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Патент USA US2137216

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Nov. 15, 1938.
o. P. MQCARTY
2,137,216
PROTECTIVE CIRCUIT
Original_ Filed Jan. 6, 1937
4
‘ T0 VOLTAGE
SENSITIVE CON7'/?0L_ FOR g
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Inventor:
Orin P McCartg,
by
is
Attorhegf
2,137,216
Patented Nov. 15, 1938
UNITED STATES PATENT ()FFICE.
2,137,216
PROTECTIVE CIRCUIT
Orin P. McCarty, Pitts?eld, Mass, assignor to
General Electric Company, a corporation of
New York
Application January 6, 1937, ‘Serial No. 119,234
Renewed March 17, 1938
15 Claims. (01. 171-—119)
My invention relates to protective circuits and
more particularly to means for protecting an im
pedance'which carries the main load current of a
transformer during operation under load of a tap
5 changer on the transformer.
Protective means of this type is disclosed and
In accordance with my invention I provide im
proved means for protecting the impedance from
the' effects of a. stopping of the ‘tap changing
switch in an “off” position.
One feature of this
means comprises relatively quick acting means
for disconnecting and effectively-by-passing the
broadly claimed in my Reissue Patent No. 20,147,
impedance when an ordinary value of line current
dated October 27, 1936. The present invention is
particularly adapted for use with‘automatic tap
10 changing voltage regulators and boosters of the
?ow passes through the impedance for substan
tially longer than a normal tap changing opera
tion combined with relatively slow acting means
for short-circuiting the impedance in case the
type disclosed and claimed in my application
current flow is so low as to fail to cause operation
Serial No. 69,789, ?led March 20, 1936 as a con
tinuation-in-part of an application ?led April 16, - of the ?rst means and yet is high enough to cause
i 1935.
This is a continuation-in-part of my appli
15 cation Serial No. 47,425, ?led October 30, 1935.
The above referred to reissue patent and patent
applications are all assigned to the assignee of this
application.
'
‘
.
_
When transformer taps are changed under load,
20 means are often provided for snapping the mov
able contact or contacts rapidly from one ?xed
contact or transformer tap to the next ?xed con
tact or transformer tap. In certain arrange
ments of this type an impedance is provided for
25 carrying the current during the switching opera
tion. Between switching operations the imped
ance is substantially short-circuited by the closed
contacts of the tap changing switch. In this way
there is always a closed conducting path for the
30 current and an objectionable momentary break
in the. circuit ‘during the switching operation is‘
eliminated.
above type takes place relatively rapidly,'I have
35 found that it is not necessary to provide an im
pedance which can carry the current for long
periods‘ or inde?nitely and it is only necessary to
provide an impedance which can carry the cur
rent safely and without overheating during the}
40 relatively short interval of time during which the
tap changing switch is going from one contact
making position to the next.
This permits a sub
stantial saving in cost as an impedance which can
carry the line current continuously is much more
45 expensive and larger than an impedance which
need only carry the line current momentarily.
It sometimes happens, however, that the mecha
nism for driving the tap changing switch gets out
of order so that for one reason or another the
50 switch will stop in an intermediate position be
tween contact making positions. When this hap—
pens the load current ?owing continuously
through the impedance causes it to overheat to
such an extent as to cause failure of the device
'55 and sometimes even ?re.
passing or short-circuiting means when an inter
rupted tap change is eventually completed.
An object of my invention is to provide a new
and improved protective means for a switching
impedance.
Another object of my invention is to ‘provide
improved means for disconnecting and ‘by-passing
or short-circuiting an impedance which is
adapted to carry the line current during a trans 25
former tap changing operation, in the event of
stopping the tap changing means in an “o "
position.
A further object of my invention is'to provide
a new and improved protective circuit and appae 30
ratus for a transformer tap changing under load
type of. regulator.
‘
As a normal tap changing operation of the
.
eventual overheating of the impedance. Another
feature is the provision of means for protecting
the transformer ‘against the effect of said by
'
My invention will be better understood from the
following description taken in connection with the
accompanying drawing and its scope will be
pointed out in the appended claims.
7
In the drawing, Fig. 1 is a diagrammatic show
ing of an embodiment of my invention and FIELZ
is a similar showing of a modi?cation thereof. -
Referring now to Fig. 1 of the accompanying
drawing, l is a switching impedance which is con
nected around a transformer tap changing switch
’2. Switch 2 is arranged to change taps on a
transformer winding 3, which is shown, by way
of example as the series winding of an auto
transformer having an exciting winding 4 6011nected across an alternating current circuit 5.
