Inverter control for support of power factor corrected loads

• US 5,784,268 A
• Filed: 09/20/1996
• Issued: 07/21/1998
• Est. Priority Date: 09/20/1996
• Status: Expired due to Fees

• Current Assignee: Powerware Corporation (Eaton Corporation PLC.)
• Original Assignee: General Signal Corporation (SPX Corporation)
• Inventors: Egstad, Chris, Mengelt, Kevin R., Steffek, Laura M.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Riley, Shawn
• Application Number:
  US08/710,656
• Time in Patent Office: 669 Days
• Field of Search: 307/43, 307/41, 307/66, 307/64, 307/85, 307/86, 307/87, 307/81, 307/55, 307/46, 363/23, 363/56, 363/55, 363/41, 363/97, 363/133, 361/88, 361/89, 361/92, 361/93, 361/111
• US Class Current: 363/55
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: H02M 7/53806 in a push-pull configuration of the parallel type

Abstract

A method for controlling an inverter to improve the ability of the inverter to support power factor corrected (PFC) loads under transient conditions is provided. Changes in the load being provided power by an inverter may cause the RMS output voltage level of the inverter to drop. A PFC load may respond to such drops in the applied voltage level by increasing its current demand. This may cause the inverter current limit to be reached, forcing the inverter to be turned off, and causing the load to be dropped. This problem is avoided by responding to such load changes by immediately increasing the RMS output voltage level of the inverter to a level sufficient to minimize the voltage drop and prevent excessive current demands from the PFC load. The output voltage level of the inverter is then gradually reduced from the high voltage level to prevent the occurrence of oscillations due to interactions between the PFC load and the inverter controller. The present invention may be applied to backup power systems employing square wave inverters. In response to load changes, or transitions between line power and inverter operation following the detection of a line power fault, the inverter pulse width is immediately increased to an approximately maximum pulse width. The pulse width is then gradually reduced in increments to the pulse width required to maintain a constant regulated output RMS voltage level on the inverter under steady state conditions.

Claims

1. A method for controlling an inverter to provide an output voltage waveform on an output of the inverter for providing power to a power factor corrected load, comprising the steps of:

(a) controlling the inverter to provide a regulated output voltage waveform at a desired steady state RMS voltage level;

(b) determining the occurrence of a transient load change condition that causes the RMS voltage level of the output voltage waveform to drop suddenly;

(c) responding to the determination of the occurrence of the transient condition by controlling the inverter to rapidly increase the RMS voltage level of the output voltage waveform to a high RMS voltage level; and

(d) controlling the inverter to gradually decrease the RMS voltage level of the output voltage waveform from the high RMS voltage level down to the desired steady state RMS voltage level.

2. The method of claim 1 wherein the step of determining the occurrence of a transient load change condition includes the steps of monitoring the output voltage waveform and determining the occurrence of a transient condition when the voltage level of the output voltage waveform drops by more than a selected amount.

3. The method of claim 1 wherein the step of determining the occurrence of a transient load change condition includes the steps of monitoring a current level on the output of the inverter and determining the occurrence of a transient condition when the current level increases by more than a selected amount.

4. The method of claim 1 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of rapidly increasing the RMS voltage level of the output voltage waveform to approximately the maximum RMS output voltage level of the inverter.

5. The method of claim 1 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of increasing the RMS voltage level of the output voltage waveform to the high RMS voltage level within a time period of one cycle of the output voltage waveform from the determination of the occurrence of the transient condition.

6. The method of claim 1 wherein the step of controlling the inverter to gradually decrease the RMS voltage level of the output voltage waveform includes the step of decreasing the RMS voltage level of the output voltage waveform from the high voltage level to the desired steady state RMS voltage level over a time period greater than one cycle of the output voltage waveform.

7. A method for controlling an inverter to provide a stepped square wave output voltage waveform having an output voltage waveform duty cycle on an output of the inverter for providing power to a power factor corrected load, comprising the steps of:

(a) regulating the output voltage waveform duty cycle to provide a desired steady state RMS voltage level on the output of the inverter;

(b) determining the occurrence of a transient load change condition that causes the RMS voltage level of the output voltage waveform to drop suddenly;

(c) responding to the determination of the occurrence of the transient condition by rapidly increasing the output voltage waveform duty cycle to a long duty cycle; and

(d) gradually reducing the output voltage waveform duty cycle from the long duty cycle to a shorter duty cycle to gradually decrease the RMS voltage level of the output voltage waveform to the desired steady state RMS voltage level.

