Note: Descriptions are shown in the official language in which they were submitted.
lOS~10 llCU-03977
This in~ention relates generally to high power
thyristors that can be used to form high-voltage solid-
~tate controllable electric valves for rectifying or
inverting bulk e]ectric power in a high voltage direct
current (HVDC) transmission system, and more particularly
it relates to a scheme for protecting such thyristors
from damage due to high-energy surges of reverse blocking
voltage.
The following U. S. patents and publications are
indicative of prior approaches in the pertinent art: ;
3,246,206 - dated April 12, 1966 - Chowdhuri; 3,629,685 - ~ ;
- dated December 21, 1971 - Johansson; 3,793,535 - dated
February 19, 1974 - Chowdhuri; General Elactric SCR Manual,
pp. 155 and 326-30 (4th ed. 1967)~
"Thyristor" is a generic name for a family of
solid-state bistable switches, including silicon controlled
rectifiers (SCRs), which are physically characterized by a
semiconductor wafer having a plurality of layers of
` alternately P- and N- type conductivities between a pair
of main current-carrying metallic electrodes (designated
the anode and the cathode, respectively). Such semiconductor
devices are equipped with suitable gating means for initiating
forward conduction between the main electrodes on receipt
of an appropriate control or trigger signal. Where high
current ratings are desired (e.g., 1,250 amps average) a
semiconductor wafer of relatively large area is used, and
to obtain high voltage ratings (e.g., 2,600 volts peak) the
base layers in the wafer are made relatively thick~ To
form a higher voltage solidstate controllable electric
valve, a plurality of such thyristors can be interconnected
in series and operated in unison. By suitably interconnecting
and arranging a plurality of such valves to form an a-c
I ~
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switch or an a~c/d-c converter, the flow of electric power
can be controlled in a high voltage alternating curre~t
circuit or in an E~VDC system.
During these cyclically recurring intervals
when the above-mentioned high voltage valve is in an off
or blocking state, the valve and its associated e~uipment
are prone to being damaged by extra high voltage surges
that may be produced at random times by a variety of
different transient disturbances, such, for example, as
lightning strokes, bushing flashovers, or inverter commu-
;~ tation failures. Such overvoltage transients can rise far
above the working or cyclical peaks of the normal system
voltage. To divert and suppress these transient surges,
a lightning arrester is commonly connected across each
valve. see, for example, U.S. patent No, 3,513,354 - dated
May 19, 1970 - Sakshaug and Kresge. To further protect the
individual thyristors from forwaxd voltage breakover, it ~-;
is also desirable to use an overvoltage responsive triggexing
scheme such as the one that is disclosed and claimed in
prior U.S. patent No. 3,662,250 - dated May 9, 1972 -
Piccone and Somos~ The present invention is concerned with
protecting the individual thyristors from damage during a
high-energy transient surge of reverse voltage on the valve.
When a thyristor is subjected to excessive voltage
in the reverse direction (anode potential negative with
respect to cathode), it can switch from a reverse blocking
state to a reverse current conducting state. This turn
on action is known as avalanche breakdown, and the critical
level of reverse voltage at which it occurs is called the
reverse breakdown voltage (V(BR)R). When turned on in this -
manner the thyristor can conduct a substantial am~unt of
` reverse current without damage, so long as the breakdown
-2-
.... "..... -. -
-` lOS~10 llCU-03977
occurs on a sin~31e-shot basis (non-repetitive) and the
reverse current does not exceed a destructively high peak
nor last longer than a relatively brief time. Transient
energy is safely dissipated within the bulk of the thyristor,
and when the surge of reverse cu~rent subsides the thyristor
fully recovers its reverse blocking capability. However,
if the surge current magnitude and duration exceeds the
energy dissipating capability of the thyristor during reverse
avalanche, it must be harmlessly diverted to prevent destruc-
tion of the thyristor. A conventional lightning arre~ter
cannot be counted on for this purpose because its operating
speed is likely to be too slow to protect either the fùll
valve or each of its constituent thyristors, especially
under a condition of unequal voltage distribution which
occurs when a very steep-front surge of abnormally high
reverse voltage is applied to the converter valve.
