Power conversion and voltage sag correction with regenerative loads

• US 7,158,393 B2
• Filed: 03/11/2005
• Issued: 01/02/2007
• Est. Priority Date: 03/11/2005
• Status: Active Grant

• Current Assignee: Rockwell Automation, Inc.
• Original Assignee: Soft Switching Technologies Corp. (Rockwell Automation, Inc.)
• Inventors: Schneider, Robert S.
• Primary Examiner(s): Vo; Tuyet
• Assistant Examiner(s): LE, TUNG X
• Application Number:
  US11/079,007
• Publication Number:
  US 20060202636A1
• Time in Patent Office: 662 Days
• Field of Search: 363 34- 40, 363/55
• US Class Current: 363/34
• CPC Class Codes: H02J 3/1842 wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
• CPC Class Codes: H02J 9/062 for AC powered loads
• CPC Class Codes: Y02E 40/20 Active power filtering [APF]

Abstract

In dynamic voltage sag correctors and other power conversion equipment having a DC bus that is subject to over-voltage conditions due to power being fed back by a regenerative load, a bus discharge switching device and a discharge resistor are connected in series across the DC bus lines. The discharge switching devices are switched on to discharge the energy storage device connected to the bus, or to separately discharge the two capacitors of a split capacitor DC bus, to eliminate the over-voltage conditions. The discharge switching devices may be switched on and off periodically with a selected duty cycle to discharge energy from the energy storage capacitor or other energy storage device through the discharge resistor at a rate which does not exceed the power rating of the discharge resistor. The discharge switching device may be switched with a fixed duty cycle or with a variable duty cycle that is based on the dynamic power dissipation characteristics of the discharge resistor.

Claims

1. A power conversion module comprising:

(a) a rectifier having rectifying devices connected to receive AC input power from AC input lines and to provide DC power on high and low DC bus lines;

(b) first and second energy storage capacitors connected in series across the DC bus lines, the connection between the capacitors defining a common node, a DC bus common conducting line connected to the common node;

(c) an inverter having inverter switching devices connected to the DC bus lines to receive DC power therefrom, the inverter switching devices switchable to provide AC output power;

(d) a first bus discharge switching device and a discharge resistor connected in series across the high DC bus line and the common line and connected in parallel with the first energy storage capacitor, and a second bus discharge switching device and a discharge resistor connected in series between the common line and the low DC bus line and connected in parallel with the second energy storage capacitor; and

(e) a DC bus control circuit connected to the DC bus lines and the common line to receive the voltage therefrom and connected to the first and second bus discharge switching devices to selectively turn the first discharge switching device on for a period of time when the voltage between the high DC bus line and the common line exceeds a selected limit and to turn the second discharge switching device on for a period of time when the voltage between the low DC bus line and the common line exceeds a selected limit, to thereby discharge the energy storage capacitors to reduce the voltage thereon.

2. The power conversion module of claim 1 wherein the rectifier comprises a pair of diodes connected together in series across the DC bus lines with a node connected therebetween which is connected to an AC input line, one of the diodes connected to conduct into the node and the other diode connected to conduct away from the node, the diodes connected across the DC bus lines to provide DC power thereto;

and wherein the inverter comprises a pair of controllable inverter switching devices connected in series across the DC bus lines and having an output node formed at the connection between the two inverter switching devices, the output node connected to one AC output line.

3. The power conversion module of claim 2 wherein the controllable inverter switching devices comprise IGBTs with anti-parallel connected diodes.

4. The power conversion module of claim 1 further including a thyristor connected in parallel with each of the discharge switching devices and a control circuit connected to the gate of each thyristor to switch the thyristors into conduction when the power conversion module is turned off to fully discharge the energy storage capacitors after turn-off of the module.

5. The power conversion module of claim 1 further including a snubber circuit connected to each discharge switching device, each snubber circuit including a diode and capacitor connected in parallel with the discharge switching device and a resistor connected between a DC bus line and the connection between the snubber diode and capacitor.

