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Патент USA US2130865

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Sept. 20, 1938.
'
T. R. WATTS ET AL
v 2,130,865
PORTABLE INSULATION TESTING EQUIPMENT
Filed May 19, 1956
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INVENTORS
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Jamé's H. Fro/res &
Thomas R. Waffs
'
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BY
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NEY ,
Sept. 20, 1938.
T. R. WATTS ET AL > -
2,130,865
PORTABLE, INSULATION 'TESTING EQUIPMENT
Filed May 19, 1956
‘2 Sheets-Sheet’ 2
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INVENTORS
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James H. fizz/res 8
Thomas /?.
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2,130,865
Patented Sept. 20, 1938
'1 UNITED, STATES PATENT OFFICE *
-
_
'.
.
'
‘2.130.805
'
ron'ranns msuml'égg v-rns'rmc squir
Thomas R. Watts, Forest Hills, and James H.
Frakes, Wilkinsburg, Pa., assignors to Westing
house Electric a Manufacturing Company, East
Pittsburgh, Pa., a corporation of Pennsylvania
Application Maylil, 1936, Serial-No.‘80,560
7 Claims. (Cl. 175-183)
The present invention relates to testing equip- and the other a ?xed resistor R4 shunted by a
ment for determining the insulation qualities of variable condenser C4, both in series with a re
high-tension bushings, transformer ‘coils and the actor Lc. A switch II in parallel with L; controls
like, and more particularly to a testing equip5 ment which is readily. portable and capable of
giving a reading of the capacitance and power-
to an indicating instrument I. through a revers
ing switch II. _
guished from removing it from the apparatus
with which it is associated and taking it to the
The bridge above described is of the so-called
Schering type, but is inverted with respect‘ to the
laboratory for testing.
usual .Schering connections, and includes the in 10
I
,
ductance In. The usual Scherlng connections
are shown in Patent 1,166,159. issued December
28, 1915. to Phillips Themas‘end assigned to the
‘character and although capable of being trans
Westinghouse Electric & Manufacturing Com- '
porteduby truck to the ?eld it was not as readily
portable, or capable of as easy manipulation as
pany. In the so-called Scherlng bridge the June
tion IQ, of the present invention, would be at
ground-potential and the Junction H2 at test po
tential.‘ These potentials are inverted, as shown
in Fig. 1, because the test specimen C1 usually
would be desirable.
,
It is an object of the present invention, there
fore, to provide a testing set for insulation and
20 dielectrics which is more readily portable than
equipment’ now available but which sacri?ces
,
,
The bridge balance conductor ll iaconnected
factor of'a test specimen in the ?eld, as distln-
For some years, equipment for testing the insulation qualities of bushings in the ?eld has been
available but it has been quite complicated in
17-5
its e?ectiveness.
has one side grounded when in service, and be 20
cause of such inversion it is unnecessary to break
none of the accuracy or practical range of meas
such ground connection, that is, the specimen
urement heretofore obtainable.
may be tested in its service position.
Other objects will’ appear from the following
25 speci?cation taken in conjunction with the ac
companying drawings, wherein: .
‘
Figure 1 is a diagrammatic sketch of the cir
cuit arrangement used in the present invention:
Fig. 2 is a schematic showing of the circuit of
30 Fig. 1 in conjunction with other parts of the
complete testing equipment;
Y
Fig. 3 is a perspective view of the encased test
ing equipment;
»
Fig. 4 is a plan view of
35 Fig. 3; and
the structureshown, in
Fig. 5 is a view showing the interior of the
testing equipment with the casing shown in ver
tical section on line V--V of Fig. 4.
_
Referring to the diagrammatic showing in Fig.
40 1, the test circuit comprises a step-up trans
1
Referring to Figs. 3, 4 and 5, the testing ap
paratus is housed in a portable metallic casing 22 25
having a removable lower portion 23, a hinged
cover 24 and a switching panel 28.
On the panel 2‘, the three adjusting pointers
38 are used to vary the resistance of Ra (see Fig.
l) and indicate the e?'ective value of such resist 30
ance. The three pointers“ vary and indicate
the eifective capacitance of C4. The indicating
instrument I! for indicating the bridge balance
is visible through. but not connected with, the
panel 26, and its sensitivity is controlled by knob 35
32 as hereinafter described.
