C Rating and battery capacity.

The capacity of a battery is indicated in Ampere-hours (Ah).  For instance, 200Ah, 80 Ah, etc.  The rate at which the battery can be discharged is indicated by the C rating.  

Batteries are usually rated as C/10, c/8, etc.  a battery with a 200 Ah capacity and a C/10 rating indicates that the battery can supply 20 A for 10 hours. 

The capacity of a battery varies inversely with the discharge rate. Batteries usually have different discharge rates for a fixed capacity. 

Combined Instrument Transformers

Combined Instrument Transformers are special measuring transformers which combine the functions of a current and a voltage transformers.  They are generally used where space is a constraint.  Typical scenarios are when measuring transformers need to be added in a functioning substation.  

Combined instrument Transformers usually have multiple secondaries for both current and voltage measurement.   These specialized transformers have been used upto 300 kV.

What are Primary and Secondary Cells ?

Primary cells are cells in which the chemical reaction which produces the power is not reversible. That is, the cell cannot be charged.  A common example is the dry cell.  Primary cells are cheaper and find application in radios, flash lights, torches, etc.    

Secondary cells are those cells in which the chemical reaction can be reversed by passing an electric current in the reverse direction.  The active material of the battery is reformed and the battery is recharged.  

Rod Earthing

Rod Earthing is an extremely popular form of Earthing as they are easy to install and retrofit.  Rod earthing involves driving metal rods made of copper, copper clad steel or stainless steel.  

The Rods used for earthing are usually about a metre long with a diameter of around 12 mm.  The rods should be driven to a depth of around 2.5 metres.  Usually the rods have threaded sections on one end so that the rods can be driven one on top of another to achieve the required resistance.

The Rods can be hammered into the ground.  This would be possible in sandy soil.  If the soil is hard, a Earthing rod driver which is a tool, usually pneumatic, which drives the earth rods into the soil is used. 

Capacitance Grading

Capacitance Grading is a method of distributing the electrostatic stress across the insulation of cables by using materials with different dielectric strength. When a cable is live, the insulation around the conductor is in a state of electrostatic stress. 

This stress is high near the centre and reduces towards the periphery. This uneven electrostatic stress can cause the failure of insulation. To prevent this kind of catastrophic failure, the insulation is graded. The permittivity of the insulating material is made to vary inversely as the distance from the centre. 

It is not possible to have a single material which has a permittivity which varies depending on the distance. Hence, the insulation is made of a number of layers of insulation made from different materials. 

Each of these materials has a different value of permittivity.   This ensures that the insulation is approximately uniform throughout the cross section of the insulation.

Rated Voltage Factor of Potential Transformers

The Rated Voltage Factor of Potential Transformers is the maximum limit of excess voltage up to which the voltage transformer can maintain its rated characteristics.  It is mentioned in percentage of the nominal voltage.  

For instance a Rated voltage factor of 1.2 would mean that the Potential Transformer can maintain its rated characteristics up to 120% of the nominal voltage.  

The following table lists the common Rated Voltage factors in Potential Transformers for different connections

Rated voltage factor	Rated time	Primary Winding Connection
1.2	Continuous	Between phases in a network Between transformer star-point and the earth in any system
1.2 1.5	Continuous for 30 seconds	Between the phase and earth in a system whose neutral is effectively earthed
1.2 1.9	Continuous for 30 seconds	Between the phase and earth in a  system whose neutral is non-effectively earthed  with automatic fault tripping
1.2 1.9	Continuous for 8 hours	Between the phase and earth in a system whose neutral is isolated without automatic fault tripping or in a system with resonant earthing without automatic fault tripping

Intersheath Grading

Intersheath Grading is a method of ensuring that the voltage gradient across the insulation of a cable does not become so steep as to cause the failure of the insulation.  The insulation of a cable is subjected to constant electrostatic stress.  This electrical stress is dependent on the voltage of the conductor.  The electrostatic stress needs to be uniform across the insulation.  Uneven electrostatic stresses can result in failure of the insulation. 

Intersheath Grading is a method of creating uniform voltage gradient across the insulation by means of separating the insulation into two or more layers by thin conductive strips.  These strips are kept at different voltage levels through the secondary of a transformer.  This ensures that all parts of the insulation are exposed to relatively the same stress.  

