Reactive power compensator

• US 5,134,356 A
• Filed: 03/07/1991
• Issued: 07/28/1992
• Est. Priority Date: 06/22/1990
• Status: Expired due to Term

• Current Assignee: Board of Regents of The University of Washington (University of Washington)
• Original Assignee: Board of Regents of The University of Washington (University of Washington)
• Inventors: Chen, Mingliang, Huang, Tony, Venkata, Subrahmanyam S., El-Sharkawi, Mohamed A., Andexler, George
• Primary Examiner(s): Stephan, Steven L.
• Assistant Examiner(s): VOELTZ, EMANUEL T
• Application Number:
  US07/665,774
• Time in Patent Office: 509 Days
• Field of Search: 323/208, 323/209, 323/210, 323/211, 323/218
• US Class Current: 323/211
• CPC Class Codes: H02J 3/1864 wherein the stepless control of reactive power is obtained by at least one reactive element connected in series with a semiconductor switch
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]

Abstract

A system and method for determining and providing reactive power compensation for an inductive load. A reactive power compensator (50,50'"'"') monitors the voltage and current flowing through each of three distribution lines (52a, 52b, 52c), which are supplying three-phase power to one or more inductive loads. Using signals indicative of the current on each of these lines when the voltage waveform on the line crosses zero, the reactive power compensator determines a reactive power compensator capacitance that must be connected to the lines to maintain a desired VAR level, power factor, or line voltage. Alternatively, an operator can manually select a specific capacitance for connection to each line, or the capacitance can be selected based on a time schedule. The reactive power compensator produces control signals, which are coupled through optical fibers (102/106) to a switch driver (110, 110'"'"') to select specific compensation capacitors (112) for connections to each line. The switch driver develops triggering signals that are supplied to a plurality of series-connected solid state switches (350), which control charge current in one direction in respect to ground for each compensation capacitor. During each cycle, current flows from ground to charge the capacitors as the voltage on the line begins to go negative from its positive peak value. The triggering signals are applied to gate the solid state switches into a conducting state when the potential on the lines and on the capacitors reaches a negative peak value, thereby minimizing both the potential difference and across the charge current through the switches when they begin to conduct. Any harmonic distortion on the potential and current carried by the lines is filtered out from the current and potential signals used by the reactive power compensator so that it does not affect the determination of the required reactive compensation.

Claims

1. A reactive power compensating system, comprising:

a. an electrical current sensor that produces a current signal corresponding to an instantaneous value of an electrical current supplied through a line to an inductive load, the electrical current generally varying periodically at a fundamental frequency;

b. a voltage sensor that produces a potential signal corresponding to an instantaneous voltage on the line, the potential signal varying according to a generally periodic cycle at the fundamental frequency;

c. filter means, connected to receive the current signal, for producing a filtered current signal from which any harmonics of the fundamental frequency that are present in the current signal are substantially filtered out;

d. processor means, connected to receive the potential signal and the filtered current signal, for determining a reactive power compensation for the inductive load as a function of the filtered current signal when the potential signal is at a predefined point in its periodic cycle and operative to produce a switching signal to select that reactive power compensation;

e. a plurality of capacitors, each selectively connectable to compensates reactance on the line; and

f. switching means, connected to the plurality of capacitors and connected to receive the switching signal from the processor means, for selectively connecting the capacitors to provide the reactive power compensation determined by the processor means in response to the switching signal, the reactive power compensation being substantially free of errors that would otherwise be caused by the harmonics of the fundamental frequency present in the electrical current in the line.

2. The reactive power compensation system of claim 1, wherein the processor means are selectively operated in one of a manual mode, an automatic mode, and a time control mode.

3. The reactive power compensation system of claim 2, wherein while operating in the automatic mode, the processor means are operative to provide reactive power compensation so as to selectively control reactive volt amperes, the voltage of the line, or a power factor associated with the line.

