Improvements in gate pulse generator for static var compensator

Abstract: A gate pulse generator for a static var compensator (SVC) constituted by a thyristor switched capacitor (TSC) comprises voltage detectors (5u, 5x) for respec­ tively detecting forward and reverse voltages (Vu, Vx) applied to antiparallel-connected thyristors (3U, 3X) in the TSC. The antiparallel-connected thyristors (3U, 3X) comprise a plurality of series-connected forward thyris­ tors (3U) and a plurality of series-connected reverse thyristors (3X). The gate pulse generator monitors the forward and reverse voltages (Vu, Vx) and determines whether a period (2 ms) in which both the forward and reverse voltages (Vu, Vx) are zero continues for a pre­ determined period (2 ms) while reverse-phase sides (3X) of the antiparallel-connected thyristors (3U, 3X) are held in conduction periods (t30 - t40), respectively. The gate pulse generator prolongs an active period (t20 - t31) of a forward conduction period signal (PHSu) by the predetermined period (2 ms). Upon switching (t30) from in-phase conduction to revese-phase conduction, even if some of the plurality of series-connected thyri­ stors (3U) are turned off, these thyristors are turned on again within the predetermined period (3U), and an overvoltage is not applied to these thyristors.

Description: The present invention relates to a static var compensator and, more particularly, to a thyristor switched capacitor and a gate pulse generator for a static var compensator used to perform var compensation in a power system or power distribution system or to suppress variations in voltage. According to a method of generating a gate pulse in a conventional static var compensator, when a circuit voltage in a power system is distorted, this voltage often fluctuates centering on 0 V, and a sufficient reverse voltage cannot be applied to a plurality of series-connected thyristors constituting a var compensator. Some of these thyristors are turned off while some are not turned off, thus causing a partial blocking state. In the partial blocking state, a high circuit voltage is applied to only the thyristors which are kept off. If this high circuit voltage exceeds a breakdown voltage of the turned-off thyristors, these thyristors may be damaged. Prior art document US-A-3 958 172 discloses a circuit for a thyristor line controller interconnecting an alternating current voltage generating power source to a load. This circuit controls the amount of power delivered to the load and reduces the fundamental frequency component to f/n, where f is the frequency of generated voltage, and n is an integral number. Further, prior art document US-A-4 337 430 describes an inductive-load power control circuit for controlling power delivery to different AC inductive loads, which loads create different respective lag angles between applied voltage and delivered current up to a known maximum lag angle. A switching circuit is connected to the load. The switching circuit is trigger-pulse-switched into conduction, and thereafter maintained in conduction so long as current through the load exceeds a certain minimum value. Additionally, a trigger-pulse generator connected to the switching circuit operates in two modes. In one mode, the conduction angle is less than a predetermined maximum width which is no less than the maximum lag angle. In this mode, a pulse is generated whose width is essentially equal tot he conduction angle. In the other mode, the conduction angle is no less than the maximum width, and the generated pulse has a fixed width substantially equaling the maximum width. It is an object of the present invention to provide a gate pulse generator which can eliminate the drawback described above, protect thyristor elements from partial blocking, and prevent damage to a thyristor valve. To solve this object the present invention provides a gate pulse generator as specified in claim 1. A gate pulse generator for a static var compensator (SVC) constituted by, e.g., a thyristor switched capacitor (TSC) comprises voltage detectors for respectively detecting forward and reverse voltages applied to antiparallel-connected thyristors in the TSC. The antiparallel-connected thyristors comprise a plurality of series-connected forward thyristors and a plurality of series-connected reverse thyristors. The gate pulse generator monitors the forward and reverse voltages and determines whether a period in which both the forward and reverse voltages are substantially zero continues for a predetermined period while reverse-phase sides of the thyristors in the TSC are held in conduction periods, (or forward-phase sides of the thyristors in the TSC are held in conduction periods) respectively. The gate pulse generator prolongs an active period of a forward conduction period signal (or reverse conduction period signal) by the predetermined period. Upon switching from in-phase conduction to reverse-phase conduction (or upon switching from reverse-phase conduction to in-phase conduction), even if some of the plurality of series-connected thyristors are turned off, these thyristors are turned on again within the predetermined period, and an overvoltage is not applied to these thyristors. