INTRODUCTION TO REACTORS:
A reactor is a coil which has large number of turns and whose ohmic resistance value is much greater. Reactors are used to limit the short circuit currents which can cause damage to the equipments of the power system. The additional reactance added in series with the system for protection, are called reactors.
A current limiting reactor is a type of reactor which limits the heavy flow of current through other sections of the system. In this way, we don’t have to shut down the whole system, we can just isolate the faulty section.
Reactors are also used to protect the circuit breakers of different ratings. They are used to limit the short circuit currents according to the capacity of circuit breakers. Therefore while doing changes in the system, we don’t have to replace the circuit breakers, instead we can add reactors and utilize the same circuit breaker, due to which, time and money, both can be saved.
Working principle of current limiting reactors:
If the reactance of a circuit during fault is X, and E voltages are given, then the short circuit current can be calculated as:
i.e, the reactance is inversely proportional to the current. If X increases, Isc decreases and vice versa. Short circuit currents depend on the generating capacity, fault point voltage and the reactance of the circuit. The figure illustrates the use of a current limiting reactor:
The rating of reactors is given in KVA and the formula for percentage reactance is:
%X= KV drop/ KV(phase voltage)
Some other uses of reactors:
- For arc suppression.
- To filter out harmonics.
- In series with low reactance auto transformers.
- In series with low reactance induction regulators.
- To protect from high voltage waves, surges and lightning.
- To control starting currents of motors.
TYPES OF REACTORS:
On the basis of construction, the reactors are of two types:
- Dry type or Air core or Open type or Unshielded type reactors.
- Oil immersed or magnetically shielded or iron core reactors.
- Air core reactors:
The reactors in which no iron core or steel core is used are called air core reactors. These reactors are only used up to 33kv.
These reactors are larger in size. Concrete slabs are arranged in the form of a circle and stranded copper coil conductors are embedded in it. These slabs provide good mechanical strength during short circuit currents.
Post insulators made of porcelain support these reactors. These are also called cast concrete type reactors. Insulated conductors are used for winding. To provide insulation between turns, glass or porcelain material is used.
These are simple, have constant current and reactance and have greater mechanical strength.
Not suitable for outdoor services, take much space due to their large size, difficult to provide cooling and can only be used up to 33kv.
- Iron core reactors:
The reactors consisting of iron core are called iron core reactors. A coil is placed inside a standard transformer tank and oil is filled for cooling and insulation purposes. Shielding is provided to prevent losses. And to prevent stray magnetic fields, the core is laminated. Shields are made in the form of short circuited rings and are earthed through end plates. The mmf produced in the rings, due to short-circuit current, keeps the flux inside the shield.
These reactors are also called oil immersed type reactors and can be used for any voltage level.
These reactors provide greater protection against short-circuit currents, have high thermal capacity, suitable for both indoor and outdoor services and can be operated at any voltage level.
They are costly, complex and difficult to repair.