Active current surge limiters with disturbance sensor and multistage current limiting

• US 8,582,262 B2
• Filed: 09/12/2011
• Issued: 11/12/2013
• Est. Priority Date: 01/31/2005
• Status: Active Grant

• Current Assignee: I-EWM Acquisation, LLC
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Divan, Deepakraj Malhar
• Primary Examiner(s): Fureman, Jared
• Assistant Examiner(s): Comber, Kevin J
• Application Number:
  US13/230,190
• Publication Number:
  US 20120063047A1
• Time in Patent Office: 792 Days
• Field of Search: 361/58, 361/93.9
• US Class Current: 361/58
• CPC Class Codes: H02H 3/025 Disconnection after limiting, e.g. when limiting is not sufficient or for facilitating disconnection
• CPC Class Codes: H02H 9/001 limiting speed of change of electric quantities, e.g. soft switching on or off progressive control of electronic switches for eliminating interferences H03K17/16
• CPC Class Codes: H02H 9/026 Current limitation using PTC resistors, i.e. resistors with a large positive temperature coefficient

Abstract

Active current surge limiters and methods of use are disclosed. One exemplary system, among others, comprises a current limiter, including an interface configured to be connected between a power supply and a load; a disturbance sensor, configured to monitor the power supply for a disturbance during operation of the load; and an activator, configured to receive a control signal from the disturbance sensor and to activate the current limiter based on the control signal.

Claims

1. A method for reducing inrush current to an electrical load in response to detection of a disturbance occurring in an input AC power supply coupled to the electrical load, comprising:

applying an input voltage to the electrical load;

monitoring the current drawn by the load;

recording peak values of the current drawn by the load;

continuously monitoring the voltage of the input AC power supply to detect a voltage sag to a predetermined level below rated voltage for short periods of time of one half cycle or more indicating a disturbance in the input AC power supply that would cause an inrush current to the electrical load upon ending of the disturbance;

in response to detection of the voltage sag, recording a peak current limit reference (I_max) corresponding to a maximum peak current value thus far recorded;

detecting when the current level exceeds the peak current limit reference (I_max) as indicating the disturbance occurring in the input AC power supply; and

adding an impedance to the electrical load in response to detecting the disturbance, wherein an inrush current to the electrical load is limited as the input voltage returns to a normal operating level voltage.

2. The method of claim 1, wherein the disturbance is sustained for a duration until the input voltage has returned to a normal operating level voltage, and further comprising the step of removing the impedance from the electrical load after the input voltage has returned to a normal operating level voltage.

3. The method of claim 1, further comprising removing the impedance when the input voltage returns to normal operating level voltage.

4. The method of claim 1, further comprising removing the impedance after a predetermined duration.

5. The method of claim 1, further comprising removing the electrical load when the inrush current exceeds a predetermined value for a predetermined duration.

6. The method of claim 1, further comprising the steps of:

establishing an allowable current limit for the electrical load by sensing and measuring the current drawn by the load at start-up; and

adding the impedance to the electrical load in response to a determination that the inrush current would exceed the allowable current limit upon ending of the disturbance.

7. The method of claim 6, wherein the allowable current limit for the electrical load is established by sensing and monitoring the current drawn by the load at startup, and storing the peak current at start-up as the allowable current limit.

8. The method of claim 6, wherein the allowable current limit at start-up is detected and stored by a peak-rectifier circuit.

9. The method of claim 6, wherein the allowable current limit is measured by an A/D converter associated with a microcontroller that effects the continuous monitoring.

10. The method of claim 9, wherein the starting current representing the allowable current limit is recorded and stored by the microcontroller as a peak inrush current.

11. The method of claim 1, wherein the disturbance comprises a voltage sag below about 90% of rated voltage for a period of time one half cycle or more.

12. The method of claim 1, wherein the step of continuous monitoring of predetermined characteristics of the input AC power supply is effected by a programmed microcomputer.

13. The method of claim 1, wherein the predetermined characteristics of the input AC power supply comprises the voltage, the current, or a combination of both.

14. The method of claim 1, further comprising the step of activating a circuit breaker configured to de-energize the electrical load when an inrush current to the electrical load exceeds a predetermined value for a predetermined duration.

