Low Resistance grounding involves connecting a low resistance in series with the grounded neutral of the three phase system. Low Resistance grounding limits permits a fault current above 10A but limits it to around 50A

Low resistance Grounding is used in situations where quick operation of ground fault relay is required. This form of earthing is used when faults need to be cleared quickly.

Low resistance grounding resistors have a time rating beyond which they may not be able to maintain thermal stability due to the heat generated by the fault currents.

Low resistance grounding ensures that equipment and conductors are not exposed to the electric and mechanical stresses during an earth fault.

However, the downside of low resistance grounding is that the system needs to be de-energized after a ground fault.

These are usually used in medium and high voltage Systems

Resistance Grounding

In Resistance Grounding, the three phase power system is grounded through a series resistance.  This resistance is intended to limit the fault current when there is an earth fault.  Resistance grounded systems are ideal as they enable quick identification of a fault and clearance.  The series resistors used to limit current are designed for thermal stability during fault conditions.  The resistors also have a time rating.  They are designed to be in circuit for a particular period of time till the fault is cleared.  Resistance grounded systems can be classified into

  • High Resistance Grounding and
  • Low Resistance Grounding

High Resistance Grounding restricts the ground fault current to less than 10A.  These systems are advantageous because the system can continue to run when there is a fault between a phase and the earth.  This ensures the system reliability and the system continues to run while the fault can be identified and rectified.  However, care must be taken to ensure that the permitted ground fault current is greater than the charging current of the line capacitances.

This is essential to ensure that there are not transient overvoltages during intermittent earth faults.  The series resistors used in High resistance grounding are designed for longer time rating as they may have to be in circuit as long as the system is running with the fault still present.

High Resistance grounding Systems are not permitted in systems which feed single phase loads.

Modern High Resistance Grounding Systems are equipped with a pulser circuit which is activated when a ground fault is detected.  This pulser circuits generates a pulsating current which can be used to identify the exact location of the ground fault with a handheld device.  This is extremely useful in identifying the fault within a short period and restoring the system.

See Also : 

Types of Earthing

A proper grounding scheme is vital component of any power system. Improperly grounded systems can result in equipment failures, overvoltages, and flashovers.  Grounding uses the earth as a return conductor in the event of a fault.  This helps to identify the fault.  Resistances can be used to limit the fault current to desired levels.  Grounding ensures system stability and prompt identification and clearing of faults.

In three phase systems, the neutral of the Star Point is usually grounded. In the case of delta connected systems, a special grounding arrangement such as Earthing Transformers or Zig-zag transformers are used.

On the basis of the grounding used, Power Systems can be classified into
  • Ungrounded Systems
  • Solidly Grounded Systems
  • Low Resistance Grounded Systems
  • High Resistance Grounded Systems

Ungrounded Systems

Ungrounded Systems can function normally in the healthy condition.  In the fault condition, as one phase gets earthed, the voltage between the other two phases and the ground increases to the line voltage(phase to phase voltage).  This places the insulation of the equipment connected to the system under excessive electrostatic stress.  Ungrounded systems are the most expensive for this reason.

Electric Equipment connected to ungrounded systems need to have insulation rated for the line voltage.  In the event of a fault on one phase, the fault current is fed by the capacitance charging current flowing the other two un-faulted phases.

This current is usually less and power can continue to flow in the other two phases.  However, if the fault is intermittent and the contact with the ground is of the make-break type.  The capacitances which form in the other two phases may charge and discharge into the system causing high overvoltages, sometimes 5 to 7 times the normal voltage.  This can cause extensive damage to other devices connected elsewhere in the system. 

While the ungrounded system can run with the other two phases even when one phase is faulty, a fault in any of the other two phases can cause a phase-to-phase short circuit via the ground.

See Also

Earthing in Three Phase Systems          Part II

Three phase High Voltage electric systems connected to transformers or generators that are solidly grounded may experience transient overvoltages during fault conditions due to the line capacitances getting charged and discharged.

Besides, the intensity of a ground fault will be greater and will be accompanied by a flashover. This may be dangerous to personnel who are in the vicinity.

The damage to the equipment is also extensive as a higher current flows in solidly grounded system. The conductors carrying the current are subjected to extensive electrical and mechanical stresses.

The ground current which flows through the soil can pose a danger to people if the step potential exceeds the safe limits.

Related Posts:

Neutral Grounding Resistors

Grounding Resistances - An introduction

Grounding Resistors are used to limit the fault current in Transformers and Alternators. When a phase to ground fault occurs, the fault current is limited only by the soil resistance. This current, which can be very high, can damage the windings.

Grounding resistances can be classified into high and low resistances.

In high resistance grounding, the fault current is limited to less than 10 amperes. While, in low resistance grounding, the current is limited to a value from 25 amperes or more.

The resistances are also categorized on the basis of time they can withstand the fault current. Typical durations are 1 second, 10 second, one minute and 10 minute rating.

The Extended Time rating resistor is used in systems where the reliability of the system is critical. This is true in petroleum industries, mines etc. In these situations, a high resistance which can sustain the fault for a long period is used. When an earth fault occurs of one phase, an alarm is generated. However, the system continues to run until the next scheduled shutdown.

Resistance grounding is not used in systems where the phase voltage exceeds 15kV for cost reasons.

Related Posts:

Disadvantages of Solid Grounded Systems

Grounding Resistances - An introduction

Shielding in Power Cables involves covering the insulation of a single cable or a group of cables by a conducting material usually a sheath which is grounded.

The objective of a shield is to ensure that the cable insulation is subjected to a uniform electric stress. The shielding also prevents transient overvoltages which are induced along the cable by ensuring a eliminating surge potentials and ensuring a uniform surge impedance.

Shielding also protects personnel from dangerous shocks which may be caused by intense electric fields.

The Energy Star Classification is a system of classification introduced by the United States Environmental Protection Agency in 1992. The Program aims at reducing energy consumption by developing more efficient electric gadgets and devices.

The program has been adopted by other countries such as New Zealand, Japan, Taiwan, the European Union and Australia.

The labelling program is voluntary. Devices which bear the Energy Star Logo consume about 20-30% less energy that normal ones.

The Energy Star labelling System is now used in numerous electric appliances such as heaters, televisions, computer monitors, lighting and even LED traffic lights.

Energy Star certified refrigerators use about 20% less power than the minimum standard. Fluorescent lighting which is standardized by Energy Star use about 75% less energy than conventional incandescent lighting.

The Energy Star standard is also being extended to commercial building which use less energy and to industrial facilities.

A Zero Watts Bulb is a bulb with low light output.  It is frequently used as a night lamp and other applications requiring low intensity lights such as.  It is called a zero watts bulb as it consumes relatively low power, around 15 W.

When all the appliances were switched off at night and only the zero watts bulb was switched on, the power consumed in the house was too small to be measured by the older electro-magnetic meters.  Hence, the moniker "zero watts".

Zero watts bulbs are increasingly being replaced by more efficient CFL bulbs which are also last longer.

Triplen Harmonics are harmonics whose orders are multiples of three.  Thus the 3rd, 6th and 9th Harmonics are known as Triplen Harmonics.  These harmonics flow through the neutral conductor of Star connected transformers, causing heating.  This heating also affects cables and causes nuisance trippings of relays.

