Systems and methods for voltage SAG compensation

1. A voltage sag compensation device, comprising:

a first capacitor having a first peak voltage and a second capacitor having a second peak voltage, the second peak voltage being lower than the first peak voltage, the first capacitor and the second capacitor being configured to be charged via an AC rectifier when the voltage sag compensation device is connected to a power line;

a first voltage sag detector configured to detect on the power line a voltage sag of a first predetermined value, and a second voltage sag detector configured to detect on the power line a voltage sag of a second predetermined value, the second predetermined value being greater than the first predetermined value;

an inverter arranged to transform DC power supplied by at least one of the first or second capacitors into AC power; and

control switching, responsive to the first voltage sag detector and to the second voltage sag detector, configured to, upon detection of a voltage sag of the first predetermined value, disconnect the power line from the AC rectifier, disconnect the power line from a load being serviced by the power line, and cause the power line to drive a gating circuit associated with the inverter such that the inverter generates AC voltage that is substantially in phase with AC voltage present on the power line which inverter generated voltage is arranged to be supplied to the load in series with the power line voltage, and further configured to, upon detection of a voltage sag of the second predetermined value, cause the inverter to receive power directly from the first capacitor.

2. The device of claim 1 , wherein the second capacitor is reinforced by the first capacitor when a voltage on the second capacitor drops below a predetermined value.

3. The device of claim 2 , wherein the second capacitor is reinforced through a field effect transistor (FET).

4. The device of claim 1 , wherein the control switching is further configured to monitor a voltage level of at least one of the first capacitor or second capacitor and to reconnect the power line to the load and disconnect the power from the gating circuit.

5. The device of claim 4 , wherein the control switching monitors a DC bus that feeds the inverter.

6. The device of claim 1 , wherein the gating circuit comprises optical isolators.

7. The device of claim 1 , further comprising an inductor connected to an output of the inverter and through which the inverter generated voltage is supplied to the load.

8. The device of claim 1 , wherein the first voltage sag detector is configured to detect a voltage sag of about 20% of nominal power line voltage.

9. The device of claim 8 , wherein the voltage sag is to about 95 volts.

10. The device of claim 1 , wherein the second voltage sag detector is configured to detect a voltage sag of about 40% of nominal power line voltage.

11. The device of claim 10 , wherein the voltage sag is to about 70 volts.

12. A voltage sag compensation system, comprising:

a capacitor charging and discharging module connected to an AC rectifier and to an inverter;

a two stage voltage detector configured to detect a degree of voltage sag that is present on a power line;

an inverter gating module selectively connectable to the power line in response to the two stage voltage detector and configured to provide gating signals to the inverter; and

a low DC bus switch out module configured to monitor an output voltage of the capacitor charging and discharging module and to disable the inverter gating module from providing the gating signals to the inverter when voltage associated with the capacitor charging and discharging module falls below a predetermined value.

13. The system of claim 12 , wherein the capacitor charging and discharging module is charged via an AC rectifier.

14. The system of claim 13 , wherein the two stage voltage detector includes a first stage and a second stage, and the first stage controls connection of the power line to the AC rectifier and to the inverter gating module.

15. The system of claim 12 , wherein the two stage voltage detector includes a first stage and a second stage, and the second stage controls which one of two capacitors in the capacitor charging and discharging module is connected directly to the inverter.

16. The system of claim 15 , wherein one of the two capacitors is reinforced by another of the two capacitors.

17. The system of claim 16 , wherein the one of the two capacitors is reinforced through a field effect transistor (FET).

18. The system of claim 12 , wherein the inverter gating module comprises optical isolators.

19. The system of claim 12 , wherein the two stage voltage detector is configured to detect a voltage sag of about 20% of nominal power line voltage.

20. The system of claim 12 , wherein the two stage voltage detector is configured to detect a voltage sag of about 40% of nominal power line voltage.

21. A method of compensating for voltage sag on a power line comprising:

charging a first capacitor and a second capacitor using an AC rectifier connected to a power line;

detecting, on the power line, a voltage sag of a predetermined amount;

disconnecting the power line from the AC rectifier and connecting the power line in such a way as to drive a gating circuit for a DC-AC inverter;

inverting DC voltage supplied by at least one of the first capacitor or the second capacitor and applying resulting AC voltage to the power line in series therewith; and

monitoring a voltage level of the DC voltage and disconnecting the power line from driving the gating circuit when the DC voltage falls below a predetermined value.

22. The method of claim 21 , further comprising detecting a voltage sag of a first predetermined value and a voltage sag of a second predetermined value greater than the first predetermined value, and selecting one of the first capacitor or the second capacitor to supply DC voltage for said inverting step based on whether a given voltage sag is at least as great as the first predetermined value or at least as great as the second predetermined value.

23. The method of claim 21 , wherein the gating circuit employs optical isolators.

24. The method of claim 21 , further comprising reinforcing a charge of the second capacitor using the first capacitor.

25. The method of claim 24 , further comprising detecting a low voltage state of the second capacitor and closing a switch to allow the first capacitor to charge the second capacitor.