FIELD OF THE INVENTION [0001] The invention relates to a method of disconnecting a thyristor-switched capacitor battery, the capacitor battery comprising a thyristor switch comprising at least one thyristor; a coil cascaded with the thyristor switch; and a capacitor cascaded with the coil. [0002] Further, the invention relates to a thyristor-switched capacitor battery comprising a thyristor switch comprising at least one thyristor; a coil cascaded with the thyristor switch; and a capacitor cascaded with the coil.

BACKGROUND OF THE INVENTION [0003] Compensation of power lines generally utilizes compensators, such as static var compensators, that generally comprise a few, for instance two or three, thyristor-switched capacitor batteries, whose battery powers to be switched are typically about 50 to 150 Mvar, the currents being 1 to 3 kA, and the busbar voltages being 12 to 18 kV. [0004] A thyristor-switched capacitor battery according to prior art is disconnected from the mains voltage at the natural zero point of the current, whereby the capacitor has a voltage equal to the peak value of the mains voltage, and thus also the greatest amount of energy possible. Thus, direct voltage charge equal to the peak value of the mains voltage remains in the capacitor. The thyristor switch must thus withstand, in total, this direct voltage remaining in the capacitor and the peak value of the mains voltage as transient recovery voltage TRV. However, since the maximum voltage of one thyristor is nowadays only about 5.2 to 8 kV and thyristors do not typically withstand any considerable overvoltages, a thyristor switch must have several cascaded thyristors due to this phenomenon occurring in the disconnection of a capacitor battery. A problem with the prior art solution is that when a thyristor switch comprises several thyristors, the summed power loss of the thyristors is great. For example in cascade connection of 10 to 30 thyristors, the summed power loss of one phase of the thyristors may be up to 50 to 150 kW. Since capacitors typically comprise at least a few capacitor batteries, the number of thyristors may be up to several hundreds, in which case their summed power loss is considerably great and economically very significant. Further, taking into account that a large number of thyristors is typically needed only in disconnecting a capacitor battery, the power loss caused by a large number of thyristors is unreasonably great compared with the utilization degree of the thyristors. [0005] Publication CA 1208 286 discloses a static compensator comprising a thyristor switch circuit which, in turn, comprises a main thyristor arranged to connect the device to the mains voltage, and an auxiliary thyristor arranged to connect a voltage limiter in parallel with the main thyristor when the voltage across the main thyristor exceeds a predetermined value. A problem with the solution according to the publication is, however, that a large amount of energy is accumulated in the switch. The power consumption is thus considerably high. In order to discharge the voltage, the switching circuit requires a large number of components, some of which are expensive.

BRIEF DESCPRIPTION OF THE INVENTION [0006] An object of the invention is thus to provide a method of disconnecting a thyristor-switched capacitor battery, and a thyristor-switched capacitor battery, by means of which above drawbacks can be alleviated. [0007] The method according to the invention is characterized by the capacitor battery further comprising an auxiliary energy source connected in parallel with the coil, the method comprising charging the auxiliary energy source until the amount of energy included in it corresponds to the amount of energy accumulated in the coil; and turning off the thyristor switch at a predetermined moment by controlling the voltage of the thyristor switch to be negative for a predetermined safety period by discharging the voltage charged in the auxiliary energy source to the coil. [0008] Further, the capacitor battery according to the invention is characterized in that the capacitor battery further comprises an auxiliary energy source connected in parallel with the coil, the capacitor battery comprising means for charging the auxiliary energy source; and means for turning off the thyristor switch at a predetermined moment by controlling the voltage of the thyristor switch (204) to be negative for a predetermined safety period by discharging the voltage charged in the auxiliary energy source to the coil. [0009] The invention is based on the thyristor-switched capacitor battery comprising a thyristor switch comprising at least one thyristor, and a capacitor, cascaded with each other, as well as a coil cascaded with the thyristor switch and the capacitor, and an auxiliary energy source, preferably an auxiliary capacitor or a battery, connected