Method and a control device for controlling a power converter configured to form a frequency droop value based on electric power supplied to alternating current system

1. A control device for controlling a power converter, the control device comprising a data processing system configured to:

form a frequency droop value (Δf drop ) based on data (U DC , I DC ) indicative of electric power (p act ) of the power converter, and

change a frequency control value (f c ) by the frequency droop value, the frequency droop value decreasing the frequency control value when a power flow direction is outwards from alternating voltage terminals of the power converter,

wherein the data processing system is configured to:

form a power control value (Δf pc ) based on data indicative of a target value (p ref ) of the electric power,

change the frequency control value by the power control value, the power control value increasing the frequency control value when the target value of the electric power corresponds to the power flow direction outwards from the alternating voltage terminals of the power converter,

deliver the frequency control value to the power converter so as to control alternating voltage frequency of the power converter,

compute a time integral of an error value proportional to a difference (p ref −p act ) between the target value of the electric power and the electric power,

correct the frequency control value with a correction value (Δf corr ) dependent on the time integral of the error value, and

limit the correction value to be at least a predetermined lower limit of the correction value and at most a predetermined upper limit of the correction value.

2. The control device according to claim 1 , wherein the data processing system is configured to form the frequency control value according to the following formulas:

f c =f 0 −Δf droop +Δf pc ,

Δ f droop =αp act , and

Δ f pc =αp ref ,

where f c is the frequency control value, f 0 is a base value of the frequency control value, p act is the electric power of the power converter, Δf droop is the frequency droop value, p ref is the target value of the electric power, Δf pc is the power control value, and α is a drooping coefficient for changing the frequency control value in accordance with the electric power.

3. The control device according to claim 1 , wherein the data processing system is configured to limit a rate of change of the power control value (Δf pc ) to be at most a predetermined upper limit of the rate of change.

4. The control device according to claim 1 , wherein the data processing system is configured to form the frequency control value according to the following formulas:

f c =f 0 −Δf droop +Δf pc +Δf corr ,

Δ f droop =αp act ,

Δ f pc =αp ref , and

Δ f corr =β I ( p ref −p act ) dt,

where f c is the frequency control value, f 0 is a base value of the frequency control value, p act is the electric power of the power converter, Δf droop is the frequency droop value, p ref is the target value of the electric power, Δf pc is the power control value, Δf corr is the correction value, α is a drooping coefficient for changing the frequency control value in accordance with the electric power, and β I is a control parameter.

5. The control device according to claim 1 , wherein the data processing system is configured to constitute a proportional and integrative controller, and the error value is an input value of the proportional and integrative controller and the correction value is an output value of the proportional and integrative controller.

6. The control device according to claim 1 , wherein the data processing system is configured to prevent an absolute value of the time integral from increasing in response to a situation in which the correction value reaches the predetermined upper limit of the correction value or the predetermined lower limit of the correction value.

7. The control device according to claim 1 , wherein the data processing system is configured to form a voltage droop value (Δu droop ) based on data indicative of reactive power (Q) of the power converter, the voltage droop value decreasing an amplitude of voltage of the power converter in response to a situation in which the power converter produces inductive reactive power.

8. A power converter comprising:

a converter stage configured to form one or more alternating voltages, and

a driver stage configured to control the converter stage to form the one or more alternating voltages in accordance with a frequency control value (f c ) expressing frequency of the one or more alternating voltages, and

a control device configured to determine the frequency control value,

wherein the control device comprising a data processing system configured to:

form a frequency droop value (Δf droop ) based on data (U DC , I DC ) indicative of electric power (p act ) of the power converter, and

change the frequency control value (f c ) by the frequency droop value, the frequency droop value decreasing the frequency control value when a power flow direction is outwards from alternating voltage terminals of the power converter,

wherein the data processing system is configured to:

form a power control value (Δf pc ) based on data indicative of a target value (p ref ) of the electric power,

change the frequency control value by the power control value, the power control value increasing the frequency control value when the target value of the electric power corresponds to the power flow direction outwards from the alternating voltage terminals of the power converter, and

deliver the frequency control value to the power converter so as to control alternating voltage frequency of the power converter,

compute a time integral of an error value proportional to a difference (p ref −p act ) between the target value of the electric power and the electric power,

correct the frequency control value with a correction value (Δf corr ) dependent on the time integral of the error value, and

limit the correction value to be at least a predetermined lower limit of the correction value and at most a predetermined upper limit of the correction value.

9. The power converter according to claim 8 , wherein the driver stage is configured to control the converter stage to form the one or more alternating voltages in accordance with the frequency control value (f c ) and a voltage control value (U c ), and the control device is a control device and configured to determine the voltage control value.

10. A method for controlling a power converter, the method comprising:

forming a frequency droop value (Δf droop ) based on data indicative of electric power (p act ) of the power converter, and

changing a frequency control value (f c ) by the frequency droop value, the frequency droop value decreasing the frequency control value when a power flow direction is outwards from alternating voltage terminals of the power converter,

wherein the method further comprises:

forming a power control value (Δf pc ) based on data indicative of a target value (p ref ) of the electric power,

changing the frequency control value by the power control value, the power control value increasing the frequency control value when the target value of the electric power corresponds to the power flow direction outwards from the alternating voltage terminals of the power converter,

delivering the frequency control value to the power converter so as to control alternating voltage frequency of the power converter,

computing a time integral of an error value proportional to a difference (p ref −p act ) between the target value of the electric power and the electric power,

correcting the frequency control value with a correction value (Δf corr ) dependent on the time integral of the error value, and

limiting the correction value to be at least a predetermined lower limit of the correction value and at most a predetermined upper limit of the correction value.

11. The method according to claim 10 , wherein the method comprises computing a time integral of an error value proportional to a difference (p ref −p act ) between the target value of the electric power and the electric power, and to correct the frequency control value with a correction value (Δf corr ) dependent on the time integral of the error value.

12. A computer program for controlling a power converter, the computer program comprising computer executable instructions for controlling a programmable processing system to:

form a frequency droop value (Δf droop ) based on data indicative of electric power (p act ) of the power converter, and

change a frequency control value (f c ) by the frequency droop value, the frequency droop value decreasing the frequency control value when a power flow direction is outwards from alternating voltage terminals of the power converter,

wherein the computer program comprises computer executable instructions for controlling the programmable processing system to:

form a power control value (Δf pc ) based on data indicative of a target value (p ref ) of the electric power,

change the frequency control value by the power control value, the power control value increasing the frequency control value when the target value of the electric power corresponds to the power flow direction outwards from the alternating voltage terminals of the power converter,

deliver the frequency control value to the power converter so as to control alternating voltage frequency of the power converter,

compute a time integral of an error value proportional to a difference (p ref −p act ) between the target value of the electric power and the electric power,

correct the frequency control value with a correction value (Δf corr ) dependent on the time integral of the error value, and

limit the correction value to be at least a predetermined lower limit of the correction value and at most a predetermined upper limit of the correction value.

13. A non-volatile computer readable medium encoded with a computer program according to claim 12 .

14. The control device according to claim 2 , wherein the data processing system is configured to limit a rate of change of the power control value (Δf pc ) to be at most a predetermined upper limit of the rate of change.

15. The control device according to claim 4 , wherein the data processing system is configured to constitute a proportional and integrative controller, and the error value is an input value of the proportional and integrative controller and the correction value is an output value of the proportional and integrative controller.