These are usually caused by power supply units for electronic appliances.  The power supply units are usually connected to only one phase.

Triplen Harmonics can be avoided by use of the delta connection for transformers and three phase loads.  Increasing the sizing of the neutral conductor is also a means of attenuating the effects of these harmonics.

The commercial Efficiency of a transformer is given by the ratio of output power to input power

Efficiency = output power in watts/input power in watts

The losses in the transformer can be classified into copper losses and iron losses. The copper losses (hysteresis and Eddy Current Losses) are independent of the load. The iron losses though are dependent on the load.

In the case of the distribution transformers, the load is continually varying. It is low in the day time and high in the evenings and night. Therefore, efficiency measured at any one point of the day would not be an accurate reflection of the transformer's capability.

Hence, We have the all day efficiency measurement of the distribution transformers

The formula for the all day efficiency of the distribution transformers is

Efficiency = Output in kWh for 24 hours/ Output in kWh for 24 hours

The All day Efficiency is always lesser than the commercial efficiency of the transformer.

Ferrite beads are series inductors which are added to electronic circuits to filter high frequency noise. The core of these inductors is made of ferrite, a ceramic magnetic materials.

The Ferrite bead offers a high impedance to High frequency signals, thus stopping them from moving further up the circuit. These signals are attenuated as heat. This makes them useful in electronic equipment to filter EMI(Electromagnetic Noise)

Common sources of Electromagnetic noise are power cables in close proximity, earthing connections,

Ferrite Beads can be found in common electronic data cables and in cellphone cables. Ferrite beads can also be connected to cellphone headphones to block high frequency signals from reaching the earphones.


In Electric Machines such as Motors and Generators, the shaft tends to maintain a distinct axial position when running. This position may be different from the position of the shaft at rest and, in the case of motors, when the machine is running with a coupled load.

The Magnetic center is caused due to the magnetic forces between the rotor and stator attract each other. These magnetic forces tend to ensure that the gap between the stator and the rotor is as small as possible. Hence, if the axial position of the machine at rest(mechanical center) is different from the magnetic center, the rotor of the machine may slightly move axially to the magnetic center when running without load.

Causes of shift of the magnetic center from the mechanical center.

There are many causes for the shift of the rotor axially when running. Some of them are

Effect of the cooling fan when running (air flow)
Different in the core stack length of the motor causes magnetic forces in the drive-end and the non-drive end to be unequal. These forces tend to balance each other by shifting the rotor axially.

In most machines, the magnetic center is indicated by an external indicator which is fitted on the stator and which points to a groove on the rotor. Correct positioning of this indicator ensures proper magnetic centering

Consequences of wrong Magnetic centering.

If the magnetic center is not set properly when the machine is reassembled after any maintenance work, the rotor may tend to shift beyond the axial limits permitted by the bearings. This is particularly true for sleeve bearings. This may cause the rotor to rub against the thrust collars of the bearing.

Adjusting the Magnetic Center

The Magnetic Center can be brought to the indicated position when the machine is at rest by either moving the bearings of by moving the stator depending on the provision of the manufacturer.

The three phase power system has been adopted universally for transmission of AC power.

The advantages of a three phase system over a single phase system are:-

Higher power/weight ratio of alternators. A three phase alternator is smaller and lighter that a single phase alternator of the same power output. Hence, it is also cheaper.

A three phase transmission system requires less copper or aluminium to transmit the same quantity of power of a specific distance than a single phase system.

Three phase motors are self-starting due to the rotating magnetic field induced by the three phases. On the other hand, a single phase motor is not self starting, it requires a capacitor and an auxilliary winding.

In Single phase systems, the instanteous power(power delivered at any instant) is not constant and is sinusoidal. This results in vibrations in single phase motors.
In a three phase power system, though, the instanteous power is always the same.

Three phase motors have better power factor compared to single phase motors.

Three phase supply can be rectified into dc supply with a lesser ripple factor.

Ampere turns is the unit of Magneto Motive Force of a magnetic circuit, the equivalent of emf in electrical circuit. The MMF is measured as the product of the dc current flowing through the circuit and the number of turns. The higher the number of turns in a coil, higher will be the magnetomotive force for the same current.

However, this relationship holds true only till the core of the coil gets saturated, after which there is no change in mmf for an increase in current.

The field strength of a coil is the magnetomotive force per unit length. This is measured as ampere-turns per metre.

Yes, Permanent magnets can lose their magnetism. There are three main causes which can affect the magnetism of a permanent magnet.

They are

Heating a magnet above the Curie Temperature (the temperature above which the magnetic properties of a material change from ferromagnetic to paramagnetic) causes the magnetic domains to be disrupted permanently. Mild heating causes a reduction in the magnetism. However, when it cools the full magnetism is restored.

Mechanical Shock:
A magnet that is subjected to shock such as being hit by a hammer or dropped from a height can lose its magnetism. However, modern magnets made from materials such as Samarium Cobalt and Neodymium can withstand shock.

An opposing magnetic field:
A demagnetising field or a field that acts in the opposite direction can also result in a loss of magnetism. Demagnetising fields are sometimes used to reduce the strength of a magnet to fit a specific application

Magnets find a wide application in the field of electrical engineering. From motors to generators and relays, the effects of magnetism are central to the application of electricity in our daily lives,

Magnets can be broadly classified into two types

Electromagnets or temporary magnets and

Permanent Magnets

Electromagnets are made by coiling a conductor around a magnetic material such as soft iron. The Electromagnet gets magnetized when current flows through the conductor and gets demagnetized when it stops. Electromagnets find extensive use in relays, cranes, in solenoid valves, etc. These magnets are characterised by low retentivity.

Permanent Magnets are magnets which retain magnetism even after the magnetizing field strength is removed. Permanent magnets are used in equipments such as speakers, data storage devices, generators, etc, etc.

While Permanent magnets too get magnetized the same way as electromagnets, they are made of special material which have very high retentivity which enable them to retain magnetism long after the magnetizing field is removed. The first permanent magnets were made from magnetite, an ore of iron which gets naturally magnetized by the earth's magnetic field.

Later, better materials such as Alnico (an alloy of Aluminium, Nickel and Cobalt) were used.

The 1970s saw the development of ceramic materials such as barium ferrite and strontium ferrite . These materials have the advantage of high formability, i.e. they can be made into any shape and size without the need for expensive machining. These magnets can also be made flexible adding the ceramic powder in a binding material such as PVC or rubber.

For applications such as the headphones for music systems, smaller and more powerful magnets are required. These magnets are made from a material known as samarium cobalt(SmCo).

Neodymium magnets are also similar to SmCo magnets, however they are cheaper. Neodymium magnets are widely used in computer hard disks. These magnets are the strongest magnets which are commercially used.

Schuko Sockets are a system of AC power plugs and connectors. Schuko sockets first originated in Germany in the early 20th century. However, now they have found wide application in almost 40 countries.

The name Schuko is derived from the German 'Schutzkontact" meaning "protective contact" a reference to the clip-shaped earth contact in contrast to the pin-type earth contacts used in other formats.

The Schuko plugs and sockets are considered safer as they are totally enclosed and the pins cannot be accessed as long as the plug is not taken out of the socket.

