U.S. patent number 3,614,464 [Application Number 04/818,216] was granted by the patent office on 1971-10-19 for arcless tap- or source-switching apparatus using series-connected semiconductors.
This patent grant is currently assigned to ITE Imperial Corporation. Invention is credited to Walter V. Chumakov.
United States Patent |
3,614,464 |
Chumakov |
October 19, 1971 |
ARCLESS TAP- OR SOURCE-SWITCHING APPARATUS USING SERIES-CONNECTED
SEMICONDUCTORS
Abstract
Tap- or source-switching apparatus employable with high-power
electrical inputs for connection to a load by means of a mechanical
switch in a manner to exhibit little or no arcing contact material
erosion or deterioration. A semiconductor switch is connected in
series with one current-carrying contact and in parallel with
another contact, and serves to establish current initiation through
the semiconductor switch and to provide current interruption across
that switch. The first of said current-carrying contacts is
arranged to be closed prior to the closing of the semiconductor
switch and the second said contact and to remain closed until after
the opening of those devices.
Inventors: |
Chumakov; Walter V. (Huntingdon
Valley, PA) |
Assignee: |
ITE Imperial Corporation
(Philadelphia, PA)
|
Family
ID: |
25224980 |
Appl.
No.: |
04/818,216 |
Filed: |
April 22, 1969 |
Current U.S.
Class: |
361/8 |
Current CPC
Class: |
H01H
9/0005 (20130101); H01H 9/542 (20130101) |
Current International
Class: |
H01H
9/00 (20060101); H01H 9/54 (20060101); H01h
009/30 () |
Field of
Search: |
;307/133,134,85,86,87,136,112 ;317/11.1,11.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hohauser; Herman J.
Claims
The embodiments of the invention is which an exclusive privilege or
property is claimed are defined as follows:
1. Electric power switching apparatus comprising:
input circuit means;
output circuit means;
first mechanical switching means having a pair of metallic
contacts, one of which is coupled to said input circuit means and
both of which are subject to arcing, erosion, deterioration and the
like if said contacts are initially engaged to couple said input
and output circuit and if said contacts are initially disengaged to
decouple said input and output circuit means;
second mechanical switching means, also having a pair of metallic
contacts, serially coupled between the other of said pair of first
mechanical switching means metallic contacts and said output
circuit means;
semiconductor switching means coupled in parallel across said
second meachanical switching means; and
timing control means for closing said semiconductor switching means
after closing of said first mechanical switching means by said
control means and prior to closing of said second mechanical
switching means by said control means to thereby effect current
initiation between said input and output circuit means through said
semiconductor switching means and for opening said semiconductor
switching means after opening of said second mechanical switching
means by said control means and prior to opening of said first
mechanical switching means by said control means to thereby effect
current interruption between said input and output circuits means
across said semiconductor switching means;
whereby said arcing, erosion, deterioration and the like is
substantially lessened.
2. Electric power switching apparatus as defined in claim 1,
wherein said timing control means is included for opening said
semiconductor switching means after closing of said second
mechanical switching means by said control means and prior to
opening of said first mechanical switching means by said control
means.
3. Electric power switching apparatus as defined in claim 1,
wherein said timing control means includes a source of control
signals for said semiconductor switching means and mechanical
linkage means operative to open and close said first and second
mechanical switching means and to condition the application of
control signals to said semiconductor switching means, all in a
predetermined time sequence.
4. Electric power switching apparatus as defined in claim 1,
wherein said input circuit means comprises a source of direct
current, wherein said semiconductor switching means includes one of
a group consisting of a transistor, a silicon-controlled rectifier,
a triac and similar conductivity controllable semiconductor devices
and wherein said timing control means is operative to close said
switching means by coupling appropriate signals to a control
electrode of the one of the semiconductor devices included.
5. Electric power switching apparatus as defined in claim 1,
wherein said input circuit means comprises a source of alternating
current, wherein said semiconductor switching means includes at
least one of a group consisting of a transistor, a
silicon-controlled rectifier, a triac and similar conductivity
controllable semiconductor device and wherein said timing control
means is operative to close said switching means by coupling
appropriate signals to a control electrode of each of the
semiconductor devices included to condition said devices to conduct
for both positive and negative cycles of an alternating current
waveform.
