A current limiting reactor is a coil which has a large inductive reactance in comparison with its ohmic resistance. It consists of a few turns of heavy copper strip, sometimes cast directly in concrete so that it can withstand large stress developed by the fault currents.
Generally, the reactance of the power system under fault conditions is low. In this condition, if a fault occurs, fault currents may rise to a dangerously high value. If we do not use the current limiting reactors to limit the amount of current, then this current will excessively overload the circuit breakers and also can damage to lines and other equipment.
A current limiting reactor performs the following functions:
- To limit the short circuit current in case of fault to protect the apparatus from excessive thermal and dynamic stress.
- To confine the effects of short circuits to the faulty sections.
- To reduce the magnitude of voltage disturbances caused by faults.
- To reduce the current up to a level so that we can handle it safely and economically.
All modern alternators have high inherent reactance to prevent the need for reactors. So we use the reactors for interconnection of large power systems only. The reactors allow free interchange of power under normal conditions, but under short circuit conditions, the disturbance is largely confined to the faulty section. Since the resistance of reactors is very small, so they have a negligible effect on the efficiency of the system.
Types of Current Limiting Reactors
There are two types of current limiting reactors. These are:
- Open Type
- Oil-immersed Type
Open Type: This type of reactors consist of bare stranded copper and are embedded in a number of specially shaped concrete slabs. The complete system is placed on a concrete box and mounted on porcelain footstep insulators.
Tough such type of reactors are very simple in construction and robust also yet they have the following disadvantages:
- It requires large space due to the magnetic field by load current, which is unrestricted.
- The coils used in this reactor are large, so it is very difficult to cool them by using fans.
- These are not useful for outdoor service.
- They can be used only up to 33kv.
Oil Immersed type: Oil-immersed reactors are used for the voltages above 33 kV. The insulation and cooling arrangements of these reactors are similar to ordinary transformers. There is a provision of iron shields or copper shields around the conductors to prevent the entering of magnetic flux into the tank wall which may cause excessive losses and heating. In these reactors, air gaps are kept to avoid saturation and to a magnetizing current of the desired value.
Location of Current Limiting Reactors
Reactors are connected
- in series with generators known as generator reactors.
- in series with feeders known as feeder reactors.
- in series with bus bars (in ring system and tie-bar system)
When reactors are connected in series with each generator, they are known as generator reactors. It protects each generator separately.
The disadvantage of this system is that if a fault occurs on any feeder, the voltage of common bus bar may drop to a very low value and the synchronized machines working on the bus bar may stop. It becomes difficult to trace out the fault, and the whole system is interrupted.
The other disadvantage is that even in normal operation, the full load current flows through the reactors, which results in voltage drop and constant power loss.
Since all modern alternators have high inherent reactance, so there is no need for external rector for these alternators. Hence this practice has become obsolete now.
When the reactors are inserted in series with the feeders, the reactors are known as feeder reactors. As we know that the short-circuits occur on feeders mostly. In this case, if a fault occurs on any feeder, the voltage drop in its reactor will not affect the bus-bars voltage so that there are very low chances to lose synchronism of the generators. Also, the fault on a feeder will not affect other feeders
Disadvantages: Even in normal operation, there is a constant voltage drop and power loss in the reactor.
Bus Bar Reactors
The above two methods have the disadvantage of considerable voltage drop and power loss in the reactors even during normal operation. We can overcome this disadvantage by installing the reactors in the bus-bars by following two methods:
- Ring system and
- Tie-Bar system.
Bus-bar Reactors (Ring System): In this scheme, bus-bar is divided into sections, and these sections are connected through reactors. Generally, one generator feeds only one feeder. Under normal operation, each generator will feed its feeder and will supply very little power to other feeders. So the voltage drop and power loss become very low due to a small amount of power flow through the reactors.
If any fault occurs on any one feeder, only one generator feeds the fault whereas the current from other generators is limited due to the presence of bus bar reactors. Therefore, when a fault occurs the healthy sections of bus-bar remain unaffected.
Bus-bar Reactor (Tie-bar system): It is clear from the Figure that there are two reactors in series between sections effectively. So the reactors of half the rating of those used in a comparable ring system are required.
Another advantage of this system is that we can connect additional generators to the system without changing the existing reactors. However, this system becomes costlier due to the requirement of an additional bus-bar, i.e. the tie-bar.
The maintenance of current limiting reactors may be carried out as per the following guidelines under the maintenance schedule:
Maintenance of Air-cooled Reactors
1. Check for rusted points.
2. Check for moisture effect.
3. Check for loose fittings or connections.
4. Blow out the dust particles with compressed air and clean with dry cloth.
5. Check for damaged supports.
6. All the parts may be checked for mechanical damage.
7. Check air passages for obstruction which limits the free circulation of air.
In addition to above mentioned points for checking under half-yearly maintenance, the following checking may be carried out:
I. The Insulation resistance between conductors and conductors to earth may be checked.
2. The filling of the compound in cable boxes may be checked.
3. Varnish coating of concrete columns should be checked and fully renewed if needed.
Maintenance of Oil-cooled Reactors.
1. Blow out the dust particles with compressed air and clean with a dry cloth.
2. Check for rusted parts, and rust must be avoided.
3. Any mechanical damage may be checked and rectified.
4. Moisture contents may be checked, and parts may be dried out.
5. Oil passage for its obstruction should be checked which should be removed for the free circulation of oil.
6. Supports for any damage or rust should be checked. It should be replaced, or rust should be removed.
7. Loose connections or fittings should be avoided.
8. Bushings for cleanliness and cracks etc. should be checked. They should be cleaned or replaced if necessary.
9 Leakage of oil, oil level or condition of oil should be checked and removed if required.
In addition to the steps mentioned above under half yearly maintenance for checking, the following steps must also be considered:
1. Insulation resistance between conductors and conductors to earth should be checked.
2. D.C. resistance of the winding should be checked.
3. The filling of the compound in the cable boxes should be checked. After checking the above mentioned steps, the faults may be rectified accordingly.
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