Low-loss control of DC-DC converters in an energy storage system

• US 7,990,117 B2
• Filed: 07/07/2008
• Issued: 08/02/2011
• Est. Priority Date: 07/06/2007
• Status: Active Grant

• Current Assignee: WEG Electric Corporation (Wpa Participações e Serviços SA)
• Original Assignee: Northern Power Systems, Inc. (Northern Power Systems Corp.)
• Inventors: Benedict, Eric L.
• Primary Examiner(s): NGUYEN, MATTHEW VAN
• Application Number:
  US12/168,355
• Publication Number:
  US 20090033304A1
• Time in Patent Office: 1,121 Days
• Field of Search: 323222-226, 323/268, 323/271, 323/282
• US Class Current: 323/271
• CPC Class Codes: H02M 3/1582 Buck-boost converters H02M3/1584 takes precedence

Abstract

An energy storage system including a buck-boost DC-DC converter operable so as to minimize switching loses. The system includes a first energy storage device, e.g., a bank of ultracapacitors, and a second energy storage device, e.g., a second bank of electrolytic capacitors. Both first and second energy storage devices are connected to the DC-DC converter. The DC-DC converter is operated in a non-standard manner so as to reduce switching and other losses. In particular, the DC-DC converter may be operated in an alternate buck mode so as to force on a switch S2 through which energy flows from the first energy storage device to the second energy storage device. The switch S2 is forced on for so long as the voltage on the second energy storage device remains within a first operating window.

Claims

1. A capacitor-based energy storage system comprising:

a first energy storage device;

a second energy storage device; and

a bi-directional DC-DC converter connecting said first energy storage device with said second energy storage device, said converter including;

an inductor, a first switch and a second diode connected to form a DC-DC boost converter;

a second switch and a first diode, said inductor, said second switch and said first diode connected to form a DC-DC buck converter, wherein said DC-DC buck converter is cross connected with said DC-DC boost converter; and

a controller controlling said first and second switches, wherein said controller forces on said second switch so as to have a duty cycle of D=1 when a voltage across said second energy storage device is within a voltage operating window.

2. A system according to claim 1, wherein said first and second energy storage devices each include at least one capacitor or capacitor-like energy storage device.

3. A system according to claim 1, wherein said controller controls said first and second switches so that said DC-DC boost converter and said DC-DC buck converter operate in accordance with conventional modes of operation until such time as voltage across said second energy storage device falls within said voltage operating window.

4. A system according to claim 1, wherein said second energy storage device has a maximum voltage level and said voltage operating window has a floor equal to 80-98% of said maximum voltage level.

5. A system according to claim 1, wherein said system is connectable to a source having a minimum target output voltage, further wherein said voltage operating window has a floor substantially equal to the minimum target output voltage of the source.

6. A system according to claim 1, wherein said first energy storage device includes a bank of ultracapacitors.

7. A system according to claim 1, wherein said second energy storage device includes a bank of electrolytic capacitors.

8. A system according to claim 1, wherein said first and second DC-DC converters include multi-phase converters.

9. A system according to claim 1, wherein said first and second switches include metal-oxide-semiconductor field-effect transistors (MOSFETs).

10. A system according to claim 1, wherein said first and second switches include insulated gate bipolar transistors (IGBTs).

11. A system according to claim 1, further including a bidirectional inverter connected between said second energy storage device and a source and a load.

12. A UPS connectable to a load and connectable to a source of power, the UPS comprising:

a. an ultracapacitor bank;

b. a second capacitor bank;

c. a bidirectional DC-DC converter connected to said ultracapacitor bank and said second capacitor bank, wherein said converter includes switches S1 and S2 for causing said converter to operate in one of conventional boost mode and conventional buck mode as function of whether said switches Si and S2 are activated or not activated;

d. a bidirectional inverter connected to said second capacitor bank and connected to said bidirectional DC-DC converter; and

e. a controller connected to said ultracapacitor bank, said second capacitor bank, said bidirectional DC-DC converter and said bidirectional inverter, wherein said controller provides control signals to said bidirectional DC-DC converter causing said converter to force on said switch S2 on for so long as voltage on said second capacitor bank remains above a first threshold, thereby causing said bidirectional DC-DC converter to operate in an alternate buck mode.

13. A UPS according to claim 12, wherein the source has a minimum target voltage output and said first threshold is substantially equal to the minimum target voltage output.

14. A UPS according to claim 12, wherein said second capacitor bank has a maximum voltage level and said first threshold is equal to 80-98% of said maximum voltage level.

15. A method of controlling a buck-boost DC-DC converter connected to a DC bus, a source and a load and having a boost converter with a switch S1 and a buck converter with a switch S2, the method comprising:

a. causing the switch S2 to turn on and operate at a duty cycle of 1 when voltage on the DC bus remains within a first operating window; and

b. operating the switches S1 and S2 when the voltage on the DC bus falls outside the operating window so that the switches S1 and S2 have a duty cycle of less than 1.

16. A method according to claim 15, wherein the source has a minimum target output voltage and the operating window has a floor voltage that is substantially equal to said minimum target output voltage.

17. A method of managing the voltages of a first energy storage device and a second energy storage device, the first energy storage device connected to the second energy storage device through a pair of cross connected DC-DC converters including a first switch and a second switch, the method comprising the steps of:

detecting a voltage on the second energy storage device;

determining if voltage on the second energy storage device is within a voltage operating window;

forcing the second switch to remain continuously on when the voltage on the second energy storage device is within the voltage operating window; and

delivering energy from the second energy storage device through the second switch to the first energy storage device.

18. A method according to claim 17, further comprising determining whether the difference between voltage on said first energy storage device and voltage on said second energy storage device is less than a first voltage threshold, and if so, and if said voltage on said second energy storage device is within the voltage operating window, turning on said second switch S2 and operating said second switch at a duty cycle of D=1.