It has heretofore been proposed to suppress
overvoltages by connecting across individual thyristors
:,, .
a voltage-variable non-linear resistor known as a metal
~ .~ . , .
oxide "varistor" (MOV). A MOV comprises a sintered body
of ceramic material (such as zinc oxide plus a small
quantity of bismuth oxide) whose resistance will switch
from an extremely high standby value for applied voltages
` of less than a so-called breakover voltage level to very
` low conducting values for voltages above the breakover
level, thereby limiting the rise of voltage to a safe
maximum level. (By definition, the peak voltage across
a varistor when conducting idling current of one milliamp
(peak) is referred to as the "breakover voltage," and the
maximum level to which the voltage rises when the varistor
is conducting the instantaneous peak current of an over-
voltage transient is referred to as the "peak clamping voltage.")
:~ . - - . . - . . . ~, .
llCU-03977
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The manner in which the current (I) in a MOV varies as a
power of the voltage (V) across it can be expressed by
the equation:
I = (V) ,
where the constant C equals the magnitude of V when I = 1
ampere, and the exponent ~ is a number greater than 10~
Such a varistor has the desirable features of
having a high degree of non-linearity, switching with
negligible delay time (less than 50 nanoseconds), having
high energy handling capability, and being capable of being
fabricated in a variety of shapes of various sizes. Its
electrical characteristics are determined by the geometry
of the body formed from the polycrystalline material and
by the composition thereof, with C being primarily a function
of the material grain size and~ being primarily a function
of the grain boundary. More information about metal oxide
varistors can be found on pages 477-81 of the SCR Manual
(5th ed.) published in 1972 by the General Electric Co.,
Electronics Park, Syracuse, N. Y. 13201, and commercially
available models are identified in the condensed specifi-
cation on page 656 of the same Manual.
MOV's that are commercially available at this
writing are not entirely satisfactory for protecting the
thyristors in a solid-state HVDC valve from high-energy
transient surges of excessive reverse blocking voltage
4~S7-a b~
A because of their tendency to be unsta~be at very high
voltage levels. By "unstable" we mean that the voltage-
current characteristic of an MOV may drift or change with
time if the varistor were repeatedly subjected to voltage
peaks higher than its breakover voltage. Such high voltage
levels also tend to cause overheating in the MOV, and this
too is a degrading influence.
~... .
;.- . ~,'- : '
105~010 llcu-03g77
Accordingly, a general objective of our inven-
tion is to provide, for protecting a thyristor from
damage due to excessive reverse blocking voltage, of an
improved protective scheme that avoids the shortcomings of
the prior art approaches in the environment of solid-
state HVDC electric valves.
In carrying out our invention in one form, we
connect across the main electrodes of a thyristor the series
combination of non-linear resistance means, which comprises
at least one metal oxide varistor, and switching means such
as a PNPN semiconductor element poled inversely with
respect to the thyristor. The switching means is operative
to switch abruptly from a normal high-resistance state to
3 a low-resistance, current conducting state if subjected to
! a voltage which attains a predetermined breakover value. -
i Its switching characteristic is coordinated with the
voltage-current characteristic of the varistor so as to `~
switch to its current conducting state in high-speed res-
, ponse to the magnitude of reverse bias voltage on the
i 20 thyristor increasing to a level which is higher than a
predetermined repetitive peak reverse voltage (VRRM) and
! which nearly equals the reverse breakdown voltage of the
thyristor. The voltage-current characteristic of the
! varistor i5 coordinated with the reverse breakdown
characteristic of the thyristor so that after a surge of
reverse bias voltage causes the aforesaid switching means
to operate, the surge current will divide between the
thyristor and the parallel-connected varistor in such
proportions that the maximum reverse current flowing
through the thyristor is lower than a predetermined
critical magnitude which might damage the same~ As
current increases to its maximum value in the varistor,
llCU~03977
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the surge o~ reverse voltage across the parallel thyristor
is safely limited to the peak clamping voltage of the MOV.
At the same time, the thyristor is conducting a significant
amount of reverse current which, while within its inherent
reverse avalanche capability, is substantial enough ~e.g.,
one-third of the total surge current at the peak magnitude
thereof) to relieve appreciably the energy dissipating
duty of the varistor. Consequently the size and the cost
of the varistor can be kept relatively small. So long as
the reverse bias voltage does not exceed VRRM, the afore-
said switching means remains in its normal high-resistance
state, whereby the varistor is not exposed to the high,
repetitive reverse voltage peaks which would otherwise
jeopardize its stability.
