U.S. patent number 3,739,272 [Application Number 05/161,190] was granted by the patent office on 1973-06-12 for filter circuit for corona detection.
This patent grant is currently assigned to Phelps Dodge Cooper Products Corporation. Invention is credited to Alexander L. McKean.
United States Patent |
3,739,272 |
McKean |
June 12, 1973 |
FILTER CIRCUIT FOR CORONA DETECTION
Abstract
A filter network for providing essentially optimum resolution of
pulse response in the detection of high frequency corona discharge,
including a high pass filter for removing the low frequency power
line signal and its harmonics, and a low pass lossy transformer
which introduces a low frequency bucking voltage in a subsequent
stage of detection to balance out any residual low frequency
signals still present, thereby providing improved resolution and
sensitivity in the detection of corona discharge.
Inventors: |
McKean; Alexander L. (Ardsley,
NY) |
Assignee: |
Phelps Dodge Cooper Products
Corporation (New York, NY)
|
Family
ID: |
22580220 |
Appl.
No.: |
05/161,190 |
Filed: |
July 9, 1971 |
Current U.S.
Class: |
324/514;
333/177 |
Current CPC
Class: |
G01R
31/58 (20200101) |
Current International
Class: |
G01R
31/02 (20060101); G01r 031/02 (); G01r
031/12 () |
Field of
Search: |
;324/54 ;333/77 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Strecker; Gerald R.
Claims
What is claimed is:
1. Apparatus for providing substantially optimum resolution of high
frequency corona discharge in an insulated cable comprising:
a high pass filter for substantially rejecting the low frequency
line signal and its harmonics present in a resultant corona
discharge; and
a low pass lossy transformer electrically coupled to said high pass
filter for introducing a bucking voltage to balance out any
residual low frequency signals resulting from the broad tuning
characteristics of the high pass filter to provide optimum
resolution and sensitivity of pulse response in the detection of
high frequency corona discharge.
2. Apparatus for providing substantially optimum resolution of high
frequency corona discharge in response to a line signal applied to
an insulated cable comprising:
a high pass filter arranged to shunt the line signal and its
harmonics to ground;
means for coupling said high pass filter to a low pass lossy
transformer to transmit the high frequency corona discharge and any
residual low frequency signals thereto;
said low pass lossy transformer including primary and secondary
windings, said primary winding having one terminal connected to
said coupling means and the other terminal connected to ground;
and
a potentiometer connected in parallel across said secondary
winding, to provide a variable output voltage pulse representative
of high frequency corona discharge between ground and the
adjustable potentiometer setting.
3. Apparatus for detection of corona discharge in an insulated
cable comprising:
a high pass filter;
a high voltage blocking capacitor connected in series with said
high pass filter;
said high pass filter and said blocking capacitor capable of being
arranged in parallel with the insulated cable being tested, said
high pass filter adapted to substantially remove the low frequency
line signal and its harmonics present in the resultant corona
discharge;
a coupling capacitor connecting said high pass filter to a low pass
lossy transformer to transmit the high frequency corona discharge
and any residual low frequency signals thereto;
said low pass lossy transformer having a secondary winding with a
limited frequency response to introduce a bucking voltage to
balance out any residual low frequency signals, thereby providing
substantially optimum resolution of the high frequency corona
discharge.
4. Apparatus for corona detection of the type having means for
applying a high voltage signal to a length of insulated cable which
is to be tested for corona discharge, a high pass filter for
rejecting low frequency signals present in a resultant corona
discharge, and a detector capable of indicating corona discharge,
wherein the improvement comprises:
a low pass lossy transformer electrically coupled to the high pass
filter for introducing a bucking voltage to balance out any
residual low frequency signals to provide optimum resolution of the
pulse response representative of high frequency corona discharge
which is transmitted to the detector.
5. Apparatus for corona detection of the type having means for
applying a high voltage signal to a length of insulated cable which
is to be tested for corona discharge, a high pass filter for
rejecting low frequency signals present in a resultant corona
discharge, and a detector capable of indicating corona discharge as
claimed in claim 4 including:
a plurality of said low pass lossy transformers connected in
cascade.