' An aluminum shell 34 having top and bottom
covers insulated therefrom, as by strips 35 se~
cured inside the shell, is secured within the cas
ing 22 by insulating brackets ll, the main body 40
former 2 the low tension winding of which is of the shell 34 constituting, with the metallic
energized from a suitable‘ alternating current ywalls of the casing 22, the standard condenser
‘
source 4, such as 115 volts. The highWension C: of Fig. 1. - winding, for the test potential, which may be of .
The bottom section 23 of the casing 22 con
45 the order of 10 kv., has one side thereof grounded ' tains the transformer I, a_ suitable fuse and ter 45
and the other connected to the junction ll of a minal block t to which may be removably con
bridge circuit, the opposite junction I! of which nected ?exible conductors (not shown) extend
is grounded... The transformer potentials may be ing upwardly 'through'the casing 22 for connec
‘ varied, of course, to obtaina desired test voltage.
tion with an outlet socket 40 on the panel 28 to
receive a plug connected to the low-potential 50
supply 4 to energize the low potential winding of
Y are respectively connected in adjacent arms of the transformer. One side of the high-tension
the bridge between the grounded junction l2 ‘and transformer winding is electrically connected to
the junctions l4 and it. Of the remaining two a spring strip 1 secured at one end to the trans
55 bridge arms, one contains a variable resistor R1. former and resiliently engaging an insulated ter
50
The insulation to be tested, here indicated as
a condenser C1, and a standard condenser C:
9,180,885
minal post'8, mounted in ‘the bottom cover of
shell 34, when the casing sections 22 and 23 are
secured together.
,
‘
'
'
and lead It: be at the same potential, to eliminate
losses between the bridge and shield, and ac
_ cording-1y the resistor 4| is of a value to balance
The electrical connections within the casing
22 are shown moreclearl'y in Fig. 2,.wherein ele
ments corresponding to those shown in the other
?gures have corresponding reference numerals or
letters.
'
-
I
Referring to Fig. 2, the casing 22 is grounded, as
the drop through the bridge circuit to the junc
tion I4.
-
A guard 33 may encase the shield 34 and lead
52 outside the casing 22. Guard 33 is grounded '
to the casing 22 and is e?ective to eliminate the
'minor electrostatic charge which would ordlnar»
ily develop on the insulation surrounding the ,10
minal [2 in Fig. 1, and one side of the high-ten . shield 54. Such charge. or the discharge thereof,
sion winding of transformer 2 is grounded to the has‘ no appreciable effect on the operation oi the
equipment, but the guard may be provided to
casing. The other side of such winding is. con
10 at 38, corresponding to the ground at bridge ter
nected to strip 1- which engages the bottom of
15 terminal 8, secured to but insulated from the
bottom of shell 34, when the two parts 22 and 23
of the casing are. together. The junction ill of
the bridge is electrically connected to'such ter
minal 8 within the shell- 34, and the junction I3
'avold a. possibly undesirable psychological eilect
on the operator.
'
15
.
It‘ is to be understood thatthe shield Maud
guard 56 are insulated from'each other and from
the lead 52, and the guard. 56 is covered with
insulation.‘ The showing of these elements is
is connected to the body,‘ or central portion, of '
merely diagrammatic, cables of such construction 20
the shell ‘by a .conductor 39. As above indicated,
being readily available.
the body of the shell and the metal casing 22 con
stitute the ?xed condenser C2 of Fig. 1.
‘ _
'
.
