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The thyristor-controlled transformer (TCT)

Another variant of the TCR is the TCT (Figure 6.9). Instead of using a separate step- down transformer and linear reactors, the transformer is designed with very high leakage reactance, and the secondary windings are merely short-circuited through the thyristor controllers. A gapped core is necessary to obtain the high leakage reactance, and the transformer can take the form of three single-phase transformers. With the arrangements in Figure 6.9 there is no secondary bus and any shunt capacitors must be connected at the primary voltage unless a separate step-down transformer is provided. The high leakage reactance helps protect the transformer against short- circuit forces during secondary faults. Because of its linearity and large thermal mass the TCT can usefully withstand overloads in the lagging (absorbing) regime.
arrangements of thyristor-controlled transformer compensator
Fig. 6.9 Alternative arrangements of thyristor-controlled transformer compensator. (a) with wye-connected reactors ant delta-connected thyristor controller; ant (b) wye-connected reactors ant thyristor controller (four-wire system).
The TCR with shunt capacitors
It is important to note that the TCR current (the compensating current) can be varied continuously, without steps, between zero and a maximum value corresponding to full conduction. The current is always lagging, so that reactive power can only be absorbed. However, the TCR compensator can be biased by shunt capacitors so that its overall power factor is leading and reactive power is generated into the external system. The effect of adding the capacitor currents to the TCR currents shown in Figure 6.4 is to bias the control characteristic into the second quadrant, as shown in Figure 6.10. In a three-phase system the preferred arrangement is to connect the capacitors in wye, as shown in Figure 6.6. The current in Figure 6.10 is, of course, the fundamental positive sequence component, and if it lies between IC max and IL max the control characteristic is again represented by equation (6.6). However, if the voltage regulator gain is unchanged, the slope reactance XS will be slightly increased when the capacitors are added.
As is common with shunt capacitor balks, the capacitors may be divided into more than one three-phase group, each group being separately switched by a circuit breaker. The groups can be tuned to particular frequencies by small series reactors in each phase, to filter the harmonic currents generated by the TCR and so prevent them from flowing in the external system. One possible choice is to have groups tuned to the 5th and 7th harmonics, with another arranged as a high-pass filter. The capacitors arranged as filters, and indeed the entire compensator, must be designed with careful attention to their effect on the resonances of the power system at the point of connection.
Voltage/current characteristics of TCR
Fig. 6.10 Voltage/current characteristics of TCR.

It is common for the compensation requirement to extend into both the lagging and the leading ranges. A TCR with fixed capacitors cannot have a lagging current unless the TCR reactive power rating exceeds that of the capacitors. The net reactive power absorption rating with the capacitors connected equals the difference between the ratings of the TCR and the capacitors. In such cases the required TCR rating can be very large indeed (up to some hundreds of MVAr in transmission system applications). When the net reactive power is small or lagging, large reactive current circulates between the TCR and the capacitors without performing any useful function in the power system. For this reason the capacitors are sometimes designed to be switched in groups, so that the degree of capacitive bias in the voltage/current characteristic can be adjusted in steps. If this is done, a smaller 'interpolating' TCR can be used.
An example is shown schematically in Figure 6.11, having the shunt capacitors divided into three groups. The TCR controller is provided with a signal representing the number of capacitors connected, and is designed to provide a continuous overall voltage/current characteristic. When a capacitor group is switched on or off, the conduction angle is immediately adjusted, along with other reference signals, so that the capacitive reactive power added or subtracted is exactly balanced by an equal change in the inductive reactive power of the TCR. Thereafter the conduction angle will vary continuously according to the system requirements, until the next capacitor switching occurs.
Hybrid compensator with switched capacitors ant 'interpolating' TCR. The switches S may be mechanical circuit breakers or thyristor switches
Fig. 6.11 Hybrid compensator with switched capacitors ant 'interpolating' TCR. The switches S may be mechanical circuit breakers or thyristor switches.

The performance of this hybrid arrangement of a TCR and switched shunt capacitors depends critically on the method of switching the capacitors, and the switching strategy. The most common way to switch the capacitors is with conventional circuit breakers. If the operating point is continually ranging up and down the voltage/ current characteristic, the rapid accumulation of switching operations may cause a maintenance problem in the circuit breakers. Also, in transmission system applications there may be conflicting requirements as to whether the capacitors should be switched in or out during severe system faults. Under these circumstances repeated switching can place extreme duty on the capacitors and circuit breakers, and in most cases this can only be avoided by inhibiting the compensator from switching the capacitors. Unfortunately this prevents the full potential of the capacitors from being used during a period when they could be extremely beneficial to the stability of the system.
In some cases these problems have been met by using thyristor controllers instead of circuit breakers to switch the capacitors, taking advantage of the virtually unlimited switching life of the thyristors. The timing precision of the thyristor switches can be exploited to reduce the severity of the switching duty, but even so, during disturbances this duty can be extreme. The number of separately switched capacitor groups in transmission system compensators is usually less than four.
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