Note: Descriptions are shown in the official language in which they were submitted.
CA 02228487 l998-02-02
s GR 95 P 3559 P
~LF,~ T~l~s Q~
Description ~ AN~L~TION
Thyristor-switched capacitor bank
The invention relates to a thyristor-switched
capacitor bank having a thyristor switch and a capacitor
5 bank.
A solid-state compensator, also called a Static
Var Compensator (SVC) comprises one or more parallel-
~ connected inductive and capacitive paths, which are
connected to the high-voltage mains power supply via a
10 dedicated transformer or else via a tertiary winding of
a m~; n~ power supply transformer. As a result of the
rated voltage on the secondary side of the transformer
being fixed, the use of a dedicated transformer offers
the capability to design the equipment optimally in terms
of its current and voltage control. Direct connection may
also be economic in medium-voltage mains power supplies
up to 30 kV.
The total amount of capacitance is provided via
permanently connected or switched capacitors (capacitor
bank), also called a Fixed Capacitor (FC), or thyristor-
switched capacitors, also called a Thyristor Switched
Capacitor (TSC). A thyristor switch which comprises a
plurality of series-connected, reverse-parallel thyris-
tor~ is normally used for this application. The capacitor
bank must now be provided with a protective inductor, in
order to limit the inrush current gradient. The use of
mechanically switched capacitors is subject to
operational limitations. In order to keep equalization
processes during switching-on as small as possible and
thus to prevent overloading, the capacitor bank must
always be discharged via a power switch during switching-
on (for example via a discharge resistor or transformer).
In comparison with this, a thyristor as a switch offers
the advantage that the capacitor bank can be connected
and disconnected from any charge state and as frequently
as desired with the m;n;mllm possible equalization
process. The controller (intelligence) which is required
, CA 02228487 1998-02-02
GR 95 P 3559 P - 2 -
for this p~rpose can easily be implcmented u~ing digital
technology.
The total amount of inductance is provided via
inductor coils. These can either be switched (Thyristor
Switched Reactor (TSR)), or else the reactive volt-
amperes at the f~ln~m~ntal frequency can be controlled
(Thyristor Controlled Reactor (TCR)) using an appropriate
controller. To this end, the entire amount of the
reactive volt-amperes emitted to the mains power supply
~rom the solid-state compensator can be adjusted
infinitely variably in terms of the capacitive or
inductive reactive volt-amperes required at the mains
power supply point.
Continuous control of a TCR path i8 always linked
to the production of h~rmo~;c currents, which must be
kept away from the transmis~ion grid by the use of
filters at the TCR connection point. The production of
harmonics can be completely prevented only by the
inductive path being operated such that it is switched
identically to the capacitive path (Thyristor Switched
Reactor (TSR)). The installed inductive volt-amperes are
then only connected or disconnected in the same way as in
the case of a thyristor-switched capacitor bank
(Thyristor Switched Capacitor (TSC)).
In principle, the solid-state compensator can
carry out various control tasks. When used in
transmission grids, the primary task is voltage control.
The solid-state compensator can thus also contribute to
limiting overvoltages at the operating frequency, can
make a contribution to improving the grid stability and
can also damp volt-ampere fluctuations between grid
sections .
The article "Statische Kompensatoren und ihre
Komponenten" [Solid-state compensators and their
components], printed in the German journal "etz", Volume
112 (1991), Issue 17, pages 926 to 930, discusses circuit
types, application and design criteria for the components
used in solid-state compenQators using thyristor
technology. The solid-state compensators which are
CA 02228487 1998-02-02
GR 95 P 3559 P - 3 -
implemented and referred to each comprise a plurality of
power-factor correctors, which are connected to a high-
voltage mains power supply by means of a transformer. The
selection and co_bination of the various power-factor
correctors depends essentially on the requirements of the
m~;n~ power supply. The following viewpoints, inter alia,
have to be considered in this case: total cost of the
compensator, 1088 assessment, reliability, maintenance
costs and the capability of the compensator to be
upgraded. For example, the SVC system at Remps
Creek/Australia comprises a thyristor-switched inductor
(TSR) and two thyristor-switched capacitor banks (TSC).
The three phases o~ each of these power-~actor correctors
are electrically connected in delta and are of identical
design.
As already mentioned, the capacitor bank of the
thyristor-switched capacitor bank (TSC) should always be
discharged during switching-on. As a rule, the capacitor
bank is disconnected from the AC mains power supply at
the current zero crossing, that i5 to say at the instant
when the mains power supply voltage is at a m~x;ml-m I~
the discharging of the capacitor bank via a discharge
circuit is a slow process in comparison with the period
of the AC voltage, then virtually twice the m~x;ml~m mains
power supply voltage occurs on the thyristor switch after
half a cycle. Relatively expensive thyristors having an
increased withstand voltage must be used for the thyris-
tor switch, or a plurality of thyristor switches must be
connected in series. If incorrect triggering of a thyris-
tor were now to occur at the least favourable point in
time, the capacitor bank would be recharged to a m~x;ml~m
of three times the mains power supply voltage amplitude.
