WO1994024622A1

WO1994024622A1 - Turnoff thyristor controlled series compensation system

Info

Publication number: WO1994024622A1
Application number: PCT/US1993/011509
Authority: WO
Inventors: Stig L. Nilsson; Narain G. Hingorani
Original assignee: Electric Power Research Institute
Priority date: 1993-04-19
Filing date: 1993-11-29
Publication date: 1994-10-27

Abstract

A series compensation system (20) is provided for compensating a transmission line (22) having a line current (IL) flowing therethrough. The system includes a capacitor (30) for placing in series with the transmission line (22), and a turnoff switching device (40) in parallel with the capacitor (30). In response to a firing command (54, 55) from a controller (70), the switching device (40) either conducts to bypass the line current (IL) around the capacitor (30), or switches off at any time during the current cycle through the switch (40) to insert the capacitor (30) in series with the transmission line (22). The switching device (40) may include a GTO thyristor, a MOS controlled thyristor, or combinations thereof with conventional thyristors. A method is also provided for compensating a transmission line (22) during heavy load periods to prevent unacceptable undervoltage conditions.

Description

TURNOFF THYRISTOR CONTROLLED SERIES COMPENSATION SYSTEM Background of the Invention

The present invention relates generally to a series compensation system for inserting a series capacitance in a transmission line during a heavy load period, and more particularly to a turnoff thyristor controlled series compensation system.

The terms "off" and "open" when used with respect to switches and thyristors herein denote a nonconducting state, whereas the terms "on" and "closed" refer to a conducting state.

Other systems have been used to solve the line compensation problem. One system based on gate turnoff (GTO) type inverters and rectifiers is described in L. Gyugyi's paper, "Solid-State Control of Electric Power in AC Transmission Systems," presented at the International Symposium on Electrical Energy Conversion in Power Systems, Capri, Italy, May 24-26, 1989. Another system previously proposed by the current inventors is disclosed in U.S. Patent

No. , to . This system improved on conventional series compensation systems by using a thyristor switch arrangement comprising a thyristor switch in parallel with a series capacitor bank. The capacitor bank comprises a plurality of parallel and series connected capacitor modules. The thyristor switch is connected in parallel with a single or several sections of parallel-connected capacitors. The thyristor switch has at least one thyristor valve, or several series connected thyristor valves. Each thyristor valve is a single pair of conventional thyristors connected in antiparallel. The number of thyristor valves in a single switch is selected to provide a voltage rating matching the voltage rating of the capacitor section across which the switch is connected. Under normal load conditions, the thyristors fire sequentially to bypass the transmission line current around the capacitor bank. If several capacitor sections and thyristor switches are used, the load current may be bypassed around certain capacitor sections by firing only a portion of the total thyristor switches. Each thyristor valve may have overvoltage protection, such as a gapless metal oxide arrestor, in parallel with the valve. Alternatively, a single arrestor may be connected across the entire thyristor switch, rather than across each thyristor valve, or combinations of the single and multiple valve arrestors may be used. The thyristor valves also include conventional snubber circuits, or current limiting reactors. Conventional overvoltage protection equipment is used for protecting the capacitors by bypassing the capacitors under system fault overcurrent conditions. Serious limitations of this earlier series compensation system stem from the operational limitations of the conventional thyristor switch employed in this system. A conventional thyristor switch can only interrupt current at a natural zero crossing of the current waveform through the switch. Figs. 8-10 are waveforms with respect to time (t) illustrating the operational limitations of this earlier series compensation system employing conventional thyristor switches.

Fig. 8 shows three concurrent graphs of the line current, the capacitor current, and the capacitor or switch voltage. The time at which the thyristor switch is turned off is shown by the vertical dashed line extending between the graphs. As shown in the voltage graph at the bottom of Fig. 8, turning on the thyristor switch causes a severe DC (direct current) offset to the AC (alternating current) voltage waveform across the capacitor and switch. This severe DC offset occurs because a conventional thyristor switch can only interrupt the current flow through the switch at a zero crossing of the current waveform, as shown by the intersection of the dashed vertical line and the top graph of Fig. 8. In some instances, the DC offset causes the peak voltage to reach up to twice the nominal value, indicated as 2pu (per unit) in Fig. 8. Thus, the capacitors in the series capacitor bank must be designed to withstand twice the rated peak voltage of the transmission line. Such required over-design drastically increases the initial cost of the capacitor bank.