The earthing lead is connected through a clip which ensures that the earthing lead makes contact before the phase and neutral leads.

The Schuko sockets can also accept C type plugs.

The Conservator is a cylindrical component of the transformer. The conservator is located at the top of the transformer. The Conservator is designed to act as a reservoir for the transformer oil. The level of the oil in the transformer can rise and fall due to temperature. The increase of temperature can be caused either by a rise in ambient temperature or due to increased load on the transformer.

An increase in temperature causes the oil in the transformer to expand. The conservator provides space for this expansion of the oil. The oil level indicator in the conservator needs to be monitored to ensure that the level of oil does not fall below the alarm limit.

As the level of oil rises and falls inside the conservator, air enters and leaves the chamber. The air may carry moisture which may cause the oil to deteriorate. Breathers filled with silica gels are provided to separate moisture from the aspirated air.

The silica gel is blue when it is dry. It turns pink when it is saturated with moisture
after which it needs to be replaced.

Proximity Effect is a phenomenon which is observed in conductors carrying alternating current. When a conductor carries ac power, the constantly varying magnetic field induces eddy currents in the nearby conductors. In conductors where the current flows in the same direction, this results in increased current density in the nearby conductors due to the changes in the current distribution across the cross-section of the conductor. Thus the resistance of the conductor increases.

In the picture, the blue zone inducts the areas with high current density, the white zone indicates low current density caused by mutual induction.

When two conductors carrying current in the same direction are located close by, the current density on the sides of the conductor adjacent to each other will be lesser than the sides on the outside.

The reduces the net current carrying capacity of the conductor. This phenomenon is not observed when dc current flows through the conductor as there is no induction in dc.

Thus the AC resistance of a conductor may be many times the DC resistance. The AC resistance is directly proportional to the frequency of the power supply. The proximity effect is an important factor considered during the design of transformers, motors and multi-core cables.

Transformer Noise is caused by a phenomenon called magnetostriction which occurs inside the transformers. Magnetostriction is a phenomenon by which a metallic objects experiences a distortion in its shape when it is placed inside a magnetic field. The objects can experience a change in the dimensions, expansion or contraction.

Inside a transformer, the core which is made in the form of laminated sheets also undergoes expansion and contraction due to the changing magnetic flux. This expansion and contraction occurs twice in an ac cycle. The fundamental frequency of the noise or vibration is double that of the frequency of the power supply. Thus a supply with a frequency of 50 Hz will cause noise or vibration whose fundamental frequency is 100 Hz.

In addition to the fundamental frequency, there are also harmonics whose frequencies are odd multiples of the fundamentals such as the 3rd harmonic, 5th harmonic, etc. A proper study of the noise and vibration spectrum is necessary to devise methods of reducing them.

Since, the core of the transformer is made of laminated steel sheets; these sheets experience unequal expansion and contraction when exposed to the magnetic flux. Hence, they rub against each other causing the distinct hum. The constant cyclic forces generated in the transformer core cause vibration which is carried to the different parts of the transformer body. In addition, they also cause noise. Thus when trying to reduce the hum of the transformer, both noise and vibration needs to be addressed. The noise of the transformer is measured in decibels (dB).

People can find the noise of a transformer disturbing and may oppose locating a transformer near their residence. In such circumstances, measures for reducing the impact of the sound may be explored.

Vibrations can be addressed by the fitment of supports or dampers. Noise can be reduced by mounting baffles and planning the location of the transformer.

Image Courtesy : http://www.flickr.com/photos/lhoon/

Substations are installations in a Transmission and Distribution System which are involved in the connection of different sections of the transmission and distribution system, usually with the transfer of electrical power from one voltage level to another.

Substations play a vital role in integrating the generating, transmitting and distributing parts of an electrical system.

A substation is generally located in an open area. The substation contains of numerous equipments such as transformers, breakers, capacitors, measuring and protection devices and so on.

Based on their functions, Substations can be classified into four types
  • Distribution Switching Substations
  • Switching Substations
  • Transmission Substation
  • Customer Substation
Distribution Substations
Distribution Substations are substations which reduce the voltage from the Transmission level, say, 132 kV to a lower level such as 66kV or 33kV. The power at this reduced voltage is then supplied for domestic consumption via distribution transformers which are located in each street. These transformers then reduce the voltage to 440V and so on.

Customer Substations
These are substations that cater to only one big consumer, usually an industry. Industrial consumers though draw power directly usually at the secondary voltage level of the distribution i.e. 66kV or 33kV

Switching Substations
These substations are involved in switching. A power system may contain numerous feeders at the same voltage levels, the switching of these feeders for maintenance and the isolation of faulty feeders is vital to the reliable functioning of the system. The Switching Substations are used in merging two or three feeders into a single feeder. Switching substations contain all the components of a substation other than transformers. such as surge arrestors, current and voltage transformers, isolators,etc.

Transmission Substations
These are substations that are connect two transmission lines. The voltage levels in these substations are 66kV or higher. Usually these substations have transformers which connect two voltage levels. The substations also contain regulating equipment such as phase-shifting transformer, VAr compensators, reactors, etc. Elaborate arrangements are made for isolation of sections of the substations for maintenance in a manner that there is no interruption to the power supply.

Image Courtesy :http://www.flickr.com/photos/vax-o-matic/

Noise and Vibration in Transformers are undesirable aspects of Transformer Operation. Noise causes disturbance in localities where the transformer is located. Vibration in transformers can affect other components connected to it. Sometimes, excessive vibration can cause components inside the transformer to come loose.

Methods of Noise Attenuation:

Location of a transformer plays a crucial part in the level of noise. A transformer located in a corner of a building with walls on three sides will have its noise amplified.

If the transformer is to be placed in a solid mass such as concrete which cannot vibrate, a solid mounting is preferred.

If the transformer is to be mounted on other surfaces such as a structural frame, a column or a platform, flexible mounting pads would be ideal

The construction of a Wall around the Transformer can help contain the transformer noise within a small area.

Use of Double Walls: Double Walls or limp walls are arrangements which contain two glass plates between which is a viscous liquid. The viscous liquid helps in damping the noise as it passes through.

A cheaper alternative would be the construction of a screening wall made of wood or concrete which can reduce the transformer noise.

Reducing Vibrations in Transformers.

Vibration pads can be used to alleviate the issue of vibration.

All the components inside the transformer should be rigidly fixed by using spacers.

All external connections such as cables, etc should be attached by means of flexible couplings.

Lightning arrestors play a vital role in any substation by protecting equipment against lightning strikes and other surges. Lightning arrestors require very less maintenance and testing.

Lightning arrestors can deteriorate over a period of time due to factors such as dust, cracks, moisture ingress, degradation of the zinc oxide elements inside, etc. This can lead to failure of the lightning arrestor. When a lightning arrestor fails, it usually explodes causing a flashover and damage to the other equipment such as PTs, CTs, etc. Hence, it is imperative that the lightning arrestors in the system are kept in a healthy condition.

The usual tests carried out on Lightning arrestors are the Insulation Resistance Tests and the Hipot Test.