in parallel with the coil. The auxiliary energy source is charged with, for example, the coil, the capacitor or a separate auxiliary transformer, until the amount of energy included in it corresponds to the amount of energy accumulated in the coil. The thyristor switch is turned off at a predetermined moment, preferably when the capacitor voltage is zero, by controlling the voltage of the thyristor switch to be negative for a predetermined safety period, which is preferably longer than the predetermined disconnecting time of the thyristor, by discharging the voltage charged in the auxiliary capacitor to the coil by means of, for example, an auxiliary switch comprising one or more thyristors. [0010] According to an embodiment of the invention, the energy accumulated in the coil is conveyed further to the capacitor or a resistor, for instance. [0011] According to a second embodiment of the invention, the thyristor-switched capacitor battery is connected to a delta or a star connection. [0012] According to a third embodiment of the invention, the thyristor-switched capacitor battery has one phase or three phases. [0013] An essential advantage of the arrangement according to the invention is that the number of thyristors included in the thyristor-switched capacitor battery can be essentially reduced, because no direct voltage component remains in the return voltage, and thus the recovery voltage is essentially lower than in a prior art solution. Hence, the summed power loss of the thyristors in the battery is significantly lower than before. This is very advantageous particularly for power transmission companies, because compensators that have the smallest losses are typically the most productive ones economically. Another advantage is that the manufacturing costs of a capacitor battery are reduced with the reduction of the number of required thyristor components. Yet another advantage is that the structure of a thyristor- switched capacitor battery according to the invention is relatively simple, and it is easy to manufacture such a battery. In addition, the reliability of a thyristor- switched capacitor battery is typically good. BRIEF DESCRIPTION OF THE FIGURES [0014] The invention will now be described in greater detail in connection with preferred embodiments, referring to the appended claims, of which: Figure 1a shows disconnection of a thyristor-switched capacitor battery according to prior art, while Figure 1b shows disconnection of a thyristor-switched capacitor battery according to an embodiment of the invention; Figure 2 shows a thyristor-switched capacitor battery; and Figures 3a, 3b and 3c show graphically some variables according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION [0015] Figure 1a presents disconnection of a thyristor-switched capacitor battery from the main voltage according to prior art, while Figure 1b presents disconnection of a thyristor-switched capacitor battery according to an embodiment of the invention from mains voltage 100. A capacitor battery according to prior art is typically disconnected from the mains voltage 100 by itself only at the zero point 104 of a current 102 in response to stopping the giving of a trigger pulse. In the figure, one disconnection moment according to the prior art solution is denoted with a reference numeral 106. In this case, peak voltage 108 remains in the capacitors. Since the maximum voltage of present-day thyristors is relatively low and they do not typically withstand significant over-voltages, several cascaded thyristors are to be used. [0016] In contrast, a thyristor-switched capacitor battery according to the invention can be disconnected from the mains voltage 100 a quarter cycle earlier, for instance, with the maximum value 110 of the current 102. No charge remains in the capacitor, and thus the instantaneous value of the capacitor voltage is zero 112, and no direct voltage component remains in the return voltage. In the figure, one disconnection moment according to a preferred embodiment of the invention is indicated with reference numeral 114. If, for example, the root-mean-square value of the network principal voltage is 14.1 kV, typically a direct voltage of 20 kV remains in a capacitor battery of prior art, in which case the voltage returning to the thyristor switch is about 40 kV. By contrast, in a capacitor battery according to the invention, no direct voltage component remains in the capacitor, so the return voltage would, in this case, be only about 20 kV. Thus, a capacitor battery according to the invention requires significantly fewer thyristors than a solution according to prior art, which enables the use of essentially less expensive switch solutions. [0017] Figure 2 shows a thyristor-switched capacitor battery according to a preferred embodiment of the invention which comprises, cascaded, a thyristor switch TV (204) comprising at least one, preferably several, for instance 5 to 20 thyristors Ti (200), T2 (202); a coil L (206) attenuating current surges; and a compensating capacitor C (208), which are connected to mains voltage Ui to U2, for example in the way shown by the figure. The mains voltage Ui to U2 is preferably the principal voltage of the network. The capacitor battery further comprises an auxiliary energy source, such as an auxiliary capacitor CA (210), connected in parallel with the coil L (206). [0018] The auxiliary capacitor CA (210) is charged for instance from the coil L (206), from the capacitor C (208) or by means of an auxiliary transformer functionally connected to the circuit, such as a voltage or current transformer, until the amount of energy accumulated in the auxiliary capacitor CA (210) corresponds to the amount of energy accumulated in the coil L (206). Preferably, a capacitor battery comprises, in accordance with Figure 2, a diode (212) and a resistor R (214) limiting the current, by means of which the auxiliary capacitor CA (210) can be charged. In order to prevent direct voltage charge from remaining in the capacitor C (208) in the disconnection, the thyristor switch TV (204) is made currentless, in other words turned off, for instance at the zero point of the mains voltage, by discharging the voltage U(CA) charged in the auxiliary capacitor to the coil L (206) for instance by means of a switch element functioning as an auxiliary switch TVA (216), which preferably comprises at least one semiconductor component, such as a thyristor or an IGTB (Insulated Gate Bipolar Transistor). The auxiliary switch TVA (216) is preferably controlled by control electronics functionally connected to it, by means of which for instance the battery disconnection command and information on the voltage of the battery at the disconnection moment are fed as input. The current pulse from the auxiliary capacitor CA (210) is preferably at least as great as the current at the coil L (206) at the disconnection moment. The voltage U(L) of the coil is thus as great as but of opposite sign to the voltage U(TV) of the thyristor switch. Thus, the thyristor switch TV (204) is switched off when the thyristor voltages turn negative for a predetermined safety period, which is preferably longer than the predetermined disconnection time of the thyristor (200, 202). The voltage U(C) of the capacitor C and the voltage Ui to Ih are zeros at this observation moment. [0019] According to one embodiment of the invention, the energy accumulated in the coil L (206) can be further conveyed to, for example, the auxiliary capacitor CA (210) or a resistor, which is preferably very small in order to make the current of the capacitor battery stop. [0020] According to one embodiment of the invention, the connection is bidirectional, in which case both the diode D (212) and the thyristor TVA (216) are connected reversely to what is shown in Figure 2. [0021] Figures 3a, 3b and 3c show graphically some variables of the solution according to a preferred embodiment of the invention shown in Figure 2, for example. Figure 3a shows the voltage U(CA) (300) of the auxiliary capacitor CA (210) as a function of time t. The auxiliary capacitor CA (210) is charged until the amount of energy in it corresponds to the amount of energy accumulated in the coil L (206). The auxiliary capacitor CA (210) is discharged at the zero point of the mains voltage at a moment tx or preferably a predetermined short time period, such as about 100 μs, before the zero point of the mains voltage. Thus, the voltage U(L) (302) of the coil L (206) changes in accordance with Figure 3b. The voltage U(L) (302) of the coil L (206) is then as high as but of opposite sign to the voltage U(TV) of the thyristor switch TV (204). Thus, a current I(TV) (304) originally coming from the thyristor switch TV (204) is preferably completely replaced with the current from the auxiliary capacitor CA (210), as shown in Figure 3c. The thyristor switch TV (204) is turned off when the voltages of the thyristors have turned negative for a safety period ti to t2, as shown in Figures 3b and 3c, the safety period being preferably longer than the disconnection time ti of the thyristor (200, 202), as shown in Figure 3b. [0022] According to one preferred embodiment of the invention, a thyristor-switched capacitor battery is connected to a delta or a star connection. [0023] According to one preferred embodiment of the invention, a thyristor-switched capacitor battery has one phase or three phases. [0024] It will be obvious to a person skilled in the art that as the technology advances, the basic idea of the invention can be implemented in a plurality of ways. The invention and its embodiments are thus not restricted to the above examples or components but may vary within the scope of the appended claims.