Single Circuit and Double Circuit Transmission Lines

Transmission lines which carry three phase power are usually configured as either single circuit or double circuit. A single circuit configuration has three conductors for the three phases.  While a double circuit configuration has six conductors (three phases for each circuit).  

Double Circuits are used where greater reliability is needed.  This method of transmission enables the transfer of more power over a particular distance.  The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view.  However, running two circuits in close proximity to each other will involve inductive coupling between the conductors.  This needs to be taken into account when calculating the fault level and while designing the protection schemes.

Double circuit transmission lines usually contain bundled conductors with the conductors placed as far as possible to minimize inductance. 

Proximity Effect

Proximity Effect refers to the phenomenon where the resistance of one conductor in a transmission line or a bus bar decreases due to the magnetizing flux from another conductor in close proximity.  

When two conductors are placed close to each other the varying magnetic flux caused by one conductor induces currents in the other conductor.  This increases the current density in the other conductor and results in increased impedance. 

The Proximity Effect increases with frequency.  Conductors are usually spaced with a sufficient distance to minimize the Proximity effect. 

A comparison of Overhead and Underground Transmission

Electric Power needs to be carried over long distances from the point of generation to the point of consumption.  This Transmission is done either through overhead lines or underground cables.  Each of these two methods of transmission has its own advantages and disadvantages. 

Overhead Transmission lines are cheaper as the insulation cost is lesser and the conductor material cost is lesser too.  They also have better heat dissipation.

However, they have significant disadvantages.  Overhead lines are vulnerable to lightning strikes which can cause interruption.  Overhead lines use  bare conductors and can cause damage if they break.  They are considered to be unsightly as they mar the scenery of the landscape.  The maintenance cost of overhead lines is more and the voltage drop in overhead lines is more.

Underground transmission due to cables is costlier than overhead transmission as the ground needs to be excavated.  This can be difficult when passing though geographic obstructions such as hills, marshes and rivers.  Special trenches need to be constructed when passing through loose soil.  Besides, heat dissipation in underground cables is an issue. Hence,  the conductors have to be thicker.  The insulation required for the cables is expensive.  Hence, it is difficult to use underground cables for voltages at HV levels (> 33 kV).  

Underground cables may have to be rerouted to accommodate other underground structures such as pipelines, sewage lines, etc.    It is necessary that the routes of underground lines are clearly marked with sign boards to prevent accidents when excavations are carried out for other reasons at a later date. 

Transposition of Conductors

          Transposition of Conductors refers to the exchanging of position of conductors of a three phase system along the transmission distance in  such a manner that each conductors occupies the original position of every other conductor over an equal distance. 

          When conductors are not transposed at regular intervals, the inductance and capacitance of the conductors will not be equal.

              When conductors such as telephone lines are run in parallel to transmission lines, there is a possibility of high voltages induced in the telephone lines.  This can result in acoustic shock or noise.  Transposition greatly reduces this undesired phenomenon.  

            In practice, however, conductors are not transposed in the transmission lines.  The transposition is done in the switching stations and the substations. 

XLPE Cables

XLPE stands for Cross linked Polyethylene.  It is a long chain polymer whose chains are cross-linked.  XLPE is a widely used material for use in cable insulation.  XLPE can be used from LV cables to cables with voltages upto 275 kV. 

The power factor of XLPE cable is very close to one.  XLPE cables have smaller charging currents and lesser dielectric loss.

XLPE cables do not require the protective metallic sheath.  They are lighter as compared to other cables and are easier to lay.

Tin Plated Copper

Copper used in bus bars is sometimes plated with Tin.  This is because copper oxidises in the presence of air.  The oxides of copper fall off the surface, this exposes more copper and further oxidation takes place.

These oxides increase the contact resistances in electrical contacts causing localized heating.  This heating causes further oxidation.  Thus, the copper is steadily eroded.

Tin plating is used protect the copper from corrosion.   Tin does not react easily with either air or water.  Tin is a soft metal which can be easily polished to give a shiny finish.   Tin Plated contacts are also used in the contacts of  isolators and switches.

Flexible Busbars

Flexible bus bars are used in Low voltage installations.  The are used in place of cables, particularly where parallel runs of cables need to be drawn to supply equipment in a particular location. 