6. Electric power switching apparatus as defined in claim 1,
wherein said input circuit means consists of one of an electric
power generating source and a terminal at which electric power is
provided and wherein said output circuit means includes a load
therefor.
7. The combination comprising:
a plurality of taps at which electric power is provided;
a load;
first mechanical switching means having a contact arm connected to
one of the plurality of said taps and having a pair of metallic
contacts, one of which is coupled to said contact arm and both of
which are subject to arcing, erosion, deterioration and the like if
said contacts are initially engaged to couple said taps and said
load means and if said contacts are initially disengaged to
decouple said taps and said load;
second mechanical switching means, also having a pair of metallic
contacts, serially coupled between the other of said pair of first
mechanical switching means metallic contacts an said load;
semiconductor switching means coupled in parallel across said
second mechanical switching means; and
timing control means for closing said semiconductor switching means
after closing of said first mechanical switching means by said
control means and prior to closing of said second mechanical
switching means by said control means to thereby effect current
initiation between said taps and said load through said
semiconductor switching means and for opening said semiconductor
switching means after opening of said second mechanical switching
means by said control means and prior to opening of said first
mechanical switching means by said control means to thereby effect
current interruption between said taps and said load across said
semiconductor switching means;
whereby said arcing, erosion, deterioration and the like is
substantially lessened.
8. The combination as defined in claim 7, wherein said timing
control means is included for opening said semiconductor switching
means after opening of said second mechanical switching means by
said control means and prior to opening of said first mechanical
switching means by said control means.
9. The combination as defined in claim 8, wherein said timing
control means is included for moving the contact arm of said first
mechanical switching means from one of the plurality of said taps
to another of the plurality of said taps, and to thereby effect
closing of said first mechanical switching means metallic
contacts.
10. Electric power switching apparatus as defined in claim 5
wherein said output circuit means includes a center tapped
transformer.
11. The combination as defined in claim 9 wherein said load
includes a center-tapped transformer.
Description
The present invention relates to electric power switching
arrangements, in general, and to arcless tap-changing equipment
employing semiconductor switching, in particular. As will become
subsequently clear, the present invention discloses a modification
of the arcless tap-changing equipment described in the copending
application Ser. No. 818,331; filed concurrently herewith, and
assigned to the same assignee as the instant invention.
In many electric relay-type devices, power switching is achieved by
the making and breaking of metallic contacts. In the "make" or
closed contact position, electric power loss and temperature rise
at the contacts are relatively low. The contact material, its
resistance, and its cooling surface area are selected so that the
temperature rise and erosion of the material during the "make"
condition are kept within prescribed limits.
However, severe problems of erosion occur in going from the "make"
to the "break" or open contact condition, and vice versa. In
particular, the interruption of electric power in going from the
"make" to the "break" condition usually causes arcing, contact
material transfer from one surface to the other or evaporation, and
an overall general deterioration of the contact surfaces. The
contact evaporation or deterioration that takes place usually
continues until the arc itself is extinguished and the temperature
between the contact surfaces decreases below a certain value.
Arcing, and the problems it cause, can also take place prior to the
closing of the contact surfaces, i.e., in going from the "break" to
the "make" condition. Localized hotspots can be created to cause
the erosion, which also may be caused by localized increase in the
contact current density immediately after the "make"
transition.
These problems are especially acute in high-power electrical source
and tap-changing equipment because of the desirability for a number
of reasons to keep the mass and the size of the switches to a
minimum, while obtaining maximum useful life of the contact
structure. These problems are particularly severe during the
interruption of highly inductive loads due to circuit voltage
buildup, elongation of the arc, and the possibility of the arc
restriking between the contact surfaces as they separate.
It is an object of the present invention, therefore, to provide
switching arrangements for tap- or source-changing equipment and
the like which cause little or no erosion or deterioration of
metallic contact switches.
It is a further object of the invention to provide such
arrangements in a manner to prevent arcing in both the "making" and
"breaking" of electrical switches.
