Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Utility Patent
1998-05-02
2001-01-02
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S511000, C324S555000
Utility Patent
active
06169406
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of Very Low Frequency (“VLF”) High-Voltage (“HV”) electrical testing of alternating current (“AC”) electrical installations and equipment which have large capacitances, for example such as VLF HV testing of characteristics and/or qualities of insulations on long lengths of buried electric power distribution cables, testing of insulation in large rotating AC machines and other AC apparatus which exhibit large capacitance values under HV test conditions. As used herein, the terms “Very Low Frequency”, “VLF” and “very low frequency” mean a frequency below about 1 Hz.
BACKGROUND
It was previous practice to test HV insulation of lengths of underground electric power distribution cables by applying HV direct current (DC) test voltages. Years ago, HV insulation for buried cables included oil-impregnated paper surrounding conductors. This insulation was encased in an outer protective jacket such as a lead sheath or other suitable protective conductive conduit. The application of HV DC test voltage to lengths of such prior buried cables would charge up the capacitance existing between their conductors and their jackets, sheaths or conduits to a peak residual voltage charge level substantially equal to the HV DC test voltage being applied. But a residual voltage charge resulting from such a test did not significantly damage cable insulation, because prior types of insulation allowed such residual charges soon to leak away.
HV DC testing of service-aged buried cable having dielectric comprising cross-linked solid polyethylene (XLPE) is undesirable for at least two reasons: (a) Such testing causes space charge to accumulate in the dielectric which results in increased electric stress locally, notably in the vicinity of “water trees” which have penetrated into the dielectric material of the buried cable. Such local electric stress therefore can result in subsequent failure in service which otherwise would not have occurred. And, (b) HV DC testing is ineffective in identifying quality problems other than major damage to the insulation. This ineffectiveness of HV DC in identifying quality problems applies to cables having other dielectric materials such as polyethylene (PE) or extruded Polyethylene rubber (EPR), not just to those having XLPE dielectric.
Various non-DC HV test systems have become commercially available in recent years. They are used instead of HV DC testing of modern buried power cables, AC machines and other HV AC electrical equipment which have large capacitances. However, these non-DC HV test systems available today do not provide advantages such as those provided by embodiments of the invention herein described.
For example, accompanying is a two-page write-up by AVO International on their BIDDLE (“TM”) Very Low Frequency Test Systems. This write-up states:
Each Biddle VLF Test System consists of
two main units: a control console and a
high-voltage, oil-filled tank (either
85 kV or 100 kV). Red warning beacon
and reusable shipping container are
available options.
Operating at a frequency of 0.1 Hertz,
these systems have the capability of
testing the largest rotating machines.
An increasing amount of test data is
proving the feasibility of substituting a
0.1-Hertz test for the normal 50/60-Hertz
test,
It is noted that Biddle's test system has specifications as follows:
SPECIFICATIONS
CAT. NO.
682983
682990
VLF Crest
85 kV
100 kV
Output Voltage
Rated Load
0.4 &mgr;F
1.0 &mgr;F
Capacitance
Rated DC
±85 kV,
±100 kV,
Output
10 mA
50 mA
Equivalent
60 kV
70 kV
50-Hz Rating*
1130 kVA
1540 kVA
Equivalent
60 kV
70 kV
60-Hz Rating*
1360 kVA
1850 kVA
*The Equivalent 50-/60-Hz Rating is the rating that would be needed to energize the rated load capacitance at the same crest test voltage as the VLF Test System.
Input Supply
240 Vac ± 10%, 1&phgr;, 50/60 Hz
with ground, NEC 50 A
277 Vac ± 10%, 1&phgr;, 50/60 Hz
with ground, NEC 50 A
Biddle's description states:
Maximum Input Current Required
At 240 V in dc mode, 45 A continuous;
in VLF mode, 70 A intermittent
At 277 V in dc mode, 40 A continuous;
in VLF mode, 60 A intermittent
Also, Biddle's description sets forth:
Dimensions
HV Power Supply, 85 kV (oil-filled)
Cat. No. 682983
42 H × 27 W × 37 D in.
106.1 H × 69.1 W × 94.1 D cm
HV Power Supply, 100 kV (oil-filled)
It is noted that Biddle's information sheets set forth the dimensions of their 85 kV (oil-filled) system but do not state the dimensions of their 100 kV system. Therefore, to provide a comparison between the Biddle system and a system embodying the present invention, the Biddle 85 kV (oil-filled) system will be used as a basis for comparison.
The “Rated Load Capacitance” of the Biddle CAT No. 682983 is 0.4 micro Farads (&mgr;F). The embodiment of the present invention described first can test loads having capacitance levels up to about 2.2 &mgr;Fs at 0.05 Hz, i.e., more than five times the testing capability of Biddle's unit. Embodiments of the invention are described later which are adapted to test loads at High Voltage at Very Low Frequencies of 0.1 Hz, 0.05 Hz, 0.02 Hz, etc.
Moreover, this Biddle unit in its Very Low Frequency (VLF) mode requires input power of 240 Volts at 70 Amperes intermittent, which represents an intermittent input power of 16.8 kilovolt Amperes (kVA). In contrast, during testing operations the sine-function test wave embodiment of the invention first described herein uses an average input of about 8 Amperes at 120 Volts, i.e., about 0.96 kVA. Its intermittent peak power is only about 1.44 kVA, which is less than about 9% of the intermittent kVA drawn by the Biddle 85 kV system. Moreover, during continuous stand-by, the disclosed method and systems require only a very modest power input, i.e., less than input into a typical household bread toaster. This stand-by input for the method and system first disclosed herein is only about 4 Amperes at 120 V, i.e., less than 0.48 kVA.
It is noted that Biddle describes their 85 kV power supply as measuring 42 inches (H) by 27 inches (W) by 37 inches (D), thereby having slightly more than 41,900 cubic inches of volume. A gallon contains 231 cubic inches. Thus, this Biddle oil-filled power supply has a volume of more than 180 gallons. Transformer oil weighs about 8 pounds per gallon. Hence, such a power supply, if filled only with transformer oil, would weigh more than 1,400 pounds. Since this HV Biddle unit also includes components of greater specific weight than its oil content, i.e., such as transformer iron and conductors of copper, its overall weight likely exceeds one Ton.
In contrast, HV units described herein have volumes less than 5½ gallons and weigh less than about 75 pounds including transformer oil and all internal components.
Typical underground electrical power distribution cables having XLPE, PE or EPR dielectric are operated by power companies at voltages usually in a range from about 5 kV up to about 35 kV. Such buried cables with a length of about 15,000 feet to about 25,000 feet (about 3 to 5 miles) often exhibit a total capacitance of about 2.2 microFarads (&mgr;F). After such a length of cable has been buried, the power company will test it before connecting it to a power distribution substation. Attempting to use HV DC test procedures for such a buried cable having XLPE, PE or EPR insulation is not advisable for reasons already explained above.
SUMMARY OF THE DISCLOSURE
A method, systems and apparatus embodying the invention enable a length of buried HV power distribution cable, for example about 3 to 5 miles long and having a capacitance of about 2.2 &mgr;F, to be tested conveniently by Very Low Frequency Sine-Wave Voltage up to a peak voltage level exceeding the cable's intended peak AC operating voltage. Moreover, the illustrative embodiments of this invention overcome or substantially reduce the inconveniences, inefficiencies and awkward handling problems associated with a heavy, bulky HV power s
Metjahic Safet
Parmelee G. Kendall
Solis Jose M.
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