4. The reactive power compensation system of claim 1, wherein the switching means are operative to connect the capacitors to provide the reactive power compensation only when an absolute value of the voltage on the line is at a maximum, as often as twice during each periodic cycle of the voltage.

5. The reactive power compensation system of claim 1, wherein the switching means are operative to connect the capacitors to provide the reactive power compensation only when an electrical current flow through the switching means is initially substantially zero as the switching means connect the capacitors.

6. The reactive power compensation system of claim 1, wherein the filter means are connected to receive the potential signal and are further operative to substantially filter out harmonics of the fundamental frequency that are present from the potential signal, producing a filtered potential signal.

7. The reactive power compensation system of claim 1, wherein the processor means include means for sampling the filtered current signal for a predetermined portion of the periodic cycle prior to the potential signal reaching its redefined point in the periodic cycle.

8. The reactive power compensation system of claim 7, wherein the predefined point in the periodic cycle occurs when the voltage on the line is equal to zero, the processor means being operative to sample and hold the filtered current signal at that instant to determine the reactive power compensation.

9. The reactive power compensation system of claim 1, further comprising feedback means for monitoring the reactive power compensation provided by the capacitors and producing a feedback signal indicative thereof that is supplied to the processor means.

10. The reactive power compensation system of claim 9, wherein the feedback means comprise a plurality of current transformers, each of which is connected to measure a reactive power compensation current provided by one of the capacitors, and in response thereto, producing the feedback signal, the processor means using the feedback signal to determine if the reactive power compensation being supplied by the capacitors is substantially equal to the required reactive power compensation determined by the processor means.

11. The reactive power compensation system of claim 10, wherein the processor means detects a malfunction in the switching means or one of the capacitors in response to the feedback signal failing to match an expected reactive power compensation, and upon detecting a malfunction, selectively causes connection of an alternative capacitor to the line to provide the reactive power compensation using a different configuration of capacitors than used before the malfunction.

12. The reactive power compensation system of claim 10, wherein in response to the potential signal, the processor means protects against an over voltage on the line.

13. The reactive power compensation system of claim 1, further comprising means for extending the time during which the switching signal is provided so that voltage variations on the line do not cause a premature interruption of a charge current flowing to the capacitors through the switching means.

14. The reactive power compensation system of claim 13, wherein the means for extending the time comprise means for producing an envelope of pulses, the pulses occurring at a frequency substantially higher than the fundamental frequency of voltage on the line.

15. A reactive power compensating system for compensating a reactive load drawing power from multiple phases of a supply, comprising:

a. a plurality of electrical current sensors, each of which is connected to sense a generally periodically varying electrical current supplied to the reactive load on a different one of the phases, producing a current signal corresponding to said electrical current;

b. a voltage sensor connected to sense a generally periodically varying voltage on each of the phases, producing a plurality of potential signals, each potential signal corresponding to the voltage on one of the phases;

c. reactive power sensing means, connected to receive the current signal and the potential signal for each phase, for determining a separate reactive power compensation for each phase, each of which can be different, as required to compensate each phase for the reactive load connected to that phase, and further operative to produce switching signals to select the reactive power compensation for each phase;

d. a plurality of capacitors assembled in arrays, each array being associated with one of the phases, each of the capacitors in the arrays being selectively connectable to provide at least a portion of the reactive power compensation required for one of the phases; and

e. switching means, electrically connected to the plurality of capacitors and connected to receive the switching signals for the reactive power sensing means, for selectively connecting the capacitors to provide the separate reactive power compensation independently determined for each phase by the reactive power sensing means, in response to the switching signals.

16. The reactive power compensating system of claim 15, wherein the reactive power sensing means comprise processor means for determining the reactive power compensation for each phase as a function of the current signal at a predetermined point in a cycle of the potential signal and for controlling the switching means using the switching signals to provide the required reactive power compensation determined for each phase at a subsequent time in the cycle.

