Switchgear and Protection – APSEEE

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Buchholz Relay

Buchholz Relay

Buchholz relay is a protective and gas actuated relay . It is used in that transformer whose winding is  immersed in oil and rating having more than 500KVA. Buchholz relay is not used in small transformer because it increase the cost. It is used to protect transformer against internal or incipient faults only. Basically, it is used for detection of winding faults.

Buchholz relay is invented by Buchholz in 1921.

Buchholz Relay Construction

Buchholz relay is installed in the pipe connecting the conservator to the main tank. There are two mercury switches used in the relay. One of the mercury switches is attached to the upper float. This switch send signal to the alarm circuit when relay operates. Whereas the other switch is mounted on lower hinged type flap. This switch send signal to the trip circuit. A release cock is mounted top of the chamber.

Buchholz Relay


Operation or Working of Buchholz Relay

When a minor fault occurs in a transformer, heat is produced, oil gets heat up. Hydrogen gas is produce in transformer. The gases being light, try to go into conservator tank but gas is trapped in Buchholz-Relay which is connected between main tank and conservator tank. It is collected in top of the chamber while passing to the conservator tank. When the predetermined amount of gas is accumulate in the upper part of the relay, oil level falls due to gas pressure. This tilt the upper float downward and closes the alarm circuit contacts through mercury switch attached to the float arm. The alarm rings bell and gives the warning signal.

In case of series fault within the transformer, large amount of gas in evolved in the main tank. In this time, lower mercury switch of the buchholz relay operates and sends the signal to the trip circuit. This completes the trip coil circuit of the circuit breaker. Thereafter, the transformer is disconnected automatically.

A release cock is provided at the top of the relay chamber so that after operation, gas pressure is released through this cock.

‘Buchholz Relay’ indicates internal faults or incipient faults in the transformer, such as inter turns fault, insulation failure. It does not respond to external faults.


Vacuum Circuit Breakers (VCBs)

Vacuum Circuit Breakers (VCBs)

Since vacuum offers highest insulating strength, so it can use as arc quenching medium. The circuit breakers that use vacuum as a arc quenching medium, known as vacuum circuit breakers (VCBs).

Construction of Vacuum Circuit Breakers

It consists of two contact one is fixed and other is movable and these contacts placed in arc shield vacuum chamber. The movable contact is connected to the control mechanism. A glass vessel or ceramic vessel is used as the body of vacuum circuit breakers.

The arc shield is provides inside surface of the insulating cover which prevent the deterioration of the internal dielectric strength.

vacuum circuit breakers (vcbs))

The pressure below 10-3mm of mercury is considered to be high vacuum. In such a low pressure mean free path of the electrons is of the order of few metres and thus when charged particles move from one electrode to other. They do not collide with residual gas molecules.

Hence, the dielectric strength of vacuum relatively higher than other medium.


When faults occurs on any part of the system breaker operates and the moving contact separates from the fixed contact. When these contacts  separate from each other, arc is struck between them. This results, hot spots are created at the contacts surface and metal vapour shoot off constituting plasma. The amount of vapour in plasma depends on vapour emission from electrodes and arc current. The arc is extinguished in vacuum because the vapours and ions produced during arc are differed in a short time and seized by the surface of moving and fixed contacts and shields. The contacts are so designed that the temperature at one point on the contact does not reach a very high value.

Applications of vacuum circuit breakers(VCBs)

  • The use of these circuit breakers found in transformer switching, capacitor bank switching etc, where high voltage and small current has interrupted.
  • These circuit breakers use in substations.


  • These circuit breakers are light in weight.
  • These are very small in size.
  • These circuit breakers have very long life.

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Induction Type Over Current Relay

Induction Type Over Current Relay

Induction type over current relay works on the induction principle. This relay operates when current in the circuit exceeds the predetermined value. These relays are used on ac circuits only.


This relay has two electromagnets.

  • Upper magnet
  • Lower magnet

The upper magnet has three limbs and primary and secondary windings are wound on central limb. This winding is connected to the CT of the line to be protected. The tapping is taken from this winding. These tappings are connected to a plug setting bridge.

The secondary is closed winding and wound on the central limb of the upper magnet and both the limbs of the lower magnet. The secondary winding is energized by induction from primary.

The aluminum disc is placed in between the upper magnet and lower magnet. The disc is free to rotate about its axis. Spiral springs are provided on disc to get controlling torque. The spindle of disc carries a moving contact which bridges two fixed contacts when the disc rotates through a pre-set angle.

induction type over current relay

The pre-set angel can be adjusted to any value between 00 and 3600. By adjusting the angle, the relay can set for any desired value.


When current flows through primary winding, the flux is set up in primary winding. When this flux links with secondary winding, an emf is induced in it according to the laws of electromagnetic induction.

Since secondary is closed, a current flows through it.

The flux is produced by the current flowing through primary and secondary windings.

These fluxes interact each other because there is a phase difference between them, this produces a driving torque on the disc.

Driving torque on the disc opposed by controlling torque provided by spiral springs.

Under normal operating conditions, controlling or restraining torque is greater than the driving torque. Therefore aluminum disc remains stationary.

