U.S. patent number 3,662,253 [Application Number 05/083,383] was granted by the patent office on 1972-05-09 for tap changing system for regulating transformers.
Invention is credited to Saburo Yamamoto.
United States Patent |
3,662,253 |
Yamamoto |
May 9, 1972 |
TAP CHANGING SYSTEM FOR REGULATING TRANSFORMERS
Abstract
A tap changing system for regulating transformers disclosed here
is of one resistor type in which vacuum switches are used for its
main arcing contacts and a semiconductor switch or a semiconductor
controlled switch bidirectionally conductive on AC is used in
series to a current limiting resistor circuit which is switchably
connected in parallel to said vacuum switches. Tap changing of one
resistor method is effected by on-off operation of the vacuum
switches, whereby on-off operation of the semiconductor switch is
automatically effected.
Inventors: |
Yamamoto; Saburo
(Higashimurayama-shi, Tokyo, JA) |
Family
ID: |
13938072 |
Appl.
No.: |
05/083,383 |
Filed: |
October 23, 1970 |
Foreign Application Priority Data
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Nov 4, 1969 [JA] |
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44/88266 |
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Current U.S.
Class: |
323/343;
336/150 |
Current CPC
Class: |
H01F
29/04 (20130101) |
Current International
Class: |
H01F
29/04 (20060101); H01F 29/00 (20060101); G05f
001/14 (); H02p 013/06 () |
Field of
Search: |
;323/43.5R,43.5S
;336/150 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,805,378 |
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May 1970 |
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DT |
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1,166,772 |
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Oct 1969 |
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GB |
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Primary Examiner: Goldberg; Gerald
Claims
Having thus described my invention, I claim:
1. A tap changing system for regulating transformers comprising a
plurality of vacuum switches interposed between a lead on the load
side and two pairs of tap selectors belonging to the odd number
taps and the even number taps of tap windings respectively, a
semiconductor switch conductive on AC, a high impedance bypass
circuit for said semiconductor switch utilizing voltage build-up
rate reduction circuit comprising a capacitor, a damping resistor
and a silicon diode, and a circuit device having therein a current
limiting resistor connected in series to said semicon-ductor
switch, said circuit device being connected between a lead on the
load side and a selector switch and. arranged to be switchable in
parallel to said two vacuum switches, wherein on-load tap changing
by one resistor method being performed during correlative switch-on
and switch-off operation of said two vacuum switches.
2. A tap changing system for regulating transformers as claimed in
claim 1, wherein said circuit device is connected stationarily in
parallel to one of the side of said two vacuum switches.
3. A tap changing system for regulating transformers comprising a
plurality of vacuum switches interposed between a lead on the load
side and two pairs of tap selectors belonging to the odd number
taps and the even number taps of tap windings respectively, a
semiconductor controlled switch conductive on AC, a high impedance
bypass circuit for said semiconductor switch utilizing voltage
build-up rate reduction circuit comprising a capacitor, a damping
resistor and a silicon diode, a gate control device for said
semiconductor controlled switch, said gate control device including
control means utilizing the secondary rise current of current
transformer inserted in the circuit of said high impedance bypass
circuit, and a circuit device having therein a current limiting
resistor connected in series to said semiconductor controlled
switch, said circuit device being connected between a lead on the
load side and a selector switch and. arranged to be switchable in
parallel to said two vacuum switches, wherein on-load tap changing
by one resistor method being performed during correlative switch-on
and switch-off operation of said two vacuum switches.
4. A tap changing system for regulating transformers comprising a
plurality of of vacuum switches interposed between a lead on the
load side and two pairs of tap selectors belonging to the odd
number taps and the even number taps of tap windings respectively,
a first semiconductor controlled switch conductive on AC, a high
impedance bypass circuit for said semiconductor switch utilizing
voltage build-u rate reduction circuit comprising a capacitor, a
damping resistor and a silicon diode, a gate control device for
said first semiconductor controlled switch, said gate control
device including control means utilizing the secondary rise current
of transformer inserted in the circuit of said high impedance
bypass circuit, a second semiconductor controlled switch conductive
on AC, gate control device for said second semiconductor controlled
switch, said gate control devices including control means utilizing
the secondary current of current transformers inserted in the
circuit of said vacuum switches, and a circuit device having
therein a current limiting resistor connected in series to said
first semiconductor controlled switch and said second semiconductor
controlled switch connected in parallel to both of said current
limiting resistor and said first semi-conductor controlled switch,
said circuit device being connected between a lead on the load side
and a selector switch and. arranged to be switchable in parallel to
said two vacuum switches switch, wherein non-arc tap changing by
one resistor method being performed during correlative switch-on
and switch-off operation of said two vacuum switches.
