U.S. patent number 3,684,949 [Application Number 05/026,754] was granted by the patent office on 1972-08-15 for voltage regulator utilizing thyristor switch means.
This patent grant is currently assigned to Sanken Electric Co., Ltd.. Invention is credited to Hirohiko Fujii, Hiroshi Kosuge, Koichi Morita, Yukio Yamachi.
United States Patent |
3,684,949 |
Yamachi , et al. |
August 15, 1972 |
VOLTAGE REGULATOR UTILIZING THYRISTOR SWITCH MEANS
Abstract
A voltage regulation system in which a transformer secondary
winding is connected in series with an A.C. line in order to
minimize the voltage variation thereof, and a voltage to be applied
to the transformer is automatically regulated in a stepwise manner
by thyristors connected to the primary winding, which thyristors
can be protected from high voltages and over-currents.
Inventors: |
Yamachi; Yukio (Tokyo,
JA), Fujii; Hirohiko (Yamato-Machi, Kita Adachi-gun,
JA), Kosuge; Hiroshi (Tokyo, JA), Morita;
Koichi (Tokyo, JA) |
Assignee: |
Sanken Electric Co., Ltd.
(Adachi-gun, JA)
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Family
ID: |
13338273 |
Appl.
No.: |
05/026,754 |
Filed: |
April 8, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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707079 |
Feb 21, 1968 |
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Foreign Application Priority Data
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Oct 20, 1967 [JA] |
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42/67209 |
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Current U.S.
Class: |
323/263;
361/3 |
Current CPC
Class: |
G05F
1/30 (20130101) |
Current International
Class: |
G05F
1/30 (20060101); G05F 1/10 (20060101); G05f
001/30 () |
Field of
Search: |
;317/11E
;323/6,9,225C,24,43.5S,45,47,62,89AG |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pellinen; A. D.
Parent Case Text
This application is a continuation-in-part of copending
application, Ser. No. 707,079 filed Feb. 21, 1968, now abandoned.
Claims
What is claimed is:
1. A voltage regulator for use between a power source and a load
comprising:
a series transformer having an iron core and primary and secondary
windings wound about said core;
said secondary winding being coupled between said source and said
load;
an excitation transformer having input means coupled across said
load and output means including an output winding having a
plurality of taps;
a plurality of thyristors each having first and second control
terminals, said first terminals being coupled to an associated one
of said taps;
means for connecting the remaining terminals of said thyristors to
said primary winding;
means for sensing the voltage across said load;
a control pulse generator coupled to said sensing means for
generating control pulses only upon the occurrence of changes in
the voltage across said load from a desired voltage level;
gating means coupled to said control pulse generator for applying
control pulse signals to the control terminals of selected ones of
said thyristors to maintain a constant voltage output across said
load;
said iron core being adapted to saturate in the presence of high
current; and inductance means coupled between the primary winding
of said series transformer and the second terminals of said
thyristors, said iron core and said inductance means protecting
said thyristors against damage from current surges which may occur
due to short-circuit conditions such as the short circuiting of
said load.
Description
This invention relates to a voltage regulator, and more
particularly to the type in which a transformer is connected in
series with an a.c. power line and the output voltage of the
transformer (hereinafter called a series transformer) is superposed
on the input voltage of the a.c. circuit, whereby the output
voltage from this circuit is allowed to be regulated.
Heretofore, a voltage regulator having such a series transformer is
well known, and FIG. 1 of the attached drawing illustrates a
circuit diagram of the conventional voltage regulator which is
operable during the loaded condition. In FIG. 1, a series
transformer is connected with an a.c. power line extending from the
power source 1 and the series transformer is composed of the
secondary winding 2, primary winding 3, and the iron core 4. The
voltage induced across the secondary winding 2 is superposed on the
input voltage from the power source 1, and the resultant output
voltage is furnished to a load 5. With this arrangement, the
induced voltage in the secondary winding 2 can be varied by
changing the voltage applied on the primary winding 3 of the series
transformer, and this voltage can be changed by the use of a
separate exciting transformer having primary winding 6, iron core
7, and a secondary winding 8 and connected in parallel with the
a.c. line. Taps 11, 11', 12, 12', 13, 13', 14, 14', 15, 15', are
provided on the secondary winding 8 of the exciting transformer,
and with these taps changed from one to another the induced voltage
in the series transformer can be changed. When the tap connection
is desired to be changed, a switch 10 is at first opened and the
connection is transferred from tap 11 to tap 12, whereupon the
local current created in the circuit is limited by a current
limiting reactor 9. Then the switch 10' is opened and the
connection is shifted from tap 11' to tap 12' at no load condition.