Harmonic Test (online test):
When the lightning arrestor is in line, a small leakage current flows through it. This current can be analysed for Harmonics. Online harmonics analysers for lightning arrestors are available. The leakage current is analysed for the presence of the 3rd Harmonic which usually indicates a failure in the near future. An arrestor thus identified can be isolated and sent for repair before any catastrophic failure can take place

The Insulation Resistance Test:
The tests are conducted with a High Voltage Meggar, usually 2500V. The value, usually in the order of megohms, is compared with the previous values and the test values of the manufacturers.

Hipot Test:
The Hipot test is conducted at about 175% of the rated voltage.

In addition to these tests, a visual inspection of the lightning arrestors for cracks, dust accumulation, broken fitments is also useful.

In the event of system overvoltages or adverse weather conditions such as thunderstorms, the lightning arrestors need to be tested more frequently

Surge capacitors are special capacitors connected to transmission lines. The function of these capacitors is to absorb the surge caused by waves which travel along the transmission lines.

The Surge Capacitor is always connected to the power supply. When the Surge appears it absorbs the surge, holds it on for sometimes and then releases it into the system.

Surge Capacitors have a very fast response time as they are continually in circuit.

However, the limitation of the Surge capacitors is that it cannot absorb a high current surge. High current surges can only be discharged by lightning arrestors.

Hence, the Surge capacitor is usually connected along with a lightning arrestor.

Capacitors are devices which store charge as an electrostatic field. When the supply connected to a capacitor is removed, the capacitor still retains the charge within itself.

Thus, when a capacitor is switched off, it still contains charge. Hence, an engineer working on a capacitor that has not been discharged can get an electric shock. It is, therefore, vital that all capacitors and other energy storage devices be discharged prior to service. Power capacitors usually have a resistor known as a bleeder resistor connected in parallel. The function of this resistor is to discharge the capacitor once the power supply has been removed.

These resistors are usually designed to reduce voltage across the capacitor to less than 50V (the permissible safe voltage for humans) within 5 minutes.Hence, service work in a capacitor should be started only after five minutes.

As a final precaution, the capacitors need to be discharged manually prior to starting the work.  (See article on Manual Discharge of capacitors)

Bulging in capacitors is caused due to pressure inside the capacitor body. This pressure can be caused due to arcing between the capacitor plates and the resultant generation of gases.

Such capacitors need to be handled carefully. Always contact the capacitor manufacturers on handling or disposing the capacitor units.

Vents are provided in some capacitors as a defence against bulging. Vents need to be periodically checked for rupture which may indicate failure.

Dry - Type:
These transformers use air as the cooling medium. These transformers are generally used for indoor applications. This kind of cooling can be applied for transformers up to 20 MVA.

These Transformers can be further classified into

a)Natural Cooled:
These transformers can be Natural Cooled with Air (Air - Natural). The natural convection of the air removes the heat generated by the transformers. The symbol for this type of cooling is AA.

b)Forced Air Cooling:
This involves cooling the windings of the transformers with forced air, usually through a fan. The symbol for Forced Air cooling in Transformer Nameplates is AF.

Gases such as Nitrogen or Sulphur HexaFluoride can also be used for cooling transformers. These transformers are also considered to be amongst the "dry" type. These transformers need to be placed in sealed containers

Oil Cooled Transformers:
These are transformers which are cooled by means of oil. These transformers are fitted with fins through which the oils pass through as they transfer the heat to the atmospheric air. The oil in the transformers needs to be periodically sampled and checked for integrity. For more on transformer oil analysis Click here.

Like their air cooled counterparts, these transformers too can be further classified on the basis of the flow of oil.

a)Oil Natural Cooling:
In this type of cooling, the oil circulates through the transformer by way of convection. The heat collected by the oil is transferred to the surrounding air by means of cooling fins. The cooling class symbol for this kind of cooling is OA

b)Forced Air Cooled:
In this kind of transformer, cooling is achieved by means of oil driven by convection. As the oil circulates through the fins, air is forced from the outside by means of fans on the fins. This enhances the process of heat exchange and increases cooling of the oil. The designation for this kind of cooling is OA/FA. The fans in this kind of cooling are controlled by a mechanism which switches them on when the transformer oil temperature increases beyond a specific limit.

c)Forced Oil, Forced Air Cooling:

This involves forcing the oil inside the transformer through a special heat exchanger by means of a pump. Air is forced through the other side of the heat exchanger by means of fans. The designation for this type of cooling is FOA (Forced Oil Air).

d)Water Cooled Transformers:
Water can also be used in the cooling of transformers. In this method, the oil which passes through the heat exchanger is cooled by water. This method of cooling is designated as FOA

Dry Type transformers use air as the cooling medium. Oil Type transformers are considered a potential fire and safety hazard. Oil Type transformers require the development and maintenance of reliable fire safety and extinction procedures

Dry Type Transformers can be located closer to the load unlike oil transformers which require special location and civil construction for safety reasons. Locating the transformers near the loads may lead to savings in cable costs and reduced electrical losses.

Oil Type transformers may require periodic sampling of the oil and more exhaustive maintenance procedures.

However, though dry type transformers are advantageous, they are limited by size and voltage rating. Higher MVA ratings and voltage ratings may require the use of oil Transformers alone.

For outdoor applications, oil filled transformers are cheaper than dry types.

Ampacity or 'ampere-carrying capacity' refers to the ability of a conductor such as wires, cables or busbars to carry current without getting damaged due to overheating.

The ampacity of a conductor should be optimimum with respect to the application. A lower ampacity would result in heating and damage to the insulation. An excessive ampacity selection would result in unnecessarily high costs.

The ampacity of any conductor depends on the following factors.
  • The temperature rating of its insulation or the insulation class.
  • The electrical properties of the conductor such as resistivity, etc.
  • The heat dissipating capacity of the conductor; this depends on the shape of the conductor, the conductor location and ambient temperature, etc.

The transformer works on the principle of mutual induction between the primary and secondary windings. The induction is caused by the constantly varying magnetic flux that links the two windings. The flux density in the windings is directly proportional to the induced voltage and inversely proportional to the frequency and the number of turns in the winding.

Magnetic Flux α Voltage/Frequency

Overfluxing is a dangerous situation in which the magnetic flux density increases to extremely high levels. The high flux density can induce excessive eddy currents in the windings and in other conductive parts inside the transformers. The heat generated by these eddy currents can damage the windings and the insulation. The high flux density also causes magnetostriction inside the transformer core and produces noise. The powerful magnetostrictive forces can also cause damage. The winding temperatures may also increase due to the heat produced.

The magnetic flux density is dependent on the current flowing through the primary windings in a transformer. This current is dependent on the voltage applied across the windings and the winding impedance. The impedance is dependent on the frequency of the applied voltage. If the nominal voltage is applied at a reduced frequency, the low inductive reactance will cause a higher current to flow through the windings.

Overfluxing is usually encountered in Transformers which are directly connected to the generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters which limit the voltage in accordance with the frequency.

Overfluxing can be prevented by the use of a Overfluxing relay. An overfluxing is an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a capacitor in series. The voltage sensing relay is connected across the capacitor. The relay operates in the event of an overfluxing and isolates the transformer

When inductive loads such as transformers or motors are switched on, a sudden rush of current into the winding is observed. This is known as the inrush current. Typically, the inrush current is around 5 to 6 times of the rated value. This is due to the absence of the back-emf when the winding when the power is first applied. This sudden surge of current causes disturbances in the system voltage and sometimes spurious operation of relays. The high current also causes stress on the windings of the machines.