; Our invention will be better understood and `
its various objects and advantages will be more fully
; appreciated from the following description taken in
conjunction with the accompanying drawings in which:
Fig. 1 is a schematic one-line diagram of a
high-voltage direct-current electric power delivery system
including converters using solid-state valves in which our
invention can be advantageously embodied;
Fig. 2 is a schematic circuit diagram of a string
of serially interconnected thristors comprising one of thee
valves used in the converters shown in block form in Fig. l;
Fig. 3 is a schematic circuit diagram of the
parallel combination of a thyristor and overvoltage pro-
tective means illustrating a preferred embodiment of our
invention and comprising one of the reiterative levels
which form the valve shown in Fig. 2;
Fig. 3a is a schematic diagram of a modified
form of switching means used in the overvoltage protective
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means which is illustrated in Fig. 3;
Fig. 4 is a chart of voltage V5. time showing
voltages across the whole valve and across certain levels
thereof during the early stages of a transient surge of
rev~Qrse blocking voltage;
Fig. 5 is a chart of current V9. voltage showing
the reverse breakdown characteristic of a typical thyristor
and the voltage-current characteristic of a typical metal
oxide varistor which can be used in practicing our inven-
tion, and
Fig. 6 is a chart of both voltage and current ~ ;
vs. time illustrating a half-cycle surge of reverse
current through the Fig. 3 circuit, which current has a
sinusoidal waveform, a peak magnitude of 400 amperes, and
a duration of 5 microseconds.
~, The first two figures of the drawings illustrate
one practical application of a protective scheme embodying
our invention. Fig. 1 is a one-line representation of
two highvoltage polyphase alternating current electric
power systems 11 and 12 which are interconnected by a d-c
link 13. The a-c system 11 comprises a bus 14 to which
electricity is supply by an appropriate source 15, and the
a-c system 12 comprises a bus 16 from which electricity is
;' delivered to a connected load circuit 17~ Additional loads
i~ (not shown) can be connected to the bus 14 of system 11,
and other sources (not shown) can be connected to the bus
~ 16 of system 12. The separate buses 14 and 16 are res-
; pectively coupled to opposite terminals of the inter-
connecting d-c link 13 by way of suitable electric power
converter stations 18 and 19.
Each of the converters 18 and 19 comprises a
conventional arrangement of power transformers, a-c/d-c
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bridges, and means for supplying firing pulses to the
controllable electric valves which are used to form
each bridge. Normally the converter 18 is opsrated in a
rectifying mode and the converter 19 is operated in an
inverting mode, whereby bulk electric power can be trans-
mitted in the form of high voltage direct currsnt from the
a-c system 11 to the a-c system 12. For more information
about the art of High Voltage Direct Current Power Trans-
mission, see the book of that title by Colin Adamson and
N. G. Hingorani (published in 1960 by Garraway, Ltd.,
London, England). ~
Each bridge in each of the converters 18 and 19 ~ -
usually comprises six identical solid-state controlled
valves arranged in a 3-phase double-way 6-pulse configura-
tion having three separate a-c terminals and a set of
positive and negative d-c terminals. In the present state
of the art, each valve will comprise a string of serially
interconnected thyristors which have individual voltage
ratings lower than the required voltage rating of the
valve. Such a valve is shown in greatly simplified form
in Fig. 2. It comprises a plurality of duplicate high-
power thristors 1, 2, 3, . . . n which are series connected
in polarity agreement betwsen the valve anode 21 and
cathode 22. One of the latter electrodes is adapted to
be connected to an a-c terminal of the bridge in which the
valve is located, and the other is connected to one of the
d-c terminals of the bridge. For higher current ratings,
additional thyristors can be respectively connected in
parallel with the thyristors shown in Fig. 2.
