Ferroresonance-suppressing static transfer switch

• US 6,560,128 B1
• Filed: 02/12/1999
• Issued: 05/06/2003
• Est. Priority Date: 02/12/1999
• Status: Expired due to Term

• Current Assignee: Perfect Galaxy International Limited
• Original Assignee: SatCon Power Systems Canada Ltd. (Satcon Technology Corporation)
• Inventors: Dewan, Shashi, Rajda, Janos
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US09/248,900
• Time in Patent Office: 1,544 Days
• Field of Search: 363/39, 363/40, 363/50, 363/53
• US Class Current: 363/50
• CPC Class Codes: H02J 3/38 Arrangements for parallely feeding a single network by two or more generators, converters or transformers
• CPC Class Codes: H02J 9/062 for AC powered loads

Abstract

A static transfer switch for reducing ferroresonance in a load consisting of a first switch for connecting and disconnecting a primary power source to and from the load, and a second switch for connecting and disconnecting a secondary power source to and from the load. Each switch includes a snubber circuit connected across the respective switch. An attenuator, consisting of a resistor in series with a capacitor, is connected between the first and second switches and ground and suppresses series ferroresonance at the load by reducing the voltage applied to the load when both the first and second switches are open and power is applied to or removed from the load.

Claims

1. A ferroresonance-suppressing static transfer switch for connecting and disconnecting a first and a second power source to and from a load, the static transfer switch comprising:

switch means configured to couple the first and second power source to the load, the switch means including a switch output for coupling the switch means to the load; and

a capacitive voltage divider coupled to the switch means and configured to attenuate the voltage applied to the load when the switch means is open for suppressing series ferroresonance at the load.

2. The static transfer switch according to claim 1, wherein the voltage divider comprises a resistive element.

3. The static transfer switch according to claim 1, wherein the voltage divider comprises a resistive element in series with a first capacitive element.

4. The static transfer switch according to claim 1, wherein the switch means includes a snubber circuit for limiting a rate of change of voltage drop across the switch means.

5. The static transfer switch according to claim 4, wherein the voltage divider comprises a resistive element in series with a first capacitive element, and the snubber circuit comprises a second capacitive element having a capacitance substantially less than a capacitance of the first capacitive element.

6. The static transfer switch according to claim 5, wherein the capacitance of the first capacitive element is at least twice the capacitance of the second capacitive element.

7. The static transfer switch according to claim 1, wherein the voltage divider is coupled to the switch output.

8. The static transfer switch according to claim 1, wherein each said power source comprises a single-phase AC power source, the load comprises a single-phase inductive load, and the switch means comprises a first switch for coupling the first power source to the load, and a second switch for coupling the second power source to the load.

9. The static transfer switch according to claim 8, wherein each said switch comprises a solid-state switch.

10. The static transfer switch according to claim 9, wherein each said solid-state switch comprises a pair of silicon-controlled rectifier switches connected back-to-back.

11. The static transfer switch according to claim 1, wherein each said power source comprises a three-phase AC power source, the load comprises a three-phase inductive load, and the switch means comprises a first switch for coupling each said first power source phase to a respective one of the load phases, and a second switch for coupling each said second power source phase to a respective one of the load phases.

12. The static transfer switch according to claim 11, wherein each said switch comprises three solid-state switches.

13. The static transfer switch according to claim 12, wherein each said solid-state switch comprises a pair of silicon-controlled rectifier switches connected back-to-back.

14. The static transfer switch according to claim 12, wherein the voltage divider comprises three voltage divider circuits, each said voltage divider circuit being coupled to a respective one of the solid state switches for suppressing the ferroresonance at each said load phase.

15. The static transfer switch according to claim 14, wherein each said voltage divider circuit is coupled between said respective solid-state switch and ground.

