CA1151240A - Thyristor switch with transient protection and rf interference suppression - Google Patents

Abstract

Title of the Invention: THYRISTOR SWITCH WITH TRANSIENT
PROTECTION AND RF INTERFERENCE
SUPPRESSION
Inventor: Gunnar Asplund ABSTRACT OF THE DISCLOSURE
A high voltage power converter has a number of thyristors interconnected. Symmetrical voltage dividers maintain equal voltage distribution across the thyristors. Each thyristor has a plurality of inductors associated therewith for reducing rapid current increases through the thyristors during firing which might injure the components and generate radio frequency interference.

Description

~5~ 40 .. . .
, Title of the Invention: THYRISTOR SWITCH WITH TRANSIENT
PROTECTION AND RF INTERFERENCE
, SUPPRESSION
;; Inventor: Gunnar AspIund ., ' . ' . l ij FIELD OF THE INVENTION
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! The present invention relates to high voltage ¦' power converter valves utilizing thyristors, and more ¦1 particularly to a voltage divider network for such thyristors I which utilize inductors for limiting abrupt current increases in the converter circuitry during ignition of the thyristors.

, BRIEF DESCRIPTION OF THE PRIOR ART
Thyristors are a well-known component for high voltage ! power converter applications. Arranging a voltage divider ¦¦ for attaining uniform voltage distribution between the ¦ thyristors of a converter switch is well established in ¦ the prior art. Such a voltage divider may consist of a series connection of a capacitor and a resistor which is~
I in turn, connected in parallel with each thyristor. During ¦1 operation of high voltage thyristor valves, !/such as in ¦~ the present application of a power converter, considerable I' radio frequency interference arises due to the generation ¦ of transient voltages and current occurring during thyristor - 1~ switching. In high power converters, this interference ¦ is so great that special measures have hitherto been required to reduce the interference, such as by utilizing phase reactoxs in the AC leads of the converter and by utilizing il1 .,i 11 ~ I

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iL1~1~40 electromagnetic screening of housings or rooms, where such converters are located. The generation of transient voltages due to external circumstances may cause the destruction of -thyrlstors. For example, in the event of a ground fault, the thyristors may be subjected to steep voltage translents which may cause component failure. Similarly, in the case of overloading caused by lightning, transient voltages of e~tremely high amplitude may cause the destruction of the thyristors. In normal operation of the converter, a last firing thyristor or thyristors may be subjected to excessive voltage transients that will terminate the component's useful-ness.

BRIEF DESCRIPTION OF THE PRESEN~ INVENTION
The present invention has as a primary object the minimization of radio frequency interference emitted by a power converter, due to the firing of thyristor components in such a converter. A further object of the invention is to main-tain voltages at thyristor terminals at a reduced level during the occurrence of transients so that the thyristors will not be destroyed by the transients.
This is basically achieved in the present invention by utilization of a plurality of series-connected thyristors, in circuit with a voltage divider including resistor and capaci-tor components. At least one inductor is connected in circuit with each thyristor for limiting the current jump upon firing of the thyristor.
According to the present invention, in a high voltage converter valve comprising at least a pair of thyristors, the improvement comprises:
a first inductor serially connected in circuit with each corresponding thyristor for limiting the rate of current 1~51;2~Q

increase in the valve during thyristor firing;
series connected resistor and first capacitor compo-nents, connected in parallel across the outward leads of each serially connected thyristor and inductor, for providing equal voltage distrlbutlon between the thyristors;
a second capacitor connected in parallel with each series connected resistor and first capacitor; and a second respective inductor connected in circuit with each corresponding first inductor and cooperating with stray capacitance for minimizing the generation of radio - frequency-interference signals from the valve.
The present invention is also characterized as fol-lows:
a high voltage thyristor power converter valve having a plurality of interconnected valve sections capable of decreasing generated radio frequency interference and rate of current increase through the valve during thyristor firing, each valve section comprising:
a thyristor connected in series with a first inductor;
- a resistor connected in series with a capacitor;
means connectlng the series connected resistor and capacitor in parallel with the series connected thyrlstor and inductor;
a second capacitor connected in parallel with the serially connected thyristor and inductor; and a second inductor connected in series between an outward terminal of the first inductor and an output terminal of a thyristor in an adjacent valve section.

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~ - 2 1~5~%40 The above-mentioned objects and advantages of the present invention will be more clearly understood when con-sidered in conjunction with the accompanying drawings, ~ in which: !