As will become clear hereinafter, arcless switching arrangements
according to the invention employ semiconductor switching means in
combination with the metallic contact switching means of the type
described above. In one embodiment of the invention a main contact
designed for continuous current-carrying duty is connected in
series with a semiconductor switch to couple a source of electric
power to a load. An auxiliary or intermittent duty contact is
connected in parallel with the semiconductor switch and arranged to
short circuit the switch when the apparatus is in the normal
current-carrying condition.
As will also become apparent, this embodiment combines numerous
advantages of metallic contact switching arrangements and
semiconductor switching arrangements, while at the same time
avoiding many disadvantages associated with switching means of
purely one type or the other.
A mechanically or electrically operable control source serves to
close the semiconductor switch after the main contact closes or
engages and to open the semiconductor switch before the main
contact opens or disengages.
Other objects and advantages of the present invention will be more
clearly understood from a consideration of the following
description of the apparatus of the invention taken in connection
with the drawings, in which:
FIG. 1a and 1b show a commonly used tap-changing arrangement
illustrating the problems caused by arcing;
FIG. 2 is a suggested tap-changing arrangement utilizing purely
semiconductor switching;
FIG. 3 shows one embodiment of arcless tap-changing equipment
constructed in accordance with the principles of the present
invention;
FIG. 3a shows a switching diagram useful in understanding of the
arcless tap-changing equipment of FIG. 3;
FIG. 4 shows another embodiment of arcless tap-changing equipment
according to the invention;
FIG. 4a shows a switching diagram useful in an understanding of the
equipment of FIG. 4;
FIGS. 5-8 show other embodiments of the arcless tap-changing
equipment; and
FIGS. 9a and 9b show switching diagrams illustrating the operation
of the equipment of FIG. 6.
Referring now more particularly to the tap-changing equipment of
FIGS. 1a and 1b, the arrangement there shown includes a three-phase
autotransformer 10, having an exciting winding 12 and a series
winding 14. A series of taps or terminals a-e are shown on the
series winding 14 and are contacted by a multiposition tap selector
switch or tap changer 16. Only one phase of the tap selector 16 is
shown for the purpose of simplicity, with its contacts A and B
being connected to taps a, b at a location which is one step before
the minimum system output condition in this buck position. As is
well known, the movement of the contacts A and B proceed, for
example, from tap a to tap b, then from tap b to tap c, etc. As is
also well known, in going from one tap to another --such as from
tap a to tap b --a point is reached prior to reaching the intended
tap at which the output voltage corresponds essentially to the
midpoint voltage between the two.
The contacts A and B are also connected, as shown, through a
switching reactor or preventive center top transformer 18 which
permits commutation of load current in the transformer from one tap
to another without the complete interruption and excessive
circulating current between taps. An output terminal 20 is
connected to the midpoint of the transformer 18 and is connected to
various utilization circuits (not shown). The tap selector 16 is
operated to move from one position to another by a reversible motor
22 and a suitable drive or gear arrangement 24.
In order to reduce arcing and contact erosion in the tap-changing
arrangement, an intermittent motion means such as a Geneva gear
arrangement 26 is normally used in the drive mechanism. Such usage
also provides for better positioning of the tap selector 16 and
also provides for a relatively fast snap action when the switch
contacts A and B are moved from one tap position to another.
The arrangement of FIG. 1 may also contain separately actuated
reversing switches 28 and 30 for reversing the connection of the
series winding 14 with respect to the exciting winding 12 when the
selector switch contacts A and B reach the tap e. Such reversing
switches serve to effectively double the control range of the
equipment since the tap selector 16 could again be actuated a given
number of times until the maximum boost position of FIG. 1b is
reached.
The illustrated tap-changing equipment of FIGS. 1 and 1b has
several distinct advantages and disadvantages. Among its advantages
are: (1) the ability to handle electric power at thousands of volts
and amperes at high efficiency and with little power loss; (2) a
relatively high short circuit and overload current capability; (3)
a moderate cost of contact surface replacement; and (4) proven
industrial application. Among its disadvantages are: (1) its
tendency to arc; (2) its large mass and limited useful life
resulting from contact wear; (3) its high initial and maintenance
cost; and (4) its limited speed of switching.
