Reactors in a power system can be located by- 1. In Series with Generators; 2. In Series with Feeders; 3. In Busbars (In Ring System or in Tie-Bar System), as explained below:
Location # 1. Generator Reactors:
When the reactors are inserted between the generator and the generator bus, as shown in Fig. 4.28 (a), the reactors are known as generator reactors. Such reactors protect the machines individually.
Since modern machines have transient reactance sufficient to protect themselves against dead 3-phase short circuit at its terminals, hence separate reactors are not required in the modern installation. However, when new machines are installed in an old power station, generator reactors may be added for the older generators. The magnitude of such reactors is very approximately about 5 per cent or 0.05 per unit.
The main disadvantage of this method is that if a short circuit occurs on one feeder, the voltage at the generator bus may drop to such a low value that the synchronous machines connected to this common bus-bar may fall out of step.
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Thus the whole system will be adversely affected. Moreover, a full-load current is always flowing through these reactors during normal operation resulting in a constant voltage drop and constant power loss.
Location # 2. Feeder Reactors:
When the reactors are connected in series with the feeders, as shown in Fig. 4.28 (b), the reactors are known as feeder reactors. In the event of fault on any one feeder, in this case, the main voltage drop is in its reactor only and the bus-bar voltage is not affected much, hence other machines continue supplying load. The other advantage is that the fault on a feeder will not affect other feeders and thus the effects of fault are localized.
The disadvantage of locating reactors in this position is that these do not provide any protection to the generators against short circuit faults across the bus-bars. This is, however, of no importance, because such faults are rare and also modern alternators have large transient reactance for their protection against short circuits.
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The other drawback is of constant voltage drop and constant power loss in reactors even during normal operation.
As we know individual feeder reactors are very expensive and must be resorted to only in the case of group feeders. The usual value of reactance for feeder circuits is of the order of 3 to 5 per cent based on the normal current loading of the circuit. Here it is to be noted that with highly inductive loads, serious interference with regulation can arise but at or near unity power factor the effect on regulation is negligible.
Location # 3. Bus-Bar Reactors:
The constant voltage drop and constant power loss in reactors may be avoided by inserting the reactors in the bus-bars, as shown in Figs. 4.28 (c) and 4.28 (d). The former is the ring system, and the latter is the tie-bar system.
(a) Bus-Bar Reactors (Ring-System):
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Bus-bar reactors are used to tie together separate bus sections. In this system sections are made of generators and feeders and these sections are connected to each other at the common bus-bar, as shown in Fig. 4.28 (c). In this system normally one feeder is fed from one generator. Under normal operating conditions small amount of power flows through the reactors, therefore, voltage drop and power loss in the reactors is low.
The reactors can, therefore, be made with a fairly high ohmic resistance and there is not much voltage drop across it. In case of fault on any one feeder, only one generator feeds the fault while the current from other generators is limited because of presence of the bus-bar reactors. Thus heavy currents and voltage disturbances caused by a short circuit on a bus section are reduced and confined primarily to that section.
Bus-bar reactors, however, do not protect the generators connected to the faulty sections. They facilitate the parallel operation of large systems and are extensively employed.
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(b) Bus-Bar Reactors (Tie-Bar System):
This is a modification of the above system. In general, with this system the voltage regulation between feeder sections is better than the above system. This system is ideally suited to the generating systems where frequently new generators are being added. In this system the generators are connected to the common bus-bar through the reactors but the feeders are fed from the generator side of the reactors.
The operation of this system is similar to ring system. It has got an additional advantage. If the number of sections is increased, the fault current will not exceed a certain value, which is fixed by the size of individual reactors. Thus the switchgear designed to operate successfully on this limiting current will continue operating successfully for any number of extensions of the sections and require no modification.