Once each cycle of the a-c system voltage, at a ~ -
time when a forward bias voltage is impressed on the illus-
trated valve (i.e., when the potential of the anode 21 is -
,
llCU-03977
l()S~O~O
positive with respect to the cathode 22), all of the consti-
tuent thyristors 1, 2, 3, ... n are fired or turned on
in unison by operation of associated control means ~not
shown) coupled to their respective gate electrodes. To
ensure proper turnon action, the valve should include
commutation transient suppressing means (not shown) such
as described and claimed in U. S. patents 3,423,664 - dated
January 21, 1969 - Dewey and 3,626,271 - dated December 7,
1971 - Dewey. Once the valve turns on, it will freely
conduct load current in a forward direction until subse-
quently turned off hy line voltage commutatlon, whereupon
it remains off until fired again one cycle later. Because
of slight discrepancies that commonly exist among the
individual switching characteristics of a plurality of
thyristors, it is standard practice to promote voltage
sharing among the respective thyristors of the valve by
connecting in shunt therewith an R-C bypass network com-
prising, across each level of the valve, a voltage equalizing
series resistor-capacitor subcircuit 23. Each subcircuit
typically includes a resistor of 10 ohms and a capacitor
of four to 10 microfarads. The benefits of such a bypass
network are more fully explained in the first-mentioned
Dewey patentD
The illustrated valve is designed to withstand -~
the voltage applied across its anode 21 and cathode 22 during
its periodic off or nonconducting intervals. At the
inverting or "receiving" end of the d-c link 13, this
voltage will forward bias the valve for most of the non-
conducting interval, whereas at the rectifying or "sending" ~y
end of the link the applied voltage reversely biases the
valve (i.e., anode potential negative with respect to
cathode). Although shown grounded in Fig. 2, it should be
llCU-03977
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understood that the cathode 22 could alternatively have
an absolute potential appreciably more negative (or more
positive3 than ground, depending on where the valve is
located in the bridge, where the bridge is position in
a pole, and whether the converter is operating in the
rectifying or inverting mode. When the illustrated
valve is reverse biased, its anode potential will be
negative with respect to ground, but if instead of being
grounded the cathode 22 were at a relatively positive
potential, the reverse-biased anode could then have a
potential which is either equal to or more positive than
ground. -
At various times during each nonconducting ~-
interval, the valve will be subjected to high peak voltages
which the associated power system normally imposes thereon.
In addition, abnormal voltage surges may be randomly pro-
duced by transient phenomena such as lightning strokes or
bushing or bus flash-overs. To help prevent damage to the
valve due to excessively high reverse or forward blocking
voltages, suitable voltage surge suppressors are commonly
used. For this purpose a lightning arrestor 24 has been
shown connected across the valve of Fig. 2. This arrestor
is relatively slow in operation, and since it is connected
across the whole valve there is no guarantee that each
constituent thyristor of the valve will not individually
be subjected to excessive voltage. To prevent destructive
breakover of any of the individual thyristors in the event
of a steep-front surge of excessive forward bias voltage,
it is therafore good practice to provide at each level
of the valve an overvoltage responsive triggering scheme
(not shown) such as described and claimed in U. S. patent
No. 3,662,250 - dated May 90 1972- Piccone and Somos.
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105~0 llCU-03977
To prever~t damage to any of the individual
thristors if turned on in the reverse breakdown mode due
to a steepfront surge of excessive reverse bias voltage,
each level of the valve is shunted by overvoltage protective
means 25. The latter means is constructed and arranged in
accordance with our present invention which will soon be
explained in conjunction with the description of Figs.
3 and 5.
In Fig. 2 the capacitors 31, 32, 33, 34 and
:
41, 42 represent distributed or ~p capacitances from
the constituent thyristors 1, 2, 3, ... n of the valve
to ground and between multiple levels of the valve. Such
stray capacitances always exist when a high voltage solid-
state valve is assembled. Their capacitive values are
characteristically small (50 to 100 picofarads) but non-
uniform. When the anode potential of the valve abruptly
changes with respect to the cathode 22, the stray capaci-
tances will significantly influence the voltage division
within the series-connected string of thyristors, and some
of these thyristors may consequently be subjected to a
disproportionately higher voltage than others. To illus-
trate this effect, assume that a fast rising voltage of
relatively negative polarity is suddenly applied to the
anode 21, thereby increasing the charge on each of the
various stray capacitances to ground (31, 32, etc.). At
each level of the valve, the magnitude of charging current
in the associated stray capacitance is equal to Cdv/dt,
where C is the stray capactive value and dv/dt is the rate
at which the voltage at that particular level is changing
with respect to ground. All o~ the resulting charging
currents will flow through the subcircuit 23 shunting
the first thyristor 1 which is closest to the anode 21
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l(~StiOll~
wh~re dvidt is highest, whereas a lower current (~qual
to the total charging current minus the increment attri-
buted to the stray capacitance 31) traverses the sub-
circuit 23 which is connected across the ne~t thyristor
2. Therefore in this example the first level of the valve
initially experiences a higher reverse bias voltage than any
of the other levelsr and there is a possibility that the
reverse breakdown voltage (V(BR)R) of thyristor 1 will be
exceeded.