Fig. 9 shows concurrent graphs of the line current, capacitor voltage, and the switch current. The time at which the conventional thyristor switch is turned on is indicated by the vertical dashed line linking the graphs together. When the thyristor switch is turned on and inserted into the line, the line current is bypassed through the thyristor switch and around the capacitor. While the thyristor switch can begin conducting at any time during the current cycle, it is preferably turned on at or close to a zero crossing of the voltage waveform, as shown by the intersection of the dashed line and the middle graph of Fig. 9. Turning the thyristor switch on at a voltage zero crossing avoids large surge currents caused by discharging the capacitor through the thyristor. In the Fig. 9 graphs, the high frequency transients known by those skilled in the art to be associated with the capacitor bypass switching operation have been omitted to simplify and clarify the graphs.

Referring to Fig. 10, concurrent voltage and current graphs are shown for two conventional thyristor switches, each in parallel with one or more series connected capacitor bank sections. Referring to the Fig. 10 graphs, the "first capacitor" refers to the bank section(s) in parallel the first switch, and the "second capacitor"¹ refers to the bank section(s) in parallel with the second switch. For the illustrated embodiment, the second switch extends across twice the number of capacitor bank sections than does the first switch. Thus, the second switch has twice the voltage magnitude of the waveform shown for the first capacitor voltage, as shown by the dashed waveforms in Fig. 10.

The Fig. 10 graphs show the performance characteristics of sequentially turning off the two thyristor switches to sequentially insert the capacitor bank sections in series with the transmission line. Initially, only the first switch is open. The first switch remains open allowing current to flow through the first capacitor bank section(s) throughout the entire time period shown in the Fig. 10 graphs. The dashed vertical line extending between each of the five graphs indicates the time of operating the second switch to open. During the time to the left of the vertical dashed line, the second switch is closed, and during the time period to the right of the dashed line the second switch is open.

As with Fig. 8, the second thyristor switch in the Fig. 10 example must be turned off at a natural zero crossing of the line current waveform. This operation causes a temporary voltage bias or DC offset component which is superimposed over the AC waveform. This undesirable DC offset transiently increases the voltage across the capacitors associated with the first switch which are already operating when the second thyristor switch is turned off (see the middle graph in Fig. 10) . The magnitude of the overvoltage experienced by the first capacitor bank section depends upon the relative size of the capacitors used in the circuit. With unequally sized capacitors, the overvoltage can be critical, often requiring operation of the overvoltage protection for the effected capacitor. The last graph in Fig. 10 illustrates the overvoltage experienced at the second capacitor bank section after the second switch is opened. The dashed lines in the middle and last graphs of Fig. 10 show the proper location of the capacitor voltage waveforms without the DC offset. Thus, a need exists for an improved series compensation system for compensating the impedance of a transmission line during a heavy load period, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.

Summary of the Invention According to one aspect of the present invention, a series compensation system is provided for compensating a transmission line having a line current flowing therethrough. The system has a capacitor for placing in series with the transmission line, and a turnoff switching device in parallel with the capacitor. The switching device turns on to bypass the line current around the capacitor, and selectively turns off to insert the capacitor in series with the transmission line so current flows through the capacitor. The switching device may be turned off at any time during a cycle of the current waveform as it flows through the switching device.

According to another aspect of the present invention, a method is provided for compensating a transmission line during heavy load periods to prevent unacceptable undervoltage conditions.

An overall object of the present invention is to provide an improved series compensation system, including an apparatus and method, for compensating the impedance of a transmission line during a heavy load period.

A further object of the present invention is to reduce the voltage stress on a capacitor which is placed in series with the transmission line to provide series compensation.

Another object of the present invention is to eliminate a DC offset component in the voltage of a capacitor when it is switched in series with a transmission line for compensation. An additional object of the present invention is to provide a series compensation system which may be used with advanced switching strategies to damp subsynchronous oscillations in the shafts of a turbine-generator unit coupled to the transmission line. Still another object of the present invention is to provide a more economical series compensation system than available in the past.

The present invention relates to the above features and objects individually as well as collectively. These and other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following description and drawings.

