Microcontroller-based tap changer controller employing half-wave digitization of A.C. signals

• US 5,581,173 A
• Filed: 11/09/1993
• Issued: 12/03/1996
• Est. Priority Date: 01/03/1991
• Status: Expired due to Term

• Current Assignee: Beckwith Electric Company Incorporated (Hubbell, Inc.)
• Original Assignee: Beckwith Electric Company Incorporated (Hubbell, Inc.)
• Inventors: Bryant, Timothy J., Yalla, Murty V. V. S., Harlow, James H., Beckwith, Robert W., Vescovi, David C., Craig, Andrew P.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Krishnan, Aditya
• Application Number:
  US08/152,001
• Time in Patent Office: 1,120 Days
• Field of Search: 323/256, 323/257, 323/216, 323/260, 323/263, 340/646, 340/659, 340/661, 361/82, 364/483, 307/31-32
• US Class Current: 323/257
• CPC Class Codes: G05F 1/147 with motor driven tap switch

Abstract

A microcontroller-based tap-changer controller including apparatus for keeping track of an electrically closed tap position and for automatically changing the tap setting of load tap-changing transformers and regulators; the tap-changer controller further utilizes the "keep-track" tap position to calculate the source voltage of the regulator for reverse power operations; and, a method for paralleling tap-changing transformers and regulators utilizing the circulating current of the units.

Claims

1. Apparatus for keeping track of an electrically closed tap position in tapchanging transformers and regulators having a microprocessor based tapchanger controller;

said tapchanging transformer including a tapchanging motor connectable to a power source, a group of electrically openable and closeable tap positions and a tapchanging mechanism;

said tapchanging motor moving said mechanism in raise and lower directions relative to said tap positions in response to respective raise and lower commands to electrically close a tap position;

a) first and second, normally open, switching means for connecting said power source to said motor,

b) said switching means closing in response to selective raise and lower commands from said controller thereby coupling said power source to said motor to move said mechanism from one tap position to another tap position in the commanded direction,

c) said switching means having a high voltage state appearing thereacross when said power source is not coupled to said motor and a low voltage state appearing thereacross when the power source is coupled to said motor,

d) means for detecting the voltage state across respective switching means as said motor is run, and means for determining the direction of operation of said tapchanging mechanism,

e) a counter contact having at least two operating states in which a change from one operating state to the other operating state indicates movement of said tapchanging mechanism;

f) means for monitoring the number of changes of state of said counter contact and for counting the number of tap-changes; and

g) means for logically combining the direction of tapchanging mechanism operation and the indication of a change in said counter contact operating state,

whereby the apparatus keeps track of the electrically closed tap position.

2. Apparatus as in claim 1 further including means for initializing said tap position identifier.

3. A tapchanger controller as in claim 2 further including a neutral tap position contact input, said neutral tap position contact input closing when the tapchanger is on a neutral position, means for resetting said tap position identifier to neutral whenever said neutral tap position contact closes.

4. Apparatus as in claim 1, further comprising

a) a controller chassis having a front panel and a display means,

b) said front panel including button interface means, and

c) said microprocessor developing a test voltage screen on said display means in response to manipulation of said button interface means to enable a bias voltage to be set to modify the measured voltages at said tap positions,

whereby said tapchanger controller is caused to effectively change said tapchanging mechanism for testing without altering any actual settings of said apparatus.

5. Apparatus for keeping track of an electrically closed tap position in tapchanging transformers and regulators having a microprocessor based tapchanger controller;

said tapchanging transformer including a tapchanging motor connectable to a power source, a group of electrically openable and closeable tap positions and a tapchanging mechanism;

said tapchanging motor moving said mechanism in raise and lower directions relative to said tap positions in response to respective raise and lower commands to electrically close a tap position;

a) raise and lower switches for connecting said source of power to said motor to thereby move said mechanism from one tap position toward a new tap position in response to a command, said switches being normally open and non-conducting,

b) means for providing a unique identifier corresponding to each tap position,

c) first raise and lower opto-isolators responsive to input command parameters to turn on the respective raise and lower switches to couple power to said motor,

d) raise and lower rectifier and filter circuitry connected to said respective raise and lower switches and providing a high voltage state when voltage from said source of power exists across a switch thereby indicating power is not coupled to said motor, and said raise and lower rectifier and filter circuitry providing, within a preset time, a low voltage state when the voltage across the respective raise and lower switch drops to a lower level thereby indicating power is coupled to said motor,