Upon completion of the transferring of the taps, current will be
flown from these circuits through the current limiting reactor 9.
However, because the direction of these currents are reversed
relative to one another, magnetic fluxes induced therein cancel
each other, and there is thus no disadvantageous effect to the
series transformer.
In this kind of the conventional voltage regulator, since
mechanical contacts are utilized, much difficulties were
experienced in maintenance and operation, rendering this kind of
construction utterly unsuitable for the voltage regulator.
In recent time, various thyristors such as reverse blocking triode
thyristors, bidirectional triode thyristors, bi-directional diode
thyristors, and so on have been developed, and many of the
mechanical switches in various applications are replaced by these
electronic switches.
In some of the applications, mere replacement of the mechanical
switches by these electronic switches will render reasonably good
results. However, in other cases where thyristors are utilized, for
instance, in the voltage regulator as in the case of the present
invention, wherein the output voltage of the series transformer is
superposed on the input voltage from the a.c. power line, some
difficult problems are encountered. The thyristors cannot be
operated at more than their rated voltage and current. For this
reason, sufficient care must be exercised to prevent the electronic
switches from carrying abnormal voltage and current.
When the contactless tap changing circuit is utilized for the
voltage regulator, each of the thyristors are connected in
parallel. As this result, the voltage is always applied across the
terminals of all of the thyristors, and if an abnormally high
voltage is generated in this circuit, other thyristors than those
to be operated at that moment might be brought into operation, with
the subsequent loss of the operation of the voltage regulator.
Moreover, when the taps of a transformer are transferred through
these thyristors, should any of the two thyristors be operated
simultaneously, a local current will flow through the circuit with
resultant damages of these thyristors and others.
For this reason, parallel operation of two thyristors at the same
time should by all means be avoided. Furthermore, when all of the
thyristors are turned off abruptly, this means that one of the
windings of the series transformer is disconnected, and high
voltage will be induced in the winding just as the case of opened
secondary winding of a current-transformer. These and other
difficulties should be overcome in the application of thyristors to
the voltage regulator, and the present invention is directed to the
solution of this problem.
Accordingly, the primary object of the present invention is the
provision of contactless voltage regulator utilizing thyristors and
which is easy to maintain, provides good stability, and is
particularly suitable for use on power distribution and
transmission lines.
A second object of the present invention is to provide a voltage
regulator in which the thyristors are protected from the
over-current which may be present in the circuit.
A third object of the present invention is to provide a voltage
regulator in which the thyristors are protected from abnormally
high voltages which may be present in the circuit.
These and other objects of the present invention can be fulfilled
by the following embodiments of the invention:
The first embodiment of the present invention is characterized in
that a series transformer having an iron core of high excitation
type with air-gaps is superposed on the a.c. power line; an
excitation transformer is connected across the a.c. power line,
secondary winding of which is connected to a plurality of taps
which, in turn, are connected to associated thyristors; said
thyristors being connected with the primary winding of said series
transformer and operable in such a manner that the voltage applied
to the primary winding of said series transformer is thereby
adjusted to a desired value, and the output voltage from this
voltage regulator can be regulated.
The second embodiment of the present invention is characterized in
that said embodiment further includes an inductive reactance
connected in series with the primary winding of the series
transformer in which a high excitation type iron core is
utilized.
