Reactive power compensating system

• US 4,677,364 A
• Filed: 01/04/1985
• Issued: 06/30/1987
• Est. Priority Date: 01/04/1985
• Status: Expired due to Term

• Current Assignee: University of Washington
• Original Assignee: United States Department of Energy
• Inventors: Williams, Timothy J., Venkata, Subrahmanyam S., El-Sharkawi, Mohamed A.
• Primary Examiner(s): Hickey, R. J.
• Application Number:
  US06/688,675
• Time in Patent Office: 907 Days
• Field of Search: 322/47, 322/29, 322/95, 322/24, 322/25, 322/28, 322/32, 322/89, 322/90, 322/59
• US Class Current: 322/47
• 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: H02P 9/46 Control of asynchronous generator by variation of capacitor
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]

Abstract

The reactive power of an induction machine is compensated by providing fixed capacitors on each phase line for the minimum compensation required, sensing the current on one line at the time its voltage crosses zero to determine the actual compensation required for each phase, and selecting switched capacitors on each line to provide the balance of the compensation required.

Claims

1. A reactive power compensating system for a three phase induction machine comprising

an array of switchable capacitors connected to a power line of each phase,

means for measuring the reactive power drawn by said induction machine,

logic decision means responsive to said measuring means for selecting the capacitors of each array required to compensate for the said reactive power drawn by said induction machine, and

means responsive to said logic decision means for switching the said selected capacitors of each array onto the power line associated with the array

wherein said reactive power measuring means is comprised of means for sensing the current through a power line of only one phase, means for converting the current sensed into a proportional voltage signal, and means for sampling and holding for at least a half cycle said proportional voltage signal when the voltage of said one phase crosses zero.

2. A reactive power compensating system as defined in claim 1 wherein each of said array of capacitors is connected in a configuration that places selected capacitors between the associated phase line and a neutral line with the three arrays thus connected in a wye configuration.

3. In a multiphase power system for an induction machine, apparatus for reactive power compensation comprising

means for sensing and holding the current amplitude of one phase line at the time the voltage of said one phase line crosses zero,

an array of compensation capacitors for each phase,

switching means for connecting each compensation capacitors to a phase,

decision logic means responsive to the current amplitude sampled and held for selecting which of said capacitors should be connected in parallel between each phase line and a neutral line,

means for timing the connection of said compensation capacitors to each of said phase lines such that connection of said capacitors is permitted to occur only at times such that the voltage of said phase is crossing zero, and

means responsive to said decision logic means and said means for timing for activating said switching means such that the compensation capacitors selected by said decision logic means are connected to each phase.

4. Apparatus as defined in claim 3 wherein said decision logic means determines a binary coded word having one bit for each capacitor to be selected, and said compensation capacitors for each phase are sized such that the capacitance each possesses is proportional to the place value of said associated binary bit within said binary coded word, whereby up to n compensation capacitors may be selected in combination to provide 2 levels of compensation.

5. Apparatus as defined in claim 4 wherein said decision logic means for said one phase stores the binary coded word determined for said one phase for use of the remaining phase lines in sequence.

6. Apparatus as defined in claim 5 wherein said multiphase power system is three phase and said timing means for each phase line controls connection of compensation capacitors in successive phases spaced 120°

apart, all in response to sampling and holding one phase from which one binary code word is determined for said one phase, and stored in separate latches for use in control of selecting and connecting compensation capacitors for the other two phases at successive 120° and

240°

phase angles with respect to said one phase.

7. Apparatus as defined in claim 6 wherein said timing means includes means for latching said binary code word for the other two phases during a half cycle following the sample and hold of said one phase, and said timing means includes means for producing timing signals at 120°

and 240°

in respect to said one phase for selecting and connecting said compensation capacitors.

8. Apparatus as defined in claim 5 including a separate fixed compensation capacitor connected to each phase line for the minimum compensation required, whereby said decision logic means is employed to determine only the additional compensation required by each phase.

9. A reactive power compensating system for an n-phase induction machine comprising

an array of switchable capacitors connected to a power line of each phase,

means for measuring the reactive power drawn by said induction machine, and

logic decision means responsive to said measuring means for selecting the capacitors of each array required to compensate for the said reactive power drawn by said induction machine, and means responsive to said logic decision means for switching the said selected capacitors of each array onto the power line associated with the array,

wherein said reactive power measuring means comprises means for sensing the current through a power line of one phase, means for converting the current sensed into a proportional voltage signal, and means for sampling and holding for at least a half cycle said proportional voltage signal when the voltage of said one phase crosses zero.