When fault occurs on the system, the value of current exceeds from pre-set value. Now, driving torque becomes greater than controlling torque.

Consequently, the disc rotates and moving contact bridges the fixed contacts and sends the signal to trip circuit.


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Valve type Lightning Arrester

Valve type Lightning Arrester

Valve type lightning arrester is extensively used on system operating at high voltage. It consists of two parts.

Series spark gaps

Non-linear resistor discs

The non-linear resistor is connected in series with the spark air gap. Both the assemblies are accommodated in tight porcelain container. It is also known as non-linear surge diverter.

The spark gap consists of a number of identical elements connected in series. Each gap consists of two electrodes with additional resistance element (grading resistor) having high ohmic value connected in parallel. Grading resistor is used to linearised the voltage distribution across the gap.

This construction is similar to that of a number of capacitors connected in series.

The spacing of the series gap is such that it will with stand the normal circuit voltage.

The non-linear resistance is usually made of silicon carbide disc. This material posses high resistance to how current and low resistance to high current. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice versa. The discs are 90mm in dia and 25mm thick. The characteristics of non-linear resistor is usually expressed as I = KVn, Where n is lie between 2 and 6 and K is constant.

Valve type Lightning ArresterValve type Lightning Arrester


Under normal conditions, when system voltage is normal, there is no affect on the spark air gap at this time, air gap remains in non-conducting state.

On the occurrence of an over voltage or voltage surge, the break down of the series spark gap takes place and the surge current start flows through the non-linear resistor to earth. Since magnitude of surge current is very large, so that non-linear resistors will offer a very low resistance path to the passage of surge. When the surge is over, the value of non-linear resistance become high and stops the flow of current through them.


  • The maintenance cost is low.
  • They do not required daily supervision.
  • The operation of this arrester is very fast.
  • The impulse ration is practically unity.
  • They provide effective protection against surges.

Disadvantages or Limitations

If moisture enters with enclosure, it affects the performance of arrester.

Applications or Uses

Power station operating on voltage upto 220KV.

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The electrical energy or electricity has become a part and parcel of our life. Electricity plays an important role of our daily’s life. The demand of electrical energy is increasing day by day. To meet this demand, the power system expands and become more complex. In this way fault level is also increased. To protect the power system against fault current some equipment are used in power system (such as Reactors). In this article we will discuss about reactors, types of reactors and locations of reactors.

What is Reactor?

Reactor is a coil of number of turns having high inductive reactance as compared to its resistance. The reactors are connected in series with the system at suitable points to limit the short circuit fault currents. Iron core and air cored reactors are used. Air cored reactors are normally of two types (i) Oil immersed type (ii) Dry type.

Location of Reactors

The reactors are connected at suitable points in power system, short circuit current limiting.  Reactors may be connected in following locations.

  • Series with each generator called Generator Reactor.
  • Series with each feeder called Feeder Reactor.
  • In Bus-Bar.
  1. Generator Reactor

When the reactors are connected in series with each generator, such type of connections is known as generator reactor.

Since in modern alternators, the reactance may as high as 2.0 P.V. So there is no need to connect reactors in series with generator.generator reactor

However, if some old machines are being used along with the modern alternators, these old machines need the reactors for limiting the short circuit current.


  • There is a constant voltage drop during normal operation.
  • It protects only generator, if fault occurs at feeder or bus-bar, it may affects the performance.
  1. Feeder Reactors

When the reactors are connected in series with each feeder, such type of arrangement are called feeder reactors.

Mostly faults are occurs in feeder, so there are many reactors are required in this circuits.

Per unit value of reactance of a feeder based on its rating may be small but we compared with the rating of whole power system. Its value becomes quite large and hence a small reactor will be required to limit the short circuit current.

feeder reactors


  • There is a constant voltage drop and constant power loss in the reactors even during normal operation.
  • If a short circuit occurs at the bus-bar, there is no protection is provided to the generator.

Bus-Bar Reactors

The generator reactors and bus-bar reactor suffer from disadvantages such as constant voltage drop and constant power loss. To overcome this disadvantages the reactor are locating in bus-bars. There are two methods of interconnecting the bus-bar through the rectors are namely

  • Ring Main System
  • Tie Bar System.

In ring main system, the bus-bar divided in two sections. This results in low power loss in the reactors.

In tie bar system, there are effectively two reactors in series between sections.





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Electrical Faults in Overhead Transmission Lines

Electrical Faults in Overhead Transmission Lines

The occurrence of faults  in overhead transmission line is common thing. In this article, we will discuss about, electrical faults in overhead transmission lines, types of electrical faults and reasons of electrical faults.

What is an electrical Fault?               

An electrical fault is any defect in the electrical circuit due to which electric current is diverted from the intented path.

Short Circuit Fault

Short circuit is a fault in which the value of current reaches upto infinity from its normal value. This type of faults results in damaging of power system equipment such as switchgears, transformers etc. If some protective steps are not taken with in time.

Open Circuit Fault

When the path of flow of current breaks, such type of fault is called open circuit faults. This fault is less dangerous than short circuit fault.

