Control of Energy Storage System Inverter System in a Microgrid Application

• US 20140218985A1
• Filed: 02/07/2013
• Published: 08/07/2014
• Est. Priority Date: 02/07/2013
• Status: Active Grant

• Current Assignee: Caterpillar Incorporated
• Original Assignee: Caterpillar Incorporated
• Inventors: Yu, Dachuan, Speckhart, Greg
• Granted Patent: US 9,042,141 B2
• Time in Patent Office: Days
• US Class Current: 363/97
• CPC Class Codes: H02J 3/241 The oscillation concerning frequency
• CPC Class Codes: H02J 3/32 using batteries with converting means
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 3/1584 with a plurality of power processing stages connected in parallel
• CPC Class Codes: H02M 7/537 using semiconductor devices only, e.g. single switched pulse inverters
• CPC Class Codes: H02M 7/797 using semiconductor devices only
• CPC Class Codes: Y02P 80/14 District level solutions, i.e. local energy networks

Abstract

A system that manages a supplemental energy source connected to a power grid uses a two stage control strategy to manage power transfers in and out of the power grid as well as in and out of an energy storage system, such as a battery bank. One stage uses a non-linear transfer function to control an output frequency of a DC-to-AC inverter to limit undesired effects of power transients that occur on the grid. A second stage uses control strategy for transferring energy between the energy storage system and an internal DC link based on a relationship between a voltage on a DC link connecting the first and second stages and a DC link reference voltage, the voltage on the DC link, and a voltage at the energy storage system. The control strategy includes rapid charging, over-charging protection, and grid transient stabilization.

Claims

1. A system for managing power transfer with a power grid comprising:

a direct current (DC) link;

an energy storage system (ESS) that stores DC power;

an energy storage controller coupled between the DC link and the ESS that manages power flow between the ESS and the DC link; and

a load controller coupled between the DC link and the power grid, the load controller including a DC-to-AC inverter, the load controller operable to adjust an output frequency of the DC-to-AC inverter using a non-linear power-frequency curve responsive to both a state of a frequency of the power grid and a power transfer state of the load controller.

2. The system of claim 1, wherein non-linear transfer curve has at least three regimes.

3. The system of claim 2, wherein the non-linear transfer curve has a first regime with a constant frequency (flat slope) active when power from the ESS is supplied to the power grid.

4. The system of claim 3, wherein the non-linear transfer curve has a second regime with a low positive slope active when charging the ESS.

5. The system of claim 4, wherein the low positive slope has a slope less than or equal to 1.

6. The system of claim 3, wherein the non-linear transfer curve has a third regime with an exponential positive slope, the third regime active when a power grid frequency rises above a target frequency during a grid transient.

7. The system of claim 1, wherein the energy storage controller is operated to sink or source current based on a relationship between a voltage on the DC link and a DC link reference voltage.

8. The system of claim 7, wherein a current limit function imposes a maximum current value and direction for the ESS controller according a current request value based on the relationship voltage on the DC link and the DC link reference voltage, the voltage on the DC link, and a voltage at the ESS.

9. A method of managing power transfer to a power grid using a bidirectional DC-to-AC inverter coupled between the power grid and an energy storage system (ESS), the method comprising:

sensing a frequency of power on the power grid;

determining a power flow at the bidirectional DC-to-AC inverter;

selecting a non-linear power-frequency curve;

setting an output frequency of the bidirectional DC-to-AC inverter using the non-linear power-frequency curve and a current power flow through the bidirectional DC-to-AC inverter.

10. The method of claim 9, wherein selecting the non-linear power-frequency curve comprises selecting the non-linear power-frequency curve having a flat slope when the power flow is from the ESS to the power grid.

11. The method of claim 10, wherein selecting the non-linear power-frequency curve comprises selecting the non-linear power-frequency curve having a portion with a positive slope when the power flow is from the power grid to the ESS during charging of the ESS.

12. The method of claim 11, wherein selecting the non-linear power-frequency curve comprises selecting the non-linear power-frequency curve having a positive exponential slope when the power flow is from the power grid to the ESS and a power grid frequency is above a specified target range of frequency.

13. A system for managing power transfer between an energy storage system component and a power grid, the system comprising:

a direct current (DC) link operatively coupled to the power grid;

a DC energy storage system (ESS);

a DC-to-DC converter coupled between the DC link and the ESS, the DC-to-DC converter adapted for bi-directional current flow; and

a controller coupled to the dc-to-dc converter that operates the DC-to-DC converter to control current magnitude and direction based on a relationship between a voltage on the DC link and a DC link reference voltage, the voltage on the DC link, and a voltage at the ESS to provide a plurality of charging and discharging modes.

14. The system of claim 13, wherein a first charging mode provides a rapid charge current into the ESS.

15. The system of claim 14, wherein a second charging mode provides a charge maintenance current from the DC link to the ESS.

16. The system of claim 15, wherein the rapid charge current is at least an order of magnitude greater than the charge maintenance current.

17. The system of claim 14, wherein the rapid charge current is capped below a level that would cause the power grid to activate an additional generator to supply power required to meet the demand of rapid charge current if not capped.

18. The system of claim 13, wherein a third charging mode provides a high current sink from the power grid to the ESS via the DC link when a power grid transient causes a high voltage condition on the DC link.

19. The system of claim 13, wherein a first discharging mode supplies current from the ESS to the DC link so that the ESS is a primary source of power on the power grid.

20. The system of claim 19, wherein a second discharging mode supplies a trickle charge current to the ESS from the DC link.