.' BRIEF DESCRIPTION OF THE FIGURES
.~ FIG. 1 is a block diagram of a portion of a power converter incorporating the present invention.
. FIG. 2 is a schematic illustration showing two inductors, connected to each thyristor, as shown in the block diagram 10 11 of FIG. 1.
' FIG. 3 is an alternate schematic showing of an inductor -, co~figuration connected with a thyristor.
l~ FIG. 4 is a partial schematic diagram illustrating j~ the filtering of radio frequency interference from a power ,~
15 1I converter of the present type, such filtering occurring in two stage s .

¦~ DETAILED DESCRIPTION OF THE: I~VENTION
li. FlG. 1 shows two thyristors, Tl and Tll, which are I Ij included in a high voltage valve which may ~ ~0 Ij comprise a large number of similarly connected series-connected '3 thyristors. The thyristors Tl and Tll are provided with ¦I respective control units, SPDl and SPDll, each of which is connected to the gate and the cathode of a corresponding I¦ thyristor. A control master SD, common to the thyristors 25 i Tl and Tll delivers a signal simultaneously to all the control 1l units for firing the thyristors, each control unit then !l i ,i _ 3 _ ,, 1, 1~5~0 supplying a control current pulse to its respective thyristor.
This described configuration including the control units and control master are well established prior art circuits.
Two inductors Ll and L2 are connected in series with the anode of thyristor Tl- The voltage divider impedances Rl, Cl and C2 are connected between the junction point Pl of the inductors and the cathode of the thyristor Tll. 3 Thyristor Tll is also provided with two inductors Lll and L12 as well as with the voltage divider impedances ` Rll, Cll and C12. In a similar manner, other thyristors would be connected in circuit with respective divider imped-ances.
The thyristors may also be provided, in a known manner, with further voltage divider impedances, for example for supplying the control units. Each thyristor may be connected i in parallel with a relatively high resistance to obtain a uniform voltage division in case of pure direct voltage application. 3 i ' I
~ In a conventional manner, the RC circuit Rl-Cl attends ~ to the voltage division during normal operation. The capacitor Cl is valued so that any non-uniformity in the voltage distribu-!' tion between the thyristors becomes so small that switching , faults will not occur in operation. The resistor Rl is valued so that the discharge current of the capacitor Cl upon firing of the thyristor is limited to a value that l is not detrimental to the thyristor.
¦ The inductor Ll and the capacitor C2 are valued so that the thyristor Tl is not subjected to an excessively . . ' .
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steep voltage transient in the event of rapid voltage drops across the switch, nor in the event that a particular thyristor ;
is fired after the other thyristors in the val~e. ~The capacitor C2, which is preferably not series-connected to ; a resistance, may suitably be given a low capacitance, which ; reduces the quantity of charge which flows from the capacitor through the thyristor upon firing thereof. This is also in agreement with the obiect of the capacitor to dampén hlgh frequency voltage components. The capacitor C2 may, ~ for example, be given a capacitance which is of the order of magnitude of 1 to 10 percent of the capacitance of the capacitor Cl. `
, The inductor L2 limits the discharging current of the capacitor C2 upon firing of the thyristor.
15 j, When the valve i8 i~ted, ~a rapid charging of a number of undampened capacitances occurs in the vicinity of the ~lve (e.g., stray capacitances~. The inductors Ll and L2 are valued so that these otherwise high curren~s are limited to a harmless level.
20 ~~ FIG. 2 shows a preferred embodiment of the inductors Ll and L2. The anode lead A-A' of the thyristor ~see also ¦~ FIG. 1~ is wound a suitable number of turns through two ring cores~Kl and K2 and forms a primary winding PL. This ' winding together with the core Kl forms the inductor Ll, j and together with the core K2 the inductor L2. The inductances of the inductors are configured to desired values by the choice of core sizes and number of winding turns. The cores Kl and K2 may, for example, be equal, in which case the . . .