The connection of the bridge-end oi’ the lead 32
is necessarily detachable to permit removal of the
casing section 23. This is accomplished. as indi
cated in Fig. 2, by a tubular portion 53 depending 25
from the bottom ofvshell 34, having an aperture
A resistor 4|, connected between the botto
25 of shell 34 and the conductor extending to bridge
junction 10, is of a value to compensate for the
potential drop of the bridge impedance devices. ‘ in line with a larger opening in the casing por
The bottom of the shell and'the top section are . tion 23. - The member 33 encloses a relatively
‘so
35.
stiff conductor 53 exposed through said aperture
and connected by conductor 53 to the bridge junc 30.
tion l4. The bridge-end of the test cable is pro
The indicating instrument I3, which may be vided with a resilient collar 33 connected to the
a‘ microammeter, is mounted on‘the top cover shield 54, and a ?ange connected to the end of
of the shell 34, as shown in Fig. 5, and is prefer . guard 56. The collar and ?ange are spaced to
ably provided with an adjustable shunt resistor engage respectively the member 53 and the cas- ' 85
connected by a conductor‘ 45 and constitute
guard electrodes at the same potential as the
bridge junction l4, by reason of resistor 4|.
_
.
' ' 3|, controlled byvknob 32 on the panel-216 (Fig.
4), to vary its sensitvity. A vacuum tube amplie
?er 42 controls the energization of the instru
ment I9; itsinput being connected to the re
40 versing switch 20 which is controlled by a handle
2| on the switching panel 26.
_
The ampli?er 42 may be of any desired type but
-preferably includes a. ?lter which will pass only
the test frequency. “ The tube circuits may be
energized by a separate battery and controlled
45
‘by a switch 43 on the instrument panel 28. A
ing 23 around the respectiveapertures therein.
The test lead‘52 projects beyond the collar 33
for engagement with the conductor terminal 58.
Accordingly, by a thrust movement the test lead
with its shield and guard may be properly con 40
nected to the test set, and may be detachably se
cured in such position by a clamping nut 3|
which pulls the ?ange of guard 33 against a
threaded nipple surrounding the opening in cas
ing portion 23.
-'
-
If at any time the bushing or other sample
voltmeter 34 for indicating the energization of
under test breaks down or is short circuited. dam
opening in the panel, as shown in Figs. 4 and 5.
All of the bridge equipment, with the exception
of the indicating instruments and C1, is mounted
comprising a strip or sheet of dielectric material
33, such as mica, having ‘on one side an electrode
the tube ?laments, maybe mounted adjacent to age might result to the bridge. In order to pre
the microammeter l9 to be visible through an ‘vent this, a discharge device 32 may be provided,
50
within the aluminum shell 34 which e?ectively
connected to the high potential junction ll of
the bridge, and electrodes on the opposite side
shields it. Fiber rods 48v '(Fig. 5) connect the connected respectively to the bridge junctions l4.
various
handles on the panel with the respective and I3. In the event of a ?ashover, the dielectric
55
bridge; control elements,_' and the indicating in- . 33 breaks down establishing a low-resistance
struments are mounted on top of the shell 34 out path shunting the various impedance devices
in the lower legs of the bridge. 03 course other
of contact with the panel. The shell 34 being cut-out
or protective devices of this general char
‘ substantially at test potential, this type ‘of con
60
struction is desirable for the protection of the
operator.
The bridge junction I 4 is connected by a con;
w
' acter may be used if desired.
7
‘ In operation, with the test lead 32 connected to
the specimen, as in Fig. 2, the ampli?er 42 ener
gized and the instrument sensitivity resistor 3|
at its lowest point of adjustment, the lead from
the low-voltage source may be plugged in the
. viewed in Fig. 3), to the ungrounded terminal of ' socket 43 (Fig. 4). The transformer 2 is thereby
the-test specimen Ci, in this case a bushing-of energized, and the high-potential test voltage is
the'condenser type. The lead 32 is relatively long impressed on the bushing throughthe bridge cir
to permit such connectionjvithout removing the cuit. The bridge may then-be balanced-by ad,
justing the resistor R: and condenser 04 to obtain
bushing from its installation.
'
A conducting shield 54 encases the lead 32, a‘ null reading on microammeter l3, the sensitivi
from within the casing section 23 to the bushing. ty control 3| being operated to maintain the indi
and the inner end of the-shield is electrically cations of the microammeter within the limits of
the scale. By reason of the plurality or. dial
connected to the bottom section or guard elec
75 trode of shell 34. It is desirable that the shield ~switches 23 and 33 for R: and G4, a desired new
ductor 53, and an insulated test lead 52,- ex
tending through the bottom of shell 34 and
through the rear of the casing. section 23 (as
16
3
2,180,885 .
curacy of reading may be obtained of the values
of these quantities required to balance the bridge.
