Inverter based storage in dynamic distribution systems including distributed energy resources
A microsource is provided, which includes an inverter, an energy storage device, and a controller. The controller calculates a maximum frequency change for the inverter based on a first comparison between a first power set point and a measured power from the inverter. The first power set point is defined based on a charge level of the energy storage device. A minimum frequency change for the inverter is calculated based on a second comparison between a second power set point and the measured power from the inverter. An operating frequency for the inverter is calculated based on a third comparison between a power set point and a measured power flow. A requested frequency for the inverter is calculated by combining the maximum frequency change, the minimum frequency change, and the operating frequency. The requested frequency is integrated to determine a phase angle of a voltage of the inverter to control a frequency of an output power of the inverter.
1. A controller for controlling a charge and/or a discharge of an energy storage device used in a distributed energy resource, the controller comprising circuitry:
to calculate a maximum frequency change for an inverter based on a first comparison between a first power set point and a measured power from the inverter, wherein the first power set point is defined based on a charge level of an energy storage device coupled to the inverter;
to calculate a minimum frequency change for the inverter based on a second comparison between a second power set point and the measured power from the inverter;
to calculate an operating frequency for the inverter based on a third comparison between a power set point and a measured power flow;
to calculate a requested frequency for the inverter by combining the calculated maximum frequency change, the calculated minimum frequency change, and the calculated operating frequency; and
to integrate the calculated requested frequency to determine a phase angle of a voltage of the inverter to control a frequency of an output power of the inverter.
2. The controller of claim 1 , wherein calculating the operating frequency comprises use of m(F o −F), where m is a slope of an F-ω characteristic, o is the power set point, and F is the measured power flow.
3. The controller of claim 1 , wherein calculating the operating frequency comprises use of m(P o −P), where m is a slope of a P-ω characteristic, P o is the power set point, and P is the measured power flow.
4. The controller of claim 3 , wherein the measured power is the measured power flow.
5. The controller of claim 1 , wherein calculating the maximum frequency change comprises subtracting the measured power from the first power set point to determine a power differential.
6. The controller of claim 5 , wherein calculating the maximum frequency change further comprises applying the determined power differential to a proportional-integral controller.
7. The controller of claim 1 , wherein calculating the minimum frequency change comprises subtracting the measured power from the second power set point to determine a power differential.
8. The controller of claim 7 , wherein calculating the minimum frequency change further comprises applying the determined power differential to a proportional-integral controller.
9. The controller of claim 1 , wherein the second power set point is defined based on the charge level of the energy storage device.
10. The controller of claim 1 , further comprising regulating an output voltage of the inverter using a voltage versus reactive power droop controller.
11. The controller of claim 1 , wherein the first power set point is set to a maximum discharge rate of the energy storage device during a transition to island mode.
12. The controller of claim 1 , wherein the first power set point is set to a preferred discharge rate of the energy storage device after a transition to island mode.
13. The controller of claim 12 , wherein the second power set point is set to the preferred discharge rate of the energy storage device after the transition to island mode.
14. The controller of claim 1 , wherein the first power set point is set to a preferred discharge rate of the energy storage device when an energy level of the energy storage device exceeds a maximum energy reserve level.
15. The controller of claim 1 , wherein the first power set point is set to approximately zero when an energy level of the energy storage device falls below a minimum energy reserve level.
16. The controller of claim 1 , further comprising measuring a current drawn by the inverter from the energy storage device.
17. The controller of claim 16 , further comprising holding a power rate from the energy storage device to a maximum power ramp rate.
18. A method of controlling a charge and/or a discharge of an energy storage device used in a distributed energy resource, the method comprising:
calculating a maximum frequency change for an inverter using a controller based on a first comparison between a first power set point and a measured power from the inverter, wherein the first power set point is defined based on a charge level of an energy storage device coupled to the inverter;
calculating a minimum frequency change for the inverter using the controller based on a second comparison between a second power set point and the measured power from the inverter;
calculating an operating frequency for the inverter using the controller based on a third comparison between a power set point and a measured power flow;
calculating a requested frequency for the inverter using the controller by combining the calculated maximum frequency change, the calculated minimum frequency change, and the calculated operating frequency; and
integrating the calculated requested frequency to determine a phase angle of a voltage of the inverter to control a frequency of an output power of the inverter.
19. A microsource, the microsource comprising:
an inverter;
an energy storage device operably coupled with the inverter; and
a controller operably coupled with the energy storage device and the inverter, the controller including circuitry configured
to calculate a maximum frequency change for the inverter based on a first comparison between a first power set point and a measured power from the inverter, wherein the first power set point is defined based on a charge level of the energy storage device;
to calculate a minimum frequency change for the inverter based on a second comparison between a second power set point and the measured power from the inverter;
to calculate an operating frequency for the inverter based on a third comparison between a power set point and a measured power flow;
to calculate a requested frequency for the inverter by combining the calculated maximum frequency change, the calculated minimum frequency change, and the calculated operating frequency; and
to integrate the calculated requested frequency to determine a phase angle of a voltage of the inverter to control a frequency of an output power of the inverter.
20. The microsource of claim 19 , further comprising a power storage device operably coupled with the inverter, wherein the controller is operably coupled with the power storage device and further includes circuitry configured to limit a power rate of the energy storage device and to control an amount of power output from the power storage device to the inverter.
21. The microsource of claim 20 , wherein the controller further includes circuitry configured to measure a current drawn by the inverter from the energy storage device to limit the power rate of the energy storage device.