Description
BRIEF DESCRIPTION OF THE INVENTION
This invention relates to an on-load tap changer for changing over
the taps of transformer, reactor or the like operated under
load.
It is well known that tap changing in a conventional type of
on-load tap changer is accompanied by arcing at the contacts in
insulating oil. In this type of on-load tap changer, the insulating
oil is deteriorated and the contacts are worn by arc, and they need
frequent check and maintenance for safe operation of the apparatus.
This is why the operating efficiency is low in the conventional
type of on-load tap changer.
In view of the foregoing, an object of this invention is to provide
a new, compact and low cost apparatus for tap changing under load
without deteriorating the insulating oil and with very little wear
of the contacts.
Another object of the invention is to provide a simpler and more
reliable apparatus for tap changing under load without causing wear
of contacts by arc.
The on-load tap changer of this invention is of one resistor type
using vacuum switches for the current switching contacts. More
specifically, the vacuum switches are used for the main current
switching contacts which are disposed between the lead on the load
side and two pairs of tap selectors belonging to the odd number of
taps and the even number of taps of tap windings respectively, a
semiconductor switch or a semiconductor controlled switch
bidirectionally conductive on AC is used for the current switching
contact of a current limiting resistor circuit switchably connected
in parallel to the vacuum switches, and said semiconductor switch
is made conducting or nonconducting automatically without resorting
to contact means, synchronizing with on-off of the two vacuum
switches. Thus the switching performance is improved, the
construction of the apparatus is simplified and the size and the
cost of the apparatus is reduced. Namely, the invention provides a
novel switching apparatus characterized by a feature that its
switching operation, based on one resistor method, is performed
safely and securely in such a short period as one cycle or so by
utilizing simple, high speed switching operation effected by
correlatively operable vacuum switches. The switching apparatus of
this invention is further characterized by a feature that another
semiconductor controlled switch is connected in parallel to the
current limiting resistor circuit comprising serially said
semiconductor controlled switch, and the firing control is done by
a gate control device using the secondary current of a current
transformer inserted into each circuits having a vacuum switch, the
firing control of which is synchronized with the vacuum switch
operation, whereby non-arc switching operation is realized.
The invention will be better understood from the following
description taken in connection with the accompanying drawings, and
its scope will be pointed out in appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are circuit diagrams showing conventional resistor
type on-load tap changers using vacuum switches,
FIGS. 3 through 6 are circuit diagrams showing the principles of an
on-load tap changer of this invention, and
FIGS. 7 through 10 are circuit diagrams showing switching devices
embodying this invention.
DETAILED DESCRIPTION OF THE INVENTION
Tap changing in conventional type of on-load tap changer is
accompanied by arcing at the contacts in insulating oil. Arcing in
insulating oil causes oil contamination and contact erosion, and
therefore they need frequent check and maintenance for safe
operation of the apparatus.
To eliminate deterioration of the insulating oil, various on-load
tap changers using vacuum switches for their current switching
circuits have been proposed. Examples of this type of on-load tap
changer are shown in FIGS. 1 and 2. Note that these apparatus are
of one resistor type.
Referring to FIG. 1, the reference T denotes a tap winding with a
plurality of taps, and S.sub.1 and S.sub.2 are tap selectors
belonging to odd number taps and even number taps respectively.
V.sub.1 and V.sub.2 are vacuum switches connected between the tap
selectors S.sub.1 and S.sub.2 and the lead L on the load side.