There are a number of methods to address this problem. Adding resistors in series to the winding and then gradually taking them out of circuit is one option. Another option is the use of softstarters which raise the terminal voltage of the machines gradually. Zero crossover switching is one method of addressing the issue of high inrush current when switching on inductive equipment. The method involves the use of Static Relays consisting of devices such as SCR or TRIACs. These devices switch on when the sinewave crosses the zero point so that the voltage is gradually increased. This, in some circumstances, reduce the inrush current.

However, this methods has its downsides too. There have been reports that if the zero cross over switching is carried out on a core that is already saturated from previous operation, extremely high currents can result.

Apart from inductive loads, inrush currents is also observed in resistive loads such as filament lamps. In filament lamps the resistance when the filament is in the cold, that is, switched off condition is lesser than the resistance when the lamp is in the switched on condition. This is due to the positive temperature co-efficient of resistance. When the lamp is switched on from the cold condition, there is a high surge of current which continues till the temperature of the resistance increases. Zero-crossover switching in resistive loads can ensure a smoother increase in current value to the steady-state condition.

This ensures a smoother increase in the current to the steady-state value.

Autotransformers are transformers which contain only one winding unlike two windings in the conventional transformer. The same winding, therefore, serves as the primary and secondary windings.

Autotransformers are advantageous over normal transformers as they are cheaper. Autotransformers are used generally for voltage conversion of equipments from one voltage to another such as from 110V to 220V or vice versa.

However, the autotransformer does not provide isolation between the primary and the secondary. Hence, there may a need to connect external filtering or suppression circuits. Thus, in the event of a failure of the insulation between the turns of the winding, there are chances of the primary voltage appearing on the secondary.

Another aspect which needs to be checked is the neutral point. If the neutral point is not at ground potential in the primary, the secondary neutral wil also not be at ground potential.

The autotransformer has higher voltage stability and better overload transformers than the ordinary transformers.

A Variac is a variable autotransformer with movable taps. Thus, it provides variable output voltage for a steady input voltage. The taps can be adjusted by a movable knob.

See Also:

Difference between Autotransformer, Variac and Dimmerstat

A scope meter is a handheld instrument that is used to see electrical parameters in graphical form. It can be described as a combination of a multimeter and an oscilloscope. It can be used to take measurements of the magnitude and frequency of a signal such as current and voltage.

A scope meter is an effective instrument in troubleshooting. As it displays the parameters in graphical form, transients and other momentary variations can be identified. This enables the identifications of improper and loose connections, grounding loops, etc which may be missed by indicating instruments.

Modern Scopemeters can also be connected to a computer by means of a USB port and the data collected can be transferred into the computer for analysis. The Scopemeter usually comes with software for this purpose. They also have recording and playback facility.

Since, scopemeters are handy and battery powered, they are portable and can be taken around easily unlike oscilloscopes.

image courtesy : http://www.aikencolon.com

An Autotransformer is a transformer with only one winding. See more. A variable autotransformer is an autotransformer with a sliding scale which can be used to adjust the transformer ratio. "Variac" and "Dimmerstat" are trade names of variable autotransformers. Hence, they are the same.

Freewheel or Fly back diodes are used across inductive components such as coils to prevent voltage spikes when the power is turned off to the devices.

When power to inductive loads such as coils and inductors is turned off, there is a sharp voltage spike. The direction of this voltage is opposite to the applied voltage in accordance with Lenz’s Law.

When a current flows through the coil of a relay, the coil gets electromagnetically charged. The energy is stored in the magnetic field around the coil. When the power supply to the coil is interrupted and the current in the coil tends to decrease, the magnetic field discharges causing a surge in the voltage.

The voltage, thus induced, can jump across the contacts of relays connected to the coils. The sparks and arcing produced can affect the life of the contacts. The voltage spikes can also damage electronic components like transistors which may be driving the relay coils.

Freewheel diodes are connected in reverse bias vis-à-vis the supply voltage. Hence, when the voltage spike appears in the opposite direction, they are short-circuited through the diode. The voltage spike is thus short-circuited across the coil. This protects the connected circuits.

Ordinary clampmeters used to measure AC currents work on the principle of electromagnetic induction caused by the alternating current flowing in the conductor which reverses direction causing a dynamically changing magnetic field. However, in DC conductors, the current flows in a fixed polarity. Consequently, the magnetic field around the conductor is fixed and does not change. Hence, a conventional clamp meter will register no reading.

A DC clampmeter works on the principle of the Hall Effect. The Hall Effect, named after Edwin Hall who discovered it 1879, states that when a conductor carrying current is placed in a magnetic field, a potential is induced across the conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. It is caused as the charge carriers, electrons or holes, experience a force known as the Lorentz force and are pushed to the sides of the conductor.

A clampmeter which works on the Hall effect has a sensor known as the Hall element. The Hall element is subjected to the magnetic field caused by the flow of current to be measured. This causes a small voltage across the Hall element. This voltage is amplified and measured.

images courtesy: commons.wikimedia.com

The Rogowski Coil is a coil which is used to measure alternating currents and fast changing current impulses.

It is named after Walter Rogowski, a German Physicist.

The Rogowski Coil consists of a coil of wire wound in a helical manner. One end of the coil is taken through the coil itself and brought to the other side. Thus, the coil has both its ends at the same side.

The coil can be wrapped around a conductor to measure the current. The Voltage induced in the coil will be proportional to the rate of change of current in the primary conductor. The voltage thus induced can be connected to an electronic integrator which will generate a signal in accordance with the changing current signal.

The advantage of the Rogowski coil over the tradition current transformer is that it can be easily wrapped around the conductor. This permits online measurements of systems without requiring a shutdown. Besides, as the Rogowski Coil has an air core instead of a metallic core, it does not get saturated while measuring large currents.

The Rogowski coil is also immune to stray electromagnetic interference to a large extent.

Rotors used in Synchronous alternators can be classified into
 1)Salient and 2)Non-Salient Pole Rotors. 

Salient pole rotors are used in application with speeds from 100 to 1500rpm. They are alternative known as "projected pole" type of rotors. 

The poles mounted on the rotor are made of laminations made of steel. The poles are connected to the rotor shaft by means of dovetail joints. Each pole has a pole shoe around which the winding is wound. The salient pole rotor is generally used in applications where the prime mover is a hydel turbine or a combustion engine which have low or medium speeds. Salient pole rotors usually contain damper windings to prevent rotor oscillations during operation. 

Non-salient pole rotors are generally used in application which operate at higher speeds, 1500rpm and above. The prime movers in these applications are generally gas or steam turbines. 

These are sometimes known as "drum rotors". 

The rotor is a cylinder made of solid forged steel. The slots on which the windings are fixed are milled on the rotor. The number of poles is usually 2 or 4 in number. 

Since these rotors are cylindrical, the windage loss is reduced. 

The noise produced is also less. These rotors have higher axial length. These rotors do not need damper windings.

Corona is a form of Partial discharge which is luminous and occassionally visible to the Naked Eye.

Corona are caused when the air around an energized conductor gets ionized, causing a discharge. Corona are caused when defects are present in the conductors such as jagged edges or cracks which cause high local electric field. A hissing sound can also be heard during the occurence of Corona.

