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Monday, 26 January 2015

Current Limiting Reactors for Power System application

"A current limiting reactor, also called a series reactor, is a coil which has high inductive reactance as compared to its resistance." 

These reactors are used to limit the short circuit current and the effect of resulting voltage disturbances during fault conditions. The short circuit currents depend upon the generating capacity, voltage at the fault point and the total reactance between the generators and the fault point.
In large interconnected systems, the total rating of the generators is very high and also, when the system is extended by the addition of more generating units, the fault currents are also increased. So the fault current to be interrupted by the same circuit breaker will become greater than the earlier value. These short circuit currents may be large enough to cause damage to the line and other equipments of the power system network.
The short circuit current can be kept within safe limits by increasing the reactance between the source and the fault. Thus, there is a need of providing a protective reactor. By including a reactor or few reactors at strategic locations, the short circuit currents at different points in the power system can be reduced. 

The reactors allow free interchange of power under normal conditions but under short circuit conditions the disturbance is limited to the faulty section. As the resistance of the reactors is very small, thus the efficiency of the system is not affected much.

Main functions of Current Limiting Reactors:

The primary functions of a current limiting reactor are:
1.      To reduce the flow of current into a short circuit so as to protect the power system apparatus and parts of the system from excessive mechanical stress and overheating.
2.      To reduce the magnitude of voltage disturbances caused by short circuits.
3.      To localize the faults by limiting the current that flows into the fault from other healthy feeders or part of the system.
4.      To reduce the duty imposed on switching equipments during short circuits.

Construction of Current Limiting Reactor:

Reactors are normally of two types:
1.      Dry type, and
2.      Oil immersed type.
In air cored dry type reactor, the core is of air and the whole construction of the reactor is free from ferromagnetic materials. The winding of the reactor is rigidly placed on glass-reinforced synthetic resin supports. Due to the absence of iron, the reactance remains fairly constant during the flow of heavy current. Dry type reactors are usually cooled by natural ventilation and sometimes provided with forced air and heat exchanger auxiliaries. These reactors occupy relatively large space and are used only up to 33 kV.
Oil immersed reactors are used for voltages above 33 kV. These reactors are similar to power transformers in several aspects. They are either without iron core or have gaped iron core. Cooling is similar to that of power transformers. The coil assembly is oil immersed and is enclosed in a tank. Laminated iron shields are provided around the outer conductors so as to avoid the entering of magnetic flux in the surrounding iron parts. Oil immersed reactors have the advantage of smaller size, high thermal capacity and higher safety against flash-overs.

Drawbacks of Reactors:

The main drawbacks of a reactor are:
1.      The total percentage reactance of the system is increased, thus causing an increase in the reactive voltage drop.
2.      The power factor is decreased.

Location of Current Limiting Reactor:

By including a reactor or few reactors at strategic locations, the short circuit currents at different points can be reduced. The reactors may be connected in –
1.      Series with the generator (Generator Reactor),
2.      Series with each feeder (Feeder Reactor),
3.      Between bus-bar sections (Bus-bar Reactor)
When a reactor is connected in between the generator and the bus, the reactor is known as generator reactor. Modern alternators are designed to have sufficiently large reactance (may be 2.0 p.u.) to protect themselves against dead 3-phase short circuits at its terminals. With such a large reactance the current during short circuits at terminals may be less than full load current therefore, externals reactors are not required. Current limiting reactors are only used with old generators having low values of reactance.
Feeder reactors are connected in series with the feeders. The advantage is that the voltage of the bus does not drop substantially in the event of fault on any one feeder. Thus, other generators continue to supply the load and other feeders are also not affected. But there is constant voltage drop and power loss during normal operation in case of feeder reactors. Cost-wise feeder reactors are expensive.
Bus-bar reactors are connected between bus sections. Two systems of bus-bar reactors are common. They are Ring system and the Tie-bar or the Star system. In the ring bus-bar system, under normal operation each generator supplies to the feeder connected to its own section and thus there will be no current through the reactors (under normal operation). Therefore there is no voltage drop or power loss in the reactor during normal operation. In case of fault on any one feeder, only one generator feeds the fault while the current from the other generator is limited because of the bus-bar reactor. This system facilitates parallel operation of systems and is extensively employed for plants of moderate output.

The tie-bar system is better and more flexible than the ring system. In this system, the generators are connected to the common bus-bar (tie-bar) through the reactors but the feeders are fed from the generator side of the reactors. 

In the ring system the short circuit current due to a fault on any bus bar section, is fed from the generators connected to other sections through one reactor, whereas in the tie-bar system the current flows through two reactors in series. Therefore this system requires only half the reactance compared to the ring system. 

In the tie-bar system, the short circuit MVA or the short circuit current is independent of the number of bus-bar sections. Thus, extra generators may be added to the system without addition of extra circuit breakers or without increasing the existing reactance.