R.sub.1 denotes a current limiting resistor switchably connected to
the vacuum switches V.sub.1 and V.sub.2 by way of a selector
S.sub.3. FIG. 1 shows the state that the vacuum switch V.sub.1 is
in the close position, V.sub.2 in the open position, selector
S.sub.3 in the close position on the side of vacuum switch V.sub.1,
and current is supplied from the tap on the side of tap selector
S.sub.1. Current supply can be changed over from the tap selector
S.sub.1 to S.sub.2 in such switching sequence that the current
limiting resistor R.sub.1 is bridged across the selector taps by
switching the selector S.sub.3 from the vacuum switch V.sub.1 to
V.sub.2, the vacuum switch V.sub.2 is opened to transfer the load
current to the circuit of the current limiting resistor R.sub.1,
and then the vacuum switch V.sub.2 is closed. In this manner, the
tap can be switched to the other side. In this switching apparatus,
however, a predischarge for a certain duration takes place
ascribable to the voltage across the taps if the switching speed of
the selector S.sub.3 is slow, or an arc is produced due to a large
leap at the contact if the switching speed of S.sub. is high. This
makes it difficult to prevent perfectly wear of the contact and
deterioration of the insulating oil. In addition, the vacuum switch
must be operated to flow or stop the sum of the load current and
the circulating current between taps according to the switching
direction in connection with rise and drop of the voltage. This
lowers the switching efficiency.
FIG. 2 is to illustrate another switching apparatus in which a
current limiting resistor R as in FIG. 1 is connected to the side
of one of the vacuum switches, and its switching operation is done
by a vacuum switch V.sub.3. FIG. 2 a shows the state that the
vacuum switch V.sub.1 is in the close position, V.sub.2 and V.sub.3
in the open position, and current is supplied from the tap on the
tap selector S.sub.1. Current supply can be switched from the tap
on the tap selector S.sub.1 to S.sub.2 in such switching sequence
that the vacuum switch V.sub.3 is closed, V.sub.1 is opened, and
then V.sub.2 is closed. To switch back from the tap on S.sub.2 side
to that on S.sub.1 side, the vacuum switch V.sub.2 is opened,
V.sub.1 is closed, and then V.sub.3 is opened. In this type of
switching apparatus, there is no deterioration of the insulating
oil by the reason as in the apparatus of FIG. 1. On the other hand,
however, increase of the necessary number of vacuum switches used
per phase causes both increase of dimension of the apparatus and
complication of the driving mechanism of the apparatus. Furthermore
the sum of the load current and the circulating current between the
taps must be switched according to the switching direction in
connection with rise and drop of the voltage as in the case of the
switching apparatus of FIG. 1.
In view of the foregoing, an object of this invention is to provide
a new, compact and low cost on-load tap changer operable without
deteriorating the insulating oil.
Another object of this invention is to provide a simpler and more
reliable on-load tap changer operable without causing wear of the
contacts due to arc. The invention will be better understood from
the following description in connection with the accompanying
drawings.
FIG. 3 is a circuit diagram showing the principles of the on-load
tap changer of this invention with respect to one phase. Referring
to FIG. 3, the reference T denotes a tap winding with a plurality
of taps, and S.sub.1 and S.sub.2 are tap selectors belonging to the
odd number taps and the even number taps respectively. V.sub.1 and
V.sub.2 are vacuum switches connected between the tap selectors
S.sub.1 and S.sub.2 and the lead L on the load side. SSS denotes a
semiconductor switch displaying bidirectional breakover on AC at a
specific voltage. For this semiconductor switch, the silicon
symmetrical switch, bidirectional triode thyristor, or
reverse-blocking triode thyristors connected reversely parallel to
each other may be used. R denotes a current limiting resistor, and
S.sub.3 is a selector for selecting the vacuum switch V.sub.1 or
V.sub.2 in parallel to the circuit having serially the resistor R
and the semiconductor switch SSS. The reference D denotes a high
impedance bypass circuit disposed in parallel to the semiconductor
switch SSS. For this circuit, a voltage build-up rate reduction
circuit which is generally used for semiconductor circuits may be
used when the tap voltage is high, or a simple high resistance
circuit may be used when the tap voltage is low. FIG. 4 shows an
example of voltage build-up rate reduction circuit in which two
pairs of circuit comprising a capacitor K connected serially to a
circuit having in parallel a silicon diode SR and a discharge
resistor Rd are connected in reversely parallel to each other. FIG.