The third embodiment of the present invention is characterized in
that when an over current flows in the circuit of the first
embodiment as a result of, for instance, short-circuit in the load,
the first half cycle of said over current is flown through the
thyristor operating in the normal condition, but the next half
cycle of the over-load current is once blocked off from all of the
thyristors, because they are operated to their non-conductive state
at that time, and when the voltage created across the thyristors
exceeds a predetermined value, only some specific thyristors having
larger capacity than others are rendered conductive, thus forming a
closed circuit through the primary winding of the series
transformer, whereby the over-current flows through these specific
thyristors and the remaining thyristors are protected from the over
current condition.
The fourth embodiment of the present invention is characterized in
that, across the ends of the primary winding of the series
transformer and the secondary winding of the exciting transformer,
there is connected a capacitor in parallel with a series connected
varister and a discharge tube, whereby the thyristors are protected
from abnormally high voltage generated within the a.c. circuit.
These and other embodiments of the present invention will be more
clearly understood from the following description when it is read
with the accompanying drawing in which,
FIG. 1 is a circuit diagram of a conventional voltage
regulator;
FIG. 2a is a circuit diagram of a voltage regulator according to
the present invention;
FIG. 2b shows a block diagram of circuits included within the
control pulse generator of FIG. 2a;
FIG. 3 is a diagram showing the connection of the thyristor;
FIG. 4 is a waveform diagram showing the control signal of the
thyristor;
FIG. 5 shows the characteristic curve of the iron core of the
transformer;
FIGS. 6a, 6b and 6c are waveform diagrams of the voltage induced in
the primary winding of the series transformer;
FIG. 7 is a circuit diagram showing another embodiment of the
present invention;
FIG. 8 is a circuit diagram showing still another embodiment of the
present invention in which an inductive reactance is provided in
the circuit;
FIG. 9 is a schematic diagram useful in explaining the operation of
the circuit shown in FIG. 8.
FIG. 10 is a waveform diagram useful in explaining the current to
be limited in the circuit of FIG. 8;
FIGS. 11a, 11b and 11c are waveform diagrams useful in explaining
an embodiment provided with an over current protecting device
according to the present invention; FIG. 12 is a circuit diagram of
an embodiment of the present invention wherein an over voltage
protecting circuit is provided;
FIGS. 12a, 12b, and 12c are waveform diagrams useful in explaining
the over voltage protecting device used in FIG. 8.
FIGS. 13a and 13b are waveform diagrams useful in explaining the
overvoltage protecting device used in FIG. 8.
Now referring to FIG. 2a which illustrates an embodiment of the
present invention, electric power is furnished from an a.c. power
source 16 to a load 21 through the secondary winding 17 of a series
transformer. The series transformer comprises a primary winding 18,
secondary winding 17, and an iron core 19. The iron core is of a
high excitation type which includes air gaps 20 within the magnetic
path, and the reason why this type of core is used will be
explained later on.
The output voltage furnished to the load 21 can be adjusted to a
desired value by varying the induced voltage in the secondary
winding 17 of the series transformer, which is in turn varied by
changing the voltage applied on the primary winding 18 of the
series transformer. To change the voltage applied on the primary
winding 18, an excitation transformer is connected across the a.c.
circuit. The excitation transformer is composed of a primary
winding 22, a secondary winding 24, and an iron core 23. A
plurality of taps are provided on the secondary winding 24 of the
excitation transformer and thyristors 25, 26, 27, 28, 29 30, are
connected as shown with these taps. The input voltage of the
primary winding 18 of the series transformer is applied through a
pair of chosen thyristors which are connected in parallel, and the
control of these thyristors is carried out by the application of a
control signal obtained from a control pulse generator 31 to the
gates of these thyristors, and this pulse generator 31 is so
arranged that it can produce control pulses by detecting the output
voltage of the a.c. power line and the line current by means of a
current transformer 32.
Thyristors 25, 26 are utilized for reversing polarity of the
voltage applied to the primary winding 18 of the series
transformer, and thyristors 27, 28, 29, 30 are used for
transferring the taps. The variation of the supply voltage to the
primary winding 18 of the series transformer depending on the
thyristors operated at that time is indicated in the following
table with the assumption that the intermediate voltage between
each of the taps of the secondary winding 24 of the excitation
transformer is 90v.