Brief Description of the Drawings Fig. 1 is a single line schematic block diagram of one form of a series compensation system of the present invention;

Fig. 2 is a schematic diagram of one form of a turnoff switch for use in the system of Fig. 1;

Fig. 3 is a schematic diagram of an alternative switch to that shown in Fig. 2; Fig. 4 is a single line diagram of an alternative switch to that shown in Fig. 2;

Fig. 5 are graphs of the concurrent voltages and currents with respect to time (t) associated with the operation of the system of Fig. 1; Fig. 6 is a schematic diagram illustrating one manner of operating the system of Fig. 1;

Fig. 7 are graphs of the concurrent voltages and currents with respect to time (t) associated with an alternative manner of operating the system of Fig. 1; and

Figs. 8-10 are graphs illustrating the operation of the earlier series compensation system having a conventional thyristor switch, discussed in the background portion above. Detailed Description of a Preferred Embodiment

Fig. 1 illustrates a series compensation system or series compensator 20 constructed in accordance with the present invention for compensating a transmission line or power line 22 having a line current I_L flowing therethrough. The transmission line 22 has a first segment 24 coupling an AC power system or grid 25 with the series compensator 20. The transmission line 22 has a second segment 26 which couples the series compensator 20 with a load 28. The load 28 may have fluctuating power requirements on a seasonal, daily, or a substantially instantaneous basis. Under heavy load conditions, the voltage at the load V_L can drop under, that is, decrease to less than, acceptable minimum limits. An undervoltage condition is caused essentially by the series inductance of the transmission line, and thus, the problem increases in severity as the length of the transmission line 22 increases. This undervoltage problem may severely limit the acceptable operating range of a practical long transmission line.

To prevent these practical limitations, the series compensator 20 electrically inserts a capacitance in series with the transmission line to effectively cancel the series inductance during a heavy load period. This series capacitance is provided by a capacitor bank 30, illustrated as having plural series connected capacitor bank sections 32, 34 and 36. Each capacitor bank section 32, 34 and 36 has one or more discrete capacitors modules 38 coupled in parallel. The number of parallel capacitors 38 within a capacitor bank section 32, 34 or 36, depends upon the current-carrying capacity required to handle the line current I_L. The number of capacitor bank sections 32, 34 or 36, or more, depends upon the required capacitance and series voltage drop across the capacitor bank 30. For example, the first capacitor bank section has a voltage drop V_C1, whereas the combined capacitor banks 34 and 36 have a total voltage drop V_C2. When series compensation is required, the line current I_L flows through the capacitor bank as indicated by the arrow labeled I_c for capacitor current. However, during normal operating conditions, the line current is bypassed around the capacitor bank 30 as a bypass or switch current I_s through the action of a turnoff switching device 40. The turnoff switching device 40 has the ability to interrupt current, and insert the capacitor bank 30 in series with the transmission line 22, at any point during the waveform of the switch current I_s. In the illustrated embodiment, the switching device comprises two turnoff switches 42 and 44. The first switch 42 is in parallel with the capacitor bank section 32, and the second switch 44 is in parallel with the two series connected capacitor bank sections 34 and 36.

Fig. 2 illustrates one embodiment of the turnoff switch 42 as having three series connected turnoff thyristor valves 45, 46 and 48. Each thyristor valve, 45, 46 and 48, has two antiparallel turnoff thyristors 50 and 52, each having turnoff capability. As a matter of terminology, a controlled thyristor referred to herein as a "turnoff thyristor" has the ability to interrupt current at any point during the waveform of the switch current I_s. In contrast, a thyristor lacking turnoff capability which can only interrupt the switch current I_s at a natural current zero, is referred to herein as a "conventional thyristor." A turnoff thyristor, such as thyristors 50 and 52, may be a gate turnoff (GTO) thyristor, a metal oxide semiconductor (MOS) controlled thyristor, or the like which are known to be structural equivalents thereto by those skilled in the art. The turnoff thyristors receive an "on" command to enter a conducting state, and an "off" command to enter a nonconducting state via firing command signals 54 and 55 received by the respective thyristors 50 and 52, illustrated for the thyristor valve section 45. The switch 42 may also include an overvoltage protection device, such as a gapless metal oxide arrestor 56, in parallel with the thyristors 50 and 52 of each valve 45, 46 and 48. Note, while only three thyristor valves are shown, the dashed lines between the valves 46 and 48 indicate that additional thyristor valves may be included to match the voltage drop V_C1 of the capacitor bank section 32. Fig. 3 shows an alternative to the Fig. 2 embodiment as a turnoff switch 42', having three or more turnoff thyristor valve sections 45, 46 and 48 as shown in Fig. 2. The switch 42', rather than having separate overvoltage protection for each thyristor valve section, switch 42' has a single arrestor 58 coupled in parallel with the series combination of thyristor valves 45, 46 and 48. While Figs. 2 and 3 are illustrated for the switch 42, it is apparent that switch 44 may be similarly constructed. For example, if each of the capacitors 38 in Fig. 1 have the same voltage drop there across, then twice as many thyristor valves are required for switch 44 than for switch 42, assuming like-rated thyristors are used therein.