5 shows a circuit having a plurality of serially connected
semiconductor switches used when the tap voltage is high. Referring
to FIG. 5, voltage build-up rate reduction circuits D.sub.1 and
D.sub.2 are connected in parallel to serially connected
semiconductor switches SSS.sub.1 and SSS.sub.2 respectively, and
branching high resistors Rs.sub.1 and Rs.sub.2 are connected in
parallel to D.sub.1 and D.sub.2. This circuit is operated as a
serial equalizing circuit of the semiconductor switching circuits
during switching operation. FIG. 5 shows an example using a
plurality of serially connected bidirectional triode thyristors.
When reverse-blocking triode thyristors are used instead of
bidirectional triode thyristors, said serial equalizing circuit
must be provided for each semiconductor switch for said operation
purpose.
The operation of this switching device will be described below.
FIG. 3 shows the state that the vacuum switch V.sub.1 is in the
close position, V.sub.2 in the open position, selector S.sub.3 in
the close position on the side of vacuum switch V.sub.2, and
current is supplied over the current from the tap on the side of
tap selector S.sub.1 . To change over the current supply from the
tap on S.sub.1 side to that on S.sub.2 side, the selector S.sub.3
is switched slowly from the side of vacuum switch V.sub.2 to the
side of V.sub.1, the vacuum switch V.sub.1 is opened and, after
leaving this state for more than half cycle, the vacuum switch
V.sub.2 is closed. An arc is drawn when the vacuum switch V.sub.1
is opened. One of the features of the vacuum switch is that the arc
voltage is as low as about 20V almost regardless of the arc
current. The capacitor K of the voltage build-up rate reduction
circuit D is charged by this low arc voltage. When the load current
reaches the next current zero point, the arc from the vacuum switch
V.sub.1 is extinguished and the load current is about to be
commutated into the voltage build-up rate reduction circuit D. At
this moment, the capacitor K which has been charged by the arc
voltage is reversely charged. In this process, the capacitor K
presents transiently a very low impedance for the period of tens
and several microseconds to several tens microseconds. (This period
depends on the capacitance of the capacitor K and the value of the
load current.) Thus the initial build-up rate of the transient
recovery voltage produced at the cutoff of the load current by the
vacuum switch can be markedly reduced by the voltage build-up rate
reduction circuit D. In this manner, the circuit D operates to let
the vacuum switch cut off the load current securely at a high
efficiency, and to facilitate the initial commutation of the load
current. Assume that this switching apparatus is not provided with
the voltage build-up rate reduction D. Then a restriking voltage
having a very high build-up rate of the power circuit is applied to
both the vacuum switch and the semiconductor switch when an
exciting current of no load transformer or a light load current of
low power factor is switched off. Without the voltage build-up rate
reduction circuit D, therefore, the vacuum switch must have a
greater breaking ability, and technical and economical sacrifice
must be made for the vacuum switch whose main purpose is to switch
the circuit of tap voltage corresponding to only a few percent of
the power circuit voltage. Furthermore it is difficult to maintain
stable switching operation if the switching apparatus has no
circuit D. Since the voltage build-up rate reduction circuit D has
a very high impedance against the power source frequency, its
terminal voltage is naturally to exceed the voltage between the
taps and rapidly increase toward the circuit voltage when the
commutated load current flows therein for the period of tens and
several microseconds to several tens microseconds from zero value.
When this overvoltage exceeds the breakover voltage of the
semiconductor switch SSS, the semiconductor switch SSS is directly
made conducting and the load current is supplied thereto via the
current limiting resistor R. The load current, upon reaching its
zero point, is cut off by the semiconductor switch SSS, and again
is commutated into the voltage build-up rate reduction circuit D to
result in an overvoltage. By this overvoltage, the semiconductor
switch SCR is made conducting again, and then the vacuum switch
V.sub.2 is closed. As a result, all the load current flows through
V.sub.2, the tap circulating current limited by the current
limiting resistor R flows in the semiconductor switch SSS and then
is cut off at the next current zero point. Thus one switching
operation is completed. In the same manner as described above, the
tap 13 changed over to the reverse side.
