WO1997006589A1

WO1997006589A1 - Thyristor switched capacitor bank

Info

Publication number: WO1997006589A1
Application number: PCT/DE1996/001345
Authority: WO
Inventors: Kadry Sadek; Marcos-Antonio Pereira
Original assignee: Siemens Aktiengesellschaft
Priority date: 1995-08-04
Filing date: 1996-07-22
Publication date: 1997-02-20
Also published as: AU6512696A; NO980034D0; NO980034L; DE19528766C1; CA2228487A1; EP0842557A1

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

description

Thyristor switched capacitor bank

The invention relates to a thyristor-switched capacitor bank with a thyristor switch and a capacitor bank.

A static compensator, also called a static var compensator (SVC), consists of one or more inductive and capacitive branches connected in parallel, which are connected to the high-voltage network via a separate transformer or via the tertiary winding of a network transformer. By using your own transformer, the nominal voltage on the secondary side enables you to optimally design the equipment with regard to its current and voltage control. A direct connection can also be economical in medium-voltage networks up to 30 kV.

The capacitive power is provided via permanently connected or switched capacitors (capacitor bank), also referred to as a fixed capacitor (FC), or thyristor-switched capacitors, also referred to as a thyristor switched capacitor (TSC). In this application, a thyristor switch is normally used, which consists of several antiparallel thyristors connected in series. The capacitor bank must then be provided with a protective choke in order to limit the inrush current steepness. The use of mechanically switched capacitors is subject to operational restrictions. In order to keep balancing processes as low as possible when switching on and thus to avoid overstressing, the capacitor bank must always be discharged via a circuit breaker when switched on (eg via a discharge resistor or converter). In contrast, a thyristor as a switch has the advantage that the capacitor bank can be used from any state of charge and as often as required with the lowest possible compensation process can be switched on and off. The "intelligence" of the control required for this is easy to implement in digital technology.

The inductive power is provided via choke coils.

These can either be switched (Thyristor Switched Reactor (TSR)) or controlled with a corresponding control in the fundamental oscillation reactive power (Thyristor Controlled Reactor (TCR)). For this purpose, the total reactive power of the static compensator delivered to the network can be continuously adjusted within the capacitive or inductive reactive power required at the network node.

The continuous regulation of a TCR branch is always associated with the generation of harmonic currents which must be kept away from the transmission network by using filters at the connection point of the TCR. The generation of harmonics can only be completely ruled out if the inductive branch is operated in the same way as the capacitive branch (Thyristor Switched Reactor (TSR)). The installed inductive reactive power is then only switched on or off as with a thyristor switched capacitor bank (Thyristor Switched Capacitor (TSC)).

The static compensator can in principle perform various control tasks. When used in transmission networks, this is primarily the task of voltage regulation. In this way, the static compensator can also help to limit operating frequency overvoltages, make a contribution to improving network stability and also dampen power fluctuations between subnetworks.

In the article "Static compensators and their components", printed in the DE magazine "etz", volume 112 (1991), number 17, pages 926 to 930, circuit types, application and design criteria of the components used by static compensators in the Thyristor technology discussed. The The realized static compensators shown each consist of several reactive power controllers which are connected to a high-voltage network by means of a transformer. The selection and the combination of the different reactive power controllers essentially depends on the requirements of the network. The following aspects are to be taken into account here, among others: total cost of the compensator, loss assessment, reliability, maintenance expenses and expansion options of the compensator. For example, the SVC plant in Kemps Creek, Australia, consists of a thyristor-connected choke (TSR) and two thyristor-switched capacitor banks (TSC). The three phases of each of these reactive power dividers are electrically connected in a triangle and are constructed identically.

As already mentioned, the capacitor bank of the thyristor-connected capacitor bank (TSC) should always be discharged when it is switched on. As a rule, the capacitor bank is at zero current crossing, i.e. at the time of maximum mains voltage, separated from the AC mains. If the discharge of the capacitor bank via a discharge circuit is a slow process in comparison to the oscillation period of the AC voltage, then twice the maximum mains voltage occurs at the thyristor switch after half an oscillation period. Relatively expensive thyristors with increased dielectric strength must be used for the thyristor switch, or several thyristor switches must be connected in series. If a misfire of a thyristor would occur at the worst time, the capacitor bank would be recharged to a maximum of three times the mains voltage amplitude.