8. The method of claim 7 wherein the step of determining the occurrence of a transient load change condition includes the steps of monitoring the output voltage waveform and determining the occurrence of a transient condition when the voltage level of the output voltage waveform drops by more than a selected amount.

9. The method of claim 7 wherein the step of determining the occurrence of a transient load change condition includes the steps of monitoring a current level on the output of the inverter and determining the occurrence of a transient condition when the current level increases by more than a selected amount.

10. The method of claim 7 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of rapidly increasing the output voltage waveform duty cycle to a duty cycle that is at least approximately 7/8 of the maximum output voltage waveform duty cycle.

11. The method of claim 7 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of increasing the output voltage waveform duty cycle to the long duty cycle within a time period of one cycle of the output voltage waveform from the determination of the occurrence of the transient condition.

12. The method of claim 7 wherein the step of gradually reducing the output voltage waveform duty cycle includes the step of reducing the output voltage waveform duty cycle from the long duty cycle to the shorter duty cycle over a time period greater than one cycle of the output voltage waveform.

13. The method of claim 7 wherein the step of gradually reducing the output voltage waveform duty cycle includes the step of reducing the output voltage waveform duty cycle at a rate of no more than approximately 1/64 of the maximum output voltage waveform duty cycle for each cycle of the output voltage waveform.

14. The method of claim 7 wherein the step of determining the occurrence of a transient load change condition includes the step of determining the occurrence of a transient load change condition when a change in the output voltage waveform duty cycle of equal to or greater than a selected threshold change amount is required in any one cycle of the output voltage waveform in order to maintain the desired steady state RMS voltage level.

15. The method of claim 14 wherein the threshold change amount is approximately 4/64 of the maximum inverter duty cycle.

16. The method of claim 7 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of rapidly increasing the output voltage waveform duty cycle to approximately the maximum output voltage waveform duty cycle.

17. The method of claim 16 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of rapidly increasing the output voltage waveform duty cycle to approximately 63/64 of the maximum inverter duty cycle.

18. A method of controlling a back up power system for providing power to a power factor corrected load from an AC power system under normal operating conditions and for providing power to the load from an inverter that is controlled to provide an output voltage waveform to the load when the AC power system fails, comprising the steps of:

(a) determining the occurrence of a failure of the AC power system;

(b) responding to the determination of the occurrence of a failure of the AC power system by controlling the inverter to provide an output voltage waveform having a desired steady state RMS output voltage level;

(c) determining the occurrence of a transient load change condition that causes the RMS voltage level of the output voltage waveform to drop suddenly;

(d) responding to the determination of the occurrence of the transient condition by controlling the inverter to rapidly increase the RMS voltage level of the output voltage waveform to a high RMS voltage level; and

(e) controlling the inverter to gradually decrease the RMS voltage level of the output voltage waveform from the high RMS voltage level down to the desired steady state RMS voltage level following the response to the determination of the occurrence of the transient condition.

19. The method of claim 18 comprising additionally the step of responding to the determination of the occurrence of a failure of the AC power system by disconnecting the AC power system from the load.

20. The method of claim 18 wherein the step of responding to the determination of the occurrence of a transient condition includes the step of rapidly increasing the RMS voltage level of the output voltage waveform to approximately the maximum RMS output voltage level of the inverter.

21. A back up power system for providing power to a power factor corrected load from an AC power system under normal operating conditions and for providing power to the load from an inverter when the AC power system fails, comprising:

(a) line fault detection means for determining the occurrence of a failure of the AC power system and for providing a line fault detection signal;

(b) means for controlling the inverter to provide an output voltage waveform having approximately the maximum RMS output voltage level of the inverter in response to the line fault detection signal, thereby providing power to the load when the AC power system fails;

(c) means for controlling the inverter to gradually decrease the RMS voltage level of the output voltage waveform from the approximately maximum RMS voltage level down to a desired steady state RMS voltage level following the response to the line fault detection signal;

(d) means for controlling the inverter to provide a regulated output voltage waveform at the desired steady state RMS voltage level;

(e) transient condition detection means for determining the occurrence of a transient load change condition that causes the RMS voltage level of the output voltage waveform to drop suddenly and for providing a transient condition signal when a load change condition is determined;

(f) means for controlling the inverter to rapidly increase the RMS voltage level of the output voltage waveform to a high RMS voltage level in response to the transient condition signal; and

(g) means for controlling the inverter to gradually decrease the RMS voltage level of the output voltage waveform from the high RMS voltage level down to the desired steady state RMS voltage level following the response to the transient condition signal.