As described above, the first object of this invention is realized
by using the circuit arrangement as in FIG. 3. The switching
operation can further be stabilized and the reliability can be
increased by the arrangement as in FIG. 6 wherein the firing
control is done by utilizing a semiconductor controlled switch in
place of the semiconductor switch SSS. Referring to FIG. 6, the
reference CT denotes a current transformer inserted into a voltage
build-up rate reduction circuit D, and C is gate control device for
a semiconductor controlled switch SCR. In this switching apparatus,
the rise of the load current commutated into the voltage build-up
rate reduction circuit D due to the cutoff the load current by the
vacuum switch is caught by the current transformer CT, and the
firing control of the semiconductor controlled switch SCR is done
by the gate control device C using the rising current of CT. In
this switching apparatus, a semiconductor switch having a control
electrode is used, and a first means in which the circuit
conduction by the load current switched under no-load state to load
state is effected by the breakover by the overvoltage produced at
the switching operation is provided, and a second means in which
the circuit conduction by the load current is made by the firing
control of the gate control device is also provided whereby the
stability and reliability of the switching operation are
increased.
FIG. 7 shows an arrangement to which the switching device as in
FIGS. 3 and 6 is applied. This arrangement is such that the circuit
of semiconductor controlled switch SCR is connected in parallel to
one of the vacuum switches which is normally closed, and the SCR
circuit is changed over to the other vacuum switch which is opened
at the switching operation. The semiconductor controlled switch SCR
is used reversely with respect to the arrangements as in FIGS. 3
and 6. FIG. 7 shows the state that the vacuum switch V.sub.1 is in
the close position, V.sub.2 in the open position, selector S.sub.3
in the close position on the side of vacuum switch V.sub.1, and
current is supplied from the tap on the side of tap selector
S.sub.1. To change over the current supply from the tap on the side
of S.sub.1 to S.sub.2, the selector S.sub.3 is switched to the side
of vacuum switch V.sub.2, and then the vacuum switch V.sub.1 is
opened and V.sub.2 is closed. When the vacuum switch V.sub.1 is
opened, the load current is commutated transiently into the voltage
build-up rate reduction circuit D, the semiconductor controlled
switch SCR is made conducting by firing control of the gate control
device C, and the load current is supplied thereto from the tap on
the side of tap selector S.sub.2. When the vacuum switch V.sub.2 is
closed, all the load current flows through V.sub.2. Thus the
switching operation is completed.
FIG. 8 shows another example of arrangement to which the invention
is applied, wherein the circuit of semiconductor controlled switch
is stationarily connected in parallel to one of the vacuum
switches. In this arrangement, the tap is changed over in the
manner as in FIG. 6 and that as in FIG. 7 alternately according to
the switching direction.