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1~51240 inductors will have the same inductance. The number of winding turns should be relatively low since the lead A-A' in typical cases is considerably thick. If the number of turns is greater than one, the inductors Ll and L2 will have no easily accessible po;nt of connection for the impedance elements Rl, Cl, C2. Instead an artificial point of connection Pl is generated by means of a secondary winding SL wound around the core Kl, which winding may be made of a relatively thin wire. One énd of the secondary winding i is connected to the conductor A-A' at a point P2 located outside the cores Kl and K2. Its other end constitutes the point of connection Pl. If the secondary winding SL
is made with the same number~of winding turns and with the ~ same direction of winding as the primary winding PL, the point Pl will always assume the same potential as the point Il o-connection of the inductors would have assumed if the ; l~ inductors had been made completely separate, that is with individual cores and windings.
~' Since conductor A-A' is normally considerably thick, '~
~ the embodiment shown in FIG. 2 involves a considerable saving , of space as compared with the alternative, similar embodiments i in which the inductors are completely separate.
An example of this latter embodiment is shown in 1 FIG. 3, in which the conductor A-A' is wound through each I core to form separate windings. This alternative is in general considerably more bulky than that shown in FIG. 2.
In the case where the inductors only have one winding turn each (conductor A-A' then being fitted right through both , cores), the alternative according to FIG. 2, however, entails ,. i .

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1~51~0 no advantages since in that case the point of connection is easily accessible ~a point on A-A' between the cores).
FIG. 4 shows a valve according to the invention, which schematically illustrates a sïngle thyristor with series inductors and voltage divider elements according to FIG. 1. As regards attenuation of radio frequency interference, the diagram according to FIG. 4 is equivalent to a valve ! according to the invention along with external capacitance Cv. The thyristor may be regarded as a generator, which generates a radio frequency alternating voltage. ~i This is filtered in two stages. The first stage consists of inductor L2 and capacitors Cl and C2 as well as resistor Rl. The second stage consists of inductor Ll and the external capacitance Cv, which consists of lead-through capacitances, stray capacitances and other external capacitances. This two-stage filtering gives a considerably higher attenuation of the radio frequency interference, especially at higher f~equencies, than a one-stage attenuation ~i~e~, with only ~i one inductor) and the same quantity of iron is used in the cores in both cases. By carrying out filtering ~irectly ~ in the ~alve, external measures for eliminating interference il ~phase inductors, screening, etc.), which were necessary in the past, may be avoided and a considerable reduction of fabricating cost may be achieved.
In the figures, the inductors associated with a thyristor are connected to the anode of the thyristor, which results in a more simple supply of the control units, for example SPDl via the voltage d;vider. Alternatively, however, the `i .

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~ - 7 , ~15~40 inductors may be connected to the other main electrode (the cathode) of the thyristor.
The above description describes how the voltage divider impedances Rl and Cl are connected between the cathode of the thyristor Tl and the point of connection of the inductors.
This embodiment is preferable since Rl and Cl then contribute to attenuate voltage oscillations between C2 and Ll. Alter-, natively, however, the series connection Rl-Cl may be con-nected in parallel with both the thyristor and both the inductors, or possibly only with the thyristor.
It should be understood that the invention is not `, limited to the exact details of constructi~n shown and des- !
cribed herein for obvious modifications will occur to persons skilled in the art.
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Claims (6)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In a high voltage converter valve having at least a pair of thyristors, the improvement comprising:
a first inductor serially connected in circuit with each corresponding thyristor for limiting the rate of current increase in the valve during thyristor firing;
series connected resistor and first capacitor com-ponents, connected in parallel across the outward leads of each serially connected thyristor and inductor, for providing equal voltage distribution between the thyristors;
a second capacitor connected in parallel with each series connected resistor and first capacitor; and a second respective inductor connected in circuit with each corresponding first inductor and cooperating with stray capacitance for minimizing the generation of radio frequency interference signals from the valve.

2. A high voltage thyristor power converter valve having a plurality of interconnected valve sections capable of decreasing generated radio frequency interference and rate of current increase through the valve during thyristor firing, each valve section comprising:
a thyristor connected in series with a first inductor;
a resistor connected in series with a capacitor;
means connecting the series connected resistor and capacitor in parallel with the series connected thyristor and inductor;
a second capacitor connected in parallel with the serially connected thyristor and inductor; and a second inductor connected in series between an outward terminal of the first inductor and an output terminal of a thyristor in an adjacent valve section.

3. The subject matter of claim 2 wherein the first inductor is connected to the anode of the thyristor to complete the series connection therebetween.

4. The subject matter of claim 2 wherein the second capacitor has a capacitance substantially lower than the first capacitor.

5. The subject matter of claim 2 wherein each in-ductor includes a respective core and a common first winding therethrough, and further wherein a second winding is wound around one of the cores, the second winding being connected between the first winding and a junction point on the con-necting means.

6. The subject matter set forth in claim 5 wherein the first and second windings have a turns ratio of unity.