SEMICONDUCTOR TYPES

The nonlinear resistance of semiconductor junctions is used in the bridge-type fault current limiter, which combines power electronics and an SC coil (21). Figure 3(b) shows the circuit of a single phase. A bias power supply provides a cur­rent in all four semiconductors. As long as the load current is less than the bias current, all four semiconductors are for­ward biased, and the ac load current flows unimpeded by the bridge circuit, assuming negligible losses in the semiconduc­tors. In the quiescent condition, each thyristor conducts half the bias current superimposed with half of the line current. When a short circuit occurs, one pair of semiconductors is turned on, and the other pair is turned off in each half cycle, automatically inserting the bridge inductor into the circuit. The inductor limits the fault current. Because the inductor carries a bias current under normal condition, use of an SC coil reduces the overall system losses. The bridge-type fault current limiter has several attractive features, such as auto­matic insertion of the current-limiting reactor, reduction of the first half-cycle short-circuit current, precise control of the amplitude of the short-circuit current, complete current inter­ruption in less than a cycle if desired, operation with multiple fast reclosures, and high efficiency. In addition, it is conceiv­able to use the controlled thyristor bridge for other transmis­sion or distribution network functions such as var control. For distribution and transmission system semiconductor SFCLs, the equivalent thyristors indicated in Fig. 3(b) are series strings of thyristors, similar to those used in static var com­pensators or electronic transfer switches. A 2.4 kV, 2.2 kA single-phase fault current limiter was successfully tested in 1995 and a 15 kV, 800 A, three-phase unit is being designed to be tested in 1998 (2).

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