U.S. patent application number 10/894568 was filed with the patent office on 2004-12-30 for on-line detection of partial discharge in electrical power systems.
Invention is credited to Ahmed, Nezar, Srinivas, N..
Application Number | 20040263179 10/894568 |
Document ID | / |
Family ID | 31186378 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040263179 |
Kind Code |
A1 |
Ahmed, Nezar ; et
al. |
December 30, 2004 |
On-line detection of partial discharge in electrical power
systems
Abstract
A method and apparatus for on-line detection of partial
discharge events in an a.c. power system, in which high frequency
electromagnetic pulses generated by partial discharge events are
detected and analyzed in the frequency domain and the time domain
to determine the type and location of the partial discharge event.
The phase relationship between the partial discharge events and the
on-line power signal is also examined to help indicate severity of
the insulation anomaly giving rise to the partial discharge
events.
Inventors: |
Ahmed, Nezar; (Chesterfield,
MI) ; Srinivas, N.; (Farmington Hills, MI) |
Correspondence
Address: |
Robert C. Collins
Reising, Ethington, Barnes, Kisselle, P.C.
P.O. Box 4390
Troy
MI
48099-4390
US
|
Family ID: |
31186378 |
Appl. No.: |
10/894568 |
Filed: |
July 20, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10894568 |
Jul 20, 2004 |
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09174032 |
Oct 16, 1998 |
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6809523 |
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Current U.S.
Class: |
324/520 |
Current CPC
Class: |
H02H 1/0015 20130101;
G01R 31/1272 20130101 |
Class at
Publication: |
324/520 |
International
Class: |
G01R 031/08 |
Claims
1. Apparatus for on-line location of partial discharge events in an
a.c. power system during operation of the system while a.c. power
is being applied through a power cable to a load, which includes: a
coupler for operative connection to the cable and responsive to
current flowing through the cable for detecting high frequency
electromagnetic pulses in the cable generated by partial discharge
events caused by continuing application of a.c. power to the
system, a spectrum analyzer including means for operating said
spectrum analyzer in a first mode of operation for analyzing pulses
from said coupler in the frequency domain to identify a frequency
at which partial discharge pulses of greatest magnitude are
received at said coupler, and in a second mode of operation in the
time domain at the frequency identified in said first mode of
operation to identify a relationship between occurrence of said
pulses and cycles of the a.c. power in the system, and means for
determining location of partial discharge events in the power
system as a function of said frequency at which partial discharge
pulses of greatest magnitude occur and said relationship between
said pulses and cycles of a.c. power.
2. The apparatus set forth in claim 1 including: a pulse phase
analyzer for determining phase angle between said pulses detected
by said coupler and a.c. power applied to the system, and means for
determining as a function of said phase angle a type of fault in
the system that causes said partial discharge pulses.
3. The apparatus set forth in claim 2 including: an antenna for
receiving electromagnetic interference from the atmosphere
surrounding the power cable, and a differential amplifier connected
between said coupler and said spectrum analyzer for subtracting
said electromagnetic interference from said partial discharge
pulses detected at said coupler.
4. The apparatus set forth in claim 1 wherein said coupler
comprises an inductive coupler for removably exteriorly surrounding
the cable to detect partial discharge current pulses in the
cable.
5. Apparatus for on-line analysis of location of partial discharge
events during operation of an a.c. power system in which a.c. power
is being applied through a cable to a load, which includes: first
means for detecting in the cable high frequency electromagnetic
pulses generated by partial discharge events in the power system
caused by continuing application of a.c. power to the system,
second means for analyzing said pulses detected by said first means
in the frequency domain to identify a frequency component of
greatest magnitude in said pulses and the frequency of said
component, third means for analyzing pulses detected by said first
means in the time domain at the frequency of said component
identified by said second means to determine a phase relationship
between said frequency component and cycles of the a.c. power in
the system, fourth means for identifying location of partial
discharge events in the power system as a function of said
frequency of said frequency component identified by said second
means and said phase relationship determined by said third means,
fifth means for determining a phase relationship between pulses
detected by said first means and the a.c. power being applied to
said system, and sixth means for determining a type of fault in
said system that causes said partial discharge events as a function
of said phase relationship determined by said fifth means.
6. The apparatus set forth in claim 5 wherein said first means
includes means for detecting said pulses within a selected
frequency range.
7. The apparatus set forth in claim 6 wherein said first means
includes means for filtering within said selected frequency range
said high frequency pulses generated by partial discharge events in
the power system against high frequency signals in the surrounding
atmosphere.
8. The apparatus set forth in claim 7 wherein said means for
filtering said high frequency pulses includes means for detecting
high frequency signals in the surrounding atmosphere within said
selected frequency range, and means for subtracting said high
frequency signals in the surrounding atmosphere from said
pulses.
9. The apparatus set forth in claim 8 wherein said selected
frequency range includes the VHF frequency range.