In order to have to dimension the thyristor switch only for a simple maximum mains voltage, which is of considerable advantage for economic reasons, the capacitor bank must be able to discharge itself quickly enough via a discharge circuit, at the longest during half a period of the alternating voltage. The duration of half a period at a frequency is AC voltage of 50 Hz 10 ms. The capacitor bank usually has a capacitance of the order of a few 100 μF. So that such a large capacitor bank can discharge at all in 10 ms, the discharge circuit must have a low resistance. A pure ohmic resistance in the discharge circuit should be, for example, only a few ohms, which in practice represents a short circuit with a correspondingly high power loss for the capacitor, which cannot be tolerated when the capacitor bank is connected to the AC voltage network.

A reactive power compensator is known from EP 0 116 275 B1, a discharge circuit with at least one inductive reactance resistor being connected in parallel to a thyristor-connected capacitor bank, and a first control unit for the thyristor switch being provided, which comprises current and voltage measurement signals from an AC voltage network to be compensated Ignition signals for the thyristor switch are generated, the discharge circuit being permanently closed and the inductive reactance variable, such that it is larger in the operating state when the thyristor switch is closed and smaller in value when the thyristor switch is open. An advantage of this embodiment is that the capacitor bank is discharged quickly and continuously after it has been switched off from the AC voltage network, without any expensive switching elements which are susceptible to faults and in the discharge circuit of the capacitor bank. A discharge circuit choke with an iron core is provided as an inductive reactive resistor. The iron core is at least largely unsaturated with the current which flows through the choke when the thyristor switch is closed and is increasingly saturated with larger currents. Their winding resistance is dimensioned so that the discharge of the capacitor bank corresponds to an RC discharge with a priori damping. Thus, the discharge circuit choke acts through the saturation properties of its iron core in the discharge circuit as a variable reactance which is greater when the capacitor is connected to the AC network, ie when the thyristor switch is closed, than when the The capacitor bank is separated from the AC voltage network when the thyristor switch is open. The difference between these two states is so considerable that in the first case only a small, unimportant current flows in the discharge case, while in the second case a large one, the capacitor bank, flows in less than half a period of time AC discharge current can flow. In addition, the discharge circuit can be permanently closed. An interruption of the charging circuit during the connection of the capacitor bank to the AC network is not necessary. The result is that the valve voltage is relatively low and costs for expensive high-voltage thyristors are saved.

The invention is based on the object of specifying a thyristor-connected capacitor bank in which the valve voltage is likewise relatively low, with no special discharge circuit being used.

According to the invention, this object is achieved by the features of claim 1.

Characterized in that the capacitor bank of a thyristor-connected capacitor bank (TSC) is divided into at least two capacitor groups connected in series, the capacitor group facing away from a capacitor group at the mains connection being provided with a parallel series circuit having a thyristor switch and a choke coil, a capacitive capacitor is obtained Voltage divider, so that the thyristor switch is loaded with a voltage value proportional to the voltage ratio. By dividing the capacitor bank into several capacitor groups, the capacitance values of which can be freely selected, the voltage of each thyristor switch corresponds to the voltage of the assigned capacitor group. The thyristor switch can thus be dimensioned for a fraction of the maximum mains voltage. Another advantage of this thyristor-switched capacitor bank according to the invention is that the capacities of an individual capacitor bank can be varied in stages which have a fraction of the total capacitance of the capacitor bank, depending on the combination of the thyristor switches that are switched on and off.

To further explain the invention, reference is made to the drawing, in which an exemplary embodiment of a thyristor-switched capacitor bank according to the invention is schematically illustrated.

FIG. 1 shows a known thyristor-switched capacitor bank, FIG. 2 shows the course of the associated thyristor voltage in a diagram over time t, whereas FIG. 3 shows the course of the associated thyristor current in a diagram over time t,

FIG. 4 shows a thyristor-switched capacitor bank according to the invention, the associated thyristor voltage being shown in a diagram over time t in FIG. 5 and the course of the associated thyristor current in a diagram over time t.

In the figures, matching parts and sizes are given the same reference numerals.