The switching mechanisms of the arrangement of FIG. 1, in addition,
are quite complex and, as noted, require careful maintenance. In
operation, the switches are normally immersed in some insulating
liquid (such as oil) to reduce the effects of arcing and to achieve
sufficient insulation or contact surface cooling. The arcing
products, though, may lead to deterioration of the insulating
properties and may very well result in flashover, especially if the
number of switch operations per unit of time is high.
FIG. 2 shows tap-changing equipment in which all the mechanically
operated parts associated with the arrangement of FIG. 1 are
replaced by controllable semiconductor switching devices --such as
silicon-controlled rectifiers and transistors. As is well known,
these devices can be brought to a conducting (or low forward
resistance) condition by application of suitable signals to their
respective control terminals. When the control signal is absent, on
the other hand, the semiconductor device will revert to a
nonconducting (or high reverse resistance) condition, thus,
effectively blocking or interrupting the main power flow from the
input terminal 32 to the output terminal 20.
The autotransformer of FIG. 2 is again shown as having five taps
a-e. The arrangement further shows inverse-- parallel connected
silicon-controlled rectifier pairs x-y connected between the
respective taps and the output terminal 20. Suitable control
circuits 42 are shown connected with the gate electrode of the
individual devices to provide control selection of the x-y pair
which is to be activated to establish the connection between an
individual one of the taps a-e and the utilization circuits
connected to the output terminal 20. Current limiting impedances
43-47 are similarly illustrated to prevent short circuit currents
from flowing between various ones of the taps a-e during switching
from one x-y pair to another. A pair of oppositely poled rectifiers
are included in each x-Y pair to enable connection to terminal 20
of both the positive and negative portions of the alternating
current signal applied to input terminal 32.
The arrangement of FIG. 2 offers the following advantages when
compared with the arrangement shown in FIG. 1: (1) it is completely
static, having no movable parts, and exhibits a switching speed in
the order of microseconds; (2) power interruption and initiation
occurs at the semiconductor junction of the devices and causes no
surface material erosion or deterioration; (3) the speed of
response is determined by the external utilization circuit, and
operation is relatively noiseless; and (4) the arrangement is one
requiring little or no maintenance. Also, it will be noted that the
arrangement of FIG. 2 is one in which no arcing problems are
created and in which convection and forced air cooling --rather
than one using and insulating liquid medium --is possible.
Among the disadvantages of the FIG. 2 arrangement, however, are:
(1) the requirement of the large number of semiconductors of high
power handling capability, although only one of them carries power
at any given time; (2) its high initial cost and high cost of
semiconductor replacement; (3) the requirement for expensive
control circuits for the individual devices; and (4) the relatively
high power loss associated with the arrangement as compared with
that for the moving contact apparatus.
Thus, it will be apparent that both the metallic contact and
semiconductor tap switching schemes offer both advantages and
disadvantages. These same advantages and disadvantages are present
when the scheme is employed to switch power sources, rather than
electrical taps, to a load. As will become clear below, tap- or
source-changing equipment according to the present invention
combines many advantages of the two switching equipment arrangement
with but few of their disadvantages.
Referring now to FIG. 3, the apparatus there shown illustrates one
embodiment of the present invention for coupling an electric power
source to a load; in part, by a metallic contact, but in a manner
in which arcing is eliminated by the use of semiconductor
switching. The principles of this and the other apparatus described
herein apply equally as well to source-changing equipments as to
tap-changing equipments. Thus, the terms "source," "power source,"
etc. as used herein may be used interchangeably with the term
"tap."
The apparatus of FIG. 3 includes a main metallic contact 50 and a
semiconductor switch 54 serially connected between the electric
power source 51 and an external load 52. An auxiliary contact 53 is
shown connected across the semiconductor switch 54. A timing
control means 55 is included and is effective to control the
conductive condition of the contacts 50 and 53 and of the switch
54, as indicated by the dotted lines. The semiconductor switch 54
in this and subsequently described embodiments of the present
invention may, for a alternating current of one source, comprise a
pair of transistors with the emitter of one connected to the
collector of the other as illustrated along side FIG. 3.
Alternatively, the semiconductor switch 54 may comprise a pair of
oppositely poled, parallel-connected silicon-controlled rectifiers.
Thirdly, a triac may be employed for the switch 54 in yet another
modification These latter two arrangements are also illustrated
alongside FIG. 3. For a direct current power source only one of
these transistors or silicon-controlled rectifiers need be
employed.