Flexible bus bars are made of flexible strips of electrolytic copper which is usually tin-plated.  These strips are enclosed in a PVC insulation.  These bus bars can slide over each other which makes them flexible.  These bus bars can be bent to a considerable extent.  The elimination of parallel runs of cables reduces costs. 

Flexible bus bars have better heat dissipation than conventional rigid bus bars.  Hence, they can carry higher currents for the same cross section. 

Since these bus bars are insulated, the clearance from the ground can be lesser as compared to normal bus bars. 

Transformer Oils.

Oil is an important component of most transformers.  Oil serves to dissipate the heat generated in the core.  It also serves to provide dielectric isolation between the windings and the transformer body.  Transformer oils need to be stable at high temperatures with excellent dielectric properties.  

Traditionally,  mineral oils have been used in transformers. The downside of mineral oil is that they are easily combustible causing transformer fires. 

Polychlorinated Biphenyls, a synthetic compound was used as a replacement for mineral oil as it is not inflammable.  However, it is highly toxic and carcinogenic, not biodegradable and can cause damage to the environment if leaked.   The use of PCBs is now banned in many countries. 

Research has led to the development of many types of transformer oil which are fire-resistant and non-polluting.  Some of these oils are based on esters which are naturally available in vegetable oil or on synthetic esters.  These oils though expensive are environment-friendly, fire resistant and have the require dielectric properties.  Their high cost is offset by the elimination of the need for building expensive vaults for the transformer to contain any fire. 

Book of the Week

Electric Motors and Drives: Fundamentals, Types and Applications (3rd Edition)
by Austin Hughes
ISBN-10: 0750647183 | ISBN-13: 978-0750647182

This is a useful and interesting book which covers fairly all the topics in the field of motors and drives. The concepts are explained clearly and the mathematics is kept to a minimum. Topics start from the basics and and cover almost all aspects of the topic. The topics covered include dc motors, induction motors theory and types as well as stepper motors. Starting methods for various motors are also adequately addressed. The book also covers drives for dc and ac motors. It also includes a selection guide for drives. Though there pictures and diagrams, more pictures could have been added. 

In all, a comprehensive book which covers all the important topics at a relatively low price. This book would be ideal for industry professionals as well as students of electrical engineering.

Permanent Magnet Synchronous Generators

Permanent Magnet Synchronous Generators are becoming the generators of choice in the wind turbine industry.  They are being increasingly used in place of induction generators (induction machines which run above the synchronous speed).  They are advantageous over induction generators as they have a higher efficiency.   Besides, they do not need a magnetizing current which needs to be fed from the grid.

The normal synchronous generator has a wound rotor with poles.  These poles in the rotor are excited by means of a dc current.  In a permanent magnetic Synchronous generator, the wound rotor poles are replaced with permanent magnets.  

The machine is often directly coupled to the wind turbine or through gears.  The AC output of the synchronous generator which has a variable frequency depending on the turbine speed is converted into DC.  This DC is converted into a sinusoidal AC voltage at the system frequency.  This voltage is then fed to the grid.

Permanent Magnet Synchronous Generators (PMSG) are costlier as the permanent magnets are made of rare earth metals.  NeFeB, an alloy of Neodymium, Iron and Boron is used to make these magnets.  The use of permanent magnets in the rotors minimizes the losses in the rotor and improve efficiency.

Lockout /Tagout

Lockout Hasp

Lockout/Tagout system is a set or procedures which prevent the accidental energization of electric equipment during maintenance.  Lockout Kits contain many types of locking systems such as 2 - Hinged Single-Pole Breaker Lockouts Breaker Lockouts which can be used to lock a breaker against operation. 

Wall Switch Lockouts prevent the operation of wall mounted switches.  Plug Lockouts prevent plugs from being connected to power sockets.  Lockout Hasps have multiple locking facility.  When more than one person is working on a machine, say, a transformer.  Each individual needs to place a lock on the Hasp.  This prevents the machinery from being energized until all people have finished their work.

Lockout Padlock with unique key

 The Lockout kit also contains a set of padlocks.  Each padlock has its own unique key.