In FIG. 1, 2 denotes a line of an electrical AC voltage network that is fed by a generator 4. A transformer 6 is connected to this line 2, to whose secondary winding a thyristor-connected capacitor bank 10 is connected by means of a mains connection 12. This thyristor-switched capacitor bank 10 consists of a thyristor switch 14 and a capacitor. capacitor bank 16, which are electrically connected in series. The thyristor switch 14 is constructed from anti-parallel thyristors 18 and 20. The ignition electrodes of these thyristors 18 and 20 are connected to a control unit (not shown in more detail) which, from signals in the network in a manner known per se and therefore also not explained in more detail when required for reactive power in the AC network, provides phase-correct pulses for the thyristors 18 and 20 of the thyristor switch 14 ¬ testifies. The transformer 6 only serves to adapt the mains voltage to the voltage which was chosen for the thyristor-switched capacitor bank 10 for economic reasons. The thyristor-switched capacitor bank 10 can also be connected directly to the network. The capacitor bank 16 can be switched on or off in a very short time by means of the thyristor switch 14. The connection is done in such a way that no compensation processes occur. Since this cannot be achieved under all operating conditions, choke coils which limit the inrush current of the capacitor bank 16 are provided. These choke coils are not shown in this illustration because of the clarity.

If the thyristor switch 14 is closed, that is to say electrically conductive, and thus the capacitor bank 16 is connected to the AC voltage network, then the voltage at the

Capacitor bank 16 at every moment of the mains voltage. If the capacitor bank 16 is separated from the AC voltage network by opening the thyristor switch 14, then the thyristor switch 14 takes over the capacitor voltage at the switching time and subsequently, with the change in the capacitor voltage and the mains voltage, the differential voltage from both. As a rule, the capacitor bank 16 is disconnected from the AC network at zero current crossing, i.e. at the time of maximum mains voltage.

Without the discharge circuit, the capacitor bank 16 would discharge only very slowly. This would have the consequence that at the time of Minimum of the mains voltage, the valve voltage u _{τh would be} approximately twice as large as the mains voltage _amplitude . For the thyristor switch 14, relatively expensive thyristors 18 and 20 with increased dielectric strength would have to be used or a plurality of thyristor switches 14 would have to be connected in series. If a misfire of a thyristor 18 or 20 of the thyristor switch 14 now occurs at the worst time, the capacitor bank 16 would be recharged to a maximum of three times the mains voltage amplitude.

_FIGS. 2 and 3 each show the time profiles of the thyristor _voltage u _τh and the thyristor _current i _τh for this thyristor-switched capacitor _bank 10 in a diagram over time t. It can be seen from these representations that the thyristor switch 14 is non-conductive during the period tl-to, since the thyristor _current i _τh is zero and the thyristor voltage u _{τh follows} the _AC voltage at the mains connection 12. At time t1, the thyristor _switch 14 becomes conductive so that the thyristor _voltage u _{τh becomes} approximately zero. Since the thyristor switch 14 has an impedance, a residual voltage is shown in the illustration in FIG. This residual voltage and the thyristor current i _τh are subject to harmonics. These harmonics are related to the settling process of the thyristor-switched capacitor bank 10. At time t3, the thyristor switch 14 again becomes non-conductive. Regardless of when the switch-off command arrives, the thyristors 18 and 20 can only interrupt the current at their next zero crossing. At this moment, the capacitor bank 16 is charged with the peak value of the mains voltage, which now remains in the form of a direct voltage at the capacitor bank 16. The difference between the mains and capacitor voltage reflects the amount of voltage at the thyristor switch 14 in the switched-off state. The voltage at the thyristor switch 14 therefore remains offset from the peak value of the mains voltage from the time t3 until the capacitor bank 16 has discharged. This turns the thyristor switch 14 increased stress (highest instantaneous value of the thyristor voltage u _τh equal to twice the peak value of the mains voltage).