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a gate pulse generator for use with a static var compensator (SVC) which is formed of a thyristor switched capacitor (TSC); and Figs. 2A to 20 are timing charts for explaining the circuit operation of the gate pulse generator shown in Fig. 1. Fig. 1 shows an arrangement of a TSC for static var compensation. Referring to Fig. 1, reference numeral 1 denotes a voltage transformer connected to power circuit PC; 2, a capacitor; 3, a thyristor switch constituted by a plurality of antiparallel-connected thyristors; 4, a series-connected reactor; 5u and 5x, voltage detectors for respectively detecting forward and reverse voltages Vu and Vx applied to thyristor switch 3; and 3U and 3X, a plurality of forward thyristors and a plurality of reverse thyristors, both of which constitute thyristor switch 3. Each of thyristors 3U and 3X comprises two or more series-connected thyristor elements. This series connection achieves a breakdown voltage higher than that of a low-breakdown thyristor element. Reference symbols FVu and FVx denote forward and reverse voltage signals generated by voltage detectors 5u and 5x in response to forward and reverse voltages Vu and Vx, respectively; Vs, a circuit voltage at the secondary winding of voltage transformer 1; Vc, a voltage applied to capacitor 2; and Ic, a charge current of capacitor 2. Detailed arrangements of voltage detectors 5u and 5x are disclosed in Fig. 10 of U.S. Patent No. 4,859,884 (Yoshino) issued on August 22, 1989. All the disclosed contents of this U.S.P. are incorporated in the specification of the present invention. Reference numerals 9u and 9x denote AND gates for logically ANDing phase signals PHu and PHx and ON signal ON for the TSC, respectively; 10u and 10x, flip-flops for generating conduction period signals PHSu and PHSx, respectively; 11u and 11x, AND gates for logically ANDing conduction period signals PHSu and PHSx from flip-flops 10u and 10x, voltage signals FVu and FVx from the thyristor switch, and ON signal ON, respectively; 12u, 12x, 13u, and 13x, one-shot circuits, respectively; 14u and 14x, AND gates for logically ANDing outputs from one-short circuits 13u and 13x and inverted signals of voltage signals FVu and FVx, respectively; and 15u and 15x, delay circuits for outputting signals b-u and b-x of logic "1" during periods (t43 and t33) between a time when outputs e14u and e14x from AND gates 14u and 14x are set at "1" over a predetermined period (2 ms) and a time when outputs e14u and e14x go to "0", respectively. Fig. 2 shows a relationship between TSC circuit voltage Vs, charge voltage Vc of capacitor 2, phase signals PHu and PHx, forward and reverse voltage signals FVu and FVx, gate pulse signals GPu and GPx, outputs a-u and a-x from one-shot circuits 13u and 13x, reset signals b-u and b-x for flip-flops 10x and 10u, and conduction period signals PHSu and PHSx. Conduction period signal PHSu is reset when both voltage detection signals FVu and FVx are kept at "0" over the predetermined period (2 ms) after conduction of the reverse thyristors is confirmed, i.e., after the reverse-phase period is set and PHx is set at "1". Upon setting (t30) of an x-phase conduction period, even if some of the plurality of series-connected thyristor elements 3u are turned off by variations in circuit PC voltage and fluctuations in capacitor voltage Vc, the OFF thyristors can be turned on again within the predetermined period (2 ms) in response to a gate pulse. That is, u-phase conduction of the antiparallel-connected thyristors is prolonged by the predetermined period (e.g., 2 ms or more). When all the plurality of u-phase series-connected thyristor elements 3u can be turned off (t31), signal PHSu goes to "0", thereby preventing partial blocking. X-phase conduction is checked by signal a-x for a predetermined period (e.g., 5 to 10 ms) from a leading edge (t30) of x-phase signal PHx (x-phase conduction period PHSx can be similarly set as in the u-phase except that the phase is inverted by 180°). Single phase conduction has been described above, but a similar operation can be performed for a 3-phase TSC. The conduction period (PHSu) continues from a time (t31) when reverse-phase (x) conduction continues over the predetermined period (2 ms) to a time when all the thyristors (3u) connected in series with their own phase (u) can be turned off. The gate pulse signal is generated within this predetermined period (2 ms) to turn on the OFF thyristors again. For this reason, in the TSC unit arranged by series-connecting the plurality of antiparallel-connected thyristors (3u and 3x), the partial blocking state can be eliminated, and damage to the thyristor valve can be prevented. A technique associated with the present invention is disclosed in the following IEEE reference: K. Murabayashi et al., "High Voltage, Large Capacity Light-triggered Thyristor Valve for Static Var Compensators" Proceedings of the International Conference on Power Conversion and Industrial Control (October 22-23, 1986; Singapore), Organized by IEEE-Institute of Electrical & Electronics Engineers (Singapore Section). All the disclosed contents of the above reference are incorporated in the specification of the present invention. Although a thyristor switched capacitor (TSC) is exemplified for a static var compensator in the embodiment of Figs. 1 and 2, the gate pulse generator of the present invention can also be applied to a var compensator using a thyristor switched reactor (TSR). Further, although the gate pulse generator of Fig. 1 is constituted by hardware logics, operation defined by the control timing shown in Fig. 2 can be reduced to practice by a software using a conventional microcomputer.