Operating Thyristors Supply voltage
__________________________________________________________________________
25, 30 -270V 25, 29 -180V 25, 28 -90V 26, 30 0 26, 29 +90V 26, 28
+180V 26, 27 +270V
__________________________________________________________________________
thus, a voltage depending on the ratio between the primary winding
18 and the secondary winding 17 of the series transformer is
produced in the secondary winding 17, and this voltage is
superposed on the input voltage 16 of the a.c. line so that a
regulated output voltage of this device is obtained.
The control of thyristor is accomplished by means of signals from
the control pulse generator 31. Said control pulse generator 31
consists of the circuits shown in detail in FIG. 2b.
The variation in voltage is detected by means of a detecting
circuit 101. In accordance with the detected value, the thyristor
to be controlled is selected and then it is conducted. When a line
voltage is of the set value in the case of FIG. 2a, the thyristors
26 and 30 are actuated. If the line voltage drops, the tap should
be stepped up, and therefore the pulse generator 102 is caused to
operate and applies a pulse to multistage, bidirectional shift
register 104. The signal passes through an OR circuit 105 and an
amplifier 106 and is applied to the gates of the thyristors 26 and
29. As a result, the line voltage varies. This variation is again
detected by the detecting circuit 101. If this value is below the
set value, the thyristors 26 and 28 are caused to operate. When the
set value is reached, balancing is carried out.
When a line voltage is high, a pulse generator 103 is caused to
operate and continues to operate until the set value is reached in
the thyristors 25 and 28 and the thyristors 25 and 29 and the
thyristors 25 and 30.
When the thyristors are to be changed over, an interrupting period
is provided as shown in FIG. 4. In other words, when a command
signal to change over the thyristors is received from the pulse
generator 102 or 103, this signal is applied to a monostable
multivibrator 107. By means of the signal from said monostable
multivibrator 107, the application of trigger signal from the
amplifier 106 to the thyristors gate is stopped.
Based on the above-described operation, the stoppage of the
thyristor action is detected by means of a zero current detecting
circuit 108 and then signals are applied to the monostable
multivibrator 107 to permit the supply of trigger signal from the
amplifier 106 to the thyristors gate.
In other words, an interrupting period is provided between a
command signal from the pulse generator 102 or 103 for changeover
of the thyristors and a zero current detecting signal.
The remarkable feature of the present invention is that the high
excitation type iron core including air-gaps within the magnetic
path is utilized for the series transformer. When the thyristors
are operating at any one pair of positions indicated in the above
table, if a thyristor, for instance, the operation of thyristor 27
is to be transferred to thyristor 28, and if the thyristor 28 is
activated while the thyristor 27 is still operating, a local
current is flown through the closed circuit formed through these
thyristors 27, 28, and the resultant current may damage these
thyristors. Though this damage might be prevented by the
utilization of far larger size of the thyristors, this would surely
be much too uneconomical.
For this reason, it is necessary that a certain interrupting period
is provided between the gate signal G1 for the thyristor 27 and the
gate signal G2 for the thyristor 28, so that the two thyristors 27,
28 are never operated at the same time. Provision of the
interrupting period, however, causes open-circuit of the primary
winding 18 of the series transformer. Besides, there are some other
cases where all of the thyristors are required to be brought into
inoperable state, and in all of these cases, the primary winding 18
of the series transformer is at the opened condition.
Now the operation of the series transformer when the primary
winding 18 is at the opened state will be more closely examined.
Supposing that an iron core of the ordinary characteristics as
shown in FIG. 5 waveform (a) is utilized, and also a comparatively
large current is flowing through the secondary winding of the
transformer, then the series transformer operates just like a
current transformer and a high voltage will be induced in the
primary winding 18. This condition is indicated in FIGS. 6a, 6b,
and 6c. When a line current as indicated in FIG. 6 (a) flows
through the a.c. circuit, the whole of this current operates as the
excitation current for the series transformer and a high magnetic
flux corresponding to this current will be created in the magnetic
core. This is because the primary winding 18 is opened and no
compensating flux can be induced in the magnetic path. As a result,
a high voltage is induced across the terminals of the primary
winding 18, the waveform of which is indicated in FIG. 6(b).