6. The power conversion module of claim 1 wherein the control circuit controls the switching of each discharge switching device during an over voltage condition between the DC bus line and the common line to which the switching device is connected to switch on and off periodically with a selected duty cycle to discharge energy from the energy storage capacitor through the discharge resistor at a rate which does not exceed the power rating of the discharge resistor.

7. The power conversion module of claim 6 wherein the control circuit switches the discharge switching device on and off during an over voltage condition with a fixed duty cycle.

8. The power conversion module of claim 6 wherein the control circuit controls the switching of the discharge switching device during an over voltage condition utilizing a variable duty cycle and period based on the dynamic power dissipation characteristics of the discharge resistor.

9. The power conversion module of claim 8 wherein the variable duty cycle and period are determined by the temperature of a model of the discharge resistor such that the discharge switching device is turned off when the model of the discharge resistor indicates that a high set point temperature has been reached, and is turned on when the model of the discharge resistor indicates that a lower set point temperature has been reached.

10. The power conversion module of claim 1 wherein each bus discharge switching device comprises multiple IGBTs connected in parallel and controlled by the control circuit to switch in common.

11. A dynamic voltage sag corrector comprising

(a) AC input and output terminals;

(b) a bypass switch connected between the input terminals and the output terminals and responsive to bypass switch control signals to open and close the bypass switch;

(c) a power conversion module connected in parallel with the bypass switch between the input terminals and the output terminals, the power conversion module comprising;

(i) a rectifier having rectifying devices connected to receive AC input power from the input terminals and to provide DC power on high and low DC bus lines;

(ii) first and second energy storage capacitors connected in series across the DC bus lines, the connection between the capacitors defining a common node, a DC bus common conducting line connected to the common node;

(iii) an inverter having inverter switching devices connected to the DC bus lines to receive DC power therefrom, the inverter switching devices switchable to provide AC output power to the output terminals;

(iv) a first bus discharge switching device and a discharge resistor connected in series across the high DC bus line and the common line and connected in parallel with the first energy storage capacitor, and a second bus discharge switching device and a discharge resistor connected in series between the common line and the low DC bus line and connected in parallel with the second energy storage capacitor; and

(v) a DC bus control circuit connected to the DC bus lines and the common line to receive the voltage therefrom and connected to the first and second bus discharge switching devices to selectively turn the first discharge switching device on for a period of time when the voltage between the high DC bus line and the common line exceeds a selected limit and to turn the second discharge switching device on for a period of time when the voltage between the low DC bus line and the common line exceeds a selected limit, to thereby discharge the energy storage capacitors to reduce the voltage thereon; and

(d) a system control circuit that receives the AC input voltage from the input terminals and determines a voltage sag condition when the AC line voltage drops below a selected low line voltage threshold level, and then provides a bypass switch control signal to the bypass switch to open the bypass switch in response to the detection of a voltage sag condition and controls the inverter of the power conversion module to provide an AC output voltage at the output terminals at approximately a pre-voltage sag condition level.

12. The sag corrector of claim 11 wherein the rectifier comprises a pair of diodes connected together in series across the DC bus lines with a node connected therebetween which is connected to an AC input terminal, one of the diodes connected to conduct into the node and the other diode connected to conduct away from the node, the diodes connected across the DC bus lines to provide DC power thereto;

and wherein the inverter comprises a pair of controllable inverter switching devices connected in series across the DC bus lines and having an output node formed at the connection between the two inverter switching devices, the output node connected to one AC output terminal.

13. The sag corrector of claim 11 wherein the controllable inverter switching devices comprise IGBTs with anti-parallel connected diodes.

14. The sag corrector of claim 11 further including a thyristor connected in parallel with each of the discharge switching devices, and a control circuit connected to the gate of each thyristor to switch the thyristors into conduction when the sag corrector is turned off to fully discharge the energy storage capacitors after turn-off of the sag corrector.

15. The sag corrector of claim 11 further including a snubber circuit connected to each discharge switching device, each snubber circuit including a diode and capacitor connected in parallel with the discharge switching device and a resistor connected between a DC bus line and the connection between the snubber diode and capacitor.

