Thyristors basics

A modification of this basic drive system uses solid-state rectifiers and thyristors to convert the wound-rotor, variable-frequency slip power first to direct current and then to hne-frequency power (60 Hz in the United States). This in turn is fed back to the power system as useful energy. 

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... 

Table 6.3 A brief comparison between a transistor and basic thyristor technology...

Figure 6.26(a) Basic IGBT or thyristor (GTO) inverter unit... 

A typical basic circuit is shown in Figure 14.3 which consists of two main parts, a three-phase bridge-connected thyristor rectifier and a three-phase bridge-connected thyristor inverter. 

Diode bridges are the simplest and are suitable where the output DC voltage is constant and related to the input AC voltage by a fixed factor. They are well suited to battery chargers, uninterruptible power supplies and cathodic protection units. Figure 15.1 shows the basic element of a three-phase diode bridge, in this case the rectifier elements Ri to and diodes, not thyristors as shown. 

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. 

The small example systems considered in this chapter are widely used basic components of power electronic systems. In power electronics, it is common to model the fast switching semiconductor devices that use various types of transistors, diodes, or thyristors simply as ideal or non-ideal switches although more sophisticated transistor models can be used and are used depending on the application and the purpose of a simulation study. 

A transient analysis in wind farms, mega solars, and smart grids requires a different approach in comparison to those in overhead lines and cables. A transient in an overhead line and cable is directly associated with traveling waves whose traveling time is in the order of 10 ps up to 1 ms in most cases, the maximum overvoltage appears within a few milliseconds. In contrast, a transient in a wind farm involving power electronic circuits is affected by the dynamic behavior of power transistors/thyristors, which is a basic element of the power electronic circuit. 

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