Lockout for Plugs

When it is not possible to lock out an equipment, Tag out procedures should be followed.  Conspicuous tags should be placed.  The Tag or the lock should only be removed by the person placing it

Lockout for MCB

         All installations should have clear Lockout/Tagout policies. Employees should be trained on proper implementation of these systems

Lockout Tag

Busbars

Strain Busbars

Busbars are electrical conductors which serve to pool up the power from different sources and distribute them to various feeders.  Busbars are generally made of copper or aluminium.  Busbars are found in substations, switchboards, distribution boards, etc. 

Busbars are sized according the current they carry.  Busbars are either flat or hollow.  This is to facilitate heat dissipation.  Busbars are supported by means of insulators.  Busbars are made flat or hollow to avoid the skin effect. 

Indoor busbars used in switchboards and distribution boards are usually flat. 

Outdoor busbars can either be hollow or strained.  Hollow busbars are rigid and are supported by means of hollow insulators.  It is easy to maintain these insulators as they are closer to the ground.  Their higher surface area minimizes the effect of corona.  They are more reliable.  However, they are expensive and require a larger area. 

Strained busbars are an overhead system of wires supported by insulators mounted usually on metallic frames.  The conductors are usually made of ACSR (Aluminium Core Steel Reinforced)

Other types of busbar include Insulated Phase busbars which consists of a rigid bar enclosed by a metallic enclosure and supported by insulator and Gas insulated busbars which consist of rigid conductors placed in a cylindrical tube filled with Sulphur hexafluoride gas)

Effect of Corona on Insulation

Corona refers to the luminous discharge when the fluid around a conductor gets ionized. 

Fractional Horse Power motors

Fractional HP Motors are motors whose power rating is less than one horse power i.e. 746 watts.  Fractional Motors range from an output of 1/20th horsepower to 1 horse power.  Motors less than 1/20th horse power are called sub-fractional horsepower motors.  

Fractional motors find wide application in automobiles for rolling up windows, windshield wipers, etc.  Induction motors, synchronous motors and dc motors can be used as Fractional HP motors.  

Fractional HP Motors also find wide application in household appliances.   Fractional Horse Power motors used in household application such as exhaust fans, blowers etc are usually single phase.  They are generally of the split phase or the capacitor run type.  

Extremely low speeds can be obtained using Fractional HP motors by means of suitable drives. 

Stepper motors and servo motors are also types of Fractional HP motors.  Fractional HP motors are also available as geared motors.

Lugs in wiring

Lugs are components which are widely used in electrical wiring.  They are used to connect cables to terminals.  Lugs enable quick disconnection of cables and reconnection.  They also protect the uninsulated ends of wires and cables.

Besides, they enable proper contact between terminals and wires.  Lugs are available in a wide range of shapes.  Some common types are the pin type lugs generally used in push type connections, fork type lugs are used in screw terminals and circular lugs. 

Lugs also serve to enable the connection of cables with large cross-sections to smaller terminals. Lugs are usually made of aluminium or copper.  They are tin plated to prevent oxidation.  The lugs are joined to the wire or cable by crimping, soldering or welding.   Some lugs are provided with PVC sheaths to protect against electric shocks.  Lugs are annealed to offer better ductility.

Circular Lug

Some lugs have inspection holes which enable full insertion of the cable into the lug .

Equipotential Bonding

A pipeline connected to the earthing grid

Equipotential bonding refers to the maintenance of all metallic objects in a vicinity in the same potential.  It is a widely followed practice in earthing.  Equipotential bonding ensures that all metallic objects are at the ground potential.  This eliminates the risk of shock occurring when someone accidentally comes in contact with objects at different potential.  An area where all the objects are kept at the ground potential is called the earthed equipotential zone. 

For instance, in a building there are many metallic fittings which are not part of the electrical distribution system such as bathroom fittings, pipes, metallic supports, steel supports used during construction etc.  Should these fittings become live due to a leak in an electric circuit, they can pose a danger of electrocution.  Hence, it is important that all metallic objects are kept at the ground potential by connecting them to the earthing grid. 

Pipes which are made of plastic or PVC need not be connected to the equipotential network.  In areas, which are likely to be wet such as bathrooms, the fittings are usually connected by an additional link to the equipotential grid.  This is known as supplementary bonding.  

Electronic devices such as computers and telecom equipment sometimes have a separate earthing pit.  This is unsafe as it permits the rise of potential between the main earthing grid and the separate electronic earthing pit in the event of lightning strike.  This can result in damage to the equipment. 

image courtesy : dehn.de

Running Three Phase motors with single Phase supply

A three phase device can be run with a single phase converter by means of a static capacitor phase converter.  The phase converter converts the single phase voltage into three voltages which can be connected to the three phases of the motor. 