The Nitrogen molecules in the air get excited and result and cause ultraviolet radiation. Corona is sometimes visible as a hazy blue light around conductors, especially during the night.

Effects of Corona
  1. Corona is accompanied by the creation of ozone and nitrogen oxides. Nitrogen oxides may react with moisture in the air and form nitric acid a potentially corrosive substance.
  2. Corona can cause damage to insulators used in High voltage applications.
  3. Corona emissions are accompanied by the generation of radio waves which can interfere with commercial radio transmission. Sometimes, Corona can also result in noise which may disturb the neighborhood.
  4. It can sometimes cause carbon deposits which may later result in arcing.
Corona can also be used to identify problems. Corona in power transmission lines indicate the presence of dirt or other substances on power lines and may require cleaning.

Corona after commission an equipment may indicate improper installation.

The degree of corona will vary with the humidity. In high humidity conditions, corona can develop into a flashover which can cause trippings and damage to equipment.

Prevention of Corona
One of the means of reducing corona in transmission lines has been to increase the radius of the conductors.  This leads to decreased electrostatic field stress on the air around the conductor and prevents the initation of the corona.  ACSR conductors ( see article on ACSR conductors) which have a greater radii than steel conductors suffer less from the effects of corona.

Partial Discharge is a type of electrical discharge which occurs in insulating materials which are located between two conductors as in cables or windings. As the name suggests, the discharge is partial without completely bridging the two conductors.

Partial discharges usually occur in voids in the insulation of cables and windings as the di-electric constant of the voids are significantly lesser than the surrounding insulating media. They can also appear across bubbles in liquid insulating media such as in breakers or transformers. In gaseous media, they can appear as corona around an electrode. Corona can be seen in wet weather in power lines.

Partial Discharge occurs usually in insulation which has been subjected to mechanical and electrostatic stress or which has been weakened by premature aging due to adverse environmental conditions. Once initiated, Partial Discharge results in the formation of electrical trees which can accelerate the failure of the cable causing a short-circuit between two conductors or an earth fault. They can appear as "tracking" or distinct pathways in the cable insulation

Partial discharge can also be caused by improperly terminated/jointed connections in HV cables and windings.

The currents which flow during Partial discharge are extremely small and last for very short periods of time of the order of nanoseconds. This makes them difficult to measure. Partial Discharges generally dissipate energy in the form of heat, light and sound.

There are a wide range of methods(Partial discharge Analtysis) to detect and monitor Partial Discharge. Acoustic sensors detect the ultrasonic frequency noises which occur when a partial discharge is taking place. There are also inductive and capacitive sensors which can detect Partial Discharges

A Flashover or an arcing fault occurs when current flows between two conductors or between the conductor and the ground or a neutral line. A sudden burst of Energy is released during an arcing fault.

The high current which flows during a flash over can cause a sudden blast of energy in the form of heat, light and sound.

Personnel who come in contact with the arc or are in the vicinity of the fault can face serious injuries such as burns, damage to sight and hearing. Sometimes, Flashovers can lead to death of personnel. The Flashover is usually accompanied by a tremendous pressure blast which may throw pieces of equipment (shrapnel) over large distances increasing the chance of injury to people nearby

Flashovers can be caused by a number of reasons such as making contact with energized equipment accidentally, dropping tools into energized equipment such as busbars, transformers when working on them. The accumulation of dirt, dust and moisture over extended periods of time can also trigger flashovers in equipment.

Flashovers can be prevented by following standard safety procedures while working on Energized equipment. Routine safety drills will also greatly reduce mistakes during work. It must be ensured that all HV equipment are insulated. The minimum line-to-line and line-to-ground clearances should be followed for all busbars. The Insulation Resistances of all cables and windings need to be checked at scheduled intervals

The relays and other protective devices should be periodically checked for correct operation.

Current limiting fuses are used in systems where high fault levels can result in excessive fault currents. The fuses function as normal fuses; however, they are designed to limit the fault current to low levels when they operate.

During normal operation, the fuse has a low resistance. However, when the fault occurs and the fuse ruptures, the heat created by the arcing inside the fuse causes the compacted quartz sand to create a high resistance environment. This quenches the arc and ensures that a very rapid fall in current.

The fault is thus cleared within the first half-cycle of the fault within 10 ms. Thus current-limiting fuses also protect systems from voltage sag in the event of a fault in one part of the system.

The current limiting fuses contains elements made of copper or silver. The elements are designed to have constrictions at a number of places which will heat up in the event of a fault. This enables quick operation. The arcing is also made to occur in a pre-determined number. The arcing which occurs in many streams enables easy quenching instead of one single arc. The quenching medium is usually compacted quartz sand.
Current limiting fuses also reduce hazards of arc-flashing, since they are extremely fast acting and also able to restrict the currents.

Arc flashing occurs when different conductors accidentally come into contact. The resulting arc can cause flashovers which generate tremendous amounts of heat causing danger to personnel nearby.

image courtesy :www.chfuses.com

Interharmonics are distortions in the current or voltage waveforms, like ordinary harmonics. However, they differ from normal harmonics in that the frequency of these waveforms are not integral to the fundamental. The waveforms are found between the normal harmonics of voltage and current.

For instance, while normal harmonics have frequencies such as 150Hz (3rd Harmononic) and 250Hz(5th Harmonic), interharmonics appearing in frequencies between these harmonics such as 160 Hz, 198 Hz, etc. However, they can appear as discrete frequencies in a spectrum. Interharmonics are believed to be caused due to transient changes in the value of current and voltage.

They often accompany normal harmonics. Fast Changes in the phase angles of currents and voltages can also cause interharmonics. Another cause of interharmonics can be asynchronous switching of semiconductor switches such as in systems which use pulse width modulation. Arcing loads, such as welding machines and arc furnaces, are also believed to cause interharmonics.

The Effects of interharmonics are saturation of current transformers, disturbance of telecommunication signals, etc. Interharmonics are known to cause low frequency mechanical oscillations.

Filters can be used to mitigate the effects of interharmonics. However, factors such as resonance, power loss, etc need to be kept in mind while making the choice of filters. Series filters are generally used against interharmonics.

Subharmonics is a term used to refer to Harmonics which have a frequency less than the fundamental frequency i.e. 50 Hz.

Gravel has many qualities which make it a preferred material for layering surfaces inside substations. Its high resistivity helps ensure that the step and touch potentials remain within limits.

It also prevents growth of weeds and small plants. It mitigates the chances of a fire in the event of oil spillage. It can be easily excavated. Besides, it also prevents the entry and movement of small animals and reptiles inside the substation.

Gravel also prevents the accumulation of water and the formation of puddles inside the substation.

All these features ensure that gravel is the material of choice for use in substations.

Water trees are tree-like defects, filled with water, which develop in the insulation of cables. The defects usually originate from defects, voids or contaminants. The trees can cause premature failure of the insulation. Water trees usually propagate in the direction of the electric field. They occur only in the presence of water in the insulation. They are usually invisible to the naked eye in the dry condition. Special dying techniques are available which can make them noticeable.