Basic to the operation of the apparatus shown in FIG. 3 is the
requirement that the semiconductors switch 54 be in its open or
nonconducting condition prior to the "making" or "breaking" of the
main contact 50. Also basic to the operation of the apparatus shown
in FIG. 3 is the requirement that the auxiliary contact 53 be
closed after the closing of the switch 54 and be opened prior to
the opening of that switch. Thus, the initiation of current flow
from the source 51 to the load 52 will occur in the semiconductor
switch 54. Similarly, the interruption of the current flow will
also occur in the switch 54, wit the overall result being that
little or not arcing will take place across the main contact 50.
This results from the fact that the voltage drop across the
semiconductor switch 54 at the point of current initiation or
current interruption is primarily controlled by the material
employed for the switch 54, and can be kept to a relatively low 1
or 2 volt value. That value is insufficient to cause significant
arcing of the contact 50 during its closing and opening.
The timing control means 55 may be of the type which is effective
to couple actuation control signals to either of the contacts 50
and 53 or the semiconductor switch 54, and to couple them in any
desired or preselected manner. FIG. 3a illustrates a switching
diagram showing the timed sequence of control signals supplied by
the timing means 55 to the apparatus of FIG. 3. FIG. 3a, for
example, illustrate the switching sequence wherein the main contact
50, the auxiliary contact 53 and the semiconductor switch 54 are
closed during the ON state of the apparatus, wherein the power
source 51 is connected to the load 52. FIG. 3a also illustrates the
switching sequence wherein the contacts 50 and 53 and the
semiconductor switch 54 are all open during the OFF state of the
apparatus, wherein the power source 51 is disconnected from the
load 52.
Considering the diagram of FIG. 3a in some detail, it will be seen
that the control signals supplied by the timing means 55
establishes the following t.sub.1 -t.sub.7 timing sequence: (1) at
time t.sub.1, no signal applied and all contacts and switches open;
(2) at time t.sub.2, a first signal applied to close main contact
50; (3) a second signal applied at time t.sub.3 to close
semiconductor switch 54, thereby connecting the power source 51 to
the load 52 to produce a voltage thereacross equal to that of the
source; (4) a third signal applied at time t.sub.4 to close the
auxiliary contact 53, thereby bypassing the semiconductor switch 54
and establishing the connection from the power source 51 to the
load 52 by means of the contacts 50 and 53; (.gamma.) at time
t.sub.5, a fourth signal applied to open the auxiliary contact 53;
(6) a fifth signal applied at time t.sub.6 to open the
semiconductor switch 54, thereby disconnecting the power source 51
from the load 52; and (7) at time t.sub.7, a sixth signal applied
to open the main contact 50. The condition where the main contact
50, the auxiliary contact 53 and the semiconductor switch 54 are
all closed (t.sub.4) constitutes the ON or "make" condition of the
apparatus. The condition where all contacts and switches are open
(t.sub.1, t.sub.7) constitutes the OFF or "break" condition of the
apparatus.
It will thus be noted that the sequence in going from the OFF or
"break" condition of the apparatus to the ON or "make" condition
requires the main contact 50 to close before the semiconductor
switch 54 and the auxiliary contact 53 are closed. The
semiconductor switch 54 is closed after the main contact 50 closes
and remains closed while the auxiliary contact 53 is closed. As an
alternate operation, the semiconductor switch 54 could be opened
after the auxiliary contact 53 closes, since that auxiliary contact
53 serves to bypass the semiconductor switch 54 in any event.
It will also be noted that the sequence in going from the ON or
"make" condition to the OFF or "break" condition is to open the
semiconductor switch 54 before the main contact 50 opens. The
auxiliary contact 53 in this respect operates to open as well
before the main contact 50 opens and, particularly, before the
semiconductor switch 54 opens.
It will further be noted that the apparatus of FIG. 3 can be
controlled to operate by the application of timing control signals
from the control means 55 as well as by the absence of signals from
the timing means 55. Thus, a signal supplied at time t.sub.5 in the
t.sub.1 -t.sub.7 sequence can serve to open the auxiliary contact
53 as well as the absence of a signal supplied to the contact 53 at
that time.