e) second raise and lower sensing opto-isolators, said raise and lower rectifier and filter circuitry being connected to said second respective raise and lower sensing opto-isolators to turn OFF said second sensing opto-isolators when said rectifier and filter circuitry provides a low voltage state thereby detecting that the motor has been energized to move said mechanism,

f) a counter contact having at least two operating states in which a change from one operating state to the other operating state indicates movement of said tapchanging mechanism,

g) means for providing an indication of the state of said counter contact as said mechanism moves from one tap position to another tap position,

h) means for monitoring the ON-OFF states of said second raise and lower sensing opto-isolators to determine the direction of tapchange, and

i) means for logically combining the outputs of said second raise and lower opto-isolators and transition information from said counter contact state to update the initial closed tap position to the new tap position

whereby the identity of each electrically closed tap position is known.

6. A method for keeping track of an electrically closed tap position of a tapchanging system in a transformer and regulator, said system including electrically openable and closeable voltage tap positions, a tapchanging mechanism driven by a tapchanging motor, switching means selectively connecting to said motor to move said mechanism in respective raise and lower directions, a counter contact having two operating states, said method consisting of the steps of:

a) selectively energizing said raise and lower switching means to cause movement of said mechanism from a closed tap position toward a new tap position such that the tapchanging transformer and regulator provides a desired output voltage,

b) detecting a change in voltage across said energized switching means and determining the direction of movement of said tapchanging mechanism by said change in voltage,

c) providing a unique identifier corresponding to each tap position,

d) counting the number of changes of state of said counter contact as said mechanism moves from a closed tap position to a new tap position, and

d) logically combining the direction of tapchanging mechanism movement, and the number of changes of state of said counter contact,

whereby said apparatus keeps track of the closed tap position.

7. A method for operation of a system including an autotransformer wherein power flow can be in a forward or reverse direction, said autotransformer having a tapped series winding comprising a group of openable and closeable tap positions, a microprocessor based controller receiving inputs from a voltage transformer, a motor driven tapchanging switching mechanism connected to said controller, and said controller including an assembly for keeping track of the closed tap positions, said method comprising the following steps:

a) combining a measure of output voltage and a measure of output load current and determining the direction of flow of real power through said autotransformer,

b) reversing the direction of motor response upon determination of a reversal of real power flow through said autotransformer,

c) entering impedance data for said autotransformer into said controller,

d) obtaining the resultant of the following parameters;

said closed tap position, said measure of voltage, said measure of current, said autotransformer impedance data, and said determination of reversal of direction of power flow to ascertain the actual voltage into which said power is flowing, and

e) operating said motor to select a tap position to regulate the output voltage to a pre-selected value.

8. A method as in claim 7 further including the steps of:

a) entering into said controller a first set of voltage and line drop compensation setpoints for use with power flowing in a first direction,

b) entering into said controller a second set of voltage and line drop compensation setpoints for use with power flowing in a second direction,

c) operating said motor for selecting said autotransformer tap positions to regulate the voltage using the respective setpoints for said first and second directions of power flow.

9. A method of paralleling tapchanging transformers and regulators using a circulating current method consisting of the steps of:

a) acquiring samples of the circulating current in said paralleled transformers,

b) performing a discrete Fourier transform on the circulating current samples to obtain circulating current phasors,

c) calculating the magnitude and polarity of said circulating current after scaling,

d) calculating a correction voltage representative of said circulating current from the circulating current phasor and polarity, and

e) moving the tap position in response to said correction voltage in a direction so to minimize the circulating current.

10. A method of operation of a system processing alternating current (AC) electrical power, said system including a tapchanging transformer having a tapped series winding comprising a group of openable and closeable tap positions, a microprocessor based controller for receiving voltage inputs from a voltage transformer, and a unipolar analog to digital converter means having a low and a high voltage reference means for receiving analog alternating current signals at the power frequency, said method consisting of the steps of:

a) obtaining periodic digital samples of said AC signals for one polarity of said AC signals and zeros for a second polarity and thereby forming samples of a modified signal related to said AC signals and conveying said samples to said processor,

b) performing a discrete Fourier transform on said modified AC signals and obtaining real and imaginary phasor components of the fundamental component of said modified signal,

c) obtaining the amplitudes of said modified AC signals and the phase angles between any two said AC signals, and

d) multiplying said amplitudes by two and obtaining the amplitudes of the corresponding unmodified AC signals.