Water trees are found more in sections of cables which are in a state of tension such as in bends. While it is possible to identify the conditions which may cause the formation of water trees, the exact mechanism and the chemical processes involved in their development is not yet fully understood. Water trees reduce the breakdown strength of the cable.

Electrical Trees are formed in the absence of water in dry conditions. They are caused by voids, impurities and defects in the insulation. High electrostatic stress which reverses direction as in AC cables can also accelerate the phenomenon. Occasionally, water trees may evolve into electrical trees. These trees are accompanied with partial discharge which may accelerate insulation failure. Electrical trees are readily visible to the naked eye.

Trees can be classified broadly into vented and bow-tie trees. Vented trees are those which originate from an electrode and reach out to another electrode. These trees grow faster as they have access to air which aids partial discharges.

Bow-tie trees are those that originate inside the insulation. Since they originate inside the insulation they do not have access to air and hence limited partial discharge occurs. They progress slower than vented trees.

image courtesy: http://www.lordconsulting.com

Monitoring Earth Faults in Ungrounded dc systems is vital to prevent any sudden tripping in the system. In ungrounded DC systems, an earth fault in one terminal will not cause any disturbance and the system will continue to run normally. However, should an earth fault occur at the other terminal also, then there will be a virtual short-circuit between the two terminals through the earth.

Such faults occur without any indication and are difficult to identify. Hence a system to monitor the earth faults in a DC system is vital.

This is a schematic of a simple design of an earth fault monitoring system for ungrounded DC systems. The system consists of two bulbs. Each bulb is connected to one terminal of the power source and the ground. Thus, one bulb is connected between the anode and the ground while another is connected across the cathode and the ground as shown in the image.

Under normal terminals, the voltage across the bulbs will be zero and they will be off. However, in the event of a ground fault between the positive terminal and the earth, the voltage across the bulb connected to the cathode will be equal to the system voltage. This will cause the lamp to glow, indicating an earth fault.

Hipot Test is a high voltage test that is used to check the integrity of insulation for high voltage equipments such as busbars, cables, motors etc. The term 'Hipot' is the shortened form of High Potential. The Hipot test is used to ensure that an insulation can withstand a high potential without risk of failure.

However, the hipot test carries with it the potential failure of the insulation during the testing process itself. Weak insulation can fail during the test. Hence, many equipment owners avoid conducting this test. The hipot test certifies that the insulation is sufficient to withstand excess voltage during operation. This is significant in situations where the failure of a machine in service can cause serious damage or downtime as compared to a failure during the testing procedure.

The Hipot test is alternatively known as Dielectric Withstand test. The test involves the application of a high voltage usually about two times the
operating voltage. Thus a 6.6kV equipment will be tested at a voltage of 13kV.

The test is conducted for 1 minute or five minute. If the hipot test is conducted on a transformer winding or an alternator winding, the test is conducted on individual phases. The phases are separated and those phases which are not subjected to the hipot voltage are grounded.

Test Procedure
Prior to commencing the hipot test, it is necessary to get the Insulation Resistance and the Polarization Index values for the insulation. This ensures that the windings are free of any moisture or contamination. A wet or contaminated winding is more vulnerable to fail during the test.

The hipot test voltage is applied to the winding terminals to be tested. The voltage is sustained for one minute or five minute and then reduced. The current during the test period is also studied. Should there be a failure during the testing. There wil be a surge in the current which will cause the MCBs in the hipot test kit to trip.

There are two methods of raising the voltage to the value of the test voltage. They are

Step Test
In this method, the test voltage is raised gradually in small incremental steps. This enables the tester to abandon the test if he suspects that any current increase which may indicate a weak winding.

Ramp test
In this method, the test voltage is raised gradually or ramped up at a specific rate. The voltage can be increased to the rated voltage along with constant monitoring of the current. The ramp method is the most effective test as it can avoid any insulation failure during the test by identifying potential weaknesses in the winding early on.

The Hipot test does not offer scope for analysis such as the Insulation Resistance or the Polarization Test. It is simple a pass-fail kind of test. It is significant in that it gives operators the confidence that the equipment is strong enough to withstand the operating voltage and transient overvoltages in the system.

The high voltage used during the test calls for high standards of safety. The area around the test location should be cleared of all items not related to the test(clutter). The area needs to be cordoned off to prevent the entry of unauthorized persons. Personnel should be stationed at the main power switch so that the switch can be turned at the first sign of any abnormality. The personnel conducting the test should be properly trained with awareness of emergency first-aid procedures in the event of an electric shock. The device which is being tested should be grounded after the test to discharge the capacitance.

image source: http://www.testequipmentconnection.com

An American company has come up with an ingenious way to generate electricity from the humble onion. According to Reuters, the new system is being installed at Gills Onions, the largest fresh onion processor in the U.S.
The system works by collecting the peels of the onions which are usually wasted. This waste is acted upon by special bacteria which generate methane. The methane is diverted to fuel cells which generate electricity. The electricity generated is estimated to be sufficient to power 360 homes.

See: http://in.reuters.com/article/entertainmentNews/idINIndia-41124320090717

Instrument Transformers are sometimes provided with fuses. These fuses are intended to isolate the transformers in the event of internal fault.

Opinion is divided over the use of these fuses. Maloperation of these fuses can cause large scale disruption of power systems by way of unintended operation of the relays that are connected to the instrument transformers. Hence, some manufacturers provide fuses in the instrument transformers only when specifically requested.

While, the fuses cannot protect the instrument transformers from burnout during normal operation as the burnout current may be extremely low, they can ensure that a fault inside the transformers does not impact the busbar or generator in which it is mounted.

Trefoil Formation refers to a method of arranging cables. The trefoil arrangement is primarily used in situations where the three phases are carried by individual cables rather than a single three phase cable.

In a three phase cable, since the individual conductors along with their insulation are placed near each other the net inductance is minimum as the magnetic field of the individual currents cancel each other out.

However, in single phase cables, when the cables are placed in a straight line the inductance is not cancelled. This can reduce the current carrying capacity of the cable by way of mutual inductance. It can also induce eddy currents in the cable sheath and metallic conduits which can cause heating. It is advisable to have conduits of non-ferrous metals.

Connecting the individual cables in the trefoil formation minimizes the magnetic field around the conductor and reduces the heating. There are special trefoil spacers which hold individual cables in place so that the magnetic fields cancel each other to the maximum.

XLPE is the acronym for Cross linked Poly-Ethylene. It is a form of polyethylene which has crosslinks which join the individual polymer chains together. Polythene is a material that has numerous applications in the modern world. However, it has the disadvantage of having a low melting point. This downside is eliminated by "cross-linking" the polymer chains. The cross linkings increase the melting point.

XLPE has many qualities which make it extremely useful for cable insulation. It is flexible permitting smaller bending radius for the cables. It is light weight and water proof. It is also tough which minimizes the need for armouring.

XLPE cables are available for a wide range of voltage ranges from 600V to 154kV.

It is easier to handle and store compared to cables with paper insulation or lead insulation.

They are relatively maintenance free and have simple terminating and jointing procedures.

Some of the other features of XLPE are
  1. It has a high softening temperature
  2. It resists aging
  3. It is light
  4. It has resistance against stress cracking.
However, XLPE does have some disadvantages such as high cost, and the formation of water trees in the insulation due to ageing which result in partial discharge. Hence, recently, another polymer known as XLVLDPE(Cross linked Very Low Density PolyEthylene) is being used for cable insulation

ACSR is an acronym for Aluminium Conductor Steel Reinforced. These conductors are widely used in High Voltage Power lines.