6. Apparatus for corona detection of the type having means for
applying a high voltage signal to a length of insulated cable which
is to be tested for corona discharge, a high pass filter means for
rejecting low frequency signals present in a resultant corona
discharge, and a detector capable of indicating a corona discharge
as claimed in claim 4 wherein:
said low pass lossy transformer includes a tertiary winding for
more selective transmission of the desired high frequency corona
discharge band to eliminate the effects of phase shift while
improving the efficiency, resolution and frequency response.
7. Apparatus for corona detection of the type having means for
applying a high voltage signal to a length of insulated cable which
is to be tested for corona discharge, a high pass filter for
rejecting low frequency signals present in a resultant corona
discharge, and a detector capable of indicating a corona discharge
as claimed in claim 6 including:
a plurality of said low pass lossy transformers connected in
cascade.
8. Apparatus for corona detection of the type having means for
applying a high voltage signal to a length of insulated cable which
is to be tested for corona discharge, a high pass filter for
rejecting low frequency signals present in a resultant corona
discharge, and a detector capable of indicating a corona discharge
as claimed in claim 4 including:
amplifier means electrically coupled to the output of said low pass
lossy transformer, adapted to match the output impedance thereof,
providing substantially distortionless transmission of the pulse
response representative of the high frequency corona discharge band
to the detector.
Description
This invention relates to corona discharge detection in
solid-dielectric or insulated cables. More specifically, this
invention relates to a power-separator filter capable of providing
a pulse response affording optimum resolution damping,
representative of "real" corona discharge in the cable system.
Ideally, insulated cables should be manufactured void-free for
stable operation and long life. If a void is present within the
length of cable being tested, application of a high voltage will
cause ionization of gas trapped within the void resulting in a high
frequency discharge known as corona discharge.
Accurate detection of corona discharge in solid-dielectric cables,
utilizing e.g. crosslinked polyethylene and rubber insulation, is
important in determining cable quality. A conventional method for
determining corona discharge utilizes a power-separator filter to
detect the major portion of the corona discharge energy occurring
predominantly in the kHz frequency range. This major portion of the
corona discharge energy is usually measured over a narrow frequency
band; detection bands at 35 kHz and 70 kHz have been found to be
practical for commercial application. Broad band detection has been
employed with some success, but increased band width significantly
decreases detector sensitivity.
It is desirable that the response of the apparatus employed for
corona discharge detection provide optimum resolution and
sensitivity. The "alpha" response, comprising one type of response
which is generally regarded as desirable and which is specified in
the cable industry, defines an initial maximum amplitude half-cycle
of an oscillatory wave, followed by highly attenuated or damped
subsequent half-cycles.
Conventional corona discharge detectors have generally employed LC
networks which produce an oscillatory response and differentiate
the desired high frequency components of the input signal.
Differentiation modifies and distorts the signal shape and
magnitude, producing both positive and negative responses. Further,
since the response is not attenuated after the initial pulse,
resolution is affected and pyramiding may occur, leading to
additive errors in determining "real" corona discharge.
Apparatus for corona discharge detection are disclosed in Letters
U.S. Pat. No. 3,015,774, issued to D. Eigen, and Letters U.S. Pat.
No. 3,229,199, issued to R. C. Mildner. Eigen discloses the use of
a conventional high pass filter for transmitting high frequency
corona discharge currents to a primary amplifier. A remote antenna
capable of detecting radiated noise and interference is coupled to
a secondary amplifier. The signals from the secondary amplifier are
utilized to buck the common noise and interference signals present
in the primary amplifier. There is no teaching in Eigen, however,
to utilize a low pass lossy transformer in the detector circuit to
eliminate residual low frequency components in detecting an optimum
pulse response, representing "real" corona discharge.