22. The backup power system of claim 21 comprising additionally means for disconnecting the AC power system from the load in response to the line fault detection signal.

23. The backup power system of claim 21 wherein the transient condition detection means includes means for monitoring the output voltage waveform and means for providing the transient condition signal when the voltage level of the output voltage waveform drops by more than a selected amount.

24. The backup power system of claim 21 wherein the transient condition detection means includes means for monitoring an output current of the inverter and means for providing the transient condition signal when the level of the output current of the inverter increases by more than a selected amount.

25. The backup power system of claim 21 wherein the means for controlling the inverter to rapidly increase the RMS voltage level of the output voltage waveform in response to the transient condition signal includes means, for rapidly increasing the RMS voltage level of the output voltage waveform to approximately the maximum RMS output voltage level of the inverter.

26. A back up power system for providing power to a power factor corrected load from an AC power system under normal operating conditions and for providing power to the load from an inverter when the AC power system fails, comprising:

(a) an inverter capable of being controlled to provide a stepped square wave output voltage waveform having an output voltage waveform duty cycle;

(b) line fault detection means for determining the occurrence of a failure of the AC power system and for providing a line fault detection signal;

(c) means for providing an output voltage waveform having approximately a maximum output voltage waveform duty cycle in response to the line fault detection signal, thereby providing power to the load at approximately a maximum RMS voltage level when the AC power system fails;

(d) means for gradually reducing the output voltage waveform duty cycle from the approximately maximum output voltage waveform duty cycle down to a shorter output voltage waveform duty cycle to thereby provide a desired steady state RMS voltage level to the load following the response to the line fault detection signal;

(e) means for regulating the output voltage waveform duty cycle to provide the desired steady state RMS voltage level to the load;

(f) transient condition detection means for determining the occurrence of a transient load change condition that causes the RMS voltage level of the output voltage waveform to drop suddenly and for providing a transient condition signal when a load change condition is determined;

(g) means for rapidly increasing the output voltage waveform duty cycle to a long duty cycle in response to the transient condition signal; and

(h) means for gradually reducing the output voltage waveform duty cycle from the long duty cycle down to a shorter duty cycle to provide the desired steady state RMS voltage level to the load following the response to the transient condition signal.

27. The backup power system of claim 26 comprising additionally means for disconnecting the AC power system from the load in response to the line fault detection signal.

28. The backup power system of claim 26 wherein the transient condition detection means includes means for monitoring the output voltage waveform and means for providing the transient condition signal when the voltage level of the output voltage waveform drops by more than a selected amount.

29. The backup power system of claim 26 wherein the transient condition detection means includes means for monitoring an output current of the inverter and means for providing the transient condition signal when the level of the output current of the inverter increases by more than a selected amount.

30. The backup power system of claim 26 wherein the means for rapidly increasing the output voltage waveform duty cycle in response to the transient condition signal includes means for rapidly increasing the output voltage waveform duty cycle to approximately the maximum output voltage waveform duty cycle.

31. The backup power system of claim 26 wherein the means for rapidly increasing the output voltage waveform duty cycle in response to the transient condition signal includes means for increasing the output voltage waveform duty cycle to a long duty cycle within a time period of one cycle of the output voltage waveform from the determination of the occurrence of the transient condition.

32. The backup power system of claim 26 wherein the means for providing an output voltage waveform having approximately the maximum output voltage waveform duty cycle in response to the line fault detection signal includes means for providing an output voltage waveform having an output voltage waveform duty cycle of at least approximately 7/8 of the maximum output voltage waveform duty cycle.

33. The backup power system of claim 26 wherein the means for gradually reducing the output voltage waveform duty cycle following the response to the line fault detection signal and the means for gradually reducing the output voltage waveform duty cycle following the response to the transient condition signal include means for reducing the output voltage waveform duty cycle at a rate of no more than approximately 1/64 of the maximum output voltage waveform duty cycle for each cycle of the output voltage waveform.

34. The backup power system of claim 26 wherein the transient condition detection means includes means for providing the transient condition signal when a change in the output voltage waveform duty cycle of equal to or greater than a selected threshold change amount is required in any one cycle of the output voltage waveform in order to maintain the desired steady state RMS voltage level.

35. The backup power system of claim 34 wherein the selected threshold change amount is approximately 4/64 of the maximum inverter duty cycle.