16. The sag corrector of claim 11 wherein the DC bus control circuit controls the switching of each bus discharge switching device during an over voltage condition between the DC bus lines and the common line to which the switching device is connected to switch on and off periodically with a selected duty cycle to discharge energy from the energy storage capacitor through the discharge resistor at a rate which does not exceed the power rating of the discharge resistor.

17. The sag corrector of claim 16 wherein the control circuit switches the discharge switching devices on and off during an over voltage condition with a fixed duty cycle.

18. The sag corrector of claim 16 wherein the control circuit controls the switching of the discharge switching device during an over voltage condition utilizing a variable duty cycle and period based on the dynamic power dissipation characteristics of the discharge resistor.

19. The sag corrector of claim 18 wherein the variable duty cycle and period are determined by the temperature of a model of the discharge resistor such that the discharge switching device is turned off when the model of the discharge resistor indicates that a high set point temperature has been reached, and is turned on when the model of the discharge resistor indicates that a lower set point temperature has been reached.

20. The sag corrector of claim 11 wherein each bus discharge switching devices comprises multiple IGBTs connected in parallel and controlled by the DC bus control circuit to switch in common.

21. A method of controlling the voltage on DC bus lines of a power conversion system having a rectifier supplying DC power to the DC bus lines and an inverter receiving the DC power from the DC bus lines and providing AC output power, wherein an energy storage device is connected across the DC bus lines to receive power from or supply power to the DC bus lines such that an over-voltage can be imposed on the energy storage device during over-voltage transients passed through the rectifier or from regenerative power fed back from the inverter to the DC bus lines, comprising:

(a) connecting a bus discharge switching device and a discharge resistor in series across the DC bus lines and in parallel with the energy storage device;

(b) detecting when an over-voltage condition occurs on the DC bus lines;

(c) during the over-voltage condition, controlling the switching of the discharge switching device to switch on and off periodically utilizing a variable duty cycle and period based on the dynamic power dissipation characteristics of the discharge resistor.

22. The method of claim 21 wherein the variable duty cycle and period are determined by the temperature of a model of the discharge resistor such that the discharge switching device is turned off when the model of the discharge resistor indicates that a high set point temperature has been reached, and is turned on when the model of the discharge resistor indicates that a lower set point temperature has been reached.

23. A power conversion module comprising:

(a) DC bus lines;

(b) a rectifier supplying DC power to the DC bus lines;

(c) an inverter having controllable switching devices receiving the DC power from the DC bus lines and providing AC output power;

(d) an energy storage device connected across the DC bus lines to receive power from or supply power to the DC bus lines such that an over-voltage can be imposed on the energy storage device during over-voltage transients passed through the rectifier or from regenerative power fed back from the inverter to the DC bus lines;

(e) a bus discharge switching device and a discharge resistor connected in series across the DC bus lines and in parallel with the energy storage device; and

(f) a DC bus control circuit connected to the DC bus lines to receive the voltage therefrom and connected to the bus discharge switching device to detect when an over-voltage condition occurs on the DC bus lines, the control circuit configured to control the switching of the discharge switching device during the over-voltage condition, to switch on and off periodically utilizing a variable duty cycle and period based on the dynamic power dissipation characteristics of the discharge resistor.

24. The power conversion module of claim 23 wherein the control circuit determines the variable duty cycle and period by the temperature of a model of the discharge resistor configured in the control circuit such that the discharge switching device is turned off when the model of the discharge resistor indicates that a high set point temperature has been reached, and is turned on when the model of the discharge resistor indicates that a lower set point temperature has been reached.

25. The power conversion module of claim 23 wherein the energy storage device is a capacitor.

26. The power conversion module of claim 23 further including a snubber circuit connected to the discharge switching device, the snubber circuit including a diode and capacitor connected in parallel with the discharge switching device and a resistor connected between a DC bus line and the connection between the snubber diode and capacitor.