17. The reactive power compensating system of claim 16, wherein for each phase, the processor means are operative to determine the reactive power compensation as often as twice each cycle of the potential signal, and operative to cause the switching means to connect the capacitors required to provide the reactive power compensation determined as often as twice each cycle.

18. The reactive power compensating system of claim 16, wherein the processor means are selectively operable in one of a manual mode, an automatic mode, and a time control mode.

19. The reactive power compensating system of claim 16, wherein the processor means are selectively operable to determine and to control on each phase one of:

a. a volt-ampere-reactive level;

b. the voltage; and

c. a power factor associated with the phase.

20. The reactive power compensating system of claim 15, wherein the reactive power sensing means comprise filter means, connected to receive the current signal and the potential signal for each phase, for producing a filtered current signal and a filtered potential signal for each phase from which any harmonics of a fundamental frequency are substantially filtered out.

21. The reactive power compensating system of claim 15, wherein the voltage sensor comprises a plurality of transformer primary windings, each of the primary windings being connected to receive a potential corresponding to the voltage on one of the phases, and secondary windings, the secondary windings providing the potential signals for the phases, so that a phasal relationship between the potential signals defines an interval prior to the predetermined point in the cycle of the potential signal during which the current signal is sampled.

22. The reactive power compensating system of claim 16, wherein the voltage sensor comprises a plurality of voltage transformers, each connected to one of the phases and operative to produce one of the potential signals, and wherein the processor means are operative to determine the predetermined point and the subsequent time in the cycle for each phase as a function of both the potential signal and a predefined time interval.

23. The reactive power compensating system of claim 15, wherein the switching means comprise a plurality of solid state switches connected in a plurality of series strings, the series string being activated in response to the switching signal to connect one of the capacitors to provide at least part of the reactive power compensation required for one of the phases.

24. The reactive power compensating system of claim 23, wherein the reactive power sensing means provide the switching signal to the solid state switches to effect the reactive power compensation required for one of the phases only when an initial electrical current through the solid state switches is substantially zero as the solid state switches are activated.

25. The reactive power compensating system of claim 23, wherein the switching means further comprise oscillator means for providing pulses at a frequency substantially higher than that of a fundamental frequency of the voltage on the phases, and wherein the switching signal is in the form of an envelope of the pulses, the pulses in the envelop repetitively gating the solid state switches so that any distortion in the voltage does not cause a premature interruption of a charge current flowing through the solid state switches to the capacitors.

26. The reactive power compensating system of claim 15, further comprising feedback means for sensing the reactive power compensation actually provided by each capacitor and producing feedback signals indicative thereof used to verify proper operation of the system.

27. The reactive power compensating system of claim 26, wherein the reactive power sensing means use the feedback signals to determine if the switching means have malfunctioned, and if so, effect a best available alternative configuration of the capacitors to provide the reactive power compensation.

28. The reactive power compensating system of claim 15, wherein the reactive power sensing means further comprise means for protecting each of the phases against an over voltage condition and against self-excitation by an inductive load, in respect to the reactive power compensation provided by the system.

29. A system for compensating reactive power loads in an alternating current power system having at least one phase, comprising:

a. voltage monitoring means for monitoring the voltage in each phase of the power system;

b. synchronization means for synchronizing to each phase in the power system without regard for any harmonics of a fundamental frequency present, and producing at least one synchronization signal in respect to each of the phases;

c. current monitoring means for monitoring the current in each phase of the power system;

d. triggering means for producing triggering signals, each triggering signal being used to select a specific reactive power compensation;

e. processor means, connected to the voltage monitoring means, the current monitoring means, and the synchronization means for;

i. determining a reactive current in each phase when the voltage in that phase is at a predetermined synchronization level;

ii. determining a reactive power compensation for each phase, based on the reactive current determined in (i); and

iii. controlling production of the triggering signals by the triggering means; and

f. compensation means, responsive to the trigger signals, for applying the reactive power compensation selected by the trigger signals, as determined by the processor means, independently to each phase.