The single phase supply is connected to two of the motor phase terminals.  The other terminal is connected to one of the single phase terminals through a capacitor.  The capacitor introduces a phase shift which causes the third phase to be out of phase by 120 degrees.  The produces the rotating magnetic field required for starting and running the three phase motor.    

In motors, the starting current is usually about six times the rated current.  Hence, a bigger value capacitor is usually used as a starting capacitor.  The starting capacitor is kept in line by means of a switch which is opened as the motor picks up speed. 

Phase Sequence Relay for motors

Phase sequence protection is an important safety for motors.  Reversing the phase sequence causes the motor to reverse its direction of rotation.  This can cause serious damage and injury to personnel if for instance, the motor is coupled to cutting equipment, or conveyor belts.  

The phase sequence indicator works by monitoring the phase sequence continually and preventing the motor from starting if the phase sequence has been reversed in the supply. 

Live Tank circuit breakers and Dead tank circuit breakers

Live Tank circuit breakers are circuit breakers in which the interrupting chamber is at the line potential. The interrupting chamber should therefore be provided with insulated supports. The centre of gravity of these circuit breakers is higher, hence live tank circuit breakers need extra support for seismic capability (ability to withstand earthquakes)

In dead tank circuit breakers, the interrupting chamber is at ground potential.  The conductors enter the interrupting chamber through insulated bushings.  Maintenance activities are easier to conduct as the interrupting chamber is at ground level. Seismic capability is higher as the interrupting chambers are at ground level. 

Live Tank Circuit Breaker

Dead Tank Circuit Breaker

Earthing Switches

Earthing switches are safety devices which are integral parts of circuit breakers.  When a circuit breaker is removed and racked out, the sections of the bus bar adjacent to the circuit breaker are automatically earthed by means of these switches.

This protects the maintenance personnel from accidental voltages.    The closing action of the earthing switch is of snap action type.  Earthing switches are usually dimensioned to withstand short circuit currents.  Earthing switches can also be motorised.

Earthing switches are usually used in conjunction with isolators. When the isolator isolates the circuits, the earthing switches make contact with the busbar and discharge any charges which may have accumulated there. 

Can a DC coil of a contactor be used with AC or vice versa ?

Both AC and DC contactors work on the principle of electromagnetic attraction.  However, there are minor constructional differences between them.  AC contactors have a shading coil which is a metallic ring with high remanence which provides magnetism during the zero crossing of the AC voltage.

DC contactors do not have this shading coil.  Hence, if DC coils are powered with an AC voltage, the contactor can chatter as the magnetism becomes zero during the zero crossing of the AC voltage.   Chattering produces an audible noise and can cause the contacts to change state causing interruption in the circuit.  

AC contactors can be used with DC voltage, in theory.  However, the presence of the shading coil in AC contactors can result in a higher drop-off voltage which can cause delay in contact operation.

Wooden Transmission Poles

Wooden Transmission Poles are used in LV and MV power transmission systems.   Wooden Poles have the advantage of being light and cheap. Wooden poles are also aesthetically more pleasing and blend better with the landscape.

Wooden Transmission Poles are generally supported by means of guy wires and usually have a metal cap on the top.  

The wood used to make these poles needs to be properly treated to prevent damage due to pests and decay.  Wooden Transmission Poles are Creosoted which means that they are pressure-treated with creosote, a chemical that provides protection against fungi, insects and marine borers.  

Wooden Poles are made from specific trees such as Pinus Sylvestris, Douglas Fir, southern Yellow Pine, etc.   The disadvantage of wooden poles is that the life of these poles cannot be predicted accurately and thus, they need to be frequently inspected.  Another downside is that these poles sometimes tend to rot at the bottom, especially, in waterlogged locations.

ETAP Software

ETAP is a software that is used for network analysis in Electrical Engineering.  It consists of a number of modules dealing with industrial distribution, transmission, arc flash analysis, etc.  ETAP stands for Electrical Transient Analysis Program. 