Because, depending on the characteristics of the iron core as
indicated in FIG. 5(a), the variation rate of the magnetic flux is
large in the region of the small current, and the value of the
voltage E=Nd.phi./dt, wherein N is the number of windings, .phi. is
magnetic flux and t is time, is also high. On the other hand, when
the current approaches the saturating region, the variation rate of
the magnetic flux is low and the induced voltage also is
decreased.
As shown in FIG. 6(b), the steep high voltage exceeds the maximum
allowable blocking voltage of the thyristors, causing the breakdown
and damage of the thyristors and other elements, which is also
accompanied by the deformation of the output voltage and other
difficulties.
According to the present invention, there is provided a device
which can overcome above described difficulties even if the primary
winding 18 of the series transformer is opened. For this purpose, a
high excitation type iron core including air-gaps in the magnetic
path is used for the series transformer. Since the iron core has a
characteristic as shown in FIG. 5 by waveform (b), the variation
rate of the magnetic flux can be maintained at a small value in the
heavy current region. This variation rate is also low throughout
the whole range of the current and straight, so that the induced
voltage E=Nd.phi./dt can be maintained low and the output waveform
can be sinusoidal as shown in FIG. 6(c).
As described above, according to the present invention a high
excitation type iron core is provided in the series transformer,
and transferring of the taps with the use of the thyristors is
thereby enabled.
Although, in the above described voltage regulator, bidirectional
triode thyristors are utilized, it is of course possible to use
various kind of thyristors, for example, the reverse blocking
triode thyristor connected in a reverse parallel combination as
shown in FIG. 3.
FIG. 7 illustrates still another embodiment of the present
invention, which is basically similar to that indicated in FIG. 2.
In this embodiment, the electric power is furnished from the power
source 16 to a load 21 through the secondary winding 17 of the
series transformer. The core 19 of the series transformer is
provided with air-gaps 20 as shown and made into high excitation
type. The input circuit for the primary winding 18 of the series
transformer includes an auto-transformer connected across the a.c.
power line. The winding 33 of the auto-transformer is provided with
a plurality of taps, and thyristors 34, 35, 36, 37, 38, are
connected with these taps to change the voltage furnished to the
primary winding 18 of the series transformer. This embodiment is
used for a comparatively low a.c. power line.
FIG. 8 illustrates still another embodiment of the invention in
which a protecting device is provided against the over current
which may be caused as a result of, for instance, a short circuit
in the load. The construction of FIG. 8 is almost similar to that
of FIG. 2, hence the same reference numbers are used for the same
circuit elements. The only difference from FIG. 2 is that an
inductive reactance 39 is used in the circuit and also an iron core
saturable at high current region is employed in the series
transformer. The characteristic curve of this iron core is the
curve b in FIG. 5. In this characteristic, the iron core is
saturated at the current I.sub.2. From the view point of the short
circuit current protection, it will be advantageous that the iron
core is made of a material having a typical rectangular hysteresis
loop.
However, as was already disclosed, since the air-gaps are provided
in the magnetic path, the flux variation is comparatively small at
the small current region. As a result, it is not proper to use an
idealistic rectangular hysteresis material for this case. It should
be noticed that, in the case of FIG. 8, because of the large
difference between the short circuit current and the rated current,
an iron core exhibiting magnetic saturation at a comparatively high
current region would be suitable to the purpose. An equivalent
circuit of this embodiment of the circuit shown in FIG. 8, when the
output terminal is shorted, is illustrated in FIG. 9.
