Multi-port grid forming control for grid interties

1. A grid forming vector current control system for controlling a grid intertie, the system comprising:

a first terminal configured to be connected to a first power grid, the first terminal comprising a first current control unit, a first virtual admittance unit, and a first phase locked loop (PLL), unit, wherein the first virtual admittance unit and the first PLL unit are configured to emulate an inertia of a first virtual synchronous machine (VSM) and a first virtual current source is connected in parallel to the first VSM;

a second terminal configured to be connected to a second power grid, the second terminal comprising a second current control unit, a second virtual admittance unit, and a second PLL unit, wherein the second virtual admittance unit and the second PLL unit are configured to emulate an inertia of a second VSM, and wherein a second virtual current source is connected in parallel to the second VSM; and

a controller configured to use transient power consumed by the first VSM to generate a power-equivalent current reference to control the second virtual current source and to use the transient power consumed by the second VSM to generate a power-equivalent current reference to control the first virtual current source.

2. The system according to claim 1 , wherein the first terminal-further comprises a first droop control unit, wherein an output signal of the first droop control unit is connected to the first virtual current source in order to emulate a first speed governor and wherein the second terminal further comprises a second droop control unit and an output signal of the second droop control unit is connected to the second virtual current source in order to emulate a second speed governor.

3. The system according to claim 1 , wherein the first terminal further comprises a first droop control unit, wherein an output signal of the first droop control unit is connected to the first PLL unit in order to emulate a first virtual mechanical speed governor and wherein the second terminal further comprises a second droop control unit and an output signal of the second droop control unit is connected to the second PLL unit in order to emulate a second virtual mechanical speed governor.

4. The system according to claim 1 , wherein:

a first factor is configured to change characteristics of the first terminal continuously from voltage source to current source by proportionally scaling the output signals of the first virtual admittance with the first factor; and

a second factor is configured to change the characteristics of the second terminal continuously from voltage source to current source by proportionally scaling the output signals of the second virtual admittance with the second factor.

5. The system according to claim 4 , wherein

the first terminal further comprises a first droop control unit;

the second terminal further comprises a second droop control unit;

at least one of the first droop control unit, the first virtual admittance unit or the first PLL unit is configured to receive the first factor in order to increase or decrease the inertial response of the first terminal to disturbances in the first grid;

at least one of the second droop control unit, the second virtual admittance unit or the second PLL unit is configured to receive the second factor in order to increase or decrease the inertial response of the second terminal to disturbances in the second grid; and

the first factor scales a nominal power of the first VSM by proportionally scaling the output signals of the first virtual admittance with the first factor, inversely proportionally scaling the proportional gain of the first PLL unit with the first factor, and proportionally scaling a first droop constant with the first factor; and

the second factor scales a nominal power of the second VSM by proportionally scaling the output signals of the second virtual admittance with the second factor, inversely proportionally scaling the proportional gain of the second PLL unit with k VSM2 , and proportionally scaling a second droop constant with the second factor.

6. The system according to claim 4 , wherein the first and second factors scale at least one of a spinning wheel factor or a transient power or the inertia.

7. The system according to claim 4 , wherein the first terminal further comprises a first droop control unit and an output signal of the first droop control unit is connected to the first virtual current source in order to emulate a first speed governor and wherein the second terminal further comprises a second droop control unit and an output signal of the second droop control unit is connected to the second virtual current source in order to emulate a second speed governor.

8. The system according to claim 4 , wherein the first terminal further comprises a first droop control unit and an output signal of the first droop control unit is connected to the first PLL unit in order to emulate a first virtual mechanical speed governor and wherein the second terminal further comprises a second droop control unit and an output signal of the second droop control unit is connected to the second PLL unit in order to emulate a second virtual mechanical speed governor.

9. The system according to claim 1 , further comprising:

a third terminal configured to be connected to a third power grid, the third terminal comprising a third droop control unit, a third current control unit, a third virtual admittance unit, and a third PLL unit, wherein the third virtual admittance unit and the third PLL unit are configured to emulate an inertia of a third VSM and a third virtual current source is connected in parallel to the third VSM; and

wherein the controller is configured to:

use the transient power consumed by the first VSM and the transient power consumed by the second VSM to generate a power-equivalent current reference to control the third virtual current source,

use the transient power consumed by the first VSM and transient power consumed by the third VSM to generate a power-equivalent current reference to control the second virtual current source, and

use the transient power consumed by the second VSM and the transient power consumed by the third VSM to generate a power-equivalent current reference to control the first virtual current source.

