Voltage supply with recovery protection for a thyristor

A voltage supply for an electric filter is disclosed. The supply comprises a high-voltage rectifier, a pulsed voltage circuit including a resonant circuit and a parallel circuit of a thyristor and a diode which controls resonance in the resonant circuit. The thyristor is fired to trigger the resonant circuit and generate an oscillation which is delivered to the filter as a pulsed voltage. In order to prevent damage to the thyristor, it is fired whenever the duration of the current flowing through the diode is shorter than the thyristor recovery time.

BACKGROUND OF THE INVENTION 
The present invention relates to a voltage supply for an electric filter. 
Electric filters usually operate with a high dc voltage obtained by 
rectification of a voltage supplied by an ac network (see, for example, 
"Siemens-Zeitschrift," 1971, No. 9, pages 567 to 572). It is known to 
superimpose on this high dc voltage supplied to the filter a pulsed 
voltage which is dependent on the operating state of the filter and can be 
generated, for example, in response to a short circuit in the filter, as 
disclosed for example in DE-OS 26 08 436 and DE-OS 30 27 172. 
Pulsed voltage sources for providing the pulsed voltage are also known, 
according to which a thyristor and a diode are connected in series with a 
dc voltage source and a transformer coupled to the filter. The resonant 
circuit formed by the transformer and the pulsed voltage source is 
triggered each time the thyristor is fired, as described for example in 
DE-OS 26 08 436, to provide an oscillation of the resonant circuit which 
is delivered to the filter as a pulsed voltage via the transformer. 
A breakdown of the electric filter resulting in a short circuit usually 
occurs at the time that the maximum voltage is applied to the filter, i.e. 
during the period the diode carries the pulsed current (oscillation), or 
shortly thereafter. Due to the filter short circuit, the resonant circuit 
oscillation is abruptly damped, i.e. the diode is blocked. Thereby, the 
maximum dc voltage is reapplied to the thyristor. If the time between 
cut-off of the thyristor by the zero crossing of the current and transfer 
of current through the diode is very short, i.e. shorter than the recovery 
time of the thyristor, the thyristor can be fired by the reapplied maximum 
dc voltage without a firing pulse. Since this firing of the thyristor 
proceeds relatively slowly, the thyristor is subjected to high thermal 
stress during this period and may be destroyed. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to protect the thyristor or a 
series and/or parallel circuit of thyristors in a filter voltage supply of 
the type described above in the event of a breakdown or short circuit in 
the electric filter. 
This and other objects are achieved in accordance with the invention by 
supplying a firing pulse to the thyristor(s) if the duration of the 
current flowing through the diode is shorter than the recovery time of the 
thyristor(s) and preventing subsequent firing pulses from firing the 
thyristor for a period of time depending on the state of the filter. The 
invention thereby limits thermal stress in the thyristor(s). 
Further in accordance with the invention, a signal is generated which is 
proportional to the duration of the current flow in the diode and is 
compared to a predetermined signal proportional to the recovery time of 
the thyristor(s) to determine if the length of time that current is 
flowing through the diode is shorter than the recovery time of the 
thyristor(s). The result of the comparison is used to control the circuit 
supplying the firing pulses to the thyristor(s). 
According to a preferred embodiment, a square wave voltage proportional in 
frequency to the diode current is generated and compared with the output 
of a multivibrator trigged by the square wave. The multivibrator in 
response to being triggered supplies an output pulse having a width or 
duration corresponding to the recovery time of the thyristor. A logic 
circuit effects the comparison and the result of the comparison is stored 
in a memory device which controls the circuit supplying the firing pulses. 
In accordance with another aspect of the invention in which a controller 
controls the high-voltage rectifier in response to filter flashovers, the 
circuit supplying the firing pulses is further controlled by the 
controller. According to the preferred embodiment described above, the 
memory device can be set by the controller. 
In accordance with still another aspect of the invention, the number of 
thyristor firing pulses supplied to the thyristor in a given time period 
is selected and used to control the period and/or amplitude of the pulsed 
voltage delivered to the filter and/or the magnitude of the high dc 
voltage delivered to the filter. 
The above and other objects, features, aspects and advantages of the 
present invention will be more readily perceived from the following 
description of the preferred embodiments thereof when considered with the 
accompanying drawings and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electric filter designated 5 in FIG. 1 is supplied in a manner known per 
se from a high-voltage rectifier 6 which is connected to ac network lines 
R', S'. Control apparatus 7 is coupled to the rectifier 6 and controls the 
high dc voltage supplied to the electric filter 5 in response to 
breakdowns, e.g. short circuits, overcurrent, etc., and is described in 
detail in the above-mentioned publication "Siemens-Zeitschrift". The 
control apparatus is not part of the invention disclosed herein and is 
therefore not described in detail. 
Also connected to the electric filter 5 is a pulsed voltage source 
comprised of a rectifier 1 which may include controlled semiconductor 
devices 11, a storage capacitor 12 connected in parallel with the 
rectifier 1, a circuit arrangement 2 connected in series with rectifier 1 
comprised of parallel-connected thyristor 22 and diode 21, and a pulse 
transformer 3 having a primary winding 31 and a secondary winding 32. The 
recitifier 1 is supplied from lines R,S,T of a three-phase network and 
with capacitor 12 provides a dc voltage to the thyristor and diode. The 
pulsed voltage U.sub.p at the secondary winding 32 is fed to the electric 
filter 5 via a coupling capacitor 4 and is applied to the filter 5 
together with the high dc voltage U.sub.gl from the high-voltage rectifier 
6. 