In FIG. 9, it will be seen that an a.c. power source 40 is
connected with the secondary winding 17 of the series transformer
through the power source impedance 41 and the output terminal is
shorted at the point 42. In this case, the short circuit current
flowing through the circuit is limited by the power source
impedance 41. By this current, a voltage is induced in the primary
winding 18 of the series transformer and a current Ip corresponding
to the short circuit current Is might appear in the circuit
containing primary winding 18 if the reactance 39 were not inserted
in the circuit, and as a result the thyristor 43 might be broken
down.
However, in accordance with the present invention, the reactance 39
is inserted in series with primary winding 18 and the thyristor 43.
Saturation characteristic as shown in FIG. 10 is used for the
series transformer. In explaining the operation, let it be assumed
that the short circuit current is Is = In Sin .omega.t, wherein In
is the maximum value of electric current, and that the number of
turns of the primary and secondary windings are equally represented
by N, the primary and secondary current will be almost equal and
represented as Is .apprxeq. Ip. Though this relation is true for
the small current region as shown in FIG. 11, the relation is not
satisfied in the larger current region. The reason of this will be
explained as follows;
Supposing the inductance of the reactance 39 is L, the induced
voltage in the reactance 39 is;
V.sub.L = di/dt
Since the thyristors 43 are conducting, most of this voltage is
applied to the primary winding 18 of the series transformer, and
the flux density of the iron core is increased by this voltage.
Supposing that the saturating flux density of the iron core of the
series transformer is Bs, and that the sectional area of the iron
core is A, and the phase angle at which the iron core is saturated
is .alpha., then
L = Inductance of non-saturable reactor
V.sub.l = induced voltage of reactor 39, and
Im = Maximum value of current.
and after the time when the phase angle becomes .alpha., the iron
core of the series transformer will be saturated. When the iron
core 19 is once saturated, the function of the series transformer
is lost and can be considered as a mere reactor. As a result, the
correspondence between the current in the primary winding Ip and
the current in the secondary winding Is will also be lost. Hence,
the peak value of the current Ip in the primary winding is
expressed by Im Sin .alpha. = N A Bs/L
This means that the short circuit current has no relation with this
value which is determined solely by the inductance L of the
non-saturable reactor and by the transformer. FIG. 11 indicates
this relation, and the current Ip is limited at the point
corresponding to the phase angle .alpha.. After this point, a
current due to the degeneration of the stored energy from the
non-saturable reactor 39 will be flown as indicated in the same
figure. Thus reason, if the surge current capacity of the
thyristors withstanding to the short circuit current is once given,
the inductance L of the non-saturable reactance 39 can be
determined as follows:
L = N A Bs/Im Sin .alpha.
When a reactance having this inductance is utilized, the thyristor
can be surely protected from the short circuit current.
Although the thyristors can be protected by the above described
protecting method, a still more economical design of the voltage
regulator will be explained.
Taking the device of FIG. 8 in consideration, when the load 21 is
shorted and a short circuit current is flown through the circuit,
one half cycle of the a.c. short circuit current limited as
described above by the reactance 39 will be flown through the now
operating thyristors. For this reason, the thyristors should have a
capacity to withstand such limited short circuit current for one
half cycle. In addition, some of the specifically selected
thyristors will be given far larger capacity than the above
described value. For instance in FIG. 8, the thyristors 26 and 30
are determined to serve as the thyristors having far more capacity
than other thyristors.
Now, assuming the thyristors 26 and 29 are operating, and a short
circuit occurs in the load 21, the short circuit current is flown
through the circuit. This short circuit current, limited by
reactance 39, flows through the thyristors 26, 29. As shown in FIG.
12(a), when the short circuit occurs at the time t.sub.1, a half
cycle of this short circuit flows through the thyristors. However,
when the a.c. short circuit current proceeds into the next half
cycle, all of the thyristors are rendered inoperable. When all of
the thyristors are rendered inoperable, that is, in a
non-conductive state, the primary winding 18 of the series
transformer is opened, and a high voltage is induced just like the
current transformer. If no appropriate measure is taken, this high
voltage will cause some of the thyristors to break down, and a
local current will flow through thus rendered conductive
thyristors.
