Methods and devices for power compensation

1. A method performed in a control device for controlling a power compensation arrangement comprising a voltage source converter and one or more power compensation branches, each power compensation branch comprising a thyristor controlled reactor, a thyristor switched reactor or a thyristor switched capacitor, the voltage source converter and the one or more power compensation branches being connected to a same busbar, wherein at least one of the one or more power compensation branches comprises a thyristor switched capacitor, the method comprising the steps of:

detecting a request in an electrical power system to which the power compensation arrangement is connected, wherein the request indicates a need of change in reactive power output of the power compensation arrangement;

determining, based on the request, whether the need of change in reactive power output of the power compensation arrangement is larger than a reactive power capacity of the voltage source converter;

providing reactive power by the voltage source converter and by one or more of the power compensation branches when it is determined that the need of change in reactive power output of the power compensation arrangement is larger than the reactive power capacity of the voltage source converter; and

compensating, by the voltage source converter, any disturbances caused by the power compensation branches when providing the reactive power to the electrical power system, wherein the compensating of disturbances comprises the steps of:

measuring a current and voltage of the thyristor switched capacitor, and a bus voltage; and

compensating, by the voltage source converter, based on the measured current and voltages and a switching time instance giving a minimal voltage of the thyristor switched capacitor, for the sum of transient currents of the thyristor switched capacitor and the voltage source converter.

2. The method as claimed in claim 1 , wherein at least one of the one or more power compensation branches comprises a thyristor controlled reactor and the compensating of disturbances comprises the steps of:

measuring a current from the thyristor controlled reactor; and

compensating, based on the measured current of the thyristor controlled reactor, harmonics caused by the thyristor controlled reactor.

3. The method as claimed in claim 2 , wherein the providing of reactive power comprises a sub-step of determining whether the power compensation branches are available to provide the reactive power and providing the reactive power based on the determination.

4. The method as claimed in claim 2 , wherein the request comprises a fault condition, a request from a voltage control, a request from a reactive power control, a request from an operator of the electrical power system or a request based on changes in the electrical power system.

5. The method as claimed in claim 1 , wherein at least one of the one or more power compensation branches comprises a thyristor switched reactor, and the compensating of disturbances comprises the steps of:

measuring a current from the thyristor switched reactor; and

compensating, based on the measured current of the thyristor switched reactor, current transients caused by the thyristor switched reactor.

6. The method as claimed in claim 5 , wherein the providing of reactive power comprises a sub-step of determining whether the power compensation branches are available to provide the reactive power and providing the reactive power based on the determination.

7. The method as claimed in claim 5 , wherein the request comprises a fault condition, a request from a voltage control, a request from a reactive power control, a request from an operator of the electrical power system or a request based on changes in the electrical power system.

8. The method as claimed in claim 1 , wherein the providing of reactive power comprises a sub-step of determining whether the power compensation branches are available to provide the reactive power and providing the reactive power based on the determination.

9. The method as claimed in claim 8 , wherein the request comprises a fault condition, a request from a voltage control, a request from a reactive power control, a request from an operator of the electrical power system or a request based on changes in the electrical power system.

10. The method as claimed in claim 1 , wherein the request comprises a fault condition, a request from a voltage control, a request from a reactive power control, a request from an operator of the electrical power system or a request based on changes in the electrical power system.

11. A control device for controlling a power compensation arrangement comprising a voltage source converter and one or more power compensation branches, each power compensation branch comprising a thyristor switched reactor, a thyristor switched reactor or a thyristor controlled capacitor, at least one of the one or more power compensation branches comprising a thyristor switched capacitor, the voltage source converter and the one or more power compensation branches being connected to a same busbar, the control device being configured to:

detect a request in an electrical power system to which the power compensation arrangement is connected, wherein the request indicates a need of change in reactive power output of the power compensation arrangement;

determine, based on the request, whether the need of change in reactive power output of the power compensation arrangement is larger than a reactive power capacity of the voltage source converter;

provide reactive power by the voltage source converter and by one or more of the power compensation branches when the control device determines that the need of change in reactive power output of the power compensation arrangement is larger than the reactive power capacity of the voltage source converter; and

compensate, by the voltage source converter, any disturbances caused by the power compensation branches when providing the reactive power to the electrical power system, by:

measuring a current and voltage of the thyristor switched capacitor, and a bus voltage; and

compensating, by the voltage source converter, based on the measured current and voltages and a switching time instance giving a minimal voltage of the thyristor switched capacitor, for the sum of transient currents of the thyristor switched capacitor and the voltage source converter.

12. The control device as claimed in claim 11 , wherein at least one of the one or more power compensation branches comprises a thyristor controlled reactor and the control device is configured to compensate disturbances by:

measuring a current from the thyristor controlled reactor; and

compensating, based on the measured current of the thyristor controlled reactor, harmonics caused by the thyristor controlled reactor.

13. The control device as claimed in claim 11 , wherein at least one of the one or more power compensation branches comprises a thyristor switched reactor and the control device is configured to compensate disturbances by:

measuring a current from the thyristor switched reactor; and

compensating, based on the measured current of the thyristor switched reactor, current transients caused by the thyristor switched reactor.

14. A computer program for a control device configured to control a power compensation arrangement comprising a voltage source converter and one or more power compensation branches, each power compensation branch comprising a thyristor controlled reactor, a thyristor switched reactor or a thyristor switched capacitor, at least one of the one or more power compensation branches comprising a thyristor switched capacitor, the voltage source converter and the one or more power compensation branches being connected to a same busbar, the computer program comprising computer program code, which, when run on the control device causes the control device to:

detect a request in an electrical power system to which the power compensation arrangement is connected, wherein the request indicates a need of change in reactive power output of the power compensation arrangement;

determine, based on the request, whether the need of change in reactive power output of the power compensation arrangement is larger than a reactive power capacity of the voltage source converter;

provide reactive power by the voltage source converter and by one or more of the power compensation branches when the control device determines that the need of change in reactive power output of the power compensation arrangement is larger than the reactive power capacity of the voltage source converter; and

compensate, by the voltage source converter, any disturbances caused by the power compensation branches when providing the reactive power to the electrical power system, by:

measuring a current and voltage of the thyristor switched capacitor, and a bus voltage; and

compensating, by the voltage source converter, based on the measured current and voltages and a switching time instance giving a minimal voltage of the thyristor switched capacitor, for the sum of transient currents of the thyristor switched capacitor and the voltage source converter.

15. A computer program product comprising the computer program as claimed in claim 14 , and a non-transitory computer readable medium on which the computer program is stored.