The winding ‘3 is connected in circuit 5 by means
of an adjustable connector 6 for selective engage
ment with the taps on the winding 3. The pur 50
pose of this is to vary the range of voltage adjust
ment produced by the tap changer 2. In the mid
position of connector 6, as shown in the drawing,
the tap changer can give equal amounts of volt
age buck and boost ‘in the circuit 5. If the con 55
2
2,137,216
nector 6 is connected to one end of the winding 3
the tap changing means will cause the autotrans
former to produce varying amounts of voltage
buck, while if the connector 6 is connected to the
other end of the winding 3 the tap changing
means will cause the autotransformer to produce
varying amounts of voltage boost. With the con
nector 6 connected to a tap intermediate the mid
tap and the end of the winding 3 the buck and
boost ratio will be 21/2 and 7 1/g, or vice versa. The
arrangement can be operated with the power flow
in the circuit 5 from right to left, as viewed in
the drawing, or from left to right.
The details of the mechanism for driving the
15 tap changing switch 2 form no part of my present
effectively short circuit the normally conduct
ing paths through the resistor I, switch 2 and
series winding 3.
The resistor I isv shown in the shape of a
hollow cylindrical member through the center II
of which is passed a bolt I2 carrying end nuts
thereon for clamping the resistor between end
conducting plates I3 mounted on a suitable in
sulating support I4. During a switching opera
tion the current flows through a conductor I5 10
to one of the plates I3 thence through the re
sistance material I to the other plate I3, from
there through a conductor I6, through the ten
sion fuse I0 and the blade switch 8 in series, and
then through a conductor IT to the other side of 15
invention and the movable contact may be oper
ated by hand if desired. However, the present
the tap changing switch 2. In normal operation, the fuse II) can carry
preferred form of automatic means for driving the
movable contact of the tap changing switch 2 in
the line current during the period of time re
quired for an ordinary tap changing operation.
response to the voltage of the circuit 5 is disclosed
in my copending application, Serial No. 69,789,
?led March 20, 1936, and assigned to the assignee
of the present application. Such anarrangement
produces an automatic voltage regulating device
25 which is well adapted for automatically holding
substantially constant voltage on branch feeder
circuits.
It is immaterial to my invention, in its broader
aspects, that the impedance I be of any partic
If, however, the tap changing switch stops in an
“off" position the current ?owing through the
30 ular type and it may be an ordinary resistor or
a self-saturating iron core reactor, or a well
known non-linear ferrc-resonant circuit or net'
work of any suitable type. However, I prefer to
use a resistor of ceramic resistance material
known as "Thyrite”, the composition of which
is disclosed and claimed in Patent No. 1,822,742,
granted September 8, 1931, on an application of
K. B. McEachron and assigned to the assignee
of the present application. Thismaterial has
the property of markedly reducing its electrical
resistance substantially instantaneously with in
creases in voltage applied thereto or increases in
current ?owing therethrough. With such a re
sistor its electrical resistance is relatively very
45 high when the contacts of the tap changing
switch 2 are closed, as the voltage across the re
sistor is then low. Consequently, the current
through the resistor is very low resulting in mini
mum heating andminimum losses in the resistor.
50 When the contacts of the tap changing switch 2
separate, the voltage across the resistor l builds
up and the current therethrough increases. This
causes its resistance to decrease greatly at the
same time so that the voltage drop across it is
55 not as great as it would be across an ordinary
linear resistance.
Consequently the switching
voltage, that is, the voltage across the contacts
of the switch 2, when they perform a tap chang
ing operation, is held to a reasonably low value.
The above characteristic of resistor I is some
times referred to as a negative resistance current
characteristic.
For protecting the resistance I against the in
jurious effect of continuous approximately full
65 load line current flow therethrough in the event
of accidental stopping of the switch 2 in an “off”
or intermediate position, as indicated by the
dotted position of the movable contact, I provide
a switch having a ?xed contact 1 and a movable
blade contact 8 which is biased towards ?xed
contact‘! by means of any suitable device, such
as a spring 9. The switch is held open by means
,of a tension fuse I I].- The connections are such
that fuse I0 is in series with resistor I, and the
75 switch, when, it closes, will ‘by-pass, bridge 01'
fuse I 0 melts it, thereby allowing the spring 9 to
cause closure of the switch by causing engage
ment between the blade contact 8 and the sta
tionary contact ‘I. By this action the resistor I
is automatically disconnected and the normal
current path through it is short-circuited. It
is to be noted that when the fuse I0 melts, the
motion of the blade 8 issuch as to increase the
distance between the fused or broken ends of the 3O
fuse I0 thereby effectively preventing any con
tinuance of an are at this point.
