Note: Descriptions are shown in the official language in which they were submitted.
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, Title of the Invention: THYRISTOR SWITCH WITH TRANSIENT
PROTECTION AND RF INTERFERENCE
, SUPPRESSION
;; Inventor: Gunnar AspIund
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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
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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
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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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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
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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
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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
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~ 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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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
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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
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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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