Current Limiting Reactors & Short Circuit Currents | Electrical Engineering

A current limiting reactor, also sometimes called a series reactor, is an inductive coil having a large inductive reactance in comparison to its resistance and is used for limiting short circuit currents during fault conditions. These are installed in feeders and ties, in generator leads, and between bus sections to reduce the magnitude of short circuit currents and the effect of the resulting voltage disturbances.

Their cost is often more than offset by the saving in circuit breaker cost as a result of the lower short circuit ratings that can be used; their use is, therefore, restricted to the interconnection of large power systems.

The reactors allow free interchange of power under normal conditions but under short circuit conditions the disturbance is confined to the faulty section. As the resistance of reactors in comparison to their reactance is very small, the efficiency of the system is not affected appreciably.

Principle of Operation of Current Limiting Reactors:

It is clear that the short circuit currents are reduced by an increase of the percentage reactance in the system. 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. The circuit breakers should have enough breaking current capacity—more than the expected fault currents.

If the fault currents are beyond the capacity of the circuit breaker, the fault currents may not be interrupted. In large interconnected systems the total rating of the generators is very high and, therefore, the fault currents are very high. There is a possibility that the circuit breakers of suitable breaking capacity may not be available.

So it becomes necessary to limit the fault currents by some means so that available or existing circuit breakers can be used safely for handling them. Further, when the system is extended by the addition of more generating units, more generating stations or by a supply from the grid, the fault currents to be interrupted by the same circuit breaker will become greater than before.

It is uneconomical to scrap the existing switchgear, which was adequate for the former output but is not of sufficient capacity to meet the extensions. In such a case there is a need of providing a protective reactor between the old system and the extensions for limiting the short circuit currents to a permissible value.

By including a reactor or a few reactors at strategic locations, the short circuit currents at different points can be reduced. Hence current limiting reactors are of considerable importance in limiting the currents so that the various circuit breakers are not called upon to break currents above their rated value.

Primary Functions of Current Limiting Reactors:

The current-limiting reactors are used to perform the following functions:

1. Protective reactors are used to reduce the flow of current into a short circuit so as to protect the apparatus from excessive mechanical stresses and from overheating and thus protect the system as a whole.

2. Protective reactors are used to reduce the magnitude of voltage disturbances caused by short circuits.

3. They also localise the fault by limiting the current that flows into the fault from other healthy feeders or parts of the system, thereby avoiding the fault from spreading. This increases the chances of continuity of supply.

4. They reduce the duty imposed on switching equipment during short circuits to be within economical ratings.

So they are used:

(i) In systems where extensions have been made and the circuit-breaker rupturing capacities have become inadequate and

(ii) In large systems, so as to limit the short circuit MVA to match with the rupturing capacity of the circuit breakers.

In general, reactors should be located at points in the network where they can be most effective. Very few occasions arise where it is necessary or desirable to introduce reactance in the generator circuit as modern alternators have sufficient inherent reactance to enable them to withstand the forces of short circuit.

However, when modern machines are to be operated in parallel with older ones, a case may arise where added reactance in the circuits of the older machines will provide protection and give them roughly the same characteristics as the new machines.

Reactors installed in individual feeder circuits are not an economical proposition as often a considerable number of feeders are involved. A good case, however, can be that of a group feeder where the insertion of additional reactance is necessary to protect a group of circuit breakers of low rupturing capacity. Similarly, an interconnector between new and old sections of an installation may profitably include a reactor and thus eliminate the need of replacing old circuit-breakers.

Drawbacks of Current Limiting Reactors:

Current limiting reactors have some drawbacks too as with the introduction of the reactors, total percentage reactance of the circuit increases, thereby causing increase in reactive volt drop and decrease in power factor due to increased angle of lag. Thus regulation becomes poorer.

Reactor Ratings:

The rating of a reactor is usually expressed in terms of percentage and on a three phase system operating at 11 kV, a 20 per cent is one which will have a voltage drop of 1,270 volts across it with full-load current flowing through it.