It is difficult to provide a fuse which will not
blow or melt when momentarily carrying full
load line current during an ordinary tap chang
ing operation and which at the same time will
melt when relatively low current is passed there
through continuously. Such relatively low cur
rent may, however, cause injury and overheating
of the resistor I if continued long enough. Con 40
sequently, there is provided suitable means in the
form of a small fusible metallic cylinder or ring
I8 surrounding the bolt I2. This fusible means
I8 responds to the temperature of the resistor
I so that if the current ?owing through the fuse
I0 is low enough not to cause it to melt but the
current is still high enough to cause overheating
of the resistor I then the fusible means I! will
melt and in so doing will spread out and inter
connect the conducting members I3, thereby
short-circuiting the resistor I. The fusible
means I8 is relatively slow acting compared to
the relatively quick acting means operated by
the fuse I0.
'
Experience has shown that vibration or sud
den J'arring will sometimes cause the switch 2
to complete a switching operation after it has
stopped in an "off" position. If this happens
afterv the protective apparatus has by-passed or
short-circuited any part of the series winding 3, 60
a heavy short-circuit current will usually be in
duced in that part of the series winding 3 by the
common winding 0. The one exception is when
the movable contact 2 makes connection to the
same tap that connector 6 is connected to for
then no part of the series winding 3 is short
circuited. If this short-circuit current persists
the transformer will be injured and its series
winding will probably be burned out.
In the modification shown in Fig. 2, I provide 70
means for protecting the transformer against
the above-described short-circuit current con—
dition. This means consists of an auxiliary cir
cuit maker and breaker for deenergizing the
shunt winding 4 of the autotransformer at times
2,187,216
,
3
If, with the parts in the positions shown in
when the series winding 3 of the autotransformer
is short-circuited by the protective means. In
this way the production of any induced voltage
in series winding 3 by transformer action is
eliminated. A simple way to construct this aux
iliary circuit maker and breaker and to secure
its proper operation is to combine it with the
tension fuse-operated switch. Thus, as shown
in Fig. 2, the upper terminal of the shunt wind
10 ing 4 of the autotransformer is connected to an
the drawing and with the circuit 5 energized, the
switch 2 is stopped in an “off” position, as indi
cated by the dotted line, the load current will be
by-passed through the switch arm 8, tension
fuse H), negative resistor l and‘ reactor is in
parallel, and the upper half of the series winding
3. When the fuse ~Ill melts, the spring 9 carries
the switch blade 8 toward contact 1, atthe same
time opening the common winding excitation 10
contacts at [8. Arcs are then drawn at the melt
auxiliary contact l8 which is normally engaged
ing fuse, and at the excitation contacts. Should
by the movable contact arm 8. When the fuse
Ill melts as a result of the stopping of the tap
changing switch 2 in an "off" position, the spring
the are at the excitation contacts tend to inter-''
rupt, the core of the transformer would imme
diately saturate from the load current passing 15'
through part of the series winding. The result is
15 9 will cause the arm 8 to separate from the
contact l8 thereby breaking the circuit through
the shunt winding 4.
Aside from the rearrangement of the tension
fuse-operated switch and its additional contact
that the arcs at the excitation contacts and at
the melting fuse do not interrupt until the switch
blade 8 reaches contact ‘I and completely by
passes the entire series Winding. The short-cir 20
20 I8, Fig. 2 differs from Fig. 1 in a number of other
cuit current from the series winding which would
tend to ?ow through the load switching by-pass
I-l9 and the tension fuse arc is limited by the
impedance of the by-pass, and consequently does
particulars. The ?rst is that the negative re
sistance current characteristic impedance, the
details of which are shown in Fig. 1 but merely
for simplicity’s sake are not shown in Fig. 2,
.not exceed a value which can easily be inter 25
has a saturable core reactor 19 connected in par
rupted under oil. The excitation are readily in—
allel therewith. For the range of current han
dled by transformer regulators of this type and , terrupts and completely removes the regulator
with the permissible voltage drop allowed in the
by-pass impedance, a reactor and negative char
. from the line.
acteristic resistor combination is less expensive
than an‘ equivalent resistor alone. This is be
cause for equal current ratings, the reactor is
less expensive than the negative resistor and with
the parallel combination the reactor carries most
of the current and the negative resistor merely
limits the high voltage peaks across-the reactor
during times when the flux in the reactor core
is below saturation values.
.