To prevent the above mentioned difficulties, the device in
accordance with the present invention is provided with
speciffically selected thyristors 26, 30 which are rendered
conductive during the next half cycle of the a.c. short circuit
current, whereby a short circuit current is flown through the
primary winding of the series transformer. This conduction of the
specific thyristors occurs at the moment t.sub.3 of FIG. 12(b) when
the value of the high voltage of a half cycle starting from the
moment t.sub.2 rises up to a predetermined value Es. When the
thyristors 26 and 30 conduct, the short circuit current flows
through the thyristors as shown in FIG. 12(c), and the over voltage
induced in the primary winding 18 of the series transformer is
thereby suppressed.
As is apparent from the above description, with the provision of
the specific thyristors of sufficient capacity to carry short
circuit current safely, all the rest of the thyristors are not
required to have so much capacity corresponding to the short
circuit current. That is, when the capacity of the specific
thyristors is determined to a value to withstand about 20 cycles of
the short circuit current, the capacity for the rest of the
thyristors can be reduced to a small one to withstand only a half
cycle of the short circuit current, and much economy of the device
can be obtained. Though in the above explanation the specific
thyristors selected are the thyristors 26 and 30, it is of course
possible to utilize another pair of the thyristors.
In FIG. 13 still another embodiment of the present invention is
illustrated. The constitution of this embodiment is basically
identical to that of FIG. 8, and like reference numbers are used
for like elements. The only difference of this embodiment from FIG.
8 is that a parallel circuit comprising a series connected
discharge tube 45 and a varister 46 as one branch of the parallel
circuit and a capacitor 44 as the other branch, is connected across
the primary winding 18 of the series transformer and the secondary
winding 24 of the excitation transformer. When the voltage
regulator is operated in the actual application, abnormal voltage
may be induced in the a.c. line, for instance, by the induction of
a surge to lightning, and the abnormal voltage is transmitted to
the secondary winding 17 and then to the primary winding 18 of the
series transformer. In other cases, such abnormal voltage may be
transmitted from the primary winding 22 to the secondary winding 24
of the excitation transformer.
As a result, the abnormal voltage enters the circuit including the
thyristors 25, 26, 27, 28, 29, 30 whereby the thyristors may be
damaged.
In order to protect the thyristors from these abnormal voltages,
the embodiment of the present invention provides a parallel circuit
comprising a discharge tube 45 and a varister 46 connected in
series and a capacitor, and connecting this parallel circuit across
the terminals of the primary winding 18 of the series transformer
and the secondary winding of the excitation transformer, the above
mentioned abnormal voltage can be minimized.
The operation of this circuit is explained as follows: When a steep
voltage as shown in FIG. 14(a) is induced, the capacitor 44 begins
to charge and flattens the wave front of this voltage. When the
voltage rises up as shown in FIG. 14(b) and reaches the discharge
voltage Vd of the discharge tube 45 at the time t.sub.1, the
discharge tube 45 starts discharging and further increase of the
height of the voltage is thereby suppressed. Since the internal
resistance of the discharge tube 45 is very small, the circuit
might be short circuited if the varister 46 were not inserted. The
varister 46 has a non-linear voltage-current characteristic, and
the current varies widely with only small changes of the voltage.
For this reason, the varister 46 can be used effectively for
absorption of the abnormal voltage, and with the provision of the
over voltage protecting circuit, the thyristors are protected from
the abnormal voltage induced in this circuit.
As described above, according to the present invention, there is
provided a voltage regulator wherein thyristors are utilized and
operated satisfactorily by the provision of the protecting
device.
The voltage regulator according to the present invention is
particularly adapted for use on distribution lines. In the
distribution lines, maintenance of the line voltage is not easy for
a place remote from the substation. In that case, the voltage
regulator automatically supplies a constant line voltage. Since the
voltage regulator according to the present invention has no
mechanical contacts, and satisfactory protection against the over
voltage and current is provided, the maintenance of the system is
extremely simple and suitable for use with distribution lines for
which constant supervision is not possible or is not desirable.
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