Grid connected inverter, and method for reducing grid frequency variation

1. A grid connected inverter, connectable to a power grid having a synchronous generator connected thereto, and operable according to an output active power command generated by a virtual synchronous generator control function, to thereby reduce grid frequency variation, the grid connected inverter comprising:

a main circuit including power semiconductor switching elements that turn on and off according to the output active power command, to perform DC-AC conversion; and

a control circuit configured to generate the output active power command, and control the main circuit based on the output active power command,

wherein the output active power command is represented by a sum of a set value of the output active power of the grid connected inverter, a value obtained by multiplying a pseudo attenuation coefficient to a frequency deviation between a grid frequency and a reference frequency, and a value obtained by multiplying a pseudo inertia coefficient to a derivative value of the grid frequency, and

wherein the control circuit adjusts the pseudo inertia coefficient after the grid frequency reaches a maximum point of frequency variation to a value smaller than the pseudo inertia coefficient before the grid frequency reaches the maximum point of frequency variation.

2. The grid connected inverter as claimed in claim 1 , wherein a renewable energy power system is connected as a DC power supply of the grid connected inverter.

3. The grid connected inverter as claimed in claim 2 , wherein the control circuit adjusts the pseudo inertia coefficient to reduce the grid frequency variation caused by sudden changes in a load connected to the power grid, or an output variation of a renewable energy power system connected as a DC power supply of the grid connected inverter.

4. The grid connected inverter as claimed in claim 3 , wherein the control circuit sets the value of the pseudo inertia coefficient after the grid frequency reaches the maximum point of frequency variation, based on an inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, and an inertia coefficient of the entire power grid after the grid frequency reaches the maximum point of frequency variation.

5. The grid connected inverter as claimed in claim 4 , wherein the control circuit computes the inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, based on a mechanical input variation of the synchronous generator, a load variation that is independent of the grid frequency, an attenuation coefficient of the synchronous generator, the pseudo attenuation coefficient, the pseudo inertia coefficient, the frequency deviation between the grid frequency and the reference frequency, the derivative value of the grid frequency, and the pseudo inertia coefficient at a time before the grid frequency reaches the maximum point of frequency variation.

6. The grid connected inverter as claimed in claim 2 , wherein the control circuit sets the value of the pseudo inertia coefficient after the grid frequency reaches the maximum point of frequency variation, based on an inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, and an inertia coefficient of the entire power grid after the grid frequency reaches the maximum point of frequency variation.

7. The grid connected inverter as claimed in claim 6 , wherein the control circuit computes the inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, based on a mechanical input variation of the synchronous generator, a load variation that is independent of the grid frequency, an attenuation coefficient of the synchronous generator, the pseudo attenuation coefficient, the pseudo inertia coefficient, the frequency deviation between the grid frequency and the reference frequency, the derivative value of the grid frequency, and the pseudo inertia coefficient at a time before the grid frequency reaches the maximum point of frequency variation.

8. The grid connected inverter as claimed in claim 1 , wherein the control circuit adjusts the pseudo inertia coefficient to reduce the grid frequency variation caused by sudden changes in a load connected to the power grid, or an output variation of a renewable energy power system connected as a DC power supply of the grid connected inverter.

9. The grid connected inverter as claimed in claim 1 , wherein the control circuit sets the value of the pseudo inertia coefficient after the grid frequency reaches the maximum point of frequency variation, based on an inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, and an inertia coefficient of the entire power grid after the grid frequency reaches the maximum point of frequency variation.

10. The grid connected inverter as claimed in claim 9 , wherein the control circuit computes the inertia of the synchronous generator before the grid frequency reaches the maximum point of frequency variation, based on a mechanical input variation of the synchronous generator, a load variation that is independent of the grid frequency, an attenuation coefficient of the synchronous generator, the pseudo attenuation coefficient, the pseudo inertia coefficient, the frequency deviation between the grid frequency and the reference frequency, the derivative value of the grid frequency, and the pseudo inertia coefficient at a time before the grid frequency reaches the maximum point of frequency variation.

11. A method for reducing grid frequency variation in a grid connected inverter, connectable to a power grid having a synchronous generator connected thereto, the method comprising:

operating the grid connected inverter according to an output active power command, generated by a virtual synchronous generator control function, and represented by a sum of a set value of the output active power of the grid connected inverter, a value obtained by multiplying a pseudo attenuation coefficient to a frequency deviation between a grid frequency and a reference frequency, and a value obtained by multiplying a pseudo inertia coefficient to a derivative value of the grid frequency; and

adjusting the pseudo inertia coefficient after the grid frequency reaches a maximum point of frequency variation to a value smaller than the pseudo inertia coefficient before the grid frequency reaches the maximum point of frequency variation, to generate the output active power command after the grid frequency reaches the maximum point of frequency variation.

12. The method for reducing grid frequency variation as claimed in claim 11 , wherein the operating operates the grid connected inverter according to

the output active power command represented by a sum of the set value of the output active power of the grid connected inverter, and the value obtained by multiplying the pseudo attenuation coefficient to the frequency deviation between the grid frequency and the reference frequency, when the frequency deviation between the grid frequency and the reference frequency is smaller than a first threshold value, and

the output active power command at a time after the grid frequency reaches the maximum point of frequency variation, when the frequency deviation between the grid frequency and the reference frequency exceeds first threshold value.

13. The method for reducing grid frequency variation as claimed in claim 12 , wherein the adjusting adjusts the pseudo inertia coefficient to reduce the grid frequency variation caused by sudden changes in a load connected to the power grid, or an output variation of a renewable energy power system connected as a DC power supply of the grid connected inverter.

14. The method for reducing grid frequency variation as claimed in claim 11 , wherein the adjusting adjusts the pseudo inertia coefficient to reduce the grid frequency variation caused by sudden changes in a load connected to the power grid, or an output variation of a renewable energy power system connected as a DC power supply of the grid connected inverter.