Systems and methods for hybrid voltage and current control in static UPS systems

1. A system comprising:

a plurality of uninterruptible power supplies (UPSs), each UPS of the plurality of UPSs comprising an inverter;

a ring bus, wherein an output of the inverter of each UPS of the plurality of UPS s is coupled to the ring bus; and

at least one controller communicatively coupled to at least one UPS of the plurality of UPSs, the at least one controller configured to:

determine a first set of gating signals for a plurality of switches in the inverter of the at least one UPS, based on a modulation function of: (i) an upper DC link voltage that flows across a first DC capacitor electrically coupled between a first switch of the plurality of switches and a neutral node and (ii) a lower DC link voltage that flow across a second DC capacitor electrically coupled between a second switch of the plurality of switches and the neutral node;

determine a bridge current applied to a circuit in the inverter of the at least one UPS, the circuit comprising an inductor, a resistor, and a capacitor;

modify the first set of gating signals to reduce the bridge current when the bridge current exceeds a first predetermined bridge current limit, wherein the modification of the first set of gating signals: (i) reduces a DC current component of a load current in the inverter of the at least one UPS and (ii) re-centers the DC current component of the load current in the inverter of the at least one UPS in less than one cycle of the load current; and

control, in response to determining that the bridge current has reached a second predetermined bridge current limit, the at least one UPS using a command voltage based on: (i) one or more reference voltage values, (ii) a voltage across the capacitor, (iii) the load current, and (iv) the bridge current.

2. The system of claim 1 , wherein the inverter of the at least one UPS comprises a single phase voltage source two level inverter.

3. The system of claim 1 , wherein the inverter of the at least one UPS comprises a single phase voltage source three level inverter.

4. The system of claim 1 , further comprising at least one circuit breaker coupled between the at least one UPS and the ring bus.

5. At least one controller for controlling a power supply system that includes a plurality of uninterruptible power supplies (UPSs), a ring bus, and at least one load electrically coupled to the plurality of UPSs and the ring bus, said at least one controller comprising:

a processor; and

a memory device communicatively coupled to the processor, the memory device storing executable instructions configured to cause the processor to:

determine a first set of gating signals for a plurality of switches in an inverter of at least one UPS of the plurality of UPSs, based on a modulation function of: (i) an upper DC link voltage that flows across a first DC capacitor electrically coupled between a first switch of the plurality of switches and a neutral node and (ii) a lower DC link voltage that flow across a second DC capacitor electrically coupled between a second switch of the plurality of switches and the neutral node;

determine when a bridge current applied to a circuit in the inverter of the at least one UPS reaches a first predetermined bridge current limit, the circuit comprising an inductor, a resistor, and a capacitor;

modify, in response to determining that the bridge current has reached the first predetermined bridge current limit, the first set of gating signals to reduce the bridge current to (i) reduce a DC current component of a load current in the inverter of the at least one UPS and (ii) re-center the DC current component of the load current in the inverter of the at least one UPS in less than one cycle of the load current; and

determine, after the bridge current has reached a second predetermined bridge current limit, a second set of gating signals based upon (i) the upper DC link voltage, (ii) the lower DC link voltage, and (iii) a command voltage, wherein said command voltage is based on at least a reference voltage value, the load current, and the bridge current, wherein the second predetermined bridge current limit is set to approximately 70% of the first predetermined bridge current limit.

6. The at least one controller of claim 5 , wherein, in a single phase voltage source three level inverter, a voltage over the capacitor is altered based on the modification of the first set of gating signals.

7. The at least one controller of claim 5 , wherein, in a single phase voltage source two level inverter, a voltage over the capacitor is altered based on the modification of the first set of gating signals.

8. The at least one controller of claim 5 , wherein the load current is a sum of a current through the at least one load and a fault current in the at least one UPS.

9. The at least one controller of claim 5 , wherein the at least one controller comprises a plurality of controllers, each controller of the plurality of controllers configured to control a different UPS of the plurality of UPSs.

10. A method of controlling a power supply system that includes a plurality of uninterruptible power supplies (UPSs), a ring bus, and at least one load electrically coupled to the plurality of UPSs and the ring bus, the method comprising:

determining when a bridge current in an inverter of at least one UPS of the plurality of UPS s reaches a first predetermined bridge current limit;

modifying, in response to determining that the bridge current has reached the first predetermined bridge current limit, a capacitor voltage in the inverter to reduce a DC component of a load current in the inverter;

determining when the bridge current in the inverter reaches a second predetermined bridge current limit, said second predetermined bridge current limit is not an overcurrent limit; and

controlling, in response to determining that the bridge current has reached the second predetermined bridge current limit, the power supply system using a command voltage, wherein the command voltage is based on: (i) one or more reference voltage values, (ii) the capacitor voltage, (iii) the load current, and (iv) the bridge current.

11. The method of claim 10 , wherein modifying the capacitor voltage comprises implementing a control algorithm that modifies gating signals for switches in the inverter when the bridge current reaches the first predetermined bridge current limit.

12. The method of claim 11 , wherein implementing the control algorithm comprises modifying the gating signals such that a magnitude of the DC component of the load current decreases.

13. The method of claim 10 , wherein the inverter of the at least one UPS is a single phase voltage source three level inverter.

14. The method of claim 10 , wherein said inverter of the at least one UPS is a single phase voltage source two level inverter.

15. The method of claim 10 , wherein modifying a capacitor voltage comprises implementing a control algorithm that reduces the DC current component of the load current through a choke associated with the at least one UPS.

16. The method of claim 10 , further comprising re-centering the DC current component in less than one cycle of the load current.

17. The system of claim 5 , wherein the inverter of the at least one UPS lacks a diode that is directly connected to either (i) the neutral node and the first switch or (ii) the neutral node and the second switch.

18. The method of claim 10 , wherein the power supply system further includes a first switching device and a second switching device, and wherein the command voltage is representative of a voltage across the second switching device.

19. The system of claim 1 , wherein the first predetermined bridge current limit is set above 150% of an overload capability for said inverter of at least one UPS of the plurality of UPSs.

20. The system of claim 1 , wherein said inverter of at least one UPS of the plurality of UPS s lacks a diode that is directly connected to the neutral node.

21. The method of claim 10 , wherein the second predetermined bridge current limit is set to approximately 70% of the first predetermined bridge current limit.