30. The system of claim 29, further comprising means for filtering out harmonic distortion of the current.

31. The system of claim 29, further comprising means for filtering out harmonic distortion of the voltage.

32. The system of claim 29, wherein the synchronization level is independent of any frequency drift of the voltage on the phases.

33. The system of claim 29, wherein the processor means are selectively operable in a manual mode, an automatic mode, or a time control mode.

34. The system of claim 29, wherein the synchronization means comprise a circuit that produces the synchronization signals in response to a phasal relationship between the voltages on the phases, the synchronization signals being used in determining both the reactive power compensation required for each phase and a time when that reactive power compensation is connected to each phase.

35. The system of claim 29, wherein the synchronization means comprise:

a. a plurality of voltage transformers, each connected to one of the phases and operative to produce a potential signal;

b. filter means for filtering out substantially all harmonics of the fundamental frequency, producing a filtered potential signal for each phase; and

c. a zero crossing detector that produces the synchronization signal for each phase as a function of the filtered potential signal.

36. The system of claim 29, wherein the processor means comprise a sample and hold circuit that responds to the synchronization means to being sampling the current in each phase upon the occurrence of one synchronization signal and holds the current sampled when another synchronization signal for that phase occurs.

37. The system of claim 29, wherein the compensation means comprise a plurality of capacitors that are selectively connected to the phase to provide the reactive power compensation determined by the processor means.

38. The system of claim 37, further comprising means for monitoring operation of the compensation means, producing a feedback signal indicating the extent of the reactive power compensation being provided by the compensation means.

39. The system of claim 29, wherein the switching means are operative to protect against a partial failure of the switching means by reconfiguring the compensation means to provide a best available reactive power compensation.

40. The system of claim 29, wherein the switching means are operative to connect the compensation means to the phases in response to the triggering signal indicating that the voltage on the phases has reached an absolute peak value, whereby an electrical current flow through the switching means as the compensation means are connected is initially substantially zero.

41. The system of claim 40, wherein the switching means include means to repetitively retrigger the switching means, so that variations in the voltage on the phase do not affect provision of the compensation.

42. An adaptive reactive power compensation system for compensating reactive power loads in an alternating current power system having at least one phase, comprising:

a. monitoring means for monitoring the voltage in each phase of the power system;

b. synchronization means for synchronizing to the voltage on each phase in the power system, producing a synchronization signal in respect to each of the phases;

c. current monitoring means for monitoring the current in each phase of the power system;

d. triggering means for producing a triggering signal for each phase;

e. processor means, connected to the voltage monitoring means, the current monitoring means, and the synchronization means for;

i. determining a reactive current in each phase when the voltage on that phase is at a predetermined synchronization level;

ii. determining a desired reactive power compensation for each phase a function of the reactive current; and

ii. controlling production of the triggering signal by the triggering means to effect the desired reactive power compensation;

f. a plurality of capacitors, each selectively connected to provide reactive power compensation to one of the phases; and

g. a plurality of switching blocks, each of which is associated with one of the phases, each switching block being connected to the triggering means and responsive to the triggering signal so that an appropriate capacitance can be connected by the switching block to each phase to provide the desired reactive power compensation determined by the processor means, the triggering signal enabling said capacitance to be connected at an appropriate time, each capacitor including means for sampling a charge current flowing to the capacitor and producing a closed loop feedback signal indicative thereof, the processor means using the closed loop feedback signal to verify that the appropriate reactive power compensation is being supplied to each phase.

43. The system of claim 42, wherein the processor means are operative to detect a failure of a component in the system if the closed loop feedback signal differs from an expected value.