ETAP also provides a real time power management software module which offers integrated power monitoring, Load flow and short circuit analysis, etc.  ETAP is particularly used in Transient Analysis which enables engineers to simulate the response of the system to transients. 

A demo version of the software can be found here.  http://www.etap.ca/software-download/ 

Contactors and IEC Classification

Selecting the right contactors and relays for use in motor control and other industrial circuits is extremely important.  It is important that the contactors are chosen keeping in mind the equipment which is to be connected to it and the current it needs to interrupt.

A contactor which is chosen for a heater circuit cannot be used in a motor circuit of the same current rating.

The International Electrotechnical Commission (IEC) has categorized contactors into the following categories.

IEC Categories	Applications
AC1	Non-inductive or slightly inductive rows
AC2	Starting of slip-ring motor
AC3	Starting of squirrel-cage motors and switching off only after the motor is up to speed. This contactor is designed to make Locked Rotor  Current and  Break the Full Load Current.
AC3	Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. (Make and Break Locked Rotor Current)
AC11	For use in Auxiliary (control) circuits

Duty Cycles of Motors

The relationship between rest and operating periods or repeatable operationg at different loads is known as a duty cycle.  It is important that motors be chosen based on the duty cycle of the equipment they are driving. 

The International Electrotechnical Commission has classified motors into various classes based on duty cycles

S1	Continuous duty	The motor works at a constant load for enough time to reach temperature equilibrium.
S2	Short-time duty	The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature.
S3	Intermittent periodic duty	Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise.
S4	Intermittent periodic duty with starting	Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise.
S5	Intermittent periodic duty with electric braking	Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods.
S6	Continuous operation with intermittent load	Sequential, identical cycles of running with constant load and running with no load. No rest periods.
S7	Continuous operation with electric braking	Sequential identical cycles of starting, running at constant load and electric braking. No rest periods.
S8	Continuous operation with periodic changes in load and speed	Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods.

source : The internet

Stray Voltages from Power Lines

When things such as Fences and pipelines are run parallel to High Voltage Power Lines.  Voltages can be induced in them.  These voltages can either be due to electromagnetic induction caused by transformer action between the pipelines and the power lines or they can be due to electrostatic coupling between the conductors and the pipeline. 

These voltages are usually very weak.  However, the voltages can rise to levels where they present a danger of electrocution. The three phases in the power lines are designed to cancel the magnetic field of one another.  Electromagnetic induction is usually caused when the loading on the three lines is not perfectly balanced.  This problem can be overcome by grounding pipelines and fences at regular intervals. 

What is meant by Right of Way of a Transmission Line

Right of Way refers to the right of the transmission or distribution utility over the strip of land the lines pass through.

Transmission companies make payments to landowners to get a right-of-way of the land beneath the transmission lines.  The length and width of the right of way depends on the voltage of the transmission lines and the height of the transmission structures.  

Right of way usually includes the right to trim trees which can be potentially dangerous by either touching the lines or falling on them.  No structure can be constructed on the right way which may  reduce the ground clearance.  

Fences or pipelines which run beneath the lines shall be grounded.  The utility reserves the right to cut any vegetation which grows beyond a specified height.  Digging the ground for trenches shall not be permitted as they may weaken the foundation of the towers. 

Hollow Insulator

Hollow Insulators are insulators which have a hollow core.  These insulators are used in substations, in circuit breakers, station posts, etc.  They are used to enclose other components such as lightning arrestors, potential transformers etc.  

They are usually made of composite materials.  However porcelain insulators are also available.  They are also lighter and have greatly reduced chances of catastrophic failure.  Hollow Insulator also have excellent seismic resistance. 

The hollow tube in these insulators can be made of FRP.  Fibre reinforced Plastic has excellent resistance to bending and can be used in cantilever-type applications. 

These insulators can be manufactured in a number of shapes such as conical, cylindrical and straight inner or outer appearance.

Arcing Grounds

Arcing Grounds is a phenomenon which is observed in ungrounded three phase systems.  In ungrounded three phase systems operating in a healthy balanced conditions, capacitances are formed between the conductors and ground.  The voltage across these capacitances is the phase voltage. 

Now, in the event of a ground fault, the voltage across the faulty conductor becomes zero while the voltages across the healthy conductors increase by a factor of 1.732. 