Mildner discloses the utilization of a pair of electrodes in
conjunction with a pair of intermediate frequency band pass
rejection filters. When a void is present in the cable being
tested, the resulting corona discharge causes an unbalance in the
capacitance of one of the electrodes, thereby producing a signal
which indicates the presence of a void. Mildner is not primarily
concerned with the optimization and sensitivity of the pulse
response or corona display.
It is an object of the present invention to provide an apparatus
which is capable of providing substantially optimum resolution and
sensitivity in the detection of "real" corona discharge.
It is a further object to provide an apparatus for eliminating any
residual low frequency components which are present in a resultant
corona discharge.
It is a further object to provide such an apparatus without
employing differentiation.
Briefly, the present apparatus provides for improved resolution of
pulse response in the detection of high frequency corona discharge.
A high pass filter rejects the low frequency line signal and its
harmonics which are present in the resultant corona discharge. A
low pass lossy transformer is electrically coupled to the high pass
filter to provide a bucking voltage which balances out or subtracts
any residual low frequency signals still present in the high pass
filter output signal, thereby providing an essentially undistorted
pulse response representing the energy and magnitude of the high
frequency corona discharge.
Other objects, aspects and advantages of the present invention will
be more fully understood when the detailed description is
considered in conjunction with the drawings as follows:
FIG. 1 shows one embodiment of the present invention employing an
oil-filled terminal system for corona detection;
FIG. 2 shows a modified low pass lossy transformer including a
tertiary winding which constitutes an additional refinement which
may be utilized to improve the quality of pulse response in corona
detection of the cable system in FIG. 1 employing an oil-filled
termination assembly; and
FIG. 3 shows a capacitance graded terminal cable system capable of
being used with the low pass lossy transformers shown in FIGS. 1
and 2.
Referring to FIG. 1, a system 10 for corona discharge detection is
shown. A series tuned high pass filter 12 is connected in parallel
with the length of insulated cable, generally indicated at 14, to
be tested for corona discharge. A low pass lossy transformer 16 (T)
is electrically coupled to the high pass filter 12 for providing a
bucking voltage to balance out or subtract any residual low
frequency components present in the input signal (V.sub.1), which
were not removed by the high pass filter 12, thereby providing a
substantially undistorted pulse response (V.sub.2) to real corona
discharge at the output terminals 18.
The output terminals 18 may be coupled to a following amplifier 20
and the amplifier output is transmitted to conventional detectors,
such as a cathode ray oscilloscope 22, and/or a recording
oscillograph 24.
Generally, the end of the insulated cable 26 to be tested is
unwound from a conventional reel 28 and arranged in an oil-filled
terminal 30. The insulated portion (cable shield) of the length of
tested cable 14 is grounded and the conducting portion (conductor)
32 of the insulated cable 26 receives a high voltage signal through
a coupling inductor or choke 34 (1,500 mH). A conventional line
voltage source 36 connected across a step-up transformer 38,
supplies the high voltage energy to the choke 34 and ultimately to
the length of tested cable 14.
A high voltage blocking capacitor 40 (0.003 uF) and the high pass
filter 12, including C.sub.f (480 uF), L.sub.f (24 mH) and its
inherent resistance R.sub.m (0.26 ohms) are connected in parallel
across the length of tested cable 14. The series tuned high pass
filter 12 has one terminal 42 grounded and the other terminal 44
coaxially coupled to the low pass lossy transformer 16 through
coupling capacitor 46 (0.001 uF) and primary shunting resistance 48
(600 ohms).
The low pass lossy transformer 16 has balanced primary and
secondary windings 50 and 52, respectively, having a 1:1 turns
ratio. Further, the secondary winding 52 has a high rejection ratio
of higher frequencies. The primary winding 50, including L.sub.p
(77 mH) and R.sub.p (7 ohms), is connected between ground and the
high pass filter 12 through coupling capacitor 46. The secondary
winding 52, including L.sub.s (75 mH) and R.sub.s (30 ohms), is
connected across a potentiometer 54 to provide a variable output
voltage pulse signal (V.sub.2) affording optimum resolution of the
real corona discharge, as measured between the grounded primary
winding and the adjustable potentiometer setting.