Referring to Fig. 4, an alternative thyristor valve 60 is shown which may be used instead of thyristor valves 45, 46 and 47. The thyristor valve 60 includes two antiparallel coupled series branches 62 and 64, with each branch having a conventional thyristor 66 without gate turnoff capability, in series with a turnoff thyristor 68 having gate turnoff capability. Several thyristor valves 60 may be coupled in series, as shown in for the thyristor valves 45, 46 and 48 in Figs. 2 and 3. The thyristor valve 60 may have optional overvoltage protection, such as the gapless metal oxide arrestor 56 in parallel with one or more branches 62 and 64, similar to the embodiments of Figs. 2 or 3, respectively.

The series compensator 20 may also include a controller 70 for supplying firing commands signals 54, 55 and 54', 55' to the switches 42 and 44, respectively. The controller 70 may receive a line status signal 72 from a line status monitoring sensor device 74. The sensor 74 may be used to detect periods of normal and heavy loads. Although the sensor 74 is illustrated as measuring a load voltage V_L, other power line parameters may be monitored to provide the line status signal 72, for instance, the line current I_L. Instead of locating the sensor 74 in the transmission line segment 26, sensor 74 may be located in the transmission line segment 24, or at the load 28. Alternatively, or in addition to the feedback provided by sensor 74, the controller 70 may receive a separate external command signal 76 from a system operator, or from some other higher-level controller (not shown) .

In operation, under normal load conditions, the controller 70 controls the switching device 40 to bypass the line current I_s through a commutating path provided by the switches 42 and 44. That is, firing command signals 54, 55 and 54', 55' are provided to turn on the thyristors 50 and 52 to conduct the switch current I_s therethrough. When the sensor 74 detects a low voltage condition indicative of a heavy load, the controller 70 responds to the line status signal by commanding the switching device 40 to be turned off so current is no longer bypassed around the capacitor bank 30.

The controller 70 may selectively either completely or partially insert the capacitor bank 30 in series with the transmission line 22 as needed to compensate the impedance of line 22. For complete insertion of the capacitor bank 30, both switches 42 and 44 are turned off. If only partial compensation is needed, only one of the switches 42 or 44 may be turned off, so the other switch remains conducting to bypass current around a portion of the capacitor bank 30. By coupling the switches 42 and 44 to capacitor bank sections having different capacitances, different levels of capacitance may be selectively inserted in the line 22 to compensate the line for corresponding different levels of load.

Using turnoff thyristors 50 and 52, the capacitor bank 30, or sections thereof, may be electrically inserted into the transmission line 22 at any time during the current cycle through the switching device 40. For example, Fig. 5 shows the line current I_L, capacitor current I_c, and capacitor voltage V_C1 waveforms for inserting the single capacitor bank section 32 in series with the line by turning off switch 42. The switch 42 is turned off at a peak in the line current cycle, as indicated the vertical dashed line connecting the three graphs of Fig. 5. Such action would be virtually impossible for a conventional thyristor switch which is limited to interrupting current only at a natural current zero crossing.

From comparing the capacitor voltage graphs in Figs. 5 and 8, it is apparent that the capacitors 38 of the capacitor bank section 32 are not subjected to the severe overvoltages experienced by capacitors in the earlier system having only a conventional thyristor switch. Using the switch device 40 of the present invention, there is no DC offset injected into the voltage waveform, and the DC component in the capacitor voltage is substantially eliminated. The capacitor voltage stresses are advantageously reduced, and the capacitors 38 need not be designed to withstand double the rated voltage, as for capacitors used in the earlier compensation systems having conventional thyristor switches. Thus, the series compensator 20 may be more economically constructed, as the capacitors require less material and physically consume less physical space.

Using the turnoff switching device 40 eliminates the problems discussed above in the background portion with respect to Fig. 10. Referring to Fig. 6, switches 42 and 44 may be turned off one at a time to sequentially insert the capacitor bank sections in the line 22. For example, if initially switch 42 is open • and switch 44 is closed, the capacitor current I_c will flow through only the first capacitor bank section 32, and then be bypassed through the second switch 44 as current I_S2. Since the thyristors 50 and 52 each have turnoff capability, switch 44 may be turned off at any time during the cycle of the switch current I_S2 flowing therethrough.