16. A ferroresonance-suppressing switching system comprising:

a load transformer;

switch means coupled to the load transformer and configured to couple a first power source and a second power source to the load transformer; and

a capacitive voltage divider coupled to the switch means and the load transformer and configured to attenuate the voltage applied to the load transformer when the switch means is open for suppressing series ferroresonance at the load transformer.

17. The switching system according to claim 16, wherein each said power source comprises a three-phase AC power source, the load transformer comprises a three-phase load transformer, and the switch means comprises a first switch for connecting and disconnecting each said first power source phase to and from a respective one of the load transformer primary phases, and a second switch for connecting and disconnecting each said second power source phase to and from a respective one of the load transformer primary phases.

18. The switching system according to claim 17, wherein the voltage divider comprises three voltage divider circuits, each said voltage divider circuit being coupled to a respective one of the load transformer primary phases.

19. The switching system according to claim 18, wherein each said voltage divider circuit comprises a resistive element in series with a first capacitive element.

20. The switching system according to claim 17, wherein each said switch comprises three solid-state switches.

21. The switching system according to claim 20, wherein each said solid-state switch includes a snubber circuit for limiting a rate of change of voltage drop across the respective solid-state switch.

22. The switching system according to claim 21, wherein the voltage divider comprises three voltage divider circuits, each said voltage divider circuit being coupled to a respective one of the load transformer primary phases and comprising a resistive element in series with a first capacitive element, and each said snubber circuit comprises a second capacitive element having a capacitance substantially less than a capacitance of the first capacitive element.

23. The switching system according to claim 17, wherein each said three-phase AC power source comprises a wye-configured three-phase AC voltage source coupled to ground, and each said voltage divider circuit is coupled between a respective one of the load transformer primary phases and ground.

24. The switching system according to claim 17, wherein the load transformer comprises a delta-wye configured load transformer.

25. A ferroresonance-suppressing static transfer switch for switching a load between a first and a second power source, comprising:

switch means configured to couple the first and second power source to the load, the switch means including a first switch input for coupling the switch means to the first power source, a second switch input for coupling the switch means to the second power source, a switch output for coupling the to the load, a first electronic switch electrically in series with the first switch input and the switch output for transferring power between the first power source and the load, and a second electronic switch electrically in series with the second switch input and the switch output for transferring power between the second power source and the load; and

a capacitive voltage divider coupled to the switch means and comprising a first snubber capacitor disposed in parallel with the first electronic switch, electrically in series with the first switch input and the switch output, a second snubber capacitor disposed in parallel with the second electronic switch, electrically in series with the second switch input and the switch output, and an attenuating capacitor coupled to the switch output, electrically in parallel with the load, the attenuating capacitor having a capacitance which is substantially greater than a capacitance of the snubber capacitors.

26. The static transfer switch according to claim 25, wherein the capacitance of the attenuating capacitor is at least twice the capacitance of the snubber capacitors.

27. A ferroresonance-suppressing switching system comprising:

a load transformer;

switch means coupled to the load transformer and configured to couple a first and a second power source to the load transformer, the switch means including a first switch input for coupling the switch means to the first power source, a second switch input for coupling the switch means to the second power source, a switch output for coupling the switch means to the load transformer, a first electronic switch electrically in series with the first switch input and the switch output for transferring power between the first power source and the load transformer, and a second electronic switch electrically in series with the second switch input and the switch output for transferring power between the second power source and the load transformer; and

a capacitive voltage divider coupled to the switch means and the load transformer and comprising a first snubber capacitor disposed in parallel with the first electronic switch, electrically in series with the first switch input and the switch output, a second snubber capacitor disposed in parallel with the second electronic switch, electrically in series with the second switch input and the switch output, and an attenuating capacitor coupled to the switch output, electrically in parallel with the load transformer, the attenuating capacitor having a capacitance which is substantially greater than a capacitance of the snubber capacitors.

28. The switching system according to claim 27, wherein the capacitance of the attenuating capacitor is at least twice the capacitance of the snubber capacitors.