U.S. patent application number 09/789875 was filed with the patent office on 2001-07-19 for method of determining contact wear in a trip unit.
Invention is credited to Andersen, Bo L..
Application Number | 20010008541 09/789875 |
Document ID | / |
Family ID | 22829819 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008541 |
Kind Code |
A1 |
Andersen, Bo L. |
July 19, 2001 |
Method of determining contact wear in a trip unit
Abstract
A method of determining contact wear in a trip unit of a circuit
breaker is presented. The trip unit includes a microcontroller and
associated memories. An algorithm (program) stored in a memory of
the trip unit measures temperatures relative to circuit breaker
contacts and cumulative energy dissipated in the breaker contacts,
and utilizes them in a variety of analysis techniques within the
trip unit to determine contact wear. These techniques include, by
way of example, differential temperature analysis, measurement of
cumulative energy dissipated in the breaker contacts, and
calculated contact wear using sampled electrical currents and
voltage and Ohm's law.
Inventors: |
Andersen, Bo L.;
(Burlington, CT) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
|
Family ID: |
22829819 |
Appl. No.: |
09/789875 |
Filed: |
February 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09789875 |
Feb 21, 2001 |
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09221884 |
Dec 28, 1998 |
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6231227 |
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Current U.S.
Class: |
374/45 ;
374/152 |
Current CPC
Class: |
H01H 2071/044 20130101;
H01H 2011/0068 20130101; H01H 71/04 20130101; H01H 1/0015
20130101 |
Class at
Publication: |
374/45 ;
374/152 |
International
Class: |
G01K 013/00; G01K
001/14; G01N 025/00 |
Claims
What is claimed is:
1. A method of detecting contact wear, at an electronic trip unit,
of at least two pairs of separable contacts of a circuit breaker,
the method comprising: sensing temperature relative to a first pair
of said at least two pairs of separable contacts to provide a first
sensed contact temperature signal indicative thereof; sensing
temperature relative to a second pair of said at least two pairs of
separable contacts to provide a second sensed contact temperature
signal indicative thereof; comparing said first and second sensed
contact temperature signals to provide a differential contact
temperature signal; and analyzing said differential contact
temperature signal to assess contact wear of said pairs of
separable contacts.
2. The method of claim 1 further comprising: sensing ambient
temperature of said circuit breaker to provide a sensed ambient
temperature signal indicative thereof; comparing at least one of
said first and second sensed contact temperature signals with said
sensed ambient temperature signal to provide at least one
differential ambient temperature signal; and wherein said analyzing
further comprises analyzing said differential contact temperature
signal and said at least one said differential ambient temperature
signal to assess contact wear of said pairs of separable
contacts.
3. The method of claim 1 wherein said analyzing comprises
statistical standard deviation analyzing of said differential
contact temperature signal relative to a mean differential
temperature.
4. The method of claim 1 wherein said analyzing comprises comparing
said differential contact temperature signal to a limit.
5. The method of claim 1 wherein said analyzing comprises
predicting a contact resistance of at least one of said pairs of
separable contacts in response to said differential contact
temperature signal and comparing said predicted contact resistance
to a limit.
6. The method of claim 5 wherein said predicting comprises modeling
resistance of said at least one of said pairs of separable contacts
and applying said differential contact temperature signal to said
modeling to provide said predicted contact resistance.
7. The method of claim 1 further comprising: displaying information
indicative of contact wear of said separable contacts in response
to said analyzing.
8. A breaker assembly comprising an electronic trip unit and a
circuit breaker having at least two pairs of separable contacts,
said breaker assembly further comprising: a sensor positioned for
sensing temperature relative to a first pair of said at least two
pairs of separable contacts to provide a first sensed contact
temperature signal indicative thereof; a sensor positioned for
sensing temperature relative to a second pair of said at least two
pairs of separable contacts to provide a second sensed contact
temperature signal indicative thereof; and a signal processor
responsive to said first sensed contact temperature signal and said
second sensed contact temperature signal, and having memory for
storing signals including program signals defining an executable
program which compares said first and second sensed contact
temperature signals to provide a differential contact temperature
signal, and analyzes said differential contact temperature signal
to assess contact wear of said at least two pairs of separable
contacts.