When the bridge isbalanced, as indicated by
the microammeter IS, the values of R1 and C4, as
read from the dials, are measures, respectively of
the capacity of the bushing in microfarads and
the power-factor of the bushing. That is, when
the bridge is balanced, the following formulae
obtain:
'
10
,
196mm- 9
and
(1)
,
i
=
.}
.
R.
—
2
Cl czR'
()
15 Cot 9 is, for all practical purposes,_equal to cos 6,
which is the power factor expression, and, in (2),
C2 and R4 being constant the capacity of the spec
imen is readily obtained from the reading of Ra.
It is assumed that the series capacitance and par
20 allel capacitance of the specimen are equal for all
practical purposes.
'
The proofs of the above formulae and state
ments are not given here, in the interest of brev- »
ity, but are well understood by those skilled in
the art.
.
When the bridge is balanced and the R3 and C4
readings taken, the bridge reversing switch 20
should be thrown, by means of handle 2| (Fig. 4) ,
to reverse the connections to the ampli?er, and
80 the bridge rebalanced, if necessary. If the read
ings are not the same with the switch 20 in both
‘Further, a testing equipment constructed, as
described, has a weight of the orderof 100 lbs.
and may be readily carried by hand or trans
ported to the ?eld in an ordinary automobile.
This renders the use of the testing equipment
much more ?exible than heretofore possible when
trucks had to be used and several men required
to move the equipment ‘into position. In spite
of the high‘ degree of portability, the accuracy of
the test results is not adversely affected and the 10
operation of the set is facilitated by the simpli
?cation of the bridge circuit and the, provision of
the corrective inductance 1s.
'
‘
In addition, the mechanical arrangement of
the parts results in a particularly compact de
sign. The instruments and dials are readily
viewed from the top of the casing, the section I!
may be readily removed to afford access to the
transformer and the bridge equipment, and the
?xed condenser plate 34 extending vertically pre
vents the accumulation of foreign matter between
1.5.
2o,
it and the wall of casing 22 which would tend to
introduce losses.
'
Quite obviously, the construction and circuit
arrangement shown may be varied without de
parting from the scope of the invention, and it
is desired that no limitations be placed on the
invention, except as imposed by the appended
claims.
We claim as our invention:
1. A device for measuring insulation qualities
positions, it indicates the existence of interfer
of a grounded specimen comprising a metallic
ence a?ecting themi'croammeter, such as an al
casing, a shell of conducting material therein
and spaced therefrom to‘ constitute a condenser
of ?xed capacity, a source of test voltage and 95
means for impressing it upon said specimen in=
eluding a bridge circuit one arm of which in
cludes said specimen and the adjacent arm con
ternating current magnetic ?eld. If the readings
35 are not the same, the values of R3 and C‘; taken
in each position of the switch 20 should be aver
aged to give the true values.
Also, errors due to interference from an elec
trostatic ?eld, such as fromv a high voltage line
40 overhead, can be eliminated by averaging the Ba
and C4 readings obtained with the low-voltage
supply reversed. This may be accomplished
merely by rotating the plug in socket 40.
It has been found in practice that electrostatic
interference may be of such magnitude that the
bridge‘cannot be balanced. That is, with the C4
dials at, zero, a null reading cannot be obtained
on the mlcreter i9. It is an important
aspect of the present invention that this condi
tion may be taken care of by the impedance In
shunted by switch II. {This impedance is so de
signed'that the value of C4 is increased by such
. an' amount that it will be su?lcient to compensate
tains said ?xed condenser, adjustable
pedance
devices mounted in said shell and connected in 40
the other two arms of said bridge and means for
operating them to balance the bridge, and
for compensating said bridge forerrors result
ing from an electrostatic charge on said speci?
men.