In order that the thyristor switch need be
designed only for the m~x;mllm mains power supply voltage
itself, which is a major advantage for economic reasons,
the capacitor bank must be able to be discharged via a
discharge circuit sufficiently quickly, at the most
within one half-cycle of the AC voltage. If the AC
voltage frequency is 50 Hz, the duration of one half-
CA 02228487 l998-02-02
GR 95 P 3559 P - 4 -
cycle i8 10 ms. The capacitor bank normally has a
capacitance in the order of magnitude of several 100 ~F.
For it to be possible for such a large capacitor bank to
be able to be discharged in 10 ms at all, the discharge
circuit must have a low impedance. A purely non-reactive
resistor in the discharge circuit will have to have, for
example, a value of only a few ohms, which, for the
capacitor, represents virtually a short circuit with a
correspondingly high power 1088, which cannot be
tolerated when the capacitor bank is connected to the AC
mains power supply.
EP 0 116 275 B1 discloses a reactive volt-ampere
compensator, a di~charge circuit having at least one
inductive impedance element being connected in parallel
lS with a thyristor-switched capacitor bank, and a first
control unit being provided for the thyristor switch,
which first control unit produces triggering signals for
the thyristor switch from current and voltage measurement
signals from an AC mains power supply which is to be
corrected, the discharge circuit being permanently closed
and the inductive impedance element being variable in
such a manner that its value is greater in the operating
state when the thyristor switch is closed and is less
when the thyristor switch is open. One advantage of this
embodiment is that rapid and continuous discharging of
the capacitor bank, after it has been disconnected from
the AC mains power supply, takes place without any
switching elements in the discharge circuit of the
capacitor bank, which switching elements would be
susceptible to defects and would be expensive. An iron-
cored discharge-circuit inductor is provided as the
inductive impedance element. The iron core is at least
largely unsaturated at that current which flows through
the inductor when the thyristor switch i8 closed, and is
increasingly saturated with greater currents. Its winding
impedance is designed such that the discharging of the
capacitor bank corresponds to an RC discharge with a
priori damping. As a result of the saturation
characteristics of its iron core, the discharge-circuit
. CA 02228487 1998-02-02
GR 95 P 3559 P - 5 -
inductor thus acts as a variable impedance element in the
discharge circuit, the impedance of which is greater when
the capacitor is being connected to the AC ~-; n~ power
supply, that is to say when the thyristor switch is
closed, than when the capacitor bank is disconnected from
the AC mains power supply with the thyristor switch open.
The difference between these two states is in this case
80 significant that only a small, insignificant current
flows in-the first-mentioned case during discharge, while
- 10 a greater current, which discharges the capacitor bank in
less than one half-cycle of the AC voltage, can flow in
the second case. In addition, the discharge circuit may
be permanently closed. There is no need for any
interruption in the charging circuit while the capacitor
bank is connected to the AC mains power supply. This
results in the thyristor voltage being relatively low,
and the costs of expensive high-voltage thyristors are
thus saved.
The invention is now based on the object of
specifying a thyristor-switched capacitor bank, in the
case of which the thyristor voltage is likewise
relatively low, no special discharge circuit being used.
This object is achieved according to the
invention by the features of Claim 1.
25As a result of the fact that the capacitor bank
of a thyristor-switched capacitor bank (TSC) is spread
into at least-two series-connected capacitor groups, that
capacitor group which is remote from a capacitor group at
the mains power supply connection being provided with a
series circuit in parallel with it, which series circuit
has a thyristor switch and an inductor coil, a capacitive
voltage divider is obtained, such that the thyristor
switch is loaded with a voltage value proportional to the
voltage ratio. As a result of the capacitor bank being
split into a plurality of capacitor groups, whose
capacitance values can be freely selected, the voltage
across each thyristor switch corresponds to the voltage
across the associated capacitor group.
The thyristor switch can thus be designed for a
, , CA 02228487 1998-02-02
GR 95 P 3559 P - 6 -
fraction of the m~Y;m~lm mains power supply voltage. A
further advantage of this thyri8tor-switched capacitor
bank according to the invention i8 that the capacitances
of an individual capacitor bank can be varied in steps,
- 5 which are a fraction of the total capacitance of the
capacitor bank, depending on the combination of the
thyristor switches which are switched on and off.
In order to explain the invention further,
reference is made to the drawing, which provides a
schematic illustration of an exemplary embodiment of a
thyristor-switched capacitor bank according to the
invention.