In the switching apparatus as shown in FIG. 3 through 8, the load
current is changed over by using the vacuum switches and
semiconductor switch and, accordingly, there is no deterioration of
the insulating oil. On the other hand, however, wear of the
contracts of the vacuum switch is unavoidable. To eliminate wear of
the contacts, the invention provides an arrangement based on the
construction of the switching apparatus as in FIG. 3 or 6. This
arrangement is such that a semiconductor controlled switch SCR
having the current limiting resistor R in series is connected in
parallel to the circuit of another semiconductor controlled switch,
and the firing control of the semiconductor controlled switch is
done synchronizing with the switching of the vacuum switch whereby
it is easily made possible to switch the vacuum switch without
causing arc. FIG. 9 shows an embodiment thereof, wherein the
components common to FIG. 6 are indicated by the identical
references. Referring to FIG. 9, SCR.sub.A denotes a semiconductor
controlled switch in which bidirectional triode thyristors similar
to SCR or reverse-blocking triode thyristors are connected in
reversely parallel to each other. CT.sub.1 and CT.sub.2 are current
transformers inserted into the circuits of vacuum switches V.sub.1
and V.sub.2 respectively, and C.sub.1 and C.sub.2 are gate control
devices using the secondary current of CT.sub.1 and CT.sub.2. The
firing control on the semiconductor controlled switch SCR.sub.A is
done by the outputs of the gate control devices C.sub.1 and C.sub.2
through a selector S.sub.4. This firing control of semiconductor
controlled switch SCR is done by the gate control device C using
the secondary current of the current transformer CT connected to
the voltage build-up rate reduction circuit D as in the apparatus
of FIG. 6. FIG. 9 shows the state that the vacuum switch V.sub.1 is
in the close position, V.sub.2 in the open position, selector
S.sub.3 in the close position on the side of V.sub.2, selector
S.sub.4 in the close position on the side of gate control device
C.sub.2, and current is supplied from the tap on the side of tap
selector S.sub.1. To change over the current supply from the tap on
the side of S.sub.1 to S.sub.2, the selector S.sub.3 is switched to
the side of vacuum switch V.sub.1, the selector S.sub.4 is switched
to the side of gate control device C.sub.1, and the semiconductor
controlled switch SCR.sub.A is kept under the gate control. Then,
by opening the vacuum switch V.sub.1, the load current is cut off
at the existing phase without causing arc and transferred to the
circuit of semiconductor controlled switch SCR.sub.A. However,
because the vacuum switch V.sub.1 has already been opened, no
firing control is effected by the gate control device C.sub.1, and
the load current of the semiconductor controlled switch SCR.sub.A
is cut off at the next current zero point and commutated into the
voltage build-up rate reduction circuit D. The rise of the load
current at this moment is caught by a current transformer CT, and
the semiconductor controlled switch SCR is fired by the gate
control device C. By this, all the load current flows in the
semiconductor controlled switch SCR. Then, when the vacuum switch
V.sub.2 is closed, all the load current flows through V.sub.2, the
circulating current between the taps flows in the semiconductor
controlled switch SCR, and it is cut off at the next current zero
point. Thus the switching operation is completed. In the same
manner as above, the tap is changed over reversely without causing
arc. The breakover characteristic of the semiconductor controlled
switch SCR used for this switching apparatus is to be determined so
that the semiconductor controlled switch SCR is made conducting
securely prior to the conduction of the semiconductor controlled
switch SCR.sub.A with respect to the external surge overvoltage and
the transient overvoltage produced in the circuit at switching
operation. In this switching apparatus, therefore, the
semiconductor controlled switch SCR breaks over always prior to
SCR.sub.A by the transient overvoltage produced at switching
operation even if both the semiconductor controlled switches
SCR.sub.A and SCR fail in firing control under no load or light
load condition. Thus switching operation with arc can be done as in
the case with the device in FIG. 3. Also, in this switching
apparatus, the semiconductor controlled switch SCR comprising
serially a current limiting resistor R can serve as an overvoltage
protective device for the switching apparatus as a whole. Therefore
it is possible to use both the semiconductor controlled switches
SCR and SCR.sub.A in the state of their being connected to the
switching circuit at all times.
