A current limiting reactor is an inductive coil having a large value of inductive reactance and is put in service for limiting short-circuit currents during a fault condition.
We install a current limiting reactor in feeders and ties, in generator leads, and between the bus sections to reduce the magnitude of short-circuit currents and also to reduce the effect of resulting voltage disturbances.
Under normal conditions, the current limiting reactor will allow free interchange of power. The resistance of the current limiting reactor is very small in comparison to its inductive reactance so it does not affect the efficiency of the system.
Why Use Current Limiting Reactor?
The current limiting reactor is of considerable importance in limiting the short-circuit current during the fault conditions.
- The short-circuit current of power systems, networks generally depends upon the generating capacity, the voltage at the fault point, and the total reactance between the fault point and the generators. There may be situations when the fault currents or the short-circuit currents are way beyond the capacity of the circuit breakers present in the system. In this case, the fault current may not be interrupted.
- In large interconnected systems, the fault current may be very high due to the large ratings of generators present in the system. Here, we may not obtain the circuit breakers with suitable breaking capacity. In such a case, we have to limit the fault current to some lower levels.
- The expansion of the system may result in the introduction of higher rating generators thereby increasing the fault level of the system. Under such conditions, it is sometimes uneconomical to replace the existing circuit breakers with higher rating breakers. So the introduction of current limiting reactors can reduce the fault level to significant levels such that the old breakers can perform their function efficiently.
Primary Functions of Current Limiting Reactor
Following are the primary functions of the current limiting reactor:
- Current limiting reactor help to reduce the flow of current in short-circuit to protect the apparatus from mechanical stress, overheating due to faults and thereby protects the whole system.
- The short-circuit causes disturbance in the voltage and the current limiting reactor helps to minimize the magnitude of the disturbance voltage.
- The current limiting reactor helps in localizing the faults by limiting the current flowing into the faulty section from other healthy sections of the system. This will avoid the fault from spreading in the system and thereby increases the chance of continuity of supply.
- The current limiting reactor will reduce the required rating of the switching equipment (such as circuit breakers).
The disadvantage of Current Limiting Reactor
The current limiting reactor will degrade the regulation of the system. As the introduction of the reactor will increase the percentage reactance of the system thereby increasing the reactive volt drop. The power factor also decreases due to an increase in the angle of lag between current and voltage.
Current Limiting Reactor Rating
The current limiting reactor rating is expressed in terms of percentage.
For an 11 kV three-phase system, a 10 percent rector rating means it will have a voltage drop of 1100 volts across it when a full load current flows through it.
Other ratings of the current limiting reactor are as follows:
Short-time Current Rating
The symmetrical RMS value of the fault current which the current limiting reactor can carry for a specified short-time duration. Such as 50 kA for 2 seconds.
The line-to-line voltage for which the current limiting reactor is designed to operate.
The fault current rating for which the current limiting reactor is capable of withstanding the mechanical and thermal stresses during a short circuit at its terminal for a designated amount of time.
Overcurrent Rating Factor
This factor is a ratio of short-time rating current to the continuous current of the reactor.
Rated Through-Put kVA
This rating of the current limiting reactor is equal to the root three times of the product of rated voltage and rated current in the case of three-phase reactors.
Design Features of Current Limiting Reactor
When limiting a short-circuit current, the reactance of the current limiting reactor should not decrease on account of saturation due to the flow of heavy current through its windings.
An air-cored current limiting reactor is generally put into application. As iron core reactors are costly and heavy and also introduces eddy current and hysteresis losses.
Types of Current Limiting Reactor
The types of current limiting reactor are as follows:
Bare Type Reactor
It consists of circular coils or bars of stranded coils installed in several specially designed concrete slabs. This arrangement of reactor provides a very rigid mechanical support against the forces which are developed during the time of faults.
A concrete base and porcelain post insulators provide the necessary insulation with the earth. This reactor is also known as dry type or concrete reactor.
Cooling is done in these current limiting reactors by natural ventilation. Sometimes forced air and heat exchanger are also employed. For large coils, we will experience difficulty in cooling with help of fans.
For outdoor services, these reactors are unsuitable. We can use them for voltage levels up to 33 kV.
Shielded or Oil-immersed Type
It consists of a cooling and insulation arrangement similar to that of an ordinary transformer. These reactors can be put into application for any voltage levels and outdoor and indoor applications.
They have a smaller size in comparison to the dry-type reactors. It provides higher safety against flash-over and higher thermal capacity.
This type of reactor has an air-cored or iron-cored construction. The iron-cored type has a saturation problem during short circuits.
Selection of Reactor
We have to consider the following aspects while selecting a current limiting reactor.
- Reactance in ohms or percentage
- Current rating
- Short-time current rating
- Voltage rating
- Rated through-put kVA
- Type of reactor
- Number of phases
- Indoor or outdoor
- Circuit characteristics such as frequency.
Location of Current Limiting Reactor
The current limiting reactor in the power system can be located along with generators, feeders, and in the bus bar which is briefly explained below.
Here we insert a current limiting reactor between the generator and the generator bus. The figure below shows the generator reactors. These reactors protect the machines or the generators individually.
Modern machines do not require the installation of reactors as their transient reactance is sufficient to protect them against a three-phase short circuit at its terminal.
With the installation of the new machine in the power system, we have to connect generator reactors for the old machines or generators. The magnitude of these reactors is 5 % or 0.05 per unit.
The flow of full load current through these reactors during the normal operation causes a constant voltage drop and power loss.
During short circuit conditions, if a short circuit occurs at any feeder, then the voltage at the generator bus may drop to such a low level. This causes the generator to fall out of step and thereby adversely affecting the whole system. This is the main disadvantage of this reactor.
We connect the feeder reactors in series with the feeder. The figure below shows the feeder reactors.
During fault in any one of the feeders, the main voltage drop is in the reactor only thereby not affecting the bus bar considerably. Therefore other machines in the system can supply power to load continuously.
When a fault occurs in any one of the feeders, the fault is localized such that other feeders are not affected.
This reactor also introduces a constant voltage drop and power loss in the system.
Both the generator and feeder current limiting reactors introduces a constant voltage drop and power loss in the system. We can eliminate this problem by introducing reactors in the bus bar.
Current Limiting Reactor for Ring System
Here, we make use of bus-bar reactors to tie together the separate bus sections. Here sections are formed of the generators and the feeders and are connected by a common bus bar as shown in the figure below.
Bus-bar reactors facilitate the parallel operation of large systems and are extensively put into operation.
Current Limiting Reactor for Tie-Bar System
This is a modified configuration of the above system. This configuration of the current limiting reactor will improve the voltage regulation between the feeder sections.
This system is suitable for the systems where generating systems are frequently added to the systems.
In this system, the generator supplies power to the feeder directly and we connect generators to the common bus bar through the reactor.
The additional advantage is that, with the increase in the number of sections, the fault current does not exceed a certain value. The size of an individual reactor fixes this value. Hence, it requires no modifications to the existing switchgear.