The arc caused between the faulty conductor and the ground gets extinguished and restarts many times, this repeated initiation and extinction of the arc across the fault produces severe voltage oscillations of the order of nearly three to four times the nominal voltage. 

This repeated arcing across the fault due to the capacitances between the conductors and the ground is known as arcing grounds.  Arcing grounds can be eliminated by the use of Peterson Coils (see Article) and Arc Suppression Coils

Impact of Pollution on Insulators

Insulators on transmission lines are subject to the deposition of pollutants from the environment.  The nature of the pollutants depends on the nature of the environment. 

In Coastal areas, the pollutants is usually sodium chloride which is deposited onto the the insulator surface.  If the humidity increases or if rainfall occurs, the sodium chloride gets moist and provides a conduction path which can result in a flash over. 

In industrial areas, the chemical salts deposited depend on the nature of the industry.  The moisture absorption by these deposits depend on the relative humidity and climatic conditions.  Some of these deposits can become acidic and can corrode the insulator surface.

The deposits on the insulator surface become conductive when they become and provide a path for the leakage current across the insulator.  When the temperature rises, dry bands form on the insulator surface.(See article on dry band formation). the voltage gradient across these bands increases until arcing occurs across the bands.  These arcs can develop into a flash over.

Post Type Insulators

Post Type insulators are usually used in substations.  These insulators are constructed in the form of a cylinder.  The circular surface of the cylinder is corrugated to increase the leakage distance.  They are mounted on pedestals.  These insulators exhibit high resistance to puncture. 

Post insulators are used in areas with heavy pollution and risk of vandalism.  Slightly damaged post insulators will still have structural integrity and good excellent properties.  Hence, there is no interruption in power.  The damaged insulator can be replaced at the next opportunity. 

Long Rod Insulators

Long Rod insulators are used in applications in place of conventional string insulators.  For HV applications, conductors are usually supported on the transmission towers by means of disc insulators in the form of strings.  The string of disc insulators provides a maximum leakage distance and prevents flashovers across the insulator.

With the use of composite material for the manufacture of insulators, the string insulators are being increasingly replaced with long rod insulators.  Long rod insulators appear similar to the string insulators.  However, they are manufactured in a single piece.  The insulator consists of a "long rod" usually of Fibre reinforced Plastic (FRP) to bear the insulator load.  This rod is designed to have high tensile strength.

The housing of the insulator is usually made of silicone rubber or similar material.  The end fittings of the insulator are directly crimped on to the FRP rod.  Long rod insulators are puncture proof and have high arc resistibility. 

Long Rod insulators are lighter than strings of disc insulators of a similar voltage rating. Long Rod Insulators are used in both HVAC and HVDC applications.

Dry Band formation and Flash overs in Insulators

When an insulator gets wet a thin film of water forms on the surface and a small leakage current starts to flow.  When this film of water evaporates due to rise in atmospheric temperature a "dry band" forms on the surface.  This dry band is formed around the insulator.  When this dry band is formed, the current flow is interrupted and a voltage gradient appears across the dry band. 

This voltage gradient exerts electrostatic stress across the surface and causes further evaporation and an increase in the width of the dry band.  This increase in width of the band causes a higher voltage gradient which causes minor arcing and can lead to flash over.

Dry band formation can be prevented by designing the insulators with  semiconductor glazing the insulator surface.  and by designing the insulator to have increased leakage distance.

Polymeric Insulators - an Overview

Since the 1960s, polymeric insulators have been used in transmission equipment in addition to conventional ceramic or glass insulators.  Polymeric insulators were originally used in areas with high pollution, high risk of vandalism and in urban areas.  Today though, their use has greatly expanded and they are used at almost all voltage levels. They are also known as composite insulators. 

Some of the advantages of polymeric insulators are

• They are hydrophobic and do not allow the accumulation of water on their surface, thus preventing surface currents and flashovers. 
• Their light weight enables easy fitment with smaller cranes and increased clearance distance between conductor and the ground.
• Their higher mechanical strength enables transmission towers to be placed at longer spans. 
• They are resistant to pollution. 
• Composite insulators do not allow the accumulation of dirt.  Hence, the cleaning and maintenance costs of the insulators are reduced. 
 