Figure 1 shows a known thyristor-switched capacitor
bank, in which
Figure 2 illustrates the behaviour of the associated
thyristor voltage in a graph plotted with
respect to time t, while, in contrast,
Figure 3 shows the behaviour of the associated thyristor
current in a graph plotted with respect to time
t,
Figure 4 shows a thyristor-switched capacitor bank
according to the invention,
Figure 5 showing the associated thyristor voltage in a
graph plotted with respect to time t, and
Figure 6 showing the behaviour of the associated thyris-
tor current in a graph with respect to time t.
Correspon~; ng parts and variables are provided
with correspo~; ng reference symbols in the Figures.
In Figure 1, 2 designates a line of an electrical
AC ~;n~ power supply, which is fed from a generator 4.
A transformer 6 is connected to this line 2, and a
thyristor-switched capacitor bank 10 i8 connected to its
secondary w; n~; ng by means of a mA; n~ power supply
connection 12. This thyristor-switched capacitor bank 10
comprises a thyristor switch 14 and a capacitor bank 16,
which are electrically connected in series. The thyristor
switch 14 is formed from reverse-parallel thyristors 18
and 20. The triggering electrodes of these thyristors 18
and 20 are connected to a control unit, which is not
CA 02228487 1998-02-02
GR 95 P 3559 P - 7 -
illustrated in more detail and which uses signals from
the m~in~ power supply in a manner which is known per se
and will therefore also not be explained in more detail
to produce pulses for the thyristors 18 and 20 of the
thyristor switch 14, which pulses are in the correct
phase required for the reactive volt-amperes in the AC
mains power supply. The transformer 6 is used only ~or
matching the mains power supply voltage to the voltage
which has been selected, for economic reasons, for the
thyristor-switched capacitor bank 10. The thyristor-
switched capacitor bank 10 can also be connected directly
to the mains power supply. The capacitor bank 16 can be
switched on or off in a very short time by m~nQ of the
thyristor switch 14. Switching-on takes place such that
any equalization processes which occur are as small as
possible. Since this cannot be achieved in all operating
conditions, inductor coils are provided, which limit the
inrush current of the capacitor bank 16. These inductor
coils are not illustrated in more detail, for the sake of
clarity in this illustration.
When the thyristor switch 14 is closed, that is
to say is electrically switched on, and the capacitor
bank 16 is thus connected to the AC m~;nQ power supply,
then the voltage across the capacitor bank 16 corresponds
to the m~;n~ power supply voltage at any instant. When
the capacitor bank 16 is disconnected from the AC mains
power supply by opening the thyristor switch 14, then the
thyristor switch 14 adopts the capacitor voltage at the
switching instants and, in consequence, with the changing
of the capacitor voltage and o~ the mains power supply
voltage, in each case adopts the difference in voltage
from both. As a rule, the capacitor bank 16 is
disconnected ~rom the AC mains power supply at the zero
crossing, that is to say at the instant when the mains
power supply voltage ig at a m~x;mnm,
Without a discharge circuit, the capacitor bank
16 would discharge only very slowly. This would result in
the thyristor voltage U~ being virtually twice as great
as the mains power supply voltage amplitude at the
CA 02228487 l998-02-02
GR 95 P 3559 P - 8 -
instant when the mains power supply voltage is at a
min;mllm Relatively expensive thyristors 18 and 20 with
an increased withstand voltage would have to be used for
the thyristor switch 14, or a plurality of thyristor
switches 14 would have to be connected in series. If
incorrect triggering of a thyristor 18 or 20 in the
thyristor switch 14 were now to occur at the least
favourable instant, then the capacitor bank 16 would be
recharged to a m~Y;mllm of three times the mains voltage
amplitude.
Figures 2 and 3 respectively show the behaviours
with respect to time of the thyristor voltage U~ and the
thyristor current i~ for this thyristor-switched
capacitor bank 10 in a graph plotted with respect to time
t. It can be seen from these illustrations that the
thyristor switch 14 is switched off during the time
period tl - tO, since the thyristor current i~ is equal
to zero and the thyristor ~oltage u~ follows the AC
voltage at the m~;n~ power supply connection 12. The
thyristor switch 14 switches on at the instant tl, 80
that the thyristor voltage u~ becomes approximately zero.
Since the thyristor switch 14 has an impedance, a resid-
ual voltage is illustrated in the illustration according
to FIG 2. This residual voltage and the thyristor current
i~ are subject to h~nmQ~;cs~ These h~mon;cs depend on
the transient process of the thyristor-switched capacitor
bank 10. The thyristor switch 14 switches off again at
the instant t3. Irrespective of when the switching-off
comm~n~ occurs, the thyristors 18 and 20 cannot interrupt
the current until their next zero crossing. At this
moment, the capacitor bank 16 is charged to the peak
value of the m~;n~ power supply voltage, and this value
is now maintained in the form of a DC voltage on the
capacitor bank 16. The difference between the mains power
supply voltage and the capacitor voltage indicates the
magnitude of the voltage across the thyristor switch 14
in the switched-off state. The voltage across the
thyristor switch 14 therefore r~m~; n ~ offset by the peak
value of the mains power supply voltage from the instant
CA 02228487 1998-02-02
GR 95 P 3559 P - 9 -
t3 until the capacitor bank 16 has been discharged. In
consequence, the thyristor switch 14 is stressed to an
increased extent (m~Y;mllm instantaneous value of the
thyristor voltage u~ is egual to twice the peak value of
the mains power supply voltage).