A firing circuit 20 supplies firing pulses to the thyristor 22, for 
example, at a periodic intervals T.sub.P (FIG. 2) of 2 ms. The firing 
pulses trigger the series resonant circuit formed by components of the 
pulsed voltage source, i.e. capacitor 12, the transformer primary winding 
31, the transformer secondary winding 32 and the coupling capacitor 4, and 
the electric filter 5. Primary current designated i.sub.1 in FIGS. 1 and 3 
flows through the primary winding 31 and induces a pulsed voltage 
designated U.sub.p in the secondary winding 32. The superposition of the 
pulsed voltage U.sub.P and the high dc voltage U.sub.gl results in the 
voltage waveform applied to the electric filter shown in FIG. 2, the 
individual pulses having a width of, for example, 200 us. Upon firing the 
thyristor 22 to trigger a one period oscillation in the resonant circuit 
during normal filter operation, thyristor 22 initially carries the current 
designated i.sub.T until the zero crossing point of the current, at which 
time diode 21 conducts the current designated i.sub.D. The oscillation 
currents i.sub.T and i.sub.D compose the primary current i.sub.1 at the 
transformer primary 31. When the diode current i.sub.D again passes 
through zero, oscillation of the resonant circuit is terminated until the 
resonant circuit is triggered by another firing pulse supplied to the 
thyristor 22. 
FIG. 4 illustrates the voltage and current relationships when a short 
circuit occurs in the filter 5. The secondary winding voltage U.sub.P 
breaks down at time t.sub.k due to a flashover and drops to zero. The 
flashover also causes the diode to block so that the diode current i.sub.D 
likewise goes to zero, and the resonant circuit oscillation is terminated. 
As a result, the full dc voltage is abruptly applied across the thyristor 
22. If the time during which the diode current flows through diode 21 is 
longer than the recovery time of the thyristor 22, the thyristor will not 
fire and there is no problem. However, if the time t.sub.x in which the 
diode current i.sub.D goes to zero is shorter than the required recovery 
time t.sub.q of the thyristor 22, the thyristor 22 will fire without a 
firing pulse. Since this process takes a relatively long time, the 
thyristor can be thermally overloaded. According to the invention, the 
period t.sub.x in which the diode current i.sub.D flows, i.e., the second 
half-wave of the primary current, is monitored. If this time t.sub.x is 
shorter than the recovery time t.sub.q of the thyristor, then the 
thyristor is immediately fired by a firing pulse so that it can again 
conduct current. Since the dc voltage at the electric filter is reduced to 
zero due to the short circuit, this additional voltage firing pulse has no 
major effect on filter operation. 
Referring to FIG. 1, the diode current i.sub.D is measured, as indicated by 
the circular connection, and supplied to a multivibrator 82 which 
generates a square wave having a pulse width or half cycle t.sub.x which 
corresponds to the spacing of the zero crossings of the current i.sub.D. 
Thus, the multivibrator 82 provides a square wave having a frequency 
proportional to the duration of the current pulses in the diode. The 
square wave signal is fed to and triggers a monostable multivibrator 83 
which produces a pulse having a width corresponding approximately to the 
recovery time t.sub.q of the thyristor 22. The output of the multivibrator 
82 and the output of the multivibrator 83 are connected to a logic circuit 
84, in which a comparison is made as to whether the signal from the 
multivibrator 82 corresponding to the duration of the current pulses 
t.sub.x is larger or smaller than the recovery time t.sub.q set in the 
monostable multivibrator 83. If the pulse width of the output signal of 
the monostable multivibrator 83 is wider than the square wave pulse width 
output signal of multivibrator 82, i.e. if the duration t.sub.x between 
the zero crossings of the diode current is shorter than the recovery time 
t.sub.q, the logic circuit 84 responds and delivers a setting signal to a 
memory device 85. The memory device transmits an immediate command via 
line 86 to the firing circuit 20 to fire the thyristor 22, and then 
disables the firing circuit 20 for a time dependent on operating data of 
the filter. The firing circuit 20 can again be enabled, for example, when 
the dc voltage at the filter reaches a given magnitude. This can be 
accomplished by having the controller 7 reset the memory 85 to enable the 
firing circuit 20. 
The number of additional firing pulses so generated per unit time can be 
determined by counting them in a counter 9 and the count used to 
optionally change the pulse firing frequency, as indicated by the line 91. 
Alternatively, it is also possible to vary the amplitude of the 
oscillation pulses by controlling the rectifier 1 and/or the magnitude. of 
the dc voltage by controlling the high voltage rectifier 6. 
Certain changes and modifications of the embodiments of the invention 
disclosed herein will be readily apparent to those skilled in the art. It 
is the applicants' intention to cover by their claims all those changes 
and modifications which could be made to the embodiments of the invention 
herein chosen for the purpose of disclosure without departing from the 
spirit and scope of the invention.