Other ratings are given below:

i. Continuous Rated Current:

It is the rms value of current which the reactor can carry continuously, with temperature rise of current carrying parts and other parts, within specified limits [e.g., 1,000 A].

ii. Rated Short-Time Current:

It is the symmetrical rms value of fault current which the reactor can carry for specified short-time duration (e.g., 60 k A for 1 second).

iii. Rated Voltage:

It is the line-to-line service voltage for which the reactor is designed.

iv. Short-Circuit Rating:

The reactors should be capable of withstanding the mechanical and thermal stresses during short circuit at its terminals for a specified period of time.

v. Rated Over-Current Factor:

It is the ratio of rated short-time current to continuous current.

vi. Rated Through-Put kVA:

It equals √3 times the product of rated voltage and rated current in case of 3-phase reactors.

Design Features of Current Limiting Reactors:

The essential requirement of a current limiting reactor is that its reactance should not decrease on account of saturation under short circuit conditions (when a large current flows through its windings) if short circuit current has to be limited to a predetermined value.

If fault current exceeds about three times rated full-load current an iron core reactor having essentially constant permeability would require large cross section of core and therefore, the reactor will turn out to be very costly and heavy. Because of this reason, air-cored reactors are usually used to limit the short circuit or fault currents.

Alternatively an iron core with an air gap is used for current limiting reactors. Moreover, the presence of an iron core introduces eddy current and hysteresis losses which means that power consumed by an iron-cored reactor is more than the power consumed in an air-cored reactor. Normally in an iron-cored reactor, the total losses are of the order of 5% of kVA rating of the reactor.

Types of Current Limiting Reactors:

There are two types of reactors in use viz, bare type and shielded type.

In the bare or unshielded type reactors, circular coils or bars of stranded copper are embedded in a number of specially designed concrete slabs. Such an arrangement affords a very rigid mechanical support against the mechanical forces developed due to the flow of short circuit currents through the reactor windings.

The mechanical forces act in the form of a couple that tries to compress the winding axially and gives rise to radial expansion and for this reason only circular coils are used as in the case of transformers. The individual turns of the winding are inclined with respect to the horizontal plane.

The necessary insulation to earth is provided by a concrete base and porcelain post insulators which also serve the purpose of supports. Such reactors are also known as cast concrete type or dry type reactors.

Dry type reactors are usually cooled by natural ventilation and are sometimes designed with forced air and heat exchanger auxiliaries.

Dry type reactors are very simple from the constructional point of view and are robust but have the following disadvantages:

(i) Large space is required because the magnetic field on account of load current is practically unrestricted.

(ii) Difficulty is experienced in cooling of large coils by fans.

(iii) These are unsuitable for outdoor services,

(iv) Their use is limited to the voltages of 33 kV.

The shielded or oil-immersed type reactors employ insulation and cooling arrangement similar to those of the ordinary transformers. In an air-cored construction, there must be laminated-iron shields or copper shields around the outside of the conductors in order to prevent the magnetic flux entering the tank walls and causing excessive losses and heating.

In iron-cored construction air gaps are introduced in the core to prevent saturation and to give a magnetizing current of the desired value. Such current limiting reactors can be used up to any voltage level, for outdoor or indoor constructions.

Other advantages of oil-immersed current-limiting reactors are:

1. They have size smaller than the dry type air-cored reactors because of ease of cooling due to oil immersion.

2. They have higher factor of safety against flash-over.

3. They have higher thermal capacity.

4. There is no magnetic field outside the tank to cause heating or magnetic forces in adjacent reactors or metal structures during short circuit.

The air-cored type, having no iron, has a constant reactance at all currents, but the reactance of the iron-core or iron-shielded types may drop by about 10 per cent due to saturation during short circuits.

Selection of Current Limiting Reactors:

The aspects to be considered in the selection of a current limiting reactor are:

(i) Reactance in ohms or in percentage.

(ii) Normal current rating,

(iii) Short-time current rating

(iv) Rated voltage

(v) Rated through-put kVA

(vi) Type (dry or oil-immersed, air-cored or iron cored)

(vii) Number of phases (single or three)

(viii) Indoor or outdoor

(ix) Circuit characteristics (frequency, voltage etc.).