting the diode bridge and a short-circuiting control unit for actuating the switch.

4. The system according to claim 3 , wherein the switch has a gate and is configured to be closed based on a voltage on the gate.

5. The system according to claim 4 , wherein the switch is a thyristor.

6. The system according to claim 3 , wherein the short-circuiting control unit is configured to close the switch based on the voltage at said terminals.

7. The system according to claim 3 , wherein the short-circuiting control unit is configured to close the switch based on the failure of a protective activity of the operation control unit.

8. The system according to claim 1 , further comprising a step up transformer between the operations control unit and the cable and a step down transformer between the cable and the electric machine.

9. The system according to claim 1 , wherein the operations control unit is a frequency converter.

10. The system according to claim 1 , wherein the cable is a subsea cable and the electric machine is a subsea electric machine.

11. The system according to claim 1 , wherein the electric machine is a motor.

12. The system according to claim 1 , further comprising the electric machine.

13. A method for protecting an operation control unit in a system for operating an electric machine,

the system having

an alternating current cable connecting the machine with the operation control unit, which alternating current cable is more than 10 kilometers long, and

an electric short-circuiting arrangement connected to terminals of the operation control unit facing the cable, said arrangement being configured to short circuit said terminals based on an overvoltage,

the method comprising the steps of:

detecting an overvoltage at the terminals of the operation control unit facing the alternating current cable leading to the electric machine, and

short-circuiting the terminals based on the detected overvoltage.

14. A system for operating an electric machine comprising:

an operation control unit controlling operation of the machine,

an alternating current cable connecting the machine with the operation control unit, and

an electric short-circuiting arrangement,

wherein the short-circuiting arrangement comprises a diode bridge connected to terminals of the operation control unit,

wherein the short-circuit arrangement comprises a switch which is a thyristor connected for short-circuiting the diode bridge and a short-circuiting control unit for actuating the switch,

wherein the switch has a gate and is configured to be closed based on a voltage on the gate,

wherein the short-circuiting control unit has two input terminals connected to two ends of the bridge legs, one connected to one side of the bridge legs and the other connected to an opposite side,

wherein the short-circuiting control unit has a control signal output, which provides the control signal that actuates the switch based on the detected DC voltage across the diode bridge legs and which control signal output is connected to the gate of the thyristor,

wherein the alternating current cable is more than 10 kilometers long, and wherein said arrangement is connected to terminals of the operation control unit facing the cable and configured to short circuit said terminals based on an overvoltage detected at said terminals.

15. The system according to claim 1 , wherein the electric short-circuiting arrangement is autonomous such that it does not require a control system but is instead self-triggering.

16. The method according to claim 13 , wherein the electric short-circuiting arrangement is autonomous such that it does not require a control system but is instead self-triggering.

17. The method according to claim 13 , wherein the short-circuiting arrangement comprises a diode bridge connected to said terminals of the operation control unit.

18. The method according to claim 13 , further comprising a step up transformer between the operations control unit and the cable and a step down transformer between the cable and the electric machine.

19. The method according to claim 13 , wherein the operations control unit is a frequency converter.

20. The system according to claim 1 , further comprising a step up transformer between the operations control unit and the cable and a step down transformer between the cable and the electric machine,

wherein the short-circuiting arrangement comprises a diode bridge connected to said terminals of the operation control unit, and

wherein the operations control unit is a frequency converter.