Description
[0001] This application is a continuation of application Ser. No.
09/174,032 filed Oct. 16, 1998.
[0002] The present invention is directed to detection of partial
discharge events in power systems such as cables, motors and
transformers, and more particularly to a method and apparatus for
detecting partial discharge events on-line while the power system
is in operation.
BACKGROUND OF THE INVENTION
[0003] Partial discharge events in high-voltage power systems, such
as high voltage power distribution cables, motors and transformers,
are high-frequency discharges that take place in small portions of
the system insulation. These discharges may have a duration on the
order often to fifteen nanoseconds, and usually occur at a peak of
the a.c. power cycle when electrical stress is highest within the
insulation. Partial discharge events generate high frequency
electromagnetic pulses that travel along the power systems.
[0004] High voltage equipment for use in electrical power systems
is conventionally tested off-line for partial discharge activity
that may indicate insulation defects and possible insulation
failure. These conventional techniques typically involve coupling a
capacitor in parallel with the equipment under test and measuring
the discharge signals across an external impedance such as a
resonant circuit. The resonant circuit expands the discharge
current pulses in the time domain so that the pulses are easier to
detect and measure. Both amplitude and phase of each partial
discharge pulse may be recorded and analyzed relative to the test
voltage. Apparatus of this character is not well suited for
detection and analysis of partial discharge events in power systems
while the systems are on-line.
[0005] It is therefore a general object of the present invention to
provide a method and apparatus for detection and analysis of
partial discharge events in an a.c. power system that are adapted
for use on-line while the system is in operation, and that may be
readily implemented for determining type and/or location of the
partial discharges as they occur.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The invention, together with additional objects, features
and advantages thereof, will be best understood from the following
description, the appended claims and the accompanying drawings in
which:
[0007] FIG. 1 is a functional block diagram of an apparatus for
on-line detection of partial discharge events in an a.c. power
system in accordance with a presently preferred embodiment of the
invention;
[0008] FIG. 2 is a schematic diagram of the inductive coupler in
FIG. 1; and
[0009] FIGS. 3A, 3B, and 3C are graphic illustrations useful in
describing the operation of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0010] The disclosure in N. H. Ahmed and N. N. E. Srinivas,
"On-line Partial Discharge Detection in Cables," I.E.E.E.
Transactions on Dielectrics and Electrical Insulation, Vol. 5, No.
2, pp 181-188 (April 1998) is incorporated herein by reference for
purposes of background.
[0011] FIG. 1 illustrates apparatus 10 in accordance with a
presently preferred embodiment of the invention for detecting
partial discharge events in an a.c. power system 12. For purposes
of illustration, power system 12 is shown as comprising a cable 14
that connects a load 16 to a power source 18. Apparatus 10 is
adapted to detect partial discharge events in cable 14 while system
12 is on-line--i.e., while power is being delivered by cable 14
from source 18 to load 16. Apparatus 10 is also adapted to detect
partial discharge events in other conventional high-voltage
equipment such as motors, transformers, gas-insulated systems and
the like.
[0012] An inductive coupler 20 is operatively coupled to cable 14
for detecting high frequency electromagnetic pulses in cable 14
generated by partial discharge events, and feeding such pulses
through an adjustable amplifier 22 to one input of a differential
amplifier 24. One presently preferred embodiment of coupler 20 is
illustrated in FIG. 2 as comprising a coil 26 mounted on a
ferromagnetic core 28. Core 28 is of annular construction, having a
hinge 30 and opposed abutting ends at 32 for opening the core so as
to encompass cable 14. The electromagnetic pulses generated by
partial discharge events in cable 14 are high-frequency
electromagnetic pulses, typically in the VHF and UHF range. The UHF
signals are dissipated very quickly in the power system, so coupler
20 preferably is adapted to be responsive to electromagnetic
signals in the VHF range, and to exclude signals outside of this
range, including the electrical power signal in cable 14 typically
at sixty hertz in the U.S.
[0013] Differential amplifier 24 has a second input that receives a
signal through an adjustable amplifier 34 from an antenna 36.
Antenna 36 may be a loop or dipole antenna adapted to be responsive
to electromagnetic interference in the surrounding atmosphere
within the frequency range of inductive coupler 20--e.g., VHF radio
signals. Within differential amplifier 24, the signals received
from antenna 36 are subtracted from those received from coupler 20,
so that the resulting output from the differential amplifier to a
pre-amplifier 38 is indicative of the high frequency signals
associated with partial discharge events from which the surrounding
electromagnetic interference has been subtracted.
[0014] The output of pre-amplifier 38 is fed through a filter 40 to
a spectrum analyzer 42. Spectrum analyzer 42 receives control
inputs from a full span control 44 and a zero span control 46 for
purposes to be described. The output of pre-amplifier 38 is also
fed through a filter 48 to a pulse phase analyzer 50. Pulse phase
analyzer 50 also receives a reference voltage 52 indicative of the
a.c. power signal in cable 14. Spectrum analyzer 42 and pulse phase
analyzer 50 are coupled to a controller 54 for controlling
operation and providing for automated partial discharge analysis.