‘
-
Another di?erence between Fig. 1 and Fig. 2 is
that
in Fig. 2, the by-pass circuit which contains
40
the fuse l0 and resistor l is connected between
the load circuit side of the tap-changing switch
2 and the electrical midpoint of the series wind
ing 3, whereas in Fig. 1 the by-pass circuit is con
45
nected across both the tap-changing switch 2 s
and the winding 3. The result is that in Fig. 2,
the maximum continuous normal voltage which
can be applied to the impedance I is one-half the
voltage of the series winding 3 whereas in Fig. 1
when the range-adjusting terminal 6 is connected
50
to either of the outermost taps of winding 3, the
full voltage of the series winding 3 can be ap
plied continuously to the by-pass impedance I.
Still another difference between the. ?gureslis
that in Fig. 2, a lightning by-pass 20 is connected
_
In Fig. 2, there is also shown a tertiary wind
ing 2| on the transformer for supplying current 30
to any suitable voltage sensitive control means
for driving the tap-changing switch 2.
>
It should be understood that in practice,‘ the
negative resistance I of ‘Fig. 2 can be constructed
as in Fig. 1 with the fusible element f8 if desired. 35
While I have shown and described particular
embodiments of my invention, it will be obvious
to those skilled in the art that changes and modi
?cations may be made in my invention and I, .
therefore,v aim in the appended claims to cover
all such changes and modifications as fali within
the true spirit and scope'of my invention.
'
What I claim as new and desire to secure by
Letters Patent of the United States is:
1. The combination, with an impedance for 45
carrying the current during operation of a trans
former tap changer under load, of protective
means for said impedance comprising anormally
open switch for short-circuiting the current path
through said impedance, means for biasing said 50
switch closed, and current responsive means for
holding said switch open unless the current
through said impedance exceeds a given value for
a given time.
2. The combination, with a circuit including an 55
impedance for carrying the current during op
eration of a transformer tap changer under load,
of protective means for said impedance compris
between the mid-point of winding 3 and the sup
ply side of the circuit. The by-pass 20 is prefer
ably made of the lightning arrester material
“Thyrite” previously referred to. With such a ing a normally open switch for bridging a por
connection
a high voltage surge, such as would tion of said circuit containing said impedance
60
result from a lightning stroke on tthe circuit 5, ' when said switch is closed, means for biasing said
switch closed, and a fusible element normally in
can readily pass through the negative characteris
tic resistors or impedances 20 and I in series as circuit with said impedance for holding said
these devices will lower their resistances very switch open unless the current therethrough ex
markedly with increase in voltage applied there- ceeds a given value for a given’ time.
3. The combination, with an‘ impedance for
to. Consequently the transformer windings are
prevented from injury due to over-voltage as a carrying the current during operation of a trans
former tap changer under load, of protective
result of such surges.
‘The switching duty imposed on the tension means for said impedance comprising a normally
fuse-operated switch in Fig. 2 is relatively greater open switch for short-circuiting the current path
through said impedance, means for biasing said
than that in Fig. 1, because this switch now actu
switch closed, and a fusible element normally in
ally removes a certain number of kilovolt am;
circuit
with said impedance for holding said
peres from the circuit. This switching principle
switch open, said switch and fusible element be—
is also somewhat different and may be describedv ing
so arranged that the closing of said switch
as follows: .
40
60
65
'
75
2,137,216
increases the distance between the terminals of
said fusible element.
'
4. The combination, with an impedance for
carrying the current during operation of a trans
former tap changer under load, of protective
means for said impedance comprising a relatively
quick acting element for short-circuiting the cur
rent path through said impedance when a current
above a predetermined value flows through said
impedance for a given time, and an additional
relatively slow acting element for short-circuit
ing said impedance if a current of less than said
predetermined value flows through said imped
ance for longer than said given time.
15
5. The combination, with an impedance for
carrying the current during operation of a trans
former tap changer under load, of protective
means for said impedance comprising a relatively
quick acting current responsive element for short
20 circuiting the current path through said imped
ance when a relatively high current ?ows through
said impedance for substantially longer than the
. time required by normal tap changing operation,
and relatively slow acting means responsive to the
25 temperature of said impedance for short-circuit
ing said impedance in case a current which is
of lower value than is required to operate said
relatively quick acting element but-which is suffi
given time for causing said switching means to
exclude said impedance.
10. In an electric circuit, a pair of spaced-con
tacts, an impedance, switching means having two
operating positions for closing circuits around
said contacts excluding and including respectively
said impedance, means responsive to current
through said impedance above a given value for a
given time for causing said switching means to
exclude said impedance, and means for short 10
circuiting said impedance when said switching
means is in a position to include said impedance.