The disadvantages of polymeric insulators are that damages in these insulators are difficult to detect. These insulators are also vulnerable to erosion and tracking on the surface.Their life expectancy is not predictable

Controlled discharging of Inductive HV Equipment

HV equipment need to be discharged prior to any maintenance work.  The discharge is usually done with an earthing rod after verifying that there is no voltage present on them with an non-contact voltage detector.

In the case of inductive equipment such as transformers and motors with high inductance value in their windings, a controlled discharge needs to be carried out.  Inductive components such as windings in transformers and motors have high inductance.  Sudden discharge of these windings will create a high discharge current and a rapid change in the flux which will result in a high voltage pulse (according to Lenz' Law). This can damage the winding insulation.

The controlled discharge is carried out with a discharge rod with a resistor in series.  The resistor used is a special non linear resistor which has a reverse temperature coefficient which means that as the temperature increases the resistance falls.  When the discharge is started, the resistance is high. As current flows through the resistor, the temperature of the resistor rises and its resistance falls.  Thus the current flow is increased.

The resistor ensures that the discharge is gradual.

SuperCapacitors - an Overview

Super capacitors are capacitors with very high capacitance values sometimes reaching up to 500 Farads.  Supercapacitors are used to store energy just like batteries.  The principle of a supercapacitor is similar to that of a capacitor.  However, the supercapacitor is built using nano-technology.  This enables the dielectric to have a very large surface area and thus store greater quantity of charge. The electrodes are usually made of activated charcoal while the dielectric is an Electrolyte soaked separator. 

Supercapacitors are advantageous over batteries in that they are lighter, more environmentally friendly and can be recycled.  Besides, supercapacitors can be  charged and discharged repeatedly unlike batteries.

Supercapacitors are also known as Ultracapacitors or Electolytic Double Layer Capacitors.   The lifetime of a supercapacitor is can be upto 100 times the lifetime of a battery. 

Supercapacitors find uses in cameras, electric automobiles, power conditioners, welders.

Supercapacitors are used as power sources in conjunction with batteries.  Supercapacitors can supply short burst of power and are useful when heavy loads are applied suddenly.  They also charge faster and absorb voltage transients better while the battery supplies the regular power requirement. 

The are also used in automobiles to where they can be charged easily and are particularly effective in recovering energy from the transmission systems through regenerative braking.

The downside of supercapacitors is the low energy to weight ratio as compared to batteries.  Further advances in technology may narrow out this difference. 

Discharging Capacitors

Capacitors are widely used in Electrical and Electronic applications.  They are used in filter circuits, for power factor correction, for starting motors and for a host of other applications.  Capacitors store electric charge and can retain it for long periods, even days and weeks.  Hence, they can cause a severe electric shock or burns when someone accidentally makes contact with them even when the equipment has been switched off.

Thus, when an equipment is being repaired, it is essential to ensure that the capacitors inside are safely discharged. 

There are three common ways of discharging capacitors.  The first is by shorting the leads with a metallic object such as a screw driver or a wire.  This method causes a rapid discharge of the capacitor as the leads are shorted.  This results in excessive current which can melt the leads of the capacitor.  The molten metal can be thrown around the equipment damaging components and even causing damage to people nearby (in the eyes and skin).  Hence, this method should not be used.

The second method is to discharge the capacitors with filament lamps.  By connecting a filament lamp of the appropriate voltage lamps, the capacitor can be discharged.  The lamps glow initially due to the presence of the charge.  The light, then slowly diminishes indicating the discharge of the potential.  If the voltage is higher than the voltage rating of the lamp, two or more lamps can be connected in series. 

The third method of discharging capacitors is by the use of resistors.  A resistor with a suitable wattage rating can gradually discharge the capacitor.  This is the most ideal method of discharging capacitors.   (See also article on Bleeder Resistors)

Tropicalization of Electric Equipment

Protecting Electric Equipment from the surrounding atmospheric conditions such as humidity, heat, cold, fungal attack, salt spray.   etc is known as the tropicalization of Electric Equipment.  Electric Equipment such as circuit breakers, contactors, PCBs are all tropicalized.  The level of Tropicalization is determined by the environment where the equipment is going to be fitted.   

One of the methods of tropicalization is known as conformal coating.  Conformal coating is done by spraying a dielectric material on the surface of the equipment to provide protection against the ingress of moisture, fungi and other elements.  Substances such as acrylic, silicone, urethane, epoxy can be used in the coating.