44. A method of compensating reactive power, comprising the steps of:

a. producing a current signal corresponding to an electrical current supplied through a line to an inductive load, the electrical current generally varying periodically at a fundamental frequency;

b. sensing a voltage on the line to produce a potential signal that varies in a generally periodic cycle at the fundamental frequency;

c. filtering the current signal to produce a filtered current signal from which any harmonics of the fundamental frequency that are present in the current signal are substantially reduced;

d. determining a reactive power compensation for the inductive load based on the filtered current signal when the potential signal is at a predefined point in its periodic cycle; and

e. selectively connecting an appropriate capacitance to provide the reactive power compensation determined for the line, the reactive power compensation being substantially free of an error that would otherwise be caused by the harmonics of the fundamental frequency present in the electrical current in the line.

45. The method of claim 44, further comprising the step of selectively compensating reactive power in one of a manual mode, an automatic mode, and a time control mode.

46. The method of claim 45, wherein while operating in the automatic mode, the reactive power compensation is provided to selectively control one of:

the voltage of the line, reactive volt amperes caused by the load on the line, and a power factor associated with the line.

47. The method of claim 44, wherein the appropriate capacitance is connected only at an instant in time when an initial charge current for the capacitance is substantially zero.

48. The method of claim 44, further comprising the steps of filtering the potential signal to substantially reduce any harmonics of the fundamental frequency that are present.

49. The method of claim 44, further comprising the steps of sampling the filtered current signal for a predetermined portion of the periodic cycle that is prior to when the potential signal reaches the predefined point in its periodic cycle, and then determining a value for the filtered current signal at that instant to determine the reactive power compensation.

50. The method of claim 44, further comprising the step of monitoring the reactive power compensation provided by the capacitance to determine if the appropriate reactive power compensation is thereby being provided.

51. The method of claim 44, further comprising the steps of detecting a malfunction of the system and providing an alternative configuration of capacitance to achieve the reactive power compensation should a malfunction occur.

52. The method of claim 44, further comprising the step of maintaining the connection of the capacitance to provide the reactive power compensation required, even though transient distortion occurs in the voltage on the line.

53. A method of compensating for a reactive load that draws power from a multi-phase supply, comprising the steps of:

a. sensing a generally periodically varying electrical current on each of the phases, producing a plurality of current signals corresponding thereto;

b. sensing a generally periodically varying voltage on each of the phases, producing a plurality of potential signals corresponding thereto;

c. in a manner unaffected by any harmonic distortion that may be present on the phases, separately determining a reactive power compensation required for each phase to compensate each phase for the reactive load connected to that phase, wherein the reactive power compensation for each phase may be different;

d. assembling a plurality of capacitors in arrays, each array being associated with one of the phases, each of the capacitors in the arrays being selectively connectable to provide at least a portion of the reactive power compensation required for one of the phases; and

3. selectively connecting the capacitors to provide the reactive power compensation determined for each phase.

54. The method of claim 53, wherein the reactive power compensation is determined for each phase as a function of the current signal at a predetermined point in a cycle of the potential signal on the phase, and the capacitors are connected to provide the reactive power compensation when the potential signal reaches a different point in the cycle.

55. The method of claim 53, wherein for each phase, the reactive power compensation is determined as often as twice each cycle of the potential signal on the phase, and the capacitors required to provide the reactive power compensation determined are connected as often as twice each cycle.

56. The method of claim 53, further comprising the step of selectively compensating the reactive power on each phase to control one of:

a. a volt-ampere-reactive level on each phase;

b. The voltage on each phase; and

c. a power factor associated with each phase.

57. The method of claim 53, further comprising the steps of filtering the current signal and the potential signal for each phase, producing a filtered current signal and a filtered potential signal, respectively, from which any harmonics of a fundamental frequency are substantially eliminated.

58. The method of claim 53, further including the step of monitoring the reactive power compensation provided to producing a feedback signal indicative thereof, for each capacitor.

59. The method of claim 58, wherein the feedback signals are used to determine if a malfunction has occurred, and if so, further comprising the step of providing a best available alternative configuration of the capacitors to effect the reactive power compensation.

60. The method of claim 53, further comprising the step of protecting each of the phase against an over voltage condition and against self-excitation by an inductive load, in respect to the reactive power compensation provided by the capacitors.