The secondary winding 52 exhibits a limited frequency response and
introduces a bucking voltage which balances out or subtracts any
residual low frequency components appearing therein while not
affecting the high frequency components being detected. Thus, the
output voltage (V.sub.2) provides a measurement of real corona
discharge.
The output terminals 18 are coupled to the input of the following
amplifier 20. The amplifier 20 has impedence matching
characteristics for maximum power transfer and for essentially
distortionless transmission of the output voltage (V.sub.2) to the
oscilloscope 22. It should be noted that various types of detectors
may be employed; it may be advantageous to use an analog charge
meter or calibrated recorder.
Referring to FIG. 2, a low pass lossy transformer 16A is shown
having an additional primary or tertiary winding 56 to eliminate
the effects of phase shift and increase the high frequency
rejection ratio while improving the efficiency and frequency
response. The primary tertiary winding 56 has only sufficient band
width to pass the low frequency signals, which serve to buck or
balance out residual low frequency components of the detected
signal at the secondary winding 52A, thereby providing a
measurement of real corona discharge (V.sub.2) across output
terminals 18A. The apparatus shown in FIG. 2 may be readily
employed in the detection of corona discharge in a cable system as
shown in FIG. 1 by simply removing the filter apparatus shown in
FIG. 1 and inserting the apparatus shown in FIG. 2 between
terminals 44 and 18.
Referring to FIG. 3, an alternate embodiment is shown for the
series tuned high pass filter 12A when used with a
capacitance-graded terminal 58, rather than with the oil-filled
terminal 30 shown in FIG. 1. The inductor L.sub.2 of the series
tuned high pass filter 12A is connected directly between the
capacitance-graded terminal 58 and the cable insulation. The
capacitance-graded terminal 58 and conductor 32A rest on insulator
60. The capacitance-graded terminal 58 may be utilized with either
of the filter apparatus embodiments shown in FIGS. 1 or 2.
It should be noted that it may be advantageous to arrange a
plurality of low pass lossy transformers in cascade to improve
efficiency and increase the high frequency rejection ratio of the
secondary winding.
During operation of the system 10 shown in FIG. 1, the line voltage
signal is applied across the step-up transformer 38, and the
resultant high voltage signal is applied to the conducting portion
32 of the length of tested cable 14 through coupling inductor 34.
Any resultant corona discharge signal will include the "real"
corona discharge, as well as low frequency components, including
the line or supply voltage signal and its harmonics. The resultant
signal is applied across the high voltage blocking capacitor 49 and
the high pass filter 12. The high pass filter 12 substantially
rejects the low frequency signal components. However, due to the
broad tuning characteristics of the high pass filter 12, residual
low frequency components remain in the resultant corona discharge
(V.sub.1).
The coupling capacitor 43 connected between one terminal 44 of the
high pass filter 12 and the primary winding 50 of the low pass
lossy transformer 16 passes the resultant corona discharge signal,
including the residual low frequency components, from the high pass
filter 12 to the transformer 16.
The low pass lossy transformer is balanced with a secondary winding
52 having a high rejection ratio of higher frequencies to provide a
bucking voltage which opposes, thereby subtracting out, the
residual low frequency components in the detected corona pulse
response to provide an output voltage pulse response (V.sub.2)
having optimum resolution representative of real corona discharge
occurring in the cable system being measured.
The pulse response representative of the high frequency corona
discharge is generally amplified and transmitted to a detector to
provide a visual indication of the type and magnitude of the real
corona discharge.
Utilization of the bucking transformer 16 coupled to the high pass
filter 12 advantageously provides detection of real corona
discharge without the attendant distortion produced by
differentiation.
It should be understood by one skilled in the art that various
modifications may be made to the present invention which are within
the spirit and scope thereof as described in the specification and
defined in the appending claims.
* * * * *