In three phase systems, a fault may involve one or more than one phases and ground. In case of earth fault current flows into earth.

In power systems, relays can detect fault and sent a signal to the circuit breakers.

Types of Electrical Faults

The faults are divided into two types.

  • Symmetrical fault.
  • Unsymmetrical fault.

Symmetrical Faults

In Symmetrical faults, when fault occurs in system, same fault current flows in all phases or three phase. In transmission line faults, 5% are symmetrical faults. The symmetrical faults are divided as follows.

All the Three Phase Short Circuited (L-L-L)

This may occur when all line conductors are short circuited due to failure of insulation.

electrical faults in overhead transmission lines

All Three Phases to Earth (L-L-L-G)

This may occur when insulators of all the three line conductors short circuited and fall into earth.

transmission line

Unsymmetrical Faults or Asymmetrical Faults

In case of unsymmetrical faults, different current flows in different phase when fault occurs. 95% faults are unsymmetrical or Asymmetrical type. These faults are divided as follows.

Single Line to Ground (L-G)

This may occur when insulation of one of the line conductor breaks and falls on the earth or ground wire.


Phase to Phase (L-L)

This may occur when one of the line conductor breaks and falls on the other line conductor.

Electrical Faults in Transmission Line

Phase to Phase and Third Phase to Earth (L-L-G)

This may occur when two line conductors are short circuited and third line conductor breaks and falls on the ground or ground wire.

Electrical Faults in Transmission Line 3

Two Phases to Earth

This may occur when insulation between two line conductors fail or when two line conductors break and fall on the ground or ground wire.

Electrical Faults in Transmission Line overhead

Reasons of Faults

  • Insulation failure or breakdown between line conductors.
  • Lightning surge.
  • Over voltage.
  • Voltage drop.
  • Unbalance current.
  • Mechanical fault in transmission lines.
  • Reversal of power.
  • Under frequency.





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Horn Gap Lightning Arrester

Horn Gap Lightning Arrester

Horn gap lightning arrester was the earliest types of lightning arrester. It is consisting of two metal rods in the shape of horn and separated by a small air gap. It is connected in shunt between each conductor and earth. The distance between two rods depends upon the supply voltage. The horns are so constructed that distance between two rods gradually increase towards the top. The horns are mounted on the insulators.

horn gap lightning arrester

One end of the horn is connected to the line through a resistance R and choke L. The resistance R helps to limit the current to a small value in the event of occurrence of voltage surge. The choke L offers small reactance at normal power frequency but a very high reactance at transient frequency.

The gap between the two rods is so set that arc may occur between gap at a voltage nearly 105 to 2 times to that of normal voltage.


Under normal conditions, there are no conduction occurs between gap. When over voltage surge occurs, spark-over takes place across the small gap. The heated air around the arc and electro-magnetic action will rise up the horn and extinguish itself. The time taken for the complete operation as usually from 3 to 5 seconds. The excess charge on the line is thus conducted through the arrester to the ground.

Advantages of Horn Gap Lightning Arrester

  • In case rod gap arrester, arc may continue even when the system attains normal frequency. But in this arrester has no such problem.
  • Series resistance helps in limiting the follow current to a small value.


  • The gap can be bridged by some external agency such as birds.
  • At different frequencies, the gap breaks at different voltages.
  • The setting of horn gap is likely to change due to corrosion. This adversely affects the performance of the lightning arrester.
  • Breakdown voltage value is affected by atmospheric conditions.

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Rod Gap Lightning Arrester

Rod Gap Lightning Arrester

Rod Gap lightning arrester is the simplest form of Lightning Arrester. It is consisting of two (1.5cm) rods which are bent almost at right angles and placed across the transformer bushing or across the string of suspension type insulator in overhead transmission system. One rod is connected to the line circuit and other rod is connected to earth. The distance between the rod gap and insulator should be more than one third of the rod length. So that the arc may not reach the insulator and damage it.

The rod gap length depends upon the operating voltage of the system. As the voltage is varied, the distance between two rods is adjusted.

Rod Gap Lightning Arrester

The gap length is so adjusted that breakdown should occur at 80% of impulse sparkover voltage in order to avoid cascading across the insulator surface of very steep fronted waves.

Under normal operating conditions, the gap remains in non conducting state. When high voltage surge occurs on the line, the gap sparks over and the surge current flows from line the earth.

Advantage of Rod Gap Lightning Arrester

  • It is cheap in cost.
  • It can be easy adjusted on site.

Disadvantage or Limitations of Rod Gap Lightning Arrester

  • When the high voltage surge occurs, the arc is established in rod gap, after the surge is over, arc in the gap continue even when the system arraigns its normal supply voltage.

When an arc is struck between the two rods or rod gap, a large amount of heat is produced. This may result the rods get damaged due to excessive heat produced by the arc.

  • The atmospheric conditions badly affect the performance of the arrester.

Uses of Rod Gap Lightning Arrester

This type of lightning arrester has many limitations and cannot be relied upon as main protection in high voltage system. So it is only used as a second line defence or back-up protection of the system.


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