Claims: A gate pulse generator for a static var compensator comprising: a thyristor switch (3) being formed of first thyristor means (3U) and second thyristor means (3X), said second thyristor means (3X) being anti-parallel connected to said first thyristor means (3U); first detector means (5u) for detecting a first voltage (Vu) applied across said thyristor switch (3); and first firing means (9u-12u); characterized in that said first thyristor means is formed of a first block of series-connected thyristors (3U) and said second thyristor means is formed of a second block of series-connected thyristors (3X), said detector means (5u) generate a first voltage signal (FVu) when the first block thyristors (3U) is forwardly biased; said first firing means (9u-12u) fire said first block thyristors (3U) in accordance with a first conduction period signal (PHSu) and said first voltage signal (FVu); and first expanding means (13x-15x; 10u) expand by a first predetermined period (2ms) an active period (t20-t31) of said first conduction period signal (PHSu) from time (t30) when said first voltage (Vu) is substantially zero. A gate pulse generator according to claim 1, characterized in that said thyristor switch (3) is provided with a capacitor (2) connected in series thereto, said thyristor switch (3) and said capacitor (2) constituting a thyristor switched capacitor (2-3). A gate pulse generator according to claim 1, characterized in that said thyristor switch (3) is provided with a reactor connected in series thereto, said thyristor switch (3) and said reactor constituting a thyristor switched reactor. A gate pulse generator according to claim 1, characterized by further comprising: second detector means (5x) for detecting a second voltage (Vx) applied across said thyristor switch (3) to generate a second voltage signal (FVx) when the second block thyristors (3X) is forwardly biased; second firing means (9x-12x) for firing said second block thyristors (3X) in accordance with a second conduction period signal (PHSx) and said second voltage signal (FVx); and second expanding means (13u-15u; 10x) for expanding by a second predetermined period (2ms) an active period (t30-t41) of said second conduction period signal (PHSx) from time (t40) when said second voltage (Vx) is substantially zero. A gate pulse generator according to claim 4, characterized by further comprising: means (100) for generating a first phase signal (PHu) when said first block thyristors (3U) is to be turned on, a second phase signal (PHx) when said second block thyristors (3X) is to be turned on, and an ON signal (ON) when said thyristor switched capacitor (2-4) is to be activated, wherein said first firing means (9u-12u) includes first generator means (9u, 10u) for generating said first conduction period signal (PHSu) when both said first phase signal (PHu) and said ON signal (ON) are generated. A gate pulse generator according to claim 5, characterized in that said second firing means (9x-12x) includes second generator means (9x, 10x) for generating said second conduction period signal (PHSx) when both said second phase signal (PHx) and said ON signal (ON) are generated. A gate pulse generator according to claim 5, characterized in that said first generator means (9u, 10u) includes means (10u) for terminating the active period (t20-t31) of said first conduction period signal (PHSu), after elapsing said first predetermined period (2ms) running from a time (t30) when both said first and second voltages (Vu; Vx) are substantially zero. A gate pulse generator according to claim 6, characterized in that said second generator means (9x, 10x) includes means (10x) for terminating the active period (t30-t41) of said second conduction period signal (PHSX), after elapsing said second predetermined period (2ms) running from a time (t40) when both said first and second voltages (Vu; Vx) are substantially zero. A gate pulse generator according to claim 4, characterized by further comprising: means (100) for generating a first phase signal (PHu) when said first block thyristors (3U) is to be turned on, a second phase signal (PHx) when said second block thyristors (3X) is to be turned on, and an ON signal (ON) when said thyristor switched capacitor (2-4) is to be activated, wherein said first expanding means (13x-15x; 10u) includes: first delay means (13x) for delaying a logical AND (e9x) of said second phase signal (PHx) and said ON signal (ON) by a first delay time (5-10ms) to provide a first delayed signal (a-x); first AND means (14x) for detecting a first logical AND (e14x) of said first delayed signal (a-x), an inverted logic level of said first voltage signal (FVu), and an inverted logic level of said second voltage signal (FVx); second delay means (15x) for delaying said first logical AND (e14x) by said first predetermined period (2ms) to provide a second delayed signal (b-x); and a first flip-flop (10u) which is set by a logical AND (e9u) of said first phase signal (PHu) and said ON signal (ON) to activate said first conduction period signal (PHSu), and which is reset by said second delayed signal (b-x) to inactivate said first conduction period signal (PHSu). A gate pulse generator according to claim 9, characterized in that said second expanding means (13u-15u; 10x) includes: third delay means (13u) for delaying a logical AND (e9u) of said first phase signal (PHu) and said ON signal (ON) by a second delay time (5-10ms) to provide a third delayed signal (a-u); second AND means (14u) for detecting a second logical AND (e14u) of said second delayed signal (a-u), an inverted logic level of said first voltage signal (FVu), and an inverted logic level of said second voltage signal (FVx); fourth delay means (15u) for delaying said second logical AND (e14u) by said second predetermined period (2ms) to provide a fourth delayed signal (b-u); and a second flip-flop (10x) which is set by a logical AND (e9x) of said second phase signal (PHx) and said ON signal (ON) to activate said second conduction period signal (PHSx), and which is reset by said fourth delayed signal (b-u) to inactivate said second conduction period signal (PHSx). A gate pulse generator according to claim 4, characterized by further comprising a computer which executes operations of the firing of said first and second firing means (9u-12u; 9x-12x) and operations of the expanding of said first and second expanding means (10u; 10x; 13x-15x; 13u-15u).

Inventors: SATO NAOKO C O INTELLECTUAL PR, SATO, NAOKO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H03K17
IPC codes: G05F1, H02M1, H03K5, H03K17, H02J3