Systems and methods for operating uninterruptible power supplies

1. An uninterruptible power supply (UPS) comprising:

a transformer configured to receive input power, said transformer comprising a primary winding, a secondary winding, and a tertiary winding;

a rectifier coupled to said secondary winding;

an inverter coupled to an output of said rectifier, wherein a connection between said rectifier and said inverter defines a DC link, said inverter configured to output a first regulated voltage; and

a voltage-boost converter coupled to said tertiary winding and configured to output a second regulated voltage that is added to the first regulated voltage.

2. The UPS according to claim 1 , further comprising a battery and a battery converter, said battery converter electrically coupled between said battery and the DC link.

3. The UPS according to claim 2 , wherein said battery converter is configured to regulate voltage on the DC link when an input voltage of the input power is outside a predefined range.

4. The UPS according to claim 2 , further comprising a bidirectional buck-boost converter coupled to a voltage-boost converter DC link and to said battery, the voltage-boost converter DC link defined by a conductor coupled between a voltage-boost converter rectifier and a voltage-boost converter inverter, said bidirectional buck-boost converter configured to:

charge said battery when an input voltage of the input power is within a predefined range; and

discharge said battery to regulate a DC link voltage of the voltage-boost converter DC link when the input voltage is outside the predefined range.

5. The UPS according to claim 1 , further comprising a controller configured to apply a control method to said inverter and said voltage-boost converter.

6. The UPS according to claim 5 , wherein said controller is configured to:

set a load voltage contribution from said voltage-boost converter at a constant value irrespective of input line conditions; and

switch said inverter to regulate the load voltage in response to disturbances.

7. The UPS according to claim 5 , wherein said controller is configured to:

vary a voltage contribution from said voltage-boost converter such that a maximum contribution occurs under low-line conditions and a minimum contribution occurs under high-line conditions; and

switch said inverter to regulate the load voltage in response to disturbances.

8. The UPS according to claim 5 , wherein said controller is configured to:

cause said inverter to provide a load voltage contribution based on an estimate of load power and line voltage; and

regulate an output voltage of said voltage-boost converter based on load current feedback.

9. The UPS according to claim 5 , wherein during a light load demand, said controller is configured to place said voltage-boost converter in a standby mode when said inverter contributes all of the power to a load.

10. The UPS according to claim 5 , wherein said controller is configured to:

control said inverter to switch at low frequencies, resulting in large voltage steps; and

control said voltage-boost converter to provide a correction voltage needed to provide a sinusoidal output voltage.

11. The UPS according to claim 1 , further comprising a bypass network that when activated, is configured to bypass said inverter and couple the input power directly to a load.

12. A method of operating a power system, said method comprising:

coupling an uninterruptible power supply (UPS) between a utility and a load, wherein the UPS includes a transformer configured to receive input power, the transformer including a primary winding, a secondary winding, and a tertiary winding;

rectifying, using a rectifier coupled to the secondary winding, the input power to regulate a DC link voltage across a DC link defined by a connection between the rectifier and an inverter;

generating a first regulated voltage using the inverter coupled to an output of the rectifier; and

generating a second regulated voltage that is added to the first regulated voltage, the second regulated voltage generated using a voltage-boost converter coupled to the tertiary winding.

13. The method according to claim 12 , further comprising regulating voltage on the DC link provided by a battery using a battery converter electrically coupled between the battery and the DC link when voltage is outside a predefined range.

14. The method according to claim 12 , further comprising:

charging the battery using a bidirectional buck-boost converter when an input voltage of the input power is within a predefined range, wherein the bidirectional buck-boost converter is coupled to a voltage-boost converter DC link and to the battery, wherein the voltage-boost converter DC link is defined by a conductor coupled between a voltage-boost converter rectifier and a voltage-boost converter inverter; and

discharging the battery to regulate a DC link voltage of the voltage-boost converter DC link when the input voltage is outside the predefined range.

15. The method according to claim 12 , further comprising:

setting a load voltage contribution from the voltage-boost converter at a constant value irrespective of input line conditions; and

switching the inverter to regulate the load voltage in response to disturbances.

16. The method according to claim 12 , further comprising:

varying a voltage contribution from the voltage-boost converter such that a maximum contribution occurs under low-line conditions and the voltage contribution is minimal under high-line conditions; and

switching the inverter to regulate the load voltage in response to disturbances.

17. The method according to claim 12 , further comprising:

causing the inverter to provide a load voltage contribution based on an estimate of load power and line voltage; and

regulating an output voltage of the voltage-boost converter based on load current feedback.

18. The method according to claim 12 , further comprising placing the voltage-boost converter in a standby mode when the inverter contributes all of the power needed to supply the load.

19. The method according to claim 12 , further comprising:

controlling the inverter to switch at low frequencies, resulting in large voltage steps; and

controlling the voltage-boost converter to provide a correction voltage needed to provide a sinusoidal output voltage.

20. The method according to claim 12 , further comprising bypassing the inverter and coupling the received input voltage directly to the load when at least one of the rectifier fails, the inverter fails, and the load has a fault causing a fuse on the load side to open.