Principles Of Current Limiting Reactors

INTRODUCTION

In electrical Engineering, a current limiting reactor is an inductive coil having a large inductive reactance in comparison to their resistance and are used to reduce short circuit current which result from plant expansions and power source additions to a level that can be adequately handled by existing distribution systems.
Reactors are used to limit the short circuit current which can lead the damage of the power system equipment.

PRINCIPLES OF CURRENT LIMITING REACTORS
The inductive reactance is chosen to be low enough for an acceptable voltage drop during normal operation but high enough to restrict a short circuit to the rating of the switch gear.

If the resistance of a circuit during fault is 'X' and 'E' voltages are given, then the short circuit current can be calculated as; Isc =E/X, i.e , the reactant is inversely proportional to the current. If 'X' increases, Isc decreases and vice-versa.

Short circuit current depend on the generating capacity, fault point voltage and the reactance of the circuit.

The rating of the reactors are given in KVA and the formula for percentage reactance is

%X = KVdrop / KV(phase voltage).

PRIMARY FUNCTIONS OF CURRENT LIMITING REACTORS

The primary functions of current limiting reactors are;

• 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 over heating.
• To localise the faults by limiting the current that flows into the fault from other healthy feeders or part of the system.

• To reduce the duty imposed on switching equipment during short circuits.

REACTOR RATING

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

Other ratings include;
• Continuous Rated Current
It is the r.m.s. value of current which the reactor can carry continuously with the temperature rise of current carrying parts within specified limits. (e.g 100A).

• Rated Short-Time Current
It is the symmetrical r.m.s value of fault current which the reactor can carry for specified short time duration (e.g, 60KVA for 1second).

• Rated Voltage
This is the line to line service voltage to which the reactor is designed.

• Short Circuit Rating
This refers to the amount of mechanical and thermal stress during short circuit conditions the reactor can withstand for a specified period of time.

So here's a little exercise to try yourself.

1. The figure below shows a power system where load at bus 5 is fed by generators at bus 1 and bus 4. The generators are rated at 150MVA, 11KV with sub-transient  reactance of 25%. The transformers are rated each at 150MVA, 11/112KV and have a leakage reactance of 8%. The lines have an inductance of 1mH/phase/km. Line L1 is 100Km long while lines L2 and L3 are each of 50Km in length. Find the fault current and MVA for a 3-phase fault at bus 5. fig. 1. Please comment below if you need a detailed solution to the exercise.

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