Electrical steel is a special type of steel used in the construction the cores of transformers and the stator of motors and generators.  This steel