Referring now to FIG. 4, the apparatus there shown is intended for
use in tap-changing equipment, but could also be used in switching
electric power sources and loads. Two pairs of interconnected taps
a--a and b--b are shown along with two contact arms A and B. The
contact arm B is connected to a center-tapped output transformer 56
by means of a sliding contact 58 and a semiconductor switch 60
connected in series, while the contact arm A is similarly connected
to the transformer 56 by means of a sliding contact 62 and a
semiconductor switch 64. A pair of auxiliary contacts 66 and 68 are
included, with the contact 66 connected across the semiconductor
switch 60 and with the contact 68 connected across the
semiconductor switch 64.
As indicated, the semiconductor switches 60 and 64 each comprise a
pair of oppositely poled, parallel-connected silicon-controlled
rectifiers, though in alternative arrangements the semiconductor
switches may be composed of the transistor or triac arrangements
illustrated alongside FIG. 3. A timing means 70 is further included
and serves to rotate the contact arms A and B from the positions
shown to controllably open and close the main contact arms A and B
as well as, by a mechanical linkage, to open and close the
auxiliary contacts 66 and 68 of the apparatus. The timing means 70
also serves to control the generation of signals in a control
source 72; which signals in turn are coupled to the gate electrodes
of the individual silicon-controlled rectifier devices to open and
close the switches 60 and 64 in response to the indication of the
timing sequence.
Considering the switching sequence diagram in FIG. 4a, it will be
seen that at time t.sub.1, the auxiliary contacts 66 and 68 and the
switches 60 and 64 are all closed. The contact arms A and B are
similarly connected to the equipotential taps a--a. At time
t.sub.2, the timing means 70 serves to open the auxiliary contact
66 while at time t.sub.3, the timing means 70 and control source 72
cooperate in opening the semiconductor switch 60. At time t.sub.4
in this equipment, the contact arm B is controlled by the timing
means 70 to move from the upper tap a to the upper tap b reaching
that point at time t.sub.5.
At time t.sub.6, the timing means 70 and control source 72
cooperate once again, but this time to close the semiconductor
switch 60, while at time t.sub.7, the timing means 70 serves to
close the auxiliary contact 66. Thus, the timing sequence (.sub.1
-t.sub.4 corresponds to the "breaking" of the apparatus described
in FIG. 3 while the sequence t.sub.4 -t.sub.7 corresponds to the
"making" of the connections as described therein. During the
interval t.sub.1 -t.sub.2, the upper tap a is connected to the
output transformer 56 and during the interval t.sub.5 -t.sub.7 the
upper tap a is disconnected from the transformer 56. During this
latter interval, however, the upper tap b is connected to the
transformer 56 by means of the main contact 58, the auxiliary
contact 66 and the semiconductor switch 60.
A similar operation results with respect to the main contact arm A.
Thus, at time t.sub.8, the timing means 70 serves to open the
auxiliary contact 68 while at time t.sub.9, the timing means 70 and
control source 72 cooperate to open the semiconductor switch 64. At
time t.sub.10, the contact arm A moves under the direction of the
timing means 70 from lower tap a to lower tap b to open the main
contact 62. At time t.sub.11, the moving contact A has reached the
lower tap b. At time t.sub.12, the timing means 70 and control
source 72 once again serve to close the auxiliary switch 64,
thereby connecting the lower tap b to the transformer 56 by way of
the sliding contact 62 and the switch 64. And, at the time
t.sub.13, the timing means 70 serves to close the auxiliary contact
68 to bypass the semiconductor switch 64 in this condition.
(In this respect it will be understood that the upper and lower
taps a may comprise a single tap, as shown for example in FIG. 9.
Similarly the upper and lower taps b may comprise a single tap. A
pair of such taps are indicated in FIG. 4 to illustrate the equal
applicability of the present invention to both of the known types
of tap-changing equipment utilizing either one-piece or two-piece
taps in their respective constructions.) It will be noted also that
although magnetic coupling is shown between upper and lower
windings of device 56, uncoupled windings, reactors or
current-limiting impedances can be used. The basic principle of
switch operation and protection, however, will remain essentially
the same.