9. The breaker assembly of claim 8 further comprising: a sensor
positioned for sensing ambient temperature of said circuit breaker
to provide a sensed ambient temperature signal indicative thereof;
wherein said processor further compares at least one of said first
and second sensed contact temperature signals with said sensed
ambient temperature signal to provide at least one differential
ambient temperature signal, and said processor further analyzes
said differential contact temperature and said at least one
differential ambient temperature signal to assess contact wear of
said at least two pairs of separable contacts.
10. The breaker assembly of claim 8 wherein said processor analyzes
said differential contact temperature signal utilizing statistical
standard deviation relative to a mean differential temperature.
11. The breaker assembly of claim 8 wherein said processor further
compares said differential contact temperature signal to a
limit.
12. The breaker assembly of claim 8 wherein said processor further
predicts a contact resistance of at least one of said pairs of
separable contacts in response to said differential contact
temperature signal to result in a predicted contact resistance and
compares said predicted contact resistance to a limit.
13. The breaker assembly of claim 12 wherein said processor models
resistance of at least one of said two pairs of separable contacts
and applies said differential contact temperature signal to provide
said predicted contact resistance.
14. The breaker assembly of claim 8 further comprising: a display
for displaying information indicative of contact wear of said
contacts.
15. A method of detecting contact wear, at an electronic trip unit,
of at least one pair of separable contacts of a circuit breaker,
comprising: sensing temperature relative to said at least one pair
of contacts to provide a first sensed contact temperature signal
indicative thereof; and processing said first sensed contact
temperature signal in accordance with a relationship of temperature
and contact wear to assess contact wear of said contacts.
16. The method of claim 15 wherein said at least one pair of said
separable contacts of said circuit breaker comprises at least two
pairs of separable contacts, said method further comprising:
sensing temperature relative to the other one of said pairs of
contacts to provide a second sensed contact temperature signal
indicative thereof; and wherein said processing comprises,
comparing said first and second sensed contact temperature signals
to provide a differential contact temperature signal, and analyzing
said differential contact temperature signal to assess contact wear
of said pairs of contacts.
17. The method of claim 15 further comprising: displaying
information indicative of contact wear of said contacts in response
to said processing.
18. The method of claim 16 wherein said analyzing comprises
statistical standard deviation analyzing of said differential
contact temperature signal relative to a mean differential
temperature.
19. The method of claim 16 wherein said analyzing comprises
comparing said differential contact temperature signal to a
limit.
20. A breaker assembly comprising an electronic trip unit and a
circuit breaker having at least two pairs of separable contacts,
said breaker assembly further comprising: means for sensing
temperature relative to a first pair of said at least two pairs of
separable contacts to provide a first sensed contact temperature
signal indicative thereof; means for sensing temperature relative
to a second pair of said at least two pairs of separable contacts
to provide a second sensed contact temperature signal indicative
thereof; and means for storing signals including program signals
defining an executable program which compares said first and second
sensed contact temperature signals to provide a differential
contact temperature signal, and analyzes said differential contact
temperature signal to assess contact wear of said at least two
pairs of separable contacts.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/221,884 filed Dec. 28, 1998, which is incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to electronic trip
units. More specifically, the present invention relates to a method
of determining contact wear of a circuit breaker at an electronic
trip unit.
[0003] Electronic trip units (trip units) are well known. An
electronic trip unit typically comprises voltage and current
sensors which provide analog signals indicative of the power line
signals. The analog signals are converted by an A/D
(analog/digital) converter to digital signals which are processed
by a microcontroller. The trip unit further includes RAM (random
access memory), ROM (read only memory) and EEPROM (electronic
erasable programmable read only memory) all of which interface with
the microcontroller. The ROM includes trip unit application code,
e.g., main functionality firmware, including initializing
parameters, and boot code. The EEPROM includes operational
parameters for the application code. An output of the electronic
trip unit actuates a circuit breaker. The circuit breaker typically
includes a pair of contacts which allows circuit current to pass
from one contact member to another contact member. When the
contacts open, circuit current is prevented from flowing from one
contact member to the other and therefore, circuit current is
prevented from flowing to a load which is connected to the
breaker.