' 11. In combination, an alternating current
closed circuit, a transformer winding connected
therein, means for changing taps on said winding,
a current conducting by-pass path for carrying
the load current of said circuit during a tap
change on said winding, said by-pass comprising
a common portion connected to a movable switch
contact and two parallel branch portions, one of 20
said branch portions being of relatively low im
pedance and being connected to a ?xed contact
normally out of contact with said movable switch
contact, the other branch portion containing an
impedance and a fuse connected to said movable 25
contact, and biasing means urging said movable
switch contact toward said ?xed contact.
12. In combination, an alternating current load
cient to cause injurious heating of said impedance circuit, a voltage regulator for said circuit com
30 continues to flow in said impedance.
prising a tap changing transformer connected 30
6. Protective means for a load current by-pass therein, an impedance connected so as normally
resistor in a transformer tap changer comprising, to carry the circuit load current during a tap
in combination, a normally open blade switch changing operation, a protective switch having a
having a ?xed. contact, a spring for urging the ' pair of normally separated contacts so connected
35 blade of said switch into engagement with said that when they engage they complete a by-pass
35
?xed contact, a tension fuse electrically and connection around said impedance, means for
mechanically connected to the blade of said ‘urging the contacts of said switch together, and a
switch for holding said switch open, a ‘conductor fuse in circuit with said impedance for holding
connecting one side of said resistor to said ?xed said switch contacts apart. said fuse being so pro
40 contact, and a conductor connecting the other
portioned as to ,melt when the tap changing mech
side of said resistor to said fuse whereby a current anism of the regulator stops in an “off” position 40
can ?ow through said resistor, fuse and blade in while said alternating current circuit is carrying
series and upon melting bf said fuse by a rela
a substantial load current whereby said impedance
tively long continued relatively high current said will be open-circuited by the melting of said fuse
45 resistor will be disconnectedand said switch will and substantially simultaneously therewith said
close thereby short-circuiting the current path protective switch will close and complete a by 45
through said resistor and fuse.
pass connection around said impedance for said
7. In combination, regulating means including circuit current.
a single movable contact switch for changing
13. In combination, an alternating current load
transformer tapsunderload,anegative resistance~ circuit, a transformer tap changing switch. a.
current characteristic resistor connected in shunt transformer having a tapped winding adapted to 50
with said means, and separate means responsive be connected in said circuit by means of said
respectively to the effects of relatively high and switch, an exciting winding on said transformer,
low currents flowing continuously through said an impedance for carrying the circuit load current
55 resistor for effectively short-circuiting it.
during a tap changing operation, means for pro
8. In combination, an alternating current cir
tecting said impedance against overloading
cuit, an auto-transformer connected therein, a should said tap changing switch stop in an inter
switch for changing taps on the series winding of mediate position. said protective means being op
said autotransformer, a resistor whose resistance erative to complete a relatively low impedance
60 decreases instantaneously with increases in cur
short circuit around at least a portion of said 60
rent therethrough connected to carry the circuit tapped winding should said tap changing switch
current during operation of said tap changing complete a tap changing operation. and means
switch, a normally open protective switch for operated by said protective means for deenergiz
short-circuiting the current path through said ing the exciting winding on said transformer.
65 resistor, a spring tending to close said protective
14. In combination, an alternating current load 65
switch, a fusible element for holding said pro
circuit, a voltage regulator for said circuit com
tective switch open against said spring, said fu
prising an autotransformer connected in said cir
sible element being normally connected in circuit cuit and provided with means for changing taps
with said resistor.
on the series winding thereof, an impedance for
70
9. In an electric circuit, a pair of spaced con
carrying the circuit load current during a tap
tacts, an impedance, switching means having two changing operation, and protective means for said
operating positions closing a circuit around said impedance operative as a result of the‘stopping of
contacts excluding and including respectively said said tap changing means in an "021,” positionwhen
impedance, and means responsive to current said circuit is carrying va substantial load current
75 through said impedance above a given value for'a for substantially simultaneously closing a rela
75
5
2,137,216
tively low impedance by-pass connection around
the normal current path through said impedance
and deenergizing the exciting winding of said
impedance operative as a result of the stopping
of said tap changing means in an “off” position
when said circuit is carrying a substantial load
transformer.
current for substantially simultaneously opening
'
‘
15. In combination, an alternating current load
circuit, a voltage regulator for said circuit com
prising an autotransformer connected in said cir
cuit and provided with means for changing taps
on the series windings thereof, an impedance for
carrying the circuit load current during a tap
changing operation, and protective means for said
the circuit through said impedance, closing a rela
tively low impedance by-pass connection around
the normal current path through said impedance
and opening the circuit of the common winding
of said transformer.
'
ORIN P. McCARTY.
10
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