15. The method of claim 1, wherein the monitoring of current drawn by the load is via a current transformer coupled to the input AC power supply and coupled to an A/D converter associated with a programmed microcontroller.

16. The method of claim 1, wherein the monitoring of the voltage of the input AC power supply and detecting the voltage sag is via an A/D converter coupled to a programmed microcontroller.

17. The method of claim 1, further comprising the step of measuring the starting current drawn by the load at start-up.

18. The method of claim 17, further comprising the step of recording and storing the starting current as a peak current value.

19. The method of claim 1, wherein a voltage sag comprises a voltage level of 90% of rated voltage.

20. The method of claim 1, wherein the impedance is a negative temperature coefficient (NTC) resistor.

21. The method of claim 1, wherein the impedance comprises a resistor and a triac.

22. A system for reducing inrush current to an electrical load in response to detection of a disturbance occurring on an input AC power supply coupled to the electrical load, comprising:

a control circuit including a programmed microcontroller configured to detect a disturbance in the input AC power supply during steady-state operation of an electrical load by continuously monitoring the voltage of the input AC power supply and the current drawn by the load and provide a control signal;

a current limiting device configured to be selectively connected between the input AC power supply and the electrical load;

an activator configured to receive the control signal from the control circuit and to insert the current limiting device based on the control signal;

wherein the microcontroller is programmed to record peak values of the current drawn by the load;

wherein the microcontroller is programmed to detect a voltage sag in the input AC power supply to a predetermined level below rated voltage for short periods of time of one half cycle or more indicating a disturbance in the input AC power supply that would cause an inrush current to the electrical load upon ending of the disturbance;

wherein the microcontroller is programmed, in response to detection of the voltage sag, to record a peak current limit reference (I_max) corresponding to a maximum peak current value thus far recorded; and

wherein the microcontroller is programmed to detect when the current level exceeds the peak current limit reference (I_max) as indicating the disturbance occurring in the input AC power supply and to provide the control signal to the activator to insert the current limiting device.

23. The system in claim 22, further comprising a circuit breaker configured to de-energize the electrical load when an inrush current to the electrical load exceeds a predetermined value for a predetermined duration.

24. The system in claim 22, wherein the disturbance is sustained for a duration until the input voltage has returned to a normal operating level voltage, and further comprising the step of removing the impedance from the electrical load after the input voltage has returned to a normal operating level voltage.

25. The system in claim 22, wherein the control circuit comprises a simulated power supply comprising a rectifier, a capacitor and a source impedance.

26. The system in claim 22, wherein the control circuit is a solid-state device.

27. The system in claim 22, wherein the current limiting device is a negative temperature coefficient (NTC) resistor.

28. The system in claim 22, wherein the current limiting device comprises a resistor and a triac.

29. The system in claim 22, further comprising a circuit for establishing an allowable current limit for the electrical load by sensing and measuring the current drawn by the load at start-up;

and

wherein the control circuit is operative to add the current limiting device to the electrical load in response to a determination that the inrush current would exceed the allowable current limit upon ending of the disturbance.

30. The system in claim 29, wherein the allowable current limit for the electrical load is established by sensing and monitoring the current drawn by the load at startup, and storing the peak current at start-up as the allowable current limit.

31. The system in claim 29, further comprising a peak-rectifier circuit for detecting and storing the allowable current limit at start-up.

32. The system in claim 29, further comprising an A/D converter associated with a microcontroller that effects the continuous monitoring and sensing and measuring the allowable current limit.

33. The system in claim 32, wherein the starting current representing the allowable current limit is recorded and stored by the microcontroller as a peak inrush current.

34. The system in claim 22, wherein the disturbance comprises a voltage sag below about 90% of rated voltage for a period of time one half cycle or more.

35. The system in claim 22, wherein the predetermined characteristics of the input AC power supply comprises the voltage, the current, or a combination of both.

36. The system in claim 22, further comprising a circuit breaker configured to de-energize the electrical load when an inrush current to the electrical load exceeds a predetermined value for a predetermined duration as determined by the control circuit.

37. The system of claim 22, wherein the monitoring of current drawn by the load is via a current transformer coupled to the input AC power supply and coupled to an A/D converter associated with the programmed microcontroller.

38. The system of claim 22, wherein the monitoring of the voltage of the input AC power supply and detecting the voltage sag is via an A/D converter coupled to the programmed microcontroller.