Thus, whether one considers the contact arm B or the contact arm A
of the arrangement of FIG 4, it will be readily apparent that the
timing sequence in each such case is to close the main contact,
close the semiconductor switch, close the auxiliary contact, open
the auxiliary contact, open the semiconductor switch and open the
main contract. Since current initiation and current interruption
are governed in both of these instances by the respective closing
and opening of the semiconductor switch, it will be seen that no
arcing problems are developed across the main contacts or, even
across the auxiliary contacts. The function of the auxiliary
contacts 66 and 68 in FIG. 4 is similar to that provided by the
contact 53 in FIG. 3 --namely, to bypass and thereby protect the
semiconductor switch under the condition at which the load is
connected either to the electric power source in the one instance
or to the tap at which exists exists electric power in the second
instance.
The tap-changing equipment illustrated in FIGS. 5-8 show
alternative arrangements of tap-changing equipment in accordance
with the invention. For the sake of simplicity, the timing control
means, or the combination of the timing control means and the
control signal source, which effects the opening and closing of the
contacts and switches employed have been omitted. In FIG. 5 the
output transformer 56 of FIG. 4 has been transferred to the input
circuit of the apparatus with all other notations being the same as
in that drawing. The connection from the contact arm (A or B) to
the output point (in this instance terminal 72) is by means of the
sliding contact and the auxiliary contact which bypasses the
respective semiconductor switch.
In FIG. 6, the apparatus shown modifies that previously described
in that a single semiconductor switch is utilized both for the
operation of the contact arm A as well as for the contact arm B.
This effects a savings of one semiconductor switch over the
previously described arrangements, and operates in the same exact
manner provided one of the auxiliary contacts 66 or 68 is
maintained in its closed position during control of the contact arm
and auxiliary contact in the other leg. A typical operation may be
as follows:
a. At time t.sub.1, the auxiliary contact 68 will be in its closed
position as well as contact arm A-62;
b. At time t.sub.2, the contact arm B moves from its tap c position
to its tap d position, thereby closing the main contact:
C. At time t.sub.3, the semiconductor switch 74 is closed thereby
initiating load current flow from the contact arm B to the terminal
72 through the sliding contacts 58, the upper winding and the
semiconductor switch 74;
d. At time t.sub.4, the auxiliary contact 66 is closed, thereby
providing the direct closed circuit path from the contact arm B to
the output terminal 72, while at the same time, forming a bypass
circuit for the semiconductor switch 74 with the auxiliary contact
68.
The sequence in going from the ON condition to the OFF condition of
the upper branch is exactly the reverse:
a. Opening the auxiliary contact 66;
b. Opening the semiconductor switch 74 to interrupt any current
flow in the upper branch of the apparatus; and
c. Opening the main contact as by moving the contact arm B from tap
d to some other tap.
The operation for the circuitry including the contact arm A is
basically the same with the modification that there the auxiliary
contact 66 is maintained closed for all operations of that circuit
and the auxiliary contact 68 is opened and closed in a
predetermined sequence to effect the coupling to and decoupling
from the output terminal 72.
A third auxiliary contact 76 is shown in dotted lines and may be
connected across the semiconductor switch 74 and controlled to
bypass that switch when the apparatus is in its ON condition. In
this manner, the semiconductor switch 74 can more easily be
bypassed under timing sequence control than is possible using the
auxiliary contacts 66 and 68.
The apparatus of FIGS. 7 and 8 show further embodiments of the
invention. The FIG. 7 apparatus, in particular, combines an
arrangement of the type described in FIG. 6 with the arrangement of
FIG. 5 with an additional provision to short circuit
current-limiting impedances when both arms are on the same tap.
This additional provision will reduce voltage drop in the apparatus
and, hence, power or voltage losses. The apparatus of FIG. 8 shows
a combined arrangement using two sets of coupling arrangements
connected in a modified manner and also could be used to short
circuit the current-limiting impedances at certain tap
positions.
