CA2228487A1 - Thyristor switched capacitor bank - Google Patents
Thyristor switched capacitor bank Download PDFInfo
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- CA2228487A1 CA2228487A1 CA002228487A CA2228487A CA2228487A1 CA 2228487 A1 CA2228487 A1 CA 2228487A1 CA 002228487 A CA002228487 A CA 002228487A CA 2228487 A CA2228487 A CA 2228487A CA 2228487 A1 CA2228487 A1 CA 2228487A1
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- thyristor
- capacitor bank
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- voltage
- power supply
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- 239000003990 capacitor Substances 0.000 title claims abstract description 118
- 230000001939 inductive effect Effects 0.000 description 7
- 230000006399 behavior Effects 0.000 description 6
- 238000000034 method Methods 0.000 description 6
- 238000007599 discharging Methods 0.000 description 4
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical group [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 230000002349 favourable effect Effects 0.000 description 2
- 230000001681 protective effect Effects 0.000 description 2
- 230000001052 transient effect Effects 0.000 description 2
- 238000004804 winding Methods 0.000 description 2
- 241000863814 Thyris Species 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 238000013016 damping Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 101150085091 lat-2 gene Proteins 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
- H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
- H02J3/1864—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein the stepless control of reactive power is obtained by at least one reactive element connected in series with a semiconductor switch
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/10—Flexible AC transmission systems [FACTS]
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Control Of Electrical Variables (AREA)
- Power Conversion In General (AREA)
Abstract
A thyristor switched capacitor bank (10) has a thyristor switch (14) and a capacitor bank (16). The capacitor bank (16) is subdivided into at least two capacitor groups (22, 24, 26) connected in series. A series mounting composed of a thyristor switch (14) and an inductance coil (30) are connected in parallel to the capacitor group (24, 26) opposed to the mains connection (12) of the first capacitor group (22). A thyristor-controlled capacitor bank (10) is thus obtained whose thyristor switch (14) needs to be adapted to only a fraction of the simple main voltage. This has considerably advantages for economic reasons.
Description
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 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 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 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 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 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 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 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 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 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 .
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.
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
~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 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 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 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 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 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 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 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 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 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 .
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.
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
Claims (5)
1. Thyristor- switched capacitor bank (10) having a thyristor switch (14) and a capacitor bank (16), characterized in that this capacitor bank (16) is spread into at least two series-connected capacitor groups (22, 24, 26), and in that a series circuit formed by a thyristor switch (14) and an inductor coil (30) is connected in parallel with the capacitor group (24, 26) which is remote from a capacitor group (22) at the mains power supply connection (12).
2. Thyristor-switched capacitor bank (10) according to Claim 1, characterized in that of the capacitor groups (22, 24, 26), that capacitor group (22) which is assigned directly to the main power supply connection (12) has the greatest capacitance value.
3. Thyristor-switched capacitor bank (10) according to Claim 1 or 2, characterized in that the capacitance values of the capacitor groups (24, 26) which are remote from the capacitor group (22) at the mains power supply connection (12) can each be selected as a function of the permissible off-state voltage of the associated thyristor switch (14).
4. Thyristor-switched capacitor bank (10) according to one of Claims 1 to 3, characterized in that the number of capacitor groups (24, 26) which are remote from the capacitor group (22) at the mains power supply connection (12) is equal to two.
5. Thyristor-switched capacitor bank (10) according to one of Claims 1 to 3, characterized in that the thyristor switch (14) is formed from reverse-parallel thyristors (18, 20).
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19528766A DE19528766C1 (en) | 1995-08-04 | 1995-08-04 | Thyristor switched capacitor bank |
| DE19528766.5 | 1995-08-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2228487A1 true CA2228487A1 (en) | 1997-02-20 |
Family
ID=7768743
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002228487A Abandoned CA2228487A1 (en) | 1995-08-04 | 1996-07-22 | Thyristor switched capacitor bank |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0842557A1 (en) |
| AU (1) | AU6512696A (en) |
| CA (1) | CA2228487A1 (en) |
| DE (1) | DE19528766C1 (en) |
| NO (1) | NO980034L (en) |
| WO (1) | WO1997006589A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA2282900C (en) | 1999-09-20 | 2011-02-01 | Bayer Inc. | Halogenated terpolymers of isobutylene, diolefin monomer and styrenic monomer |
| EP2147492A2 (en) * | 2007-05-18 | 2010-01-27 | ABB Technology AG | Static var compensator apparatus |
| CN105656058A (en) * | 2016-03-14 | 2016-06-08 | 重庆明斯克电气有限责任公司 | Dynamic switching circuit of electromechanical synchronous switch of capacitor and control method of dynamic switching circuit |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE378719B (en) * | 1973-12-13 | 1975-09-08 | Asea Ab | |
| DE2804481A1 (en) * | 1978-01-31 | 1979-08-02 | Licentia Gmbh | Control circuit with bidirectional thyristor arrangement - controlling reactive power of capacitor connected to AC mains |
| EP0116275B1 (en) * | 1983-02-08 | 1987-07-15 | BBC Aktiengesellschaft Brown, Boveri & Cie. | Reactive power compensator |
-
1995
- 1995-08-04 DE DE19528766A patent/DE19528766C1/en not_active Expired - Fee Related
-
1996
- 1996-07-22 EP EP96924762A patent/EP0842557A1/en not_active Withdrawn
- 1996-07-22 CA CA002228487A patent/CA2228487A1/en not_active Abandoned
- 1996-07-22 AU AU65126/96A patent/AU6512696A/en not_active Abandoned
- 1996-07-22 WO PCT/DE1996/001345 patent/WO1997006589A1/en not_active Ceased
-
1998
- 1998-01-05 NO NO980034A patent/NO980034L/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| NO980034L (en) | 1998-04-03 |
| WO1997006589A1 (en) | 1997-02-20 |
| AU6512696A (en) | 1997-03-05 |
| DE19528766C1 (en) | 1997-01-16 |
| NO980034D0 (en) | 1998-01-05 |
| EP0842557A1 (en) | 1998-05-20 |
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Legal Events
| Date | Code | Title | Description |
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| FZDE | Discontinued |