39. The system of claim 22, wherein the microcontroller is programmed to measure the starting current drawn by the load at start-up.

40. The system of claim 39, wherein the microcontroller is programmed to record and store the starting current as a peak current value.

41. A multi-step active current surge limiting apparatus for reduction of inrush current to an electrical load in response to detection of a disturbance in an input AC power supply coupled to the electrical load, comprising:

a current-limiting circuit coupled between the input power supply and the electrical load, the current-limiting circuit comprising;

(a) a current limiting impedance comprising (i) first resistor, (ii) a second resistor in series with the first resistor, and (iii) a switch in parallel with the second resistor that is actuated by a first control signal to bypass the second resistor, and

(b) a relay in parallel with the current limiting impedance and having contacts that are in an open position to couple the input AC power supply to the electrical load through the current limiting impedance and close in response to a second control signal to couple the input AC power supply to the electrical load and bypass the current limiting impedance; and

a control circuit that provides the first control signal and the second control signal to provide multiple levels of impedance to the inrush of current to the electrical load in response to predetermined conditions.

42. The apparatus of claim 41, wherein the control circuit comprises a programmed microprocessor connected to receive signals representing the voltage of the input power supply provided from an analog-to-digital (A/D) converter, the microprocessor programmed to identify when the voltage falls outside a nominally acceptable boundary defined by a preset limit, and to provide the first control signal and the second control signal.

43. The apparatus of claim 41, wherein the control circuit comprises a programmed microprocessor connected to receive signals representing the voltage of the input power supply provided from an analog-to-digital (A/D) converter, the microprocessor providing the first control signal to cause the relay to open and then close after a predetermined time after the voltage returns to a normal level.

44. The apparatus of claim 41, wherein the disturbance in the input power supply is a voltage disturbance, a current disturbance, or a combination thereof.

45. The apparatus of claim 41, wherein the switch is a semiconductor switch.

46. The apparatus of claim 45, wherein the semiconductor switch is selected from the group comprising a thyristor and a triac.

47. The apparatus of claim 41, wherein the switch is in the open position and the relay is in the open position at start-up so that the first resistor and the second resistor provide maximum impedance.

48. The apparatus of claim 41, wherein the switch is in the closed position and the relay is in the open position so that only the first resistor provides a current limiting impedance.

49. The apparatus of claim 41, wherein after a start-up condition where the switch and the relay are open so that the first resistor and the second resistor provide a current limiting impedance, the control circuit closes the switch to allow a greater current to flow and thereafter closes the relay to bypass the current-limiting circuit.

50. The apparatus of claim 41, further comprising a selectively actuatable circuit breaker coupled prior to the current-limiting circuit that is responsive to a trip signal to provide further protection for the load and for the apparatus if the sensed current remains excessive for a predetermined period.

51. The apparatus of claim 41, wherein the control circuit is operative to turn on the switch at a predetermined first phase angle (α

) in the cycle of the input AC power supply so as to allow additional current to flow to the load, and is operative to turn off the switch as a predetermined second phase angle (β

) in the cycle of the input AC power supply.

52. The apparatus of claim 51, wherein the predetermined first phase angle (α

) in the cycle of the input AC power supply is determined when the line voltage V_line exceeds the effective DC bus voltage V_dc at the load.

53. The apparatus of claim 52, wherein the predetermined second phase angle (β

) in the cycle of the input AC power supply is determined when the line voltage V_line is about equal to the effective DC bus voltage V_dc at the load.

54. The apparatus of claim 41, wherein the switch is controlled by the control circuit so as to control the average current applied to the load capacitance and minimize recovery time.

55. The apparatus of claim 51, wherein the predetermined first phase angle (α

) in the cycle of the input AC power supply is automatically changed by the control circuit to maintain limited line current as a function of capacitor charging.

56. The apparatus of claim 41, wherein the resistors in the current-limiting circuit only operate during transients.

57. The apparatus of claim 41, wherein the apparatus provides a soft start process for equipment without built-in startup protection, by (a) initially coupling the first resistor and the second resistor to the equipment at startup, (b) sensing the inrush current with a current sensor, (c) turning on the switch when the current is at an allowable level or after a preset time period to allow additional current to flow, and (d) closing the relay after the current level again subsides or after a second preset time period.