[0004] Circuit breaker contact wear, is a frequently occurring yet
difficult to measure or predict problem because it is affected by a
variety of factors. Contact wear is affected by the cumulative
energy dissipated through arcing as breakers are opened. However, a
single severe over-current fault can destroy contacts more quickly
than several smaller faults, even though the smaller faults may add
up to the same total energy dissipated. For example, some types of
faults have more severe affects on contact wear than others, ground
faults will destroy contacts more quickly than manual openings.
Contacts are not generally easily inspected without costly
disassembly and power down. However, if not detected contact wear
may result in loss of power. The only current solution to this is
defensive preventative maintenance whether required or not.
BRIEF SUMMARY OF THE INVENTION
[0005] The above discussed and other drawbacks and deficiencies are
overcome or alleviated by a method of detecting contact wear, at an
electronic trip unit, of at least two pairs of separable contacts
of a circuit breaker, the method comprising sensing temperature
relative to a first pair of said at least two pairs of separable
contacts to provide a first sensed contact temperature signal
indicative thereof, sensing temperature relative to a second pair
of said at least two pairs of separable contacts to provide a
second sensed contact temperature signal indicative thereof,
comparing said first and second sensed contact temperature signals
to provide a differential contact temperature signal, and analyzing
said differential contact temperature signal to assess contact wear
of said pairs of separable contacts.
[0006] The above discussed and other drawbacks and deficiencies are
also overcome or alleviated by a breaker assembly comprising an
electronic trip unit and a circuit breaker having at least two
pairs of separable contacts, said breaker assembly further
comprising a sensor positioned for sensing temperature relative to
a first pair of said at least two pairs of separable contacts to
provide a first sensed contact temperature signal indicative
thereof, a sensor positioned for sensing temperature relative to a
second pair of said at least two pairs of separable contacts to
provide a second sensed contact temperature signal indicative
thereof, and a signal processor responsive to said first sensed
contact temperature signal and said second sensed contact
temperature signal, and having memory for storing signals including
program signals defining an executable program which compares said
first and second sensed contact temperature signals to provide a
differential contact temperature signal, and analyzes said
differential contact temperature signal to assess contact wear of
said at least two pairs of separable contacts.
[0007] The above discussed and other features and additional
advantages of the present invention will be appreciated and
understood by those skilled in the art from the detailed
description and drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Referring now to the drawing wherein the FIGURE is a
schematic block diagram of an electronic trip unit of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0009] It is therefore seen to be desirable to detect contact wear
in an electronic trip unit. In a preferred embodiment of the
present invention, a contact wear detection algorithm (program) is
initialized in the microcontroller of the trip unit for detecting
contact wear. The contact wear detection algorithm (1) measures
temperatures of arcs in close proximity to circuit breaker
contacts, and/or (2) calculates and stores cumulative energy
dissipated in the breaker contacts as a result of open and close
operations. A variety of analysis techniques are utilized within
the trip unit to determine contact wear. An accurate assessment of
contact wear is yielded by these methods, separately or in
combination.
[0010] The electronic trip unit of the present invention comprising
voltage, current, and temperature sensors which provide analog
signals indicative of the power line signals, contact temperatures,
and ambient temperatures. The analog signals are converted by an
A/D (analog/digital) converter to digital signals which are
processed by a microcontroller. The trip unit further includes RAM
(random access memory), ROM (read only memory) and EEPROM
(electronic erasable programmable read only memory) all of which
communicate with the microcontroller. The ROM includes trip unit
application code, e.g., main functionality firmware, including
initializing parameters, and boot code. The application code
includes code for the contact wear detection algorithm of the
present invention. The EEPROM includes operational parameters,
e.g., code for setting user defined thresholds for the contact wear
detection algorithm for the application code. These parameters may
be stored in the trip unit at the factory and are selected to meet
customers' requirements, but can also be remotely downloaded.
[0011] Temperature and electrical analysis is used to develop
thermodynamic and electrical models of frame geometries of circuit
breakers. These models provide the contact wear algorithm with the
nominal operating parameters required to predict contact resistance
and heat rise over ambient temperatures as a function of current
flow through the breakers as the contacts wear. Alarms can be
generated when (1) contact heat rise over ambient temperature
deviates from stored nominal values, or (2) when calculated contact
resistance (R=V/I phase corrected) deviates from stored specified
maximum values. Thereby indicating that maintenance or replacement
of the breaker is required due to contact wear.