I
.2. A device for measuring insulation
ties
of a grounded specimen comprising a metallic
casing, a shell of conducting material therein. and
spaced therefrom to constitute a condenser of
?xed capacity, a source of test voltage and means
for impressing it upon said specimen including
a bridge circuit one arm of which includes said
specimen and the adjacent arm contains said
ror interference values usually encountered. For
example, if it is increased by .3 mid, which is
equivalent to 3% power factor, the value will be
?xed condenser, adjustable impedance devices in
sumciently larger than errors usually encountered
for compensating said bridge for errors resulting
from an electrostatic charge on said specimen
comprising an impedance element of ?xed value
connected in additive relation to an impedance
to permit the desired compensation. In reading
the power factor from the dials of C4, therefore,
with switch ?- open, the following formula ob
tains:
.
\
'
device in one of the said other two arms.
.
P. F- =C4+ C4 (with240 reversed) _ .3 mi‘LX 10
The R4 values are read as usual.
in accordance with the foregoing, a testing
equipment is provided which will read directly
power-factors up to 10%. Above this value, it is
necessary to revert to a correction chart or curve
because the assumption that power factor is equal
to the reading of C4 no longer holds with sum;
- cient accuracy.
In the great majority of cases.
however, the power factor will be below 10 %, and
the reading may be taken directly from the C4
dials.
,
._
the other two arms of said bridge and means for
operating them to‘balance the bridge, and means
.'
3. A device for measuring insulation qualities
of a'grounded specimen comprising a metallic
casing, a sheet of conducting material therein
and spaced therefrom to constitute a condenser
of ?xed capacity, a source of test voltage and
means for impressing it upon said specimen in
cluding a bridge vcircuit one arm of which in
cludes said specimen and the adjacent arm con
tains said ?xed condenser, adjustable impedance 70
devices in the other two arms of said bridge and
means for operating them to balance the bridge.
and means for compensating said bridge for
errors resulting from an electrostatic charge on
said specimen comprising an impedance element 75
4
_ of fixed value connected ingadditive relation to an
impedance device in one of the said other two
arms, and means for switching said element'out
of said relation.
6
‘
g
‘
4. A device for measuring insulation qualities
of a grounded specimen comprising ametallic
casing, a shell of conducting material therein
and spaced therefrom to constitute a condenser
of fixed capacity, va source of test voltage and
10 means for impressing it upon said specimen in
cluding a bridge circuit one arm of which in
cludes said specimen and the adjacent arm con
of a grounded insulation specimen comprising an
inverted Schering bridge circuit, means for im
pressing a high-potential test voltage on one
junction thereof, the opposite junction being
grounded, a shielded test‘ cable extending from
an adiacenfbridge junction, a shell of conduct
ing material surrounding said bridge and elec-‘
trically connected to‘ the fourth terminal of said
bridge, a grounded casing enclosing said shell
constituting therewith a condenser oil known 10
value, and means for impressing said test voltage
on the shield of said cable including an im
device having a voltage drop approxi
tains said ?xed condenser, adjustable resistors. pedance
mating the drop‘oi said bridge circuit.
'
.
and condensers in the other two legs for balanc
15 in: said bridge, and'means for over-compensat
ing said balancing means for errors resulting
from an electrostatic charge on said specimen.
'i. In a device for measuring the insulation
qualities ‘of .a test specimen comprising abridge»
15.
circuit, means for impressing a test voltage on
5. In a device for measuring the power-factor one junction thereof, adjustable impedance de
‘ of a grounded insulation specimen comprising‘ an vices in the arms of the bridge adjacent to said
junction. a‘standard oi’ comparison in a'third 20
no inverted vBch‘erlng bridge circuit, means for im
’ pressing a high-potential test voltage on one arm of said bridge, means including a shielded
for connecting a test specimen in the fourth
junction thereof, the opposite junction of which cable
arm
of
the bridge, the junction between said
is to be grounded,‘ a shielded test cable extending
from an adjacent bridge junction, a shell of con vthird and fourth arms being electrically con
ducting material surrounding' said bridge and nected to the source of said test voltage, and 25
electrically connected to the fourth terminal of means for compensating said bridge for errors
said bridge. a grounded casing enclosing said resulting from an electrostatic charge on the
1
shell constituting therewith a condenser of specimen.
THOMAS R'WA'I‘I‘S.
known value. and means for ‘impressing said test
a voltage on the shield of said cable.
6. In a device for measuring the power-factor
30
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