Aluminium is a good conductor of electricity besides being cheap. However, its mechanical properties are not desirable. It is soft and cannot be hardened. It also has low tensile strength

This problem is resolved by providing a core of steel stranded cables inside an outer layer of aluminium stranded cables. The steel imparts excellent mechanical properties. Due to the skin effect, the bulk of the power is transmitted through the outer aluminium layer of the conductor which have better conductivity.

The amount of aluminium and steel strandings can be adjusted depending on the requirement for mechanical strength vis-a-vis electrical conductivity.

The conductors are sometimes impregnated with grease to protect against corrosion.

The strength of ACSR conductors is greater than that of copper conductors. The ACSR conductors also have a higher corona limit as they have a higer diameter.

The Efficiency of the transformer is the given by

(Power output/Power Input)*100

The Efficiency of the transformer is affected by the losses inside the transformer.

These losses can be categorized into three types
  1. Copper losses,
  2. Core Losses and
  3. Stray losses

Copper Losses
These are losses caused by the heating of the conductor when current is passes through it. They are also known as I2R losses as the heat generated is proportional to the formula


Core Losses
Core losses are the losses which occur in the core of the transformer. There are two kinds of core losses, They are Hysteresis Loss and Eddy Current Losses

Hysteresis losses occur when the magnetic orientation of the molecules inside the core are reversed when the magnetic field changes. This reversal of orientation of the molecules results in the generation of heat.

Eddy current losses occur due to circulating currents in the form of eddies which are generated in the core. These eddy currents generate heat.

Stray losses
These are losses which occur due to the leakage of the magnetic flux of the transformer. This leakage can cause eddy currents in the fitments of the transformer such as the tank, channels, bolts, etc.

Power quality is a vital attribute of any distribution and transmission system. It is the duty of the utility to deliver power to the consumer that is reliable and consistent at a predetermined voltage and frequency. However, the quality of power is affected by numerous issues such as sags, swells, over voltage, etc. The following are some of the problems that affect power quality.

Voltage Sags
They are also known as voltage dips. These are voltage drops which last for a short period, from 8milliseconds to 60 seconds. They are caused by momentary faults in the transmission lines which get cleared quickly. The drop in voltage may range from 10 to 90 percent.

Voltage Swells
These are momentary increases in the voltage exceeding 110% which last less than a minute. They can be caused by phase to ground faults.

These are caused by a drop in the voltage to less than 90% of the voltage for more than 1 minute. They are also referred to as brownouts. Overloading of the distribution system can cause undervoltage.

They are similar to voltage swells except that they can last for more than a minute. The voltage exceeds 110%. Overvoltages are caused by poor voltage regulation. They can also be caused by switching of capacitors or by sudden load reduction. Wrong setting of transformer taps can also lead to overvoltages.

Interruptions are defined as the drop of voltage to 10%. Interruptions are further classified into momentary interruptions lasting from 8 milliseconds to 3 seconds, temporary interruptions lasting from 3 seconds to 1 minute and sustained interruptions or long duration interruptions which last for more than a minute. Power interruptions can cause loss to production lines in industries. Some of the ways of mitigating the effects of interruptions are by installing a UPS, an emergency genset or by having power fed from two different feeders.

See Article on Transients

Unbalanced Voltages
When the voltages in all the three phases are not balanced, this situation occurs. This is caused by single phase overloading of one phase. Loads such as arc furnaces also lead to unbalanced loading. Unbalanced loading can be harmful for loads such as transformers and motors where the unbalanced currents which flow as a result of the unbalanced voltages can cause heating.

See Article on Harmonics

Voltage Fluctuation
These are caused by quick variations in the voltage between 90% to 105% of the rated value. Arc furnaces and welding transformers can cause voltage fluctuations. Voltage fluctuations can cause “flicker” in incandescent lamps

A Transient can be defined as a momentary change in a parameter such as voltage, current or frequency that occurs when a system changes from one steady state operating condition to another. Transients occur during switching on and switching off loads, during starting motors, etc.

Transients can be classified into

Impulsive and
Oscillatory transients

Impulsive Transients
They are usually caused by lightning. These transients do not have any impact on the system frequency. They cause a very sharp change in either the voltage or the current. However, the change is in only one direction, the positive or the negative side i.e. they are unidirectional. They are characterized by a sharp rise followed by a decay. For instance a 1.5 x 60 microsecond, 2500V surge cause the voltage to rise to 2500V in a period of 1.5 microseconds. This will be followed by a decay to 50% of the voltage value in 60 microseconds.

The impulsive transient may appear differently in the waveform from different points in the system as it is a fast-changing phenomenon. This is because the transient can be modified by various components of the power system.

Oscillatory Transients
They differ from impulsive transients in that they are bi-directional, they occur on both the positive and negative sides of the waveform. Oscillatory transients can be classified into High, Medium, and Low Frequency Transients depending on the primary frequency of the transient.

High frequencies, with a primary frequency greater than 500 kHz, are caused by a reaction of the system to an impulsive transient. Impulsive transient can excite the natural frequency of the power system which can cause oscillations.

Medium Frequency Transients, with a primary frequency ranging from 5 to 500 kHz, are known as Medium frequency transients. They are generally caused by switching capacitor banks or charging large cables.

Low frequency transients have a primary frequency less than 5 kHz. They are usually caused by transformer energization and Ferro resonance.

Reclosers are switching equipment used in transmission lines. A recloser can sense an overcurrent condition by itself and isolate the system. It does not need an external relay to operate it. When the overcurrent condition is cleared, it closes automatically in a pre-determined sequence.

Reclosers are useful in distribution lines, where some of the overcurrent conditions are temporary in nature and clear quickly. They ensure that the system is normalized quickly.

After isolating a system, the recloser closes again after a preset time to see if the fault has been cleared. If the fault persists, the recloser trips again. It closes again for a few more times to see if the fault has cleared. Then it locks the system and permanently isolates it. The recloser then may need to be reset.

Like circuit breakers, reclosers consist of an interrupting mechanism with an insulating medium for arc extinction. They also contain coils for opening and closing, besides sensing transformers.

The tripping coils of reclosers are powered by the fault current which flows through the system. Unlike circuit breakers, where the closing coil is powered by an auxiliary supply, the reclosers are powered by a transformer which is located on the source side.

image courtesy: www.energobit.com

Skin Effect refers to the tendency of alternating current(AC) to flow along the outer surface(skin) of the conductor rather than through the entire cross-section of the conductor.

Skin Effect is caused due to eddy currents form due to the magnetic fields created when current flows through the conductor. These eddy currents are strongest near the centre. The magnetic fields oppose the flow of the current. Hence, the current finds it easier to flow across the periphery of the conductor.

Conductors which carry AC such as busbars in substations are made hollow for this reason as current flows only along the surface. The Skin effect becomes more pronounced at higher frequencies. That is why radio antennae are made hollow.

Conductors in overhead lines and in cables are generally made of strands instead of one solid conductor.

Skin Effect does not occur when conducting DC.