Thyristors commutation

The thyristor is a semiconductor device made of germanium or silicon wafers and comprises three or more Junctions, which can be switched from the OFF state to the ON state or vice versa. Basically it is a ptipn junction, as shown in Figure 6.20(a) and can be considered as composed of two transistors with npn and pnpjunctions, as illustrated in Figure 6.20(b). It does not turn ON when it is forward biased, unlike a diode, unless there is a gate firing pulse. Thyristors are forced commutated (a technique... 

The use of SCRs in an inverter circuit is intricate because of the absence of a natural commutation. Now only a forced commutation is possible, as it is connected to a d.c. source which provides no current zeros and hence facilitates no natural commutation. A forced commutation calls for a separate switching circuit, which is cumbersome, besides adding to the cost. As a result of this feature, they are also called forced commutated thyristors. 

Once fired, a thyristor cannot be controlled. Ft requires a forced commutation to switch it off and (he gate control is quite cumbersome. To swdtch OFT-, (he conducting current Is reduced to less than its holding current. The commutation circuitry is ihcrctbre highly complex an[Pg.118]

The power-conditioning system (built by GE) is based on GTO thyristor technology and features rapid (4.2 ms) response and bi-directional four-quadrant operation (charge, discharge, and VAR capable), 12-pulse waveform (low distortion), and is self-commutating. It is rated at 1.6-MVA peak (10 s) and at 1-MW continuous power [23]. 

The basic circuit of a thyristor bridge is almost the same as that for a diode bridge. The essential differences are the replacement of the diode elements by thyristor elements, the inclusion of a controlled firing system for the thyristor gates, and in some cases the application of forced commutation circuits, see Figure 15.1. 

Control of the triggering pulses to the thyristors needs to be carefully managed when the commutation is in Modes 2 and 3, otherwise the operation of the bridge may become unstable, see Chapter 7 of Reference 1. 

Figure 15.4 Trapezoidal current in the supply side of a six-pulse thyristor bridge, with the commutation angle u = 20°.

High-power inverters were initially developed for the long-distance transmission of power from a three-phase source to a remote three-phase sink using a DC overhead transmission line or cable. Early DC power transmission used mercury arc thyratrons (gas-filled values or tubes), which functioned in a manner very similar to the early types of thyristors. The on state of the valves was controllable, but the off state was determined by natural commutation made available by the sinusoidal voltages of the sink power system, see Reference 13. A brief description of three-phase inverters follows. 

At the same time development of semiconductor switching devices advanced. Gate commutated devices based on Thyristor technology and then later increased power capabihty for transistor type devices has enabled faster device switching and multi-device structures capable of complex switching patterns to not only prove more dynamic control but improve harmonics and power control as seen in PWM drives apphed today. 

Another popular rectifier circuit is the full-controlled three-phase full wave rectifier. This circuit is more expensive because six thyristors are used. However, the form factor is much better, about 1.01, and the ripple current is 360 Hz. The higher frequency makes it easier to filter the ripple current. The half-controlled three-phase bridge rectifier circuit may require armature current smoothing reactors to reduce the ripple current. Another problem associated with the non-uniform DC input to the motor is the commutation. The motor must commutate under a relatively high degree of leakage reactance. 

The switches turn off naturally, that is, load commutation takes place. This means thyristors can be... 

Most modem extmders are fitted with steplessly variable speed drives, such as a.c. commutator or thyristor controlled d.c. motors, to allow precise selection of the optimum screw speed for the particular application. 

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