FIG. 10 shows another embodiment of the invention in which the
circuit arrangement using semiconductor controlled switch as in
FIG. 9 is used. The switching device of FIG. 10 is such that a pair
of circuit devices comprising a semiconductor controlled switch SCR
and a semiconductor switch SSS having a voltage build-up rate
reduction circuit D and a current limiting resistor R are connected
in parallel to vacuum switches V.sub.1 and V.sub.2 respectively,
whereby the load current is changed over. The circuit devices in
FIG. 10 are indicated by the references with the appendixes A and a
on the side of vacuum switch V.sub.1, and B and b on the side of
vacuum switch V.sub.2. Further description of these circuit devices
is omitted. The references S.sub.4 and S.sub.5 denote selectors for
switching the output circuits of the gate control device according
to the switching direction. The figure shows the state that the
vacuum switch V.sub.1 is in the close position, V.sub.2 in the open
position, the selector S.sub.4 in the close position on the side of
gate control device Cb, the selector S.sub.5 in the close position
on the side of gate control device C.sub.2, and current is supplied
from the tap on the side of selector S.sub.1. To change over the
current supply from the tap on the side of selector S.sub.1 to
S.sub.2, the selector S.sub.4 is changed over to the side of gate
control device C.sub.1, the selector S.sub.5 to the side of gate
control device Ca, thereby placing the semiconductor controlled
switch SCR.sub.A under the firing control. Then, when the vacuum
switch V.sub.1 is opened, the load current is cut off at the
existing phase without arc, and is transferred to the circuit of
the semiconductor controlled switch SCR.sub.A. This load current is
cut off at the next current zero point and then is commutated into
the voltage build-up rate reduction circuit Da. Thus the
semiconductor controlled switch SCR.sub.B is made conducting by the
firing control of gate control device Ca, and the load current is
supplied from the tap on the side of tap selector S.sub.2.
Following this operation, when the vacuum switch V.sub.2 is closed,
all the load current flows through the switch V.sub.2. Thus the
switching operation is completed. In the same way as above, the tap
is changed over reversely without causing arc. The normal switching
operation in this switching device can be done without arc as in
the foregoing manner, without the aid of the semiconductor switches
SSS.sub.a and SSS.sub.b comprising serially current limiting
resistors Ra and Rb. Now assuming that this switching apparatus is
not provided with the circuit of semiconductor switch having said
current limiting resistor, there is possibility of causing
short-circuit between the taps if both the switches SCR.sub.A and
SCR.sub.B should fail in firing control and SCR.sub.A and SCR.sub.B
become conducting at random due to the transient overvoltage
produced at switching operation. In this switching apparatus,
however, the switching operation with arc as in the case with the
device of FIG. 3 can be done by the use of semiconductor switches
SSS.sub.a and SSS.sub.b comprising serially a current limiting
resistor R, even in the event that the semiconductor controlled
switches SCR.sub.A and SCR.sub.B fail in firing control, therefore,
the semiconductor controlled switches SCR.sub.A and SCR.sub.B can
be used in the state of their being stationarily connected to the
switching circuit.
The on-load tap changers embodying this invention as have been
illustrated in FIGS. 3 through 8 are advantageous in the following
points in comparison with the conventional type of on-load tap
changers shown in FIGS. 1 and 2.
1. A semiconductor switch is used for the switching contact of the
current limiting resistor circuit and thus the switching operation
is done without contact and arc.
2. Unlike the conventional switching method, the semiconductor
switch which serves as the current limiting resistor contact is
made conducting and non-conducting automatically immediately after
the vacuum switches which serve as the main contacts have been
operated correlatively.
3. Switching operation is done in a very short period of time by
virtue of high speed switching performed by two correlatively
operable vacuum switches.
4. Current cutoff by the vacuum switch is done once against only
the load current irrespective of the direction of voltage rise or
drop.
Also, the load tap changers of this invention as shown in FIGS. 9
and 10 have the following features compared with the conventional
type of on-load tap changer.
1. On-off of the vacuum switch can be done without arc by simple
circuit arrangement.
2. Firing control of each semiconductor controlled switch is
performed automatically and at a high speed only depending on the
electrical conditions of the switching circuit.
3. It is possible to use the semiconductor controlled switches in
the state of their being connected to the switching circuit at all
times.
4. The switching period is very short, and the device is protected
against overvoltage due to external surge. Therefore, it is hardly
probable that the device fails in switching due to misoperation of
the semiconductor controlled switches.
As has been described above, the on-load tap changer of this
invention makes various advantages available; for example, two
vacuum switches are sufficient per phase, circuit construction is
simple, and switching performance is excellent. The on-load tap
changer of this invention can therefore be used in many ways for
any capacity at reduced costs, such as for line voltage regulator
of high voltage distribution system and super-high voltage
transformer of large capacity.
While the principles of the invention have been described above in
connection with specific embodiments, and particular modifications
thereof, it is to be clearly understood that this description is
made only by way of example and not as a limitation on the scope of
the invention.
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