THYRISTOR VALVE FOR A HIGH VOLTAGE CONVERTOR

VALVE A THYRISTORS POUR CONVERTISSEUR A HAUTE TENSION

English Abstract

ABSTRACT OF THE DISCLOSURE:

In a high voltage thyristor valve, a voltage
divider is connected in parallel with each thyristor. A
capacitor for supplying the control device of the thyristor
is connected to a tap on the voltage divider and arranged
to be charged from the voltage divider. A resistor is con-
nected in series with the capacitor, this resistor ins-
tantaneously providing a supply voltage to the control
device of the thyristor when an overvoltage appears across
the thyristor.

Claims

The embodiments of the invention in which an ex-
clusive property or privilege is claimed are defined as fol-
lows:
1. Thyristor valve for a high voltage convertor
having a plurality of series-connected thyristors each pro-
vided with a control device, and having a voltage divider
connected in parallel with each thyristor, each thyristor
being also provided with a capacitor, supplied from a tap
on the voltage divider connected in parallel with this
thyristor, for supplying the control device of this thyristor,
characterised in that each capacitor is connected in series
with a resistor to form a series-connection having two
terminals, said terminals being connected to the voltage
divider for charging the capacitor and to a supply connection
means of the control device for feeding a supply voltage to
this control device, whereby in response to a step voltage
across the thyristor, a supply voltage is immediately
delivered to the control device, which supply voltage having
a sufficient magnitude to enable firing of the thyristor
and a prevention of a destruction of this thyristor through
said overvoltage.
2. Thyristor valve according to claim 1, charac-
terised in that a diode is connected in parallel with the
resistor for a rapid discharge of the capacitor to the
control device.

Description

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The present invention relates to a -thyristor
valve for a high voltage convertor having a plurality
of series-connected thyristors and having voltage dividers
connected in parallel with the thyristors. The thyristors
are provided with capacltors ~or supplying -their control
devices, these capaci-tors being supplied from taps on the
voltage dividers.
Thyristor valves of the above-mentioned kind are
previously known. In certain cases a very rapid ignition of
at least one or a few of the thyristors in the valve is
required, for example in the event of a transient overvolt-
age. If such an overvoltage occurs at a moment when the
above-mentioned capacitor is not charged, it will take a,
certain time, because of the impedance of -the voltage di-
lS vider, before the capacitor is charged -to such a -voltage
level that the con-trol device becomes operational and igni-
tion o~ the thyristor may take place. In the event of
rapidly growing overvol-tages in connection with valves of
this kind, thyristors may therefore become destroyed be-
fore being ignited by their control devices.
The invention aims to provide a thyristor valveof the above-mentioned kind, in which a suEficien-t supply
volta~e to the control device for ignition of the -thyristor
always exists before the voltage across the thyristor has
increased to a harmful level.
More particularly, -the present invention relates
to a thyristor valve for a high voltage convertor having
a plurality of series-connec-ted thyristors each provided
with a control device, and having a voltage divider con-
nected in parallel with each -thyristor, each thyristor
being also provided wi-th a capaci-tor r supplied from a tap
on the voltage divider connected in parallel with this
thyristor, for supplying the control device of this
; thyristor. The thyristor valve is characterised in that
each capacitor is connected in series with a resistor to
form a series-connection having two terminals, these


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terminals being connected to the voltage divider for
charging the capacitor and -to a supply connec-tion means
of the control device for feeding a suppl.y voltage to this
con-trol device, whereby in response to a step voltage
across the thyristor, a supply vol-tage is immediately
delivered to the control device, which supply voltage
having a sufficient magnitude to enable firing of the
thyristor and a prevention of a destruction of this
thyristor through the overvoltage. In a preferred embodi-
men-t, a diode is connected in parallel with the resistor
for a rapid discharge of -the capacitor to -the control
device.
In the following~ a preferred embodiment of the
presen-t invention will be described with reference to the
. accompa.nying figure, which figure shows a thyristor valve,
including the characteristics of the invention, with a
thyristor having an associated control device and voltage
divider.
The thyristor T is one of the series-connected
thyristors which are included in the valve. The -thyristor
is provided with a control device SD, connected to -the
gate of the thyristor, for ignition of the thyristor. The
ignition of the thyristor is normally carried out upon the
receipt of an optical ignition signal via the photo-diode
FD. To protect the thyristor in the event of an overvoltage
in the o~f-s-tate, the con-trol device is also adapted to
ignite the thyristor as soon as the off state voltage of
the thyristor reaches a predetermined level.
A voltage divider, consisting of the capacitor
C1, the resistor R1 and the ~ener diode ~D, is connected
in parallel with the thyristor. Capacitor C2, which
constitutes an energy storage for the con-trol device, is
charged from the voltage divider via a diode Dl. In
series with the capacitor C2 there is arranged a resistor
R2 and in parallel with the resistor there is arranged a
diode D2, through which the capacitor can be rapidly

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discharged to the control device via -the supply connection S
of said control device. In the event of a rela-tively slowly
growing overvol-tage, the capacitor C2 has time to be charged
to a sufficient voltage level for opera~tion of the control
device before the thyris-tor voltage reaches harmful values,
and upon ignition of the thyris-tor the capacitor is rapidly
discharged through diode D2 to the control device and via
the control device to the gate of the thyristor. In the
event of a very rapidly growing overvoltage, the capacitor
will not have time to be charged sufficiently rapidly.
However, a voltage drop is obtained instantaneously across
the resistor R2~ and thus supply voltage is delivered to
the control device. By adjusting the resistance of the
resistor R2 in a suitable manner to the other impedances of
the voltage divider circuit, it may be ensured that sufficient
supply voltage is always delivered to the control device
before the thyristor voltage has time to assume a detrimental
level.
The diode D2 may in certain cases be omitted, for
example when the resistor R2 has low resistance.




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