Figure 4 shows one embodiment of a thyristor-
switched capacitor bank 10 according to the invention. In
the case of this thyristor-switched capacitor bank 10,
the capacitor bank 16 is split into, for example, three
series-connected capacitor groups 22, 24 and 26. A series
circuit 28 formed by a thyristor switch 14 and an
inductor coil 30 is in each case electrically connected
in parallel with the capacitor groups 24 and 26. The
capacitor group 22, which is assigned directly to the
mains power supply connection 12 of the thyristor-
switched capacitor bank 10, has the greatest capacitance
value of the capacitor groups 22, 24, 26. These capacitor
groups 22, 24, 26 form a capacitive voltage divider. The
m~; mllm voltage load on the thyristors 18 and 20 of the
thyristor switches 14 can be predetermined by the
selection of the capacitance values of the individual
capacitor groups 24 and 26.
FIGs 5 and 6 respectively show the behaviours
with respect to time of the thyristor voltage u~ and of
the thyristor current i~ for the e-mbodiment according to
the invention of a thyristor-switched capacitor bank 10
according to FIG 4, in each case in a graph plotted with
respect to time. It can be seen from these illustrations
that the thyristor switch 14 is switched off during the
time period tl - tO, since the thyristor current i~ is
equal to zero and the thyristor voltage u~ follows the AC
voltage at the mains power supply connection 12. The
thyristor switch 14 switches on at the instant tl, 80
that the thyristor voltage u~ becomes zero. In this
state, the thyristor switch carries the thyristor current
i~, which is subject to harmonics because of the
transient process. The thyristor switch 14 is switched
off again at the instant t4. When the current i~ in the
thyristor switch 14 reaches zero, the thyristor switch 14
CA 02228487 l998-02-02
GR 95 P 3559 P - 10 -
is switched off. The voltage across the capacitor bank 16
starts from zero and builds up, which results in a shift
in its behaviour. The peak value of the voltage u~ in the
first half-cycle thus reaches twice the nom;nAl value of
the voltage at the mains power supply connection 12. This
is immediately followed by the ;mm~;ate discharging of
the capacitor bank 16 by deliberate triggering of the
thyristor switches 14 by means of a plurality of current
pulses. In con8equence, the shift in the voltage across
the thyristor switch 14 is immediately cancelled out.
The refinement according to the invention of the
thyristor-switched capacitor bank 10 achieves the
following advantages:
a) The capacitance of a thyristor-switched capacitor
bank 10 can be varied in steps which are a fraction
of the total capacitance of this capacitor bank,
dep~n~ing on the combination of thyristor switches
14 which are switched on and off.
b) The thyristor switches 14 need not be designed for
the voltage of the entire capacitor bank of the
thyristor-switched capacitor bank 10, but
corresponding to the voltage of the associated
capacitor group 24 or 26.
c) In the event of a fault in the triggering of a
thyristor switch 14, said thyristor switch 14 can
now be protected by controlled switching-on. This is
now acceptable with regard to mains supply
operation, since the resultant change in the
capacitance of the capacitor bank of the thyristor-
switched capacitor bank 10 is limited to the effect
of a single capacitor group 22. As a result of the
controlled reduction in the voltage shift in a few
cycles after the thyristor switch 14 is switched off
each time (Figures 5 and 6), protective triggering
is accordingly necessary only if a triggering fault
takes place during this short time. In consequence,
the thyristors 18 and 20 need no longer be designed
for three times the normal operating voltage.
In comparison with the prior art mentioned
CA 02228487 1998-02-02
.
GR 95 P 3559 P - 11 -
initially, the thyristors 18 and 20 of each thyristor
switch 14 without a discharge circuit can be designed for
a fraction of the mains power supply ~oltage itself,
which is a major advantage for economic reasons.
CA 02228487 1998-02-02
GR 95 P 3559 P
List of reference symbols
2 Line of an AC main~ power supply
4 Generator
6 Transformer
Thyristor-switched capacitor bank
12 Mains power ~upply connection
14 Thyristor switch
16 Capacitor bank
18, 20 Thyri~tor
22, 24, 26 Capacitor group
28 Serie3 circuit
Inductor coil
U~ Thyristor voltage
i~ Thyristor current