The timing sequences for these arrangements can be such as to
activate the upper rightmost semiconductor switch in the opening
and closing of the main contact 58 and, then the leftmost upper
semiconductor switch in the next sequence. Then the rightmost lower
switch can be brought into play with the main contact 62 and then
the leftmost lower semiconductor switch. The precise switching
diagram illustrating the operation of these apparatus will be
readily apparent to one skilled in the art. In employing the FIG. 7
apparatus, however, it must be remembered that operation of the
parallel-connected semiconductor switch in the leftmost portion
requires the maintaining of the auxiliary contact in the rightmost
portion in the other branch to be closed in much the same manner as
described with respect to FIG. 6. As indicated in FIG. 7, the
bypass auxiliary contact is included to further improve the bypass
operation of the semiconductor switch during the time at which
current is flowing from the contact arm to the output terminal.
FIG. 9a shows a switching sequence diagram when apparatus of the
type shown in FIG. 6 is employed. It will be understood in this
switching diagram that the bypass contact 76 of that apparatus is
included. It will be seen from this switching diagram that the
basic sequence is the same as the sequences described with respect
to the other FIGS. of the drawings. Namely, in "making" the
connection, the main contact is first closed (t.sub.6), then the
semiconductor switch (t.sub.7), then the bypass contact (t.sub.8)
and lastly the contact completing the circuit from the tap to the
output terminal (t.sub.9). It will also be seen that in this
parallel arrangement of the semiconductor switch, the auxiliary
contact in the branch not connected between the tap and the output
point is maintained closed during this operation i.e., the
auxiliary contact 68. In "brekaing" the connection, the sequence is
the reverse.
FIG. 9b shows the switching diagram for the instance in which the
bypass contact 76 is omitted. Here, too, it will be seen that the
sequence is to close the main contact (t.sub.5), close the
semiconductor switch (t.sub.6), and close the auxiliary contact
connecting the tap to the output terminal while the auxiliary
contact in the other branch is closed (t.sub.7). Again, in
"breaking" the connection, the sequence is reversed.
In all these arrangements of FIGS. 5-8, it should be noted that the
initiation of current flow from the tap is upon the closing of the
semiconductor switch --whether the current flow be from the main
contact arm directly to the output terminal (as in FIG. 5) or from
the contact arm of one branch to the contact arm circuit of the
other branch (as shown in FIG. 6). Also, it should be noted that
the interruption of current occurs upon the opening of the
semiconductor switch in all of these arrangements. All of these
arrangements will therefore serve to prevent arcing across the main
contacts of the described equipment when connected in the manner
shown in the respective drawings.
As was previously mentioned, the combination of the metallic
contact switching arrangement and the semiconductor switching
arrangement as illustrated in the foregoing FIGS. offer a number of
advantages over the purely metallic contact or semiconductor
switching arrangements. Some of these are as follows:
1. Arcing is eliminated, and in a manner which permits convection
or forced air cooling of a contact surface. 2. The resulting wear
of the contact is largely mechanical in nature, thereby reducing
the need for contact maintenance while at the same time increasing
the useful life of the contact appreciably. In addition, it no
longer is necessary for the contact material to be arc resistant,
enabling the material to be selected on the basis of best wear and
conductivity. 3. Physical size and mass of the contact can be
reduced, and the operating mechanism simplified thus permitting
higher speed of switching or control. 4. Few power semiconductors
are needed, with the utilization factor of them being high. At the
same time, the semiconductor devices are not subjected to voltage
or current stresses in those instances in which the semiconductor
is not in service when the main contact is engaged to couple the
source of power to a load. This increases reliability and the life
of the semiconductors employed. 5. Service between the source and
the load can be maintained even if the semiconductor devices fail.
It is also possible in this respect to replace the semiconductors
while maintaining service, merely by freezing the main contact in
place and operating a semiconductor disconnect switch.
While the foregoing arrangements have been described in an
environment in which the power source was an alternating current
source and where the tap employed was to connect an alternating
current source to a load, the principles of the present invention
can also be applied for switching of direct current sources. In
such case it is only necessary to employ one instead of the two
cooperating semiconductor devices shown in the drawings. The
polarity of the device employed and its manner of the connection
within the apparatus will, of course, depend upon whether the
source is a positive voltage source or negative voltage source. In
the special case of silicon-control rectifier devices furthermore,
special commutating circuits known to those in the art would be
necessitated in order to negate the load current flowing to allow
the rectifier device to recover its blocking ability.
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