Thus, whenever the switch 44 is turned off, the DC offset voltage for V_C1 and V_C2 waveforms is controlled so that it equals zero, and none of the capacitor bank sections 32, 34 or 36 see any overvoltage, such as that shown in Fig. 10 for the earlier system. Rather, the capacitor voltage waveforms for V_C1 and V_c2 are symmetrical, such as the waveform in the bottom graph of Fig. 5. Even if the capacitors within each capacitor bank section 32, 34 or 36 are unequal in size, no overvoltage is experienced. In this manner, operation of the series compensator 20 continues without the need for operation of the overvoltage protection, such as arrestors 56 or 58, or other overvoltage protection devices (not shown) coupled directly to the capacitor bank 30.

In an alternative mode of operation, the capacitor bank voltage may be modulated as shown by the line current I_L, capacitor current I_c, capacitor voltage V_C1, and switch current I_S1 waveforms in Fig. 7. In this method of operation, the switch current I_S1 may be interrupted by turning switch 42 off at a controlled time ti before a current zero of the line current waveform. The line current then flows through the capacitor bank section 32 until bypassed by turning on switch 42 when the voltage across the capacitor V_C1 reaches zero again at a later time t₂. That is, between t₀ and t_lf t₂ and t₃, and after time t.-, , current is bypassed around the capacitor bank section 32 and through the switch 42. Between times t_x and t₂, and times t₃ and t₄ current flows through the capacitor bank section 32. In a steady-state case, the times t₂ and t₄ at which the current is bypassed through switch 42 are after the line current I_L has reversed polarity, and has the same magnitude as it did when the switch 42 was turned off at times t_x and t₃. As will be apparent to those skilled in the art, this illustration is a slight oversimplification of the actual behavior of system 20 during modulation, but it is a close approximation to the actual situation if the injected capacitor voltage V_C1 is small. In the graphs of Fig. 7, the time intervals between t_x and t₂, and between t₃ and t_« are expanded to more clearly illustrate this action. Furthermore, the high frequency transients known by those skilled in the art to be associated with the capacitor bypass switching operations have been omitted to simplify and clarify the graphs of Fig. 7.

It is also apparent that other more complex modulation schemes may be implemented with the series compensator 20. For example, the modulation may be preformed with unequal pulses where one of the switching times t_x or t₂ is closer to the current zero crossing than the other. Such complex modulation schemes with unequal pulses may be useful in creating subsynchronous resonance (SSR) damping signals for damping undesirable subsynchronous resonance frequencies on the AC grid 25. Referring again to Fig. 4, an operational advantage of mixing turnoff thyristors 68 with conventional thyristors 66 is illustrated. An alternative method of operating the thyristor valve 60 is illustrated by assuming the valve 60 is coupled in parallel with the capacitor bank section 32, instead of switch 42. First, commutation of the line current to capacitor bank section 32 begins by turning off the turnoff thyristors 68. Next, the balance of the capacitor voltage is taken up by turning on the conventional thyristors 66 after the switch current I_s has reached a current zero. It is apparent to those skilled in the art that such a thyristor valve 60 may need careful matching of device characteristics and εmibber circuits (not shown) to be feasible.

Furthermore, in some implementations the combined thyristor valve 60 may be more economically initially constructed, and more efficient in operation than the straight turnoff valves 45, 46 and 48.

In manufacturing a series compensation system, the costs of the valves used in the switching device are dominant, and the ability to reduce the voltage seen by the valves, such as voltage V_C1 across the bank section 32, results in a significant cost savings. By avoiding the undesirable DC offset voltage experienced using a conventional system, the capacitor bank 30 may also be more economically constructed. Different operation strategies also dictate the required rating of the series compensator 20.

For example, there are complex operational tradeoffs between the various short circuit operation strategies. During a short circuit event on the transmission line 32, the tradeoff involves either bypassing the capacitor bank 30 or inserting the capacitor bank 30 in series with the line. If the switching device 40 is conducting during a short circuit event, the thermal stresses on the power semiconductor devices 50 and 52, or 66 and 68, may be substantial. If the switches conduct during a system short circuit, there may be no need for the capacitor bank to have separate overvoltage protection, such as spark gaps, or metal oxide arrestors (not shown) . If during a short circuit event the switches 42 and 44 are not conducting, the voltage across the capacitor bank 30 may be substantial, even if the surge voltage is limited by spark gaps or metal oxide arrestors. Given the illustrated structure of the series compensator 20, both operating strategies are possible, that is, either letting short circuit current flow through the capacitor bank or through the switching device 40, and economic considerations will most likely determine which route is pursued.