FIG. 4 shows an embodiment of a thyristor-switched capacitor bank 10 in accordance with the invention. In this thyristor-connected capacitor bank 10, the capacitor bank 16 is divided into, for example, three capacitor groups 22, 24 and 26 connected in series. A series circuit 28 of a thyristor switch 14 and a choke coil 30 are electrically connected in parallel to the capacitor groups 24 and 26. The capacitor group 22, which is directly assigned to the mains connection 12 of the thyristor-switched capacitor bank 10, has the largest capacitance value of the capacitor groups 22, 24, 26. These capacitor groups 22, 24, 26 form a capacitive voltage divider. The maximum voltage load of the thyristors 18 and 20 of the thyristor switches 14 can be predetermined by the choice of the capacitance values of the individual capacitor groups 24 and 26.

5 and 6 each show in a diagram over time the time profiles of the thyristor _voltage u _τh and the thyristor _current i _τh for the embodiment of a thyristor-switched capacitor bank 10 according to FIG. 4. It can be seen from these representations that the thyristor switch 14 is non-conductive during the period t1-tO, since the thyristor _current i _τh is zero and the thyristor _voltage u _{τh follows} the AC voltage at the mains connection 12. At time t1, the thyristor _switch 14 becomes conductive, so that the thyristor _voltage u _{τh becomes} zero. In this state, the thyristor _switch carries the thyristor _current i _τh , which is subject to harmonics as a result of the settling process. At time t4, the thyristor switch 14 becomes non-conductive again. When the zero value of the current i _τh in the thyristor _switch 14 is reached, this is switched off. The voltage at the capacitor bank 16 starts from zero and builds up, as a result of which a shift in its course is caused. The peak value of the voltage u _τh therefore reaches twice the nominal value of the voltage at the mains connection 12 in the first half period. Immediately thereafter, the capacitor bank 16 is immediately discharged by the targeted ignition of the thyristor switches 14 by means of several current pulses. As a result, the voltage shift at the thyristor switch 14 is immediately canceled.

The following advantages are achieved by this embodiment of the thyristor-connected capacitor bank 10 according to the invention:

a) The capacitance of a thyristor-connected capacitor bank 10 can be varied in stages which have a fraction of the total capacitance of this capacitor bank, depending on the combination of thyristor switches 14 that are switched on and off. b) The thyristor switches 14 do not have to be designed for the voltage of the entire capacitor bank of the thyristor-connected capacitor bank 10, but rather according to the voltage of the assigned capacitor group 24 or 26. c) In the event of an ignition fault of a thyristor switch 14, this can now be protected by a controlled connection. This is now acceptable in view of the network operation, since the resulting change in the capacitance of the capacitor bank of the thyristor-connected capacitor bank 10 is limited to the effect of a single capacitor group 22. Due to the controlled reduction of the voltage shift in a few periods after each switching off of the thyristor switch 14 (FIGS. 5 and 6), protective ignition is only necessary if an ignition fault occurs within this short time. The thyristors 18 and 20 therefore no longer have to be designed for three times the normal operating voltage. Compared to the prior art mentioned at the beginning, the thyristors 18 and 20 of each thyristor switch 14 can be dimensioned without a discharge circuit for a fraction of the simple mains voltage, which is of considerable advantage for economic reasons.

Claims

claims

1. Thyristor switched capacitor bank (10) with a thyristor switch (14) and a capacitor bank (16), characterized in that this capacitor bank (16) is divided into at least two capacitor groups (22, 24, 26) connected in series, and that to one A series circuit of a thyristor switch (14) and a choke coil (30) is connected in parallel at the capacitor group (24, 26) facing away from the capacitor connection (12).

2. Thyristor-switched capacitor bank (10) according to claim 1, d a d u r c h g e k e n e z e i c h n e t that of the capacitor groups (22,24,26) the capacitor group (22) directly assigned to the mains connection (12) has the largest capacitance value.

3. Thyristor switched capacitor bank (10) according to claim 1 or 2, characterized in that the capacitance values of the capacitor group (22) at the mains connection (12) facing away capacitor groups (24,26) each depending on the permissible reverse voltage of the associated thyristor switch ( 14) can be selected.

4. Thyristor switched capacitor bank (10) according to one of claims 1 to 3, characterized in that the number of capacitor groups (22) at the mains connection (12) facing away from capacitor groups (24, 26) is equal to two.

5. Thyristor-switched capacitor bank (10) according to one of the

Claims 1 to 3, so that the thyristor switch (14) is constructed from antiparallel thyristors (18, 20).