Protection of Reactors

Unit iiProtection of reactors ,boosters and capacitorProtection of ReactorsINTRODUCTION 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

Generator Reactors When reactor is connected between bus bar and generator, it is called a generator reactor. This reactor can also be connected in series with the generator. When a new generator is connected with an old generator, a reactor is added in series with the old generator to provide protection. The value of this reactor depends on the impedance of that generator. Its pu value should be 0.05 or 0.06Placement of reactors in power system

4Feeder reactors:

It is when a reactor is connected in series with a feeder Usually short circuits occur on feeders therefore, feeder reactors are very important. In the absence of feeder reactors, if a fault occurs on the nearest generating station, the bus bar voltage will be reduced to zero and the connected generators will lose their synchronismAdvantages: The voltage drop across a reactor during faulty conditions will not affect the voltage of bus bar, therefore, there are less chances of losing synchronism. A fault on a feeder will not affect any other feeder.Disadvantages: Every feeder needs a reactor hence the number or reactors increases.If the number of generators increases, then the size of the reactor should also be increased. During normal operation, full load current passes through the reactor which causes continuous power loss. Reactors should be connected according to the power factor of the feeders to regulate proper voltages. Feeder reactance should be about 0.05 to 0.12 pu.Bus bar reactorsThese reactors are connected with bus bars. Bus bar reactors divide the bus bar in smaller sections. If the voltage level is same, no current passes through these reactors and every section act as an independent bus bar.If a fault occurs on a section of bus bar, the reactor prevents the fault from reaching to other sections and only the fault section is affected. Hence a bus bar should be large enough to protect the system but it should not disturb the synchronism of the system. A reactor which drops the voltage about 30 to 50% at full current is suitable. However the reactance of a sinlge bus bar reactor should be about 0.3 to 0.5 pu.Ring Main system

In this system, a bus bar is divided into different sections and these sections are connected together through a reactor. Each feeder is fed by a separate generator and during normal operation each generator supplies power to its respective load due to which very less power loss occurs in the reactors.Tie bar system

In this system, the generators are connected to a common bus bar through the reactors and feeders are fed through the generator side of the reactors. This system is very efficient in case of larger systems. The reactance of the reactors in this case is half as compared to the ring system reactanceShunt reactors:

These reactors are connected between phases or between a phase and a neutral. Reactors are added in order to compensate the effect of line capacitance in long transmission lines.Neutral Reactors

These reactors are used to earth the neutral terminal of a system. They provide a passage for the current in case of a single phase to ground fault. Peterson coil is also a neutral reactor. In normal conditions, no current passes through this reactor. These reactors are used below 70kV.Reactors in power systemREACTORS Series REACTOR Shunt REACTOR PURPOSE To reduce short circuit current PURPOSE To reduce over voltage ZZActs as inductive load and reduce high voltage by absorbing MVAR.Acts as inductance and opposes the flow of short circuit current. AREA OF APPLICATION Tie Lines AREA OF APPLICATION Bus , Lines, Tertiary winging of ICT Classification of shunt reactorsShunt Reactors Dry Type (system voltage Below 72.5 KV) Oil immersed Type (system voltage 72.5 KV & above) Core less Gapped Core Air Core Star connected with neutral grounding Range 30 to 300 MVAR Connected at the terminals of transmission line Delta connected Range below 30 MVAR Connected at the tertiary winding of transformer Switch on / off type Permanently connected type with thyristor controlled Current-limiting reactor

This reactor is series connected to the transmission line or to the feeder for limiting the current under system fault conditions to levels compatible with the protection equipment of the circuit. It is a very cost effective solution, as it eliminates the need for upgrading the entire switching and protection system when the short-circuit power of the system is increased.This reactor is designed to offer specified impedance and to withstand the rated and fault (short-time) currents during a specified period of time.Current-limiting reactor seismic design

Working principle of current limiting reactorsIf the reactance of a circuit during fault is X, and E voltages are given, then the short circuit current can be calculated as:Isc= E/Xi.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)

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.Dry type or Air core or Open type or Unshielded type reactorsThe 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.

Advantages:These are simple, have constant current and reactance and have greater mechanical strength.Disadvantages: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.Advantages: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.Disadvantages:They are costly, complex and difficult to repairBOOSTER IN PROTECTION

Three phase booster transformer

Protection of capacitor banksShunt capacitor banks (SCBs) are widely used in transmission and distribution networks to produce reactive power support.increased power factor, reduced losses, improved systemcapacity and better voltage level at load points.Types of Protection arrangementsElement Fuse.Unit fuse. Bank Protection.Bank Protection schemes