Having illustrated and described the principles of our invention with respect to a preferred embodiment, it should be apparent to those skilled in the art that our invention may be modified in arrangement and detail without departing from such principles. For example, other arrangements of the capacitor bank 30 and the turnoff switching device 40 may be used, as well as other arrangements of turnoff thyristors alone, or in combination with conventional thyristors. We claim all such modifications falling within the scope and spirit of the following claims.

Claims

1. A series compensation system for compensating a transmission line having a line current flowing therethrough, the system comprising: a capacitor for placement in series with the transmission line; and a turnoff switching device in parallel with the capacitor for bypassing line current around the capacitor, and for selectively inserting the capacitor in series with the transmission line during a cycle of the line current through the switching device.

2. A series compensation system according to claim 1 wherein the turnoff switching device comprises a turnoff thyristor valve having two antiparallel turnoff thyristors.

3. A series compensation system according to claim 2 wherein the turnoff thyristor valve comprises a gate turnoff thyristor.

4. A series compensation system according to claim 2 wherein the turnoff thyristor valve comprises a metal oxide semiconductor controlled thyristor.

5. A series compensation system according to claim 2 wherein the turnoff thyristor valve has two conventional thyristors, with one conventional thyristor in series with a one of the turnoff thyristors, and the other conventional thyristor in series with the other turnoff thyristor.

6. A series compensation system according to claim 1 further including: at least two series capacitors for placement in series with the transmission line; and at least two turnoff switching devices, each in parallel with at least one of the series capacitors for selectively inserting one or more of the capacitors in series with the transmission line during a cycle of the line current through the switching device.

7. A series compensation system according to claim 1 wherein: the system further includes at least two series capacitors for placing in series with the transmission line; and the turnoff switching device is in parallel with at least two of the series capacitors.

8. A series compensation system according to claim 1 wherein the turnoff switching device comprises a plurality of series connected turnoff thyristor valves, each having two antiparallel turnoff thyristors.

9. A series compensation system according to claim 8 further including at least one overvoltage protection device in parallel with at least one of the plurality of series connected thyristor valves.

10. A series compensation system for compensating a transmission line having a line current flowing therethrough, the system comprising a capacitor and a turnoff switching device in parallel with the capacitor for selectively inserting the capacitor in series with the transmission line at any time under a substantially normal peak capacitor voltage.

11. A series compensation system according to claim 10 wherein the turnoff switching device comprises a gate turnoff thyristor.

12. A series compensation system according to claim 10 wherein the a turnoff switching device comprises a metal oxide semiconductor controlled thyristor.

13. A series compensation system according to claim 10 wherein the turnoff switching device includes a gate turnoff thyristor in series with a conventional thyristor.

14. A method of compensating a transmission line having a line current flowing therethrough, comprising the steps of: providing a capacitor in series with the transmission line and a turnoff switching device in parallel with the capacitor; and selectively switching the turnoff switching device to a nonconducting state at any time during a current cycle through the switching device to allow the line current to flow through the capacitor.

15. A method of compensating according to claim 14 further including the step of selectively switching the turnoff switching device to a conducting state to bypass the line current through the switching device.

16. A method of compensating according to claim 14 wherein: the providing step comprises providing the switching device with a turnoff thyristor in series with a conventional thyristor; and the step of selectively switching the turnoff switching device to a nonconducting state to insert the capacitor in the line comprises the steps of: switching the turnoff thyristor to a nonconducting state before a current waveform through the switching device has reached a zero crossing; and after the current waveform through the switching device has reached a zero crossing, switching the conventional thyristor to a nonconducting state.

17. A series compensation system for compensating a transmission line having a line current flowing therethrough, the system comprising: a capacitor for placement in series with the transmission line; a sensor for sensing a parameter of power flow through the transmission line; a controller responsive to the sensor for generating a switch control signal; and a turnoff switching device in parallel with the capacitor, the turnoff switching device responsive to the switch control signal for bypassing line current around the capacitor, and for selectively inserting the capacitor in series with the transmission line at any time during a cycle of the line current through the switching device.

18. A series compensation system according to claim 17 wherein the turnoff switching device comprises a gate turnoff thyristor.

19. A series compensation system according to claim 17 wherein the turnoff switching device comprises a metal oxide semiconductor controlled thyristor.

20. A series compensation system according to claim 17 wherein the turnoff switching device includes a gate turnoff thyristor in series with a conventional thyristor.