10. The system according to claim 1 , wherein the first terminal further comprises a first droop control unit and an output signal of the first droop control unit is connected to the first virtual current source in order to emulate a first speed governor.

11. The system according to claim 1 , wherein the first terminal further comprises a first droop control unit and an output signal of the first droop control unit is connected to the first PLL unit in order to emulate a first virtual mechanical speed governor.

12. A method for controlling a grid intertie, the method comprising:

providing a first terminal configured to be connected to a first power grid, the first terminal comprising a first current control unit, a first virtual admittance unit, and a first phase locked loop (PLL) unit;

emulating, with the first virtual admittance unit and the first PLL unit, an inertia of a first virtual synchronous machine VSM;

providing a first virtual current source connected in parallel to the first VSM;

providing a second terminal configured to be connected to a second power grid, the second terminal comprising a second current control unit, a second virtual admittance unit, and a second PLL unit;

emulating, with the second virtual admittance unit and the second PLL unit, an inertia of a second VSM;

providing a second virtual current source connected in parallel to the second VSM; and

using transient power consumed by the first VSM to generate a power-equivalent current reference to control the second virtual current source and using transient power consumed by the second VSM to generate a power-equivalent current reference to control the first virtual current source.

13. The method according to claim 12 , wherein the first terminal further comprises a first droop control unit and wherein the method further comprises connecting an output signal of the first droop control unit to the first virtual current source in order to emulate a first speed governor; and

wherein the second terminal further comprises a second droop control unit and wherein the method further comprises connecting an output signal of the second droop control unit to the second virtual current source in order to emulate a second speed governor.

14. The method according to claim 12 , wherein the first terminal further comprises a first droop control unit and wherein the method further comprises connecting an output signal of the first droop control unit to the first PLL unit in order to emulate a first virtual mechanical speed governor; and

wherein the second terminal further comprises a second droop control unit and wherein the method further comprises connecting an output signal of the second droop control unit to the second PLL unit in order to emulate a second virtual mechanical speed governor.

15. The method according to claim 12 , comprising:

applying a first factor to change characteristics of the first terminal continuously from voltage source to current source by proportionally scaling the output signals of the first virtual admittance with the first factor; and

applying a second factor to change characteristics of the second terminal continuously from voltage source to current source by proportionally scaling the output signals of the second virtual admittance with the second factor.

16. The method according to claim 15 , comprising:

feeding the first factor into at least one of the first droop control unit, the first virtual admittance unit or the first PLL unit in order to increase or decrease the inertial response of the first terminal to disturbances in the first grid;

feeding the second factor into at least one of the second droop control unit, the second virtual admittance unit or the second PLL unit in order to increase or decrease the inertial response of the second terminal to disturbances in the second grid;

scaling, with the first factor, a nominal power of the first VSM by proportionally scaling the output signals of the first virtual admittance with the first factor, inversely proportionally scaling the proportional gain of the first PLL unit with the first factor, and proportionally scaling a first droop constant with the first factor; and

scaling, with the second factor, a nominal power of the second VSM by proportionally scaling the output signals of the second virtual admittance with the second factor, inversely proportionally scaling the proportional gain of the second PLL unit with the second factor, and proportionally scaling a second droop constant with the second factor.

17. The method according to claim 15 , comprising scaling, with the first and second factors, at least one of a spinning wheel factor or a transient power or the inertia.

18. The method according to claim 12 , further comprising:

providing a third terminal configured to be connected to a third power grid, the third terminal comprising a third current control unit, a third virtual admittance unit, and a third PLL unit;

emulating, with the third virtual admittance unit and the third PLL unit, an inertia of a third VSM;

providing a third virtual current source connected in parallel to the third VSM;

using the transient power consumed by the first VSM and the transient power consumed by the second VSM to generate a power-equivalent current reference to control the third virtual current source;

using the transient power consumed by the first VSM and transient power consumed by the third VSM to generate a power-equivalent current reference to control the second virtual current source; and

using the transient power consumed by the second VSM and the transient power consumed by the third VSM to generate a power-equivalent current reference to control the first virtual current source.