Fig. 4 demonstrates what can happen in a valve when
subjected to a surge of reverse bias voltage which rises at a
very high rate. It is assumed that such a surge is super-
imposed on the normal maximum reverse blocking voltage of
the valve. It is further assumed that the latter voltage
is 240 kv, that there are of the order of 240 thyristors in
series in the valve, and therefore that the working peak
reverse voltage per thyristor (VRwM) is approximately lkv.
In Fig. 4 the broken line 51 represents a transient surge of
reverse bias voltage on the valve, which voltage is increas-
ing in magnitude at a constant rate of minus 1,200kv per
mi¢rosecond. If this increasing voltage were shared uni-
formly by all levels of the valve, the reverse blocking ~-
voltage on each thyristor would rise at the rate of minus
5kv per microsecond, as is depicted by the solid line 52 in
Fig. 4~ It will be observed that at this average rate of rise,
the reverse blocking voltage 52 reaches a magnitude of 2kv
after 200 nanoseconds which is approximately the time re-
quired for the valve voltage 51 to attain the threshold or ~;
spark-over magnitude of the lightning arrestor 24 (assumed
to be 480 kv~, and after an additional 100 nanoseconds,
which approximates the operting time T of the lightning
oo ~v/~ s
A arrestor for a voltage surge of l,20Q ]~v ~ , the thyristor
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. .- -: :
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volta~e 52 is still at a safe level oE 2.5kv
In practice, solne oE the thyristors (such as
thyristor 1 in the example given above) can experience a
much faster than average rise in reverse blocking voltage, and
a rate 15 -time higher as shown by the line 53 in Fig. 4.
If the reverse bias voltage on the thyristor 1 continued to
increase at this higher rate, it would reach a dangerously
high level of nearly Skv in only 50 nanoseconds, as is indi-
cated by the straight line extension 53a of line 53. This
is far in advance of the lightning arrestor operation.
However, as the reverse bias voltage on thyristor 1 approaches
3kv~ this thyristor has an avalanche breakdown which, in
combination with the operation of the overvoltage protective
means 25, thereafter clamps this voltage (as indicated by the
bend 53b in the line 53) to a safe maximum level.
A preferred embodiment of the reverse overvoltage ;~
protective means 25 is shown in Fig. 3 It comprises voltage-
variable non-linear resistance means 61, switching means -~
62, and wires 63, 64, and 65 which connect the resistance
means 61 and the switching means 62 in series with each other
and in parallel with the associated thyristor 1. Preferably
f'~'P/I/
A the switching means 62 comprises a ~ semiconductor element
which is poled inversely to the main thyristor l, and in Fig.
3 it is shown simply as an auxiliary thyristor whose gate
electrode is connected either internally or directly
axtsrnally to its cathode, This auxiliary thyristor 62 has
a normal high-resistance state. It is operative to switch
abruptly to a low-resistance, forward current conducting
state if subjected to a voltage of a predetermined break-
over value VBo(s). The switching characteristic of the
auxiliary thyristor 62 is coordinated with the voltage-
current characteristic of the non-linear resistance
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means 61 so that the thyristor ~2 will switch to its current
conducting state in high-speed response to the magnitude of
the reverse bias voltage across the main thyristor 1
increasing above VRRM and nearly equaling V(B~)R.