[0012] The frame geometry of a circuit breakers may affect the rate
at which heat is thermodynamically conducted away from the circuit
breaker contacts and are modeled or experimentally determined for
each model of breaker at rated current ranges. As contact wear
resistance increases the temperature across the contacts during
closed operation of the circuit breaker will increase with the
contacts acting as electrical resistors dissipating electric energy
as heat. This in turn has an accelerating affect on the rate of
wear of the contacts. If undetected this will eventually lead to
the mechanical and/or electrical failure of the breakers leading to
a power outage.
[0013] Referring to the FIGURE, a general schematic of an
electronic trip unit of the present invention is generally shown at
30. Trip unit 30 comprises a voltage sensor or sensors 32 which
provides analog signals indicative of voltage measurements on a
signal line 34 and a current sensor or sensors 36 which provides
analog signals indicative of a current measurements on a signal
line 38. The analog signals on lines 34 and 38 are presented to an
A/D (analog/digital) converter 40, which converts these analog
signals to digital signals. The digital signals are transferred
over a bus 42 to a microcontroller (signal processor) 44, such
being commercially available from the Hitachi Electronics
Components Group (Hitachis H8/300 family of microcontrollers). Trip
unit 30 further includes RAM (random access memory) 46, ROM (read
only memory) 48 and EEPROM (electronic erasable programmable read
only memory) 50 all of which communicate with the microcontroller
44 over a control bus 52. It will be appreciated that A/D converter
40, ROM 48, RAM 46, or any combination thereof may be internal to
microcontroller 44, as is well known. EEPROM 50 is non-volatile so
that system information and programming will not be lost during a
power interruption or outage. Data, typically status of the circuit
breaker, is displayed by a display 54 in response to display
signals received from microcontroller 44 over control bus 52. An
output control device 56, in response to control signals received
from microcontroller 44 over control bus 52, controls a circuit
breaker 58 via a line 60.
[0014] A plurality of temperature sensors 66-69 are located within
circuit breaker 58. Temperature sensors 66-68 are each located in
close proximity to contacts for phase A, B and C, respectively. The
exact location of the sensor is not critical as it will be
different for various circuit breakers. What is important is that
these temperature sensors 66-68 be located relative to their
respective contacts to provide an indication of temperature at that
contact. Temperature sensor 69 is also located in circuit breaker
58, however it is located away from the contacts of the circuit
breaker to sense ambient temperature within the circuit breaker
itself. The temperature sensors 66-69 may be simple thermocouple
devices which provide an analog signal indicative of the sensed
temperature. These temperature sensed analog signals on lines 71-74
are presented to A/D converter 40, where they are converted to
digital signals. These digital signals are then transferred over
bus 42 to microcontroller 44 and processed in accordance with the
present invention.
[0015] Calibration, testing, programming and other features are
accomplished through a communications I/O port 62, which
communicates with microcontroller 44 over control bus 52. A power
supply 63 which is powered by the service electricity, provides
appropriate power over a line 64 to the components of trip unit 30.
ROM 48 includes trip unit application code, e.g., main
functionality firmware, including initializing parameters, and boot
code. The application code includes code for a contact wear
detection algorithm in accordance with the present invention.
[0016] EEPROM 50 includes operational parameter code, e.g., code
for setting user defined thresholds for the contact wear detection
algorithm. These parameters may be stored in the trip unit at the
factory and are selected to meet customers' requirements, but can
also be remotely downloaded as described hereinafter. The contact
wear detection algorithm is run in real-time and is initiated
preferably from the boot code at start up.
[0017] The contact wear detection algorithm (program) of the
present invention calculates differential temperatures between each
contact sensor 66-68 and the ambient sensor 69, and differential
temperatures between the contacts sensors 66-68, i.e., the
difference between sensor 66 (phase A) and sensor 67 (phase B), the
difference between sensor 67 (phase B) and sensor 68 (phase C), and
the difference between sensor 68 (phase C) and sensor 66 (phase A).
The contact wear detection algorithm estimates resistance of
contacts based on contact heat rise over ambient temperature and
compares the results to a stored table of expected heat rises as a
function of current. For example, if current in phase A is 400
amps, ambient temperature 90 degrees, and contact temperature of
phase A is 140 degrees, then heat rise over ambient is 140-90=50
degrees. If the stored table in this example shows the expected
heat rise at 400 amps current to be only 30 degrees, and if an
alarm threshold is set to allow only a 10 degree deviation (or 40
degrees) then an alarm will be issued.