Controller 54 receives operator input 56, and is coupled to a
display 58 for displaying signal information to the operator.
[0015] Spectrum analyzer 42 is initially operated in a so-called
full span mode for detecting and analyzing input information by
amplitude or magnitude as a function of frequency over the entire
frequency range set by full span control 44. A typical output of
analyzer 42, under full span control in the frequency domain, is
illustrated in FIG. 3A. The partial discharge events result in
signal peaks at multiple frequencies in the range of 200 KHz to 200
MHZ. In the specific example illustrated in FIG. 3A, the peak of
greatest magnitude is at 24 MHz. These peaks or lines are
indicative of the partial discharge activity in the cable. It is a
characteristic of the electromagnetic pulses generated by partial
discharge events that there is more attenuation at the higher
frequencies than at the lower frequencies as the signal pulses
travel through the cable. Consequently, receipt of signals
predominately in the lower frequency range, as illustrated in FIG.
3A, indicates that the insulation anomaly that is causing the
partial discharge events is fairly far away from the location of
coupler 20. On the other hand, lines or peaks of greater magnitude
at the high frequency end of the spectrum would indicate that the
insulation anomaly is closer to the inductive coupler. Depending
upon the type of cable involved, the apparatus of the present
invention can pinpoint the location of the partial discharge
activity to within fifty feet.
[0016] Spectrum analyzer 42 is then operated in the so-called zero
span mode to isolate signal activity at one or more of the peaks
illustrated in FIG. 3A. For example, FIG. 3B illustrates partial
discharge pulse amplitude as a function of time (i.e., in the time
domain) at the 24 MHz frequency illustrated in FIG. 3A. The
illustration of FIG. 3B has a time duration of 50 milliseconds,
which corresponds to three cycles of the sixty Hz power signal in
cable 40. It will be seen in FIG. 3B that partial discharge events
take place alternately at the positive and negative peaks of the
power signal. The occurrence of partial discharge events at both
the positive and negative peaks of the power signal indicates that
the insulation anomaly in question is near the middle of the
insulation between the center conductor and the outer sleeve or
shield of the cable. If partial discharge events take place only at
the positive peaks of the a.c. signal, this indicates that the
insulation anomaly is near the center conductor, while the
occurrence of partial discharge events on only the negative peaks
indicates that the insulation anomaly is near the shield. Thus, the
output of spectrum analyzer 42 in the full-span or frequency domain
mode of operation, and in the zero-span or time domain mode of
operation, indicates location of the insulation anomaly both
longitudinally and radially of the cable.
[0017] Pulse phase analyzer 50 receives from filter 48 the high
frequency electromagnetic pulses generated by the partial discharge
events, and receives a reference voltage 52 indicative of the power
signal in cable 14. Analyzer 50 analyzes the phase angle of the
partial discharge signals versus the reference voltage. FIG. 3C
illustrates this relationship of pulse count in pulses per second
versus partial discharge magnitude in millivolts versus phase
angle. The information provided at pulse phase analyzer 50 helps
determine the type of insulation anomaly that causes the partial
discharge events. Pulse phase analyzer 50 provides: (1) phase angle
data indicating the angle at which partial discharge occurs. For
example, if the partial discharge occurs at 90.degree. phase angle,
this means that the source of the partial discharge event is in
air, such as near the termination ends of the cable; (2) whether
partial discharge occurs at the positive, negative or both peaks of
the a.c. signal. This helps analyze anomaly type, as discussed
above; (3) the pulse magnitude indicates the severity of the
problem.
[0018] There have thus been disclosed a method and apparatus for
on-line detection of partial discharge events in a.c. power systems
that distinguish the partial discharges from surrounding
electromagnetic interference. Spectrum analyzer 42 analyzes the
detected signals as a function of frequency. One or more frequency
lines can then be examined in the zero-span mode. Partial discharge
signals occur at the peak of the operating voltage, while noise has
no pattern to follow in the zero-span mode. When the partial
discharge frequencies are identified, the signals at one or more
frequencies are analyzed in the time-domain mode. The phase angle
pattern analyzed at pulse phase analyzer 50 determines if the
partial discharge signal is generated in the equipment under test
or adjacent equipment. For example, if the partial discharge events
take place at or near 90.degree. phase angle to the a.c. signal,
this means that the partial discharges are generated in the cable
under test. If the events occur at plus or minus 120.degree.0 phase
angle, this means that the events are occurring in adjacent cables.
The pulse count and magnitude at phase angle analyzer 50 are used
to indicate the severity of the problem. The filter system
disclosed allows discrimination between signals associated with
partial discharge events and electromagnetic interference in the
surrounding atmosphere.
* * * * *