The non-linear resistance means 61 of the over-
voltage protective means 25 comprises at least one MOV 66; as
illustrated in Fig. 3 we prefer to use two duplicate MOVs
66 in series. The voltage-current characteristic of this
means in coordinated with the reverse breakdown character-
istic of the main thyristor 1 so that when the auxiliary
thyristor 62 switches to its current conducting state due
to a surge of reverse bias voltage, surge current will divide ~
between the resistance means 61 and the main thyristor 1 in ~ <
such proportions that the maximum value of reverse current
in the main thyristor 1 does not exceed a predetermined criti- ~:
cal magnitude which can damage this component~
For a better general understanding of the requisite
coordination between the main thyristor and the parallel non-
linear resistance means, a typical example will now be con-
sidered with the aid of Figs. 5 and 6. In Fig~ 5 the curve
71 represents the voltage~current characteristic of the non- :
linear resistance means 61 (which actually comprises two
MOVæ 66 in series), and the curve 72 represents the reverse
breakdown characteristic of the main thyristor 1. In this ~ :
example the main thyristor is assumed to have a reverse
breakdown level V(BR)~ of approximately 2~8kv, and if the
instantaneous magnitude of a surge of non-repetitive reverse
bias voltage increases beyond this level, the main thyristor
will experience an avalanche breakdown which causes it to
switch from a reverse blocking state to a reverse current
conducting state. The working peak reverse voltage across
the main thyristor is assumed to be in a range of approxi-
mately 1.0 to 1.2kv, and the respective peak reverse voltage
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1()5~0~0
(VRRM) is assumed to be 2.6]cv which i5 more than twice VR~M.
(The relatively high magnitude of VRRM is due to transient
commutation voltage overshoot associated with those levels
of the valve whose thyristors have the smallest reverse re-
covery currents.)
The voltage-current characteristic 71 of the non-
linear resistance means 61 is defined by the equation
I = (~)~. Proper selection of the exponent alpha and the
constant C is based on certain additional given information
about the avalanche capability of the main thyristor and
about the nature of the non-repetitive reverse overvoltage
transients to which the thyristor is exposed. For the
present example, it will be assumed that such transients in-
volve a half-cycle surge of reverse current having a sinusoidal
waveform and a period of S microseconds during which or upon -
the expiration of which the surge either is suppressed by
spark over of the associated lightning arrestor or otherwise
subsides. During this period, on a non-repetitive, single-
shot basis, the main thyristor can safely conduct reverse
current up to a predetermined critical maximum or peak value
IRM~ Typically IRM = 200 amps, but in some cases it can be
much higher or lower than this magnitude~ In the illustrated
example, the reverse voltage across the thyristor at 200
amps is approximately 4050 volts. It is further assumed that
at this voltage the worst-case peak magnitude of surge
current which is to be divided between the main thyristor
and the parallel non-linear resistance means is 500 amperes.
Consequently a first constraint on the design of the resistance
means 61 is that it conduct more current than the difference
(e.g., 300 amps) between this peak magnitude of surge current
(500 amps) and IRM (200 amps) when the magnitude of voltage
across it is the same (e.g., 4050 volts) as the voltage across
llCU-03977
1()5~010
the thyristor when conducting IRM. In other words, in this
particular example the non-linear resistance means should
have a voltage drop less than 4050 volts when conducting 300
amps. The 300-amp, 4050-volt point is shown at X in Fig.
5.
A second constraint in the selection of the exponent
alpha and the constant C of the non-linear resistance means
61 is that this means must not conduct all of the surge
current at the peak magnitude thereof~ The main thyristor
can absorb an appreciable portion of the reverse current
surge~ As a minimum, current in the main thyristor should
equal 1% of the worst-case peak magnitude of surge current,
which in the present example is at least 5 amperes. For
the illustrated raverse breakdown characteristic curve 72,
the voltage across the main thyristor is approximately 3600 `
vol~s when conducting reverse current of 5 amps, and conse~
quently the magnitude of current in the non-linear resistance
means must be no greater than 495 amps at the same voltage.
The 495-amp~ 3600-volt point is shown at Y in Fig. 5. Be-
tween the points X and Y is a range 73 in which the non-linear
resistance means 61 will divert from the main thyristor 1
between approximately 60 and 99% of the total surge current
flowing through these paralleled elements at the 500-amp
peak magnitude thereof.
Preferably the parameter C of the MOV is given a
magnitude approximating the reverse breakdown level of the
main thyristor, whereby current in the non-linear resistance
means 61 of Fig, 3 will have a magnitude of 1 ampere when
the reverse bias voltage across the main thyristor has
increased to the vicinity of V(BR)R~ Any magnitude within a
range of approximately plus and minus 15 percent of V(BR)R
is satisfactory for C, and a magnitude below this range can
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105~0~0
be used if desixed.