[0018] Also, OHM's law resistance-in-contact=voltage-across-contact
divided by current-through-contact (AC phase adjusted) is used to
calculate the contact resistance which is compared against a stored
maximum allowable value. Thereby allowing for alternate means of
assessing this parameter for each breaker contact.
[0019] In accordance with another embodiment of the present
invention a statistical standard deviation analysis of these
differential temperatures relative to predefine differential
temperature means (arithmetic) is used to identify eminent severe
failures, (such as defined in U.S. patent application Ser. No.
09/221,243 entitled Method of Statistical Analysis In An
Intelligent Electronic Device, filed concurrently herewith, which
is herein incorporated by reference.) Alternatively, these
differential temperatures are compared to pre-set maximum
acceptable values and an alarm is used when a maximum valve is
exceeded. In still another alternative, the circuit breaker
geometry is thermodynamically molded, i.e., current through the
circuit breaker contacts, contact temperatures, ambient
temperatures, and a maximum acceptable contact resistance constant
are used to calculate a predicted contact resistance. An alarm is
issued when the predicted contact resistance exceeds the maximum.
Thermodynamic and electrical modeling of the circuit breaker will
be readily apparent to one of ordinary skill in the art, using
basic thermodynamic and electrical equations and known modeling
tools. The method of such modeling is not critical to the present
invention, rather this is simply another method for comparing the
sensed temperatures to benchmarks or limits for assessing contact
wear.
[0020] In accordance with still another embodiment of the present
invention, for each trip event and manual opening (such can be
detected as set forth in U.S. patent application Ser. No.
09/221,244), entitled Method of Detecting Manual Trips In An
Intelligent Electronic Device, filed concurrently herewith, which
is incorporated herein by reference) of an energized breaker a
measure of the energy dissipated as breakers are opened is
calculated as (I.sup.2) (T), where I is the contact current and T
is the contact temperature. This energy dissipation is calculated
and then summed up in registers of the microcontroller for each
contact and for each fault type, e.g., short-time, long-time,
ground fault, instantaneous, and manual, to provide cumulative
fault energy by fault type or total.
[0021] The cumulated fault energy by fault type or total is
compared to the thresholds (which may be set by the user) with
alarms being issued when the threshold is exceeded. Also, empirical
constants may be assigned to the cumulate fault energy for
different fault types to make, e.g., ground faults more severe than
manual openings.
[0022] In addition to detecting contact wear, the present invention
can be used to develop a history of contact wear progression over
time. As contact temperatures across the contacts increases,
contact wear will also increase. This information can be used to
predict how much of a contact's life is used up (or remain).
[0023] A priority ranking of maintenance tasks for maintaining
circuit breakers may be established based on this information,
i.e., which circuit breaker will require maintenance first due to
contact wear. Many large facilities have hundreds of circuit
breakers to maintain. Users typically overhaul a certain percentage
of their circuit breakers annually. Therefore accurately
prioritizing the order in which individual circuit breaker problems
should be addressed will allow for more effective use of limited
resources, and help decrease facility down time.
[0024] All of the aforementioned limits or settings are preferably
stored in EEPROM 50 and can be altered by downloading desired
settings via communications I/O port 62. This would include
remotely downloading such data when the unit is connected to a
system computer (not shown), either directly, over the telephone
lines, or any other suitable connection. It may also be preferred
that EEPROM 50 comprises a flash memory whereby such data is
flashed, as is well known.
[0025] In terms of communicating contact wear information, this can
occur in several ways: (1) generating an event message to be
transmitted via a network connection to an attached computer (not
shown) or other central monitoring device (not shown); (2)
displaying a message on display 54 of the trip unit or breaker; or
(3) closing a relay contact which in turn may be used to operate a
horn, warning light or other alarm (not shown). Contact wear
information may also be displayed (or printed) in the form of a
log. Information of, e.g., accelerated contact wear, is useful as
an aid in determining the cause or root (i.e., systemic root cause)
of a problem that may otherwise be difficult to determine.
[0026] While preferred embodiments have been shown and described,
various modifications and substitutions maybe made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
* * * * *