Frequency responsive charge sustaining control of electricity storage systems for ancillary services on an electrical power grid
Systems, apparatus, and methods are provided for maintaining the state of charge of energy storage devices such as batteries, flywheel, capacitors, or other technologies that are energetically coupled with the electricity grid to support ancillary services. To reliably respond to requests to regulate the grid, the charge on the energy storage device is sustained or restored to a specified range in a manner that optimizes the readiness of the energy storage device to supply ancillary services in light of the condition of the grid. A state of charge (SOC) of the energy storage device and the grid frequency may be monitored. When a request from the operator to regulate the grid frequency is not being serviced, the charge of the energy storage device may be increased or decreased so that the charge may be sustained within the specific range. Once the SOC falls outside of the first range, charge may be added to or removed from the energy storage device when the grid frequency has appropriate values, e.g. if the grid frequency is respectively above a first setpoint or below a second setpoint.
1. A method of responding to changes in the operating frequency of an electricity grid by means of energy storage devices that are energetically coupled with the electricity grid, the method comprising:
determining whether the operating frequency of the electricity grid is within a predetermined range of a nominal operating frequency;
if the operating frequency is determined to be outside said range, regulating the grid frequency by transferring energy between the energy storage device and the electricity grid;
if the operating frequency is within said range, determining whether a state of charge (SOC) of the energy storage device is within predetermined limits; and
transferring energy between the energy storage device and the electricity grid to bring the SOC within said predetermined limits when the operating frequency is within said range and the SOC of the energy storage device is outside said predetermined limits.
2. The method of claim 1 , further comprising:
monitoring the state of charge (SOC) with an energy storage device management system from which an SOC value is received; and
monitoring the grid frequency with a transducer from which the grid frequency is received, wherein the SOC value and the grid frequency are received at a programmable logic control.
3. The method of claim 1 , wherein energy is transferred between the grid and the energy storage device to regulate the frequency at a rate that is based upon the operating frequency of the grid.
4. The method of claim 3 , wherein said rate is a linear function of the operating frequency of the grid.
5. The method of claim 3 , wherein said rate is a non-linear function of the operating frequency of the grid.
6. The method of claim 5 , wherein said non-linear function is a polynomial of an order higher than one.
7. The method of claim 1 , wherein energy is transferred between the grid and the energy storage device to bring the SOC within said predetermined limit at a rate that is based upon the SOC.
8. The method of claim 1 , wherein energy is transferred between the grid and the energy storage device to bring the SOC within said predetermined limit at a rate that is based upon the operating frequency of the grid.
9. The method of claim 1 , wherein energy is transferred between the grid and the storage cells to bring the SOC within said predetermined limit at a rate that is based upon the capacity of the energy storage device.
10. A method of responding to changes in the operating frequency of an electricity grid by means of energy storage devices that are energetically coupled with the electricity grid, the method comprising:
determining whether a parameter of the operating frequency of the electricity grid is within a predetermined range;
if the operating frequency parameter is determined to be outside said range, adjusting the grid frequency by transferring energy between the energy storage devices and the electricity grid;
if the operating frequency is within said range, determining whether a state of charge (SOC) of the energy storage devices is within predetermined limits and whether the operating frequency is within a predetermined band around a nominal operating frequency; and
transferring energy between the energy storage device and the electricity grid to bring the SOC within said predetermined limits when the operating frequency is within said predetermined band and the SOC of the energy storage device is outside said predetermined limits.
11. The method of claim 10 , wherein said parameter is the instantaneous value of the operating frequency.
12. The method of claim 11 , wherein said predetermined band is equal to said predetermined range.
13. The method of claim 11 , wherein said predetermined band is smaller than said predetermined range.
14. The method of claim 10 , wherein said parameter is the rate of change of the operating frequency.
15. A system for responding to changes in the operating frequency of an electricity distribution grid, comprising:
a least one energy storage device;
a converter that, in response to a command to add energy to or absorb energy from the grid, selectively couples said storage cell to the grid to transfer energy between the energy storage device and the grid; and
a state-of-change (SOC) maintenance system that determines whether the operating frequency of the electricity grid is within a predetermined range of a nominal operating frequency, determines whether the SOC of the energy storage device is within predetermined limits, and controls said converter transfer energy between the energy storage device and the electricity grid to bring the SOC within said predetermined limits when the operating frequency is within said range and the SOC of the energy storage device is outside said predetermined limits.
16. The system of claim 15 , wherein said SOC maintenance system includes a transducer that provides data pertaining to the operating frequency of the grid.
17. The system of claim 15 , wherein said SOC maintenance system includes a controller that instructs the converter to connect and disconnect the energy storage device and the grid.
18. The system of claim 17 , wherein said controller instructs the converter to transfer energy at a designated rate.
19. The system of claim 18 , wherein the controller instructs the converter to transfer energy between the grid and the energy storage device at a rate that is based upon the SOC.
20. The system of claim 18 , wherein the controller instructs the converter to transfer energy between the grid and the energy storage device at a rate that is based upon the operating frequency of the grid.
21. The system of claim 20 , wherein said rate is a non-linear function of the operating frequency of the grid.
22. The system of claim 21 , wherein said non-linear function is a polynomial of an order higher than one.
23. The system of claim 18 , wherein the controller instructs the converter to transfer energy between the grid and the energy storage device at a rate that is based upon the capacity of the energy storage device.
24. The system of claim 17 , further including a storage cell management system that provides data to the controller pertaining to the state of charge of the energy storage device.