To obtain the particular voltage-current characteris-
tic 71 shown in FigO 5, we have chosen a constant C of approx-
imately 2800 and an d of approxima-tely 15070 These para-
meters satisfy the various constraints discussed aboveO
(Persons skilled in the art will understand that if the
non-linear resistance means 61 actually comprises a serial
combination of two or more MOVs 66 among which the total
voltage V is equally divided, the magnitude of C for each
of the individual MOVs has to be correspondingly reduced.)
In this example the 500-amp peak magnitude of the reverse
surge current is reached at a safe peak clamping voltage of
approximately 4040 volts, at which point the amount of
current flowing through the main thyristor 1 is approximately
three-fifths as much as the current in the parallel non-
linear resistance means 61.
The breakover voltage VBO(v) of a non-linear resis-
tance means having the values of d and C that are specified
in the preceding paragraph is approximately 1800 volts
which is much lower than the given value o~ VRRMO To save the
resistance means 61 from exposure to this higher repetitive
peak reverse voltage, our overvoltage protective means 25
includes the switching means 62 which in Fig. 3 is shown as
an auxiliary thyristorO The breakover current of the
auxiliary thyristor 62 should be the same as the idling
current of the non-linear resistance means 61, namely one
milliamp (peak). This is the threshold magnitude of current
which must flow through the switching means in order to
switch from its normal high-resistance state to a low-
resistance, current conducting state when the forward bias
voltage across the auxiliary thyristor attains its breakover
value VBO(s). In accordance with our invention, the sum of
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10~010
VBO(s) and VBO(v) exceeds VRRM and nearly equals V(BR)R.
As has been shown by way o~ exa~ple in Fig. 5, VBo(s) is
approximately 1000 volts, whereby the auxiliary thyristor
62 breaks over when a surge of non-repetitive reverse
voltage has attained a magnitude of approximately 2800 volts.
The switching time of the auxiliary thyristor plu9 the delay
time of the metal oxide varistors 66 will approximately
match the reverse breakdown delay time of the main thyristor,
and consequently the peak voltage clamping action of the MOVs
and the avalanche action of the main thyristor take place
concurrently~ ;
If the inherent breakover current of the element
used for the switching means 62 were lower than one milliamp,
it is desirable to connect a calibrating resistor 67 of
appropriate size in parallel circuit relationship therewith
so as to increase the current flowing between conductors
64 and 65 to substantially one milliamp at the breakover
point of the auxiliary thyristor. This option is indicated
by the broken lines in Fig~ 3.
In lieu of the Fig. 3 arrangement of an auxiliary
thyristor having a VBo of 1000 volts, the switching means
62 can comprise an auxiliary thyristor 62a of slightly
higher VBo value, with the gate electrode of the latter
thyristor being connected to its anode by way of an addi-
tional PNPN se~iconductor element 68 as is shown in FigO 3a.
The breakover voltage value of the additional element 68 is
1000 volts. The Fig. 3a arrangement enhances the di/dt
capability of the overvoltage protective means and is
particularly desirable at ambient temperatures higher than room
temperature.
Fig. 6 shows the division of current between the
main thyristor I (iR) and the overvoltage protective means
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llCU-03977
105~010
25 (iMov) for a revers~ overvoltage transient V which is
assumed to rise from zero at a rate of approximately 20 ~v/,~
to a relatively flat peak magnitude of 4 kv and then to
subside after a period of approximately 5 microseconds.
The main thyristor and the non-linear resistance means in the
overvoltage protective means have the exemplary charac-
teristics illustrated in Fig. 5. ~he maximum or peak of iR
is approximately one-third the 410-amp peak magnitude of the
sum (I) of the reverse surge currents iR and iMoV, and the
peak of iMov is approximately two-thirds that of I~ Thus
the surge energy dissipation duty is shared jointly by the
main thyristor in its reverse avalanche state and by the
parallel non-linear resistance means. The limited amount
of energy that the main thyristor has to dissipate is suf-
ficiently small to prevent self destruction, yet it is
large enough to relieve the non-linear resi~tance means of
the burden of absorbing the total surge energy.
While we have shown and described a preferred
form of our invention by way of illustration, many modifi-
cations will probably occur to those skilled in the art.
For example, the overvoltage protective means 25 could be
advantageously used to protect other types of devices or
switches having characteristics similar to the main
thyristors 1, 2, etc., previously described. We therefore
intend by the claims which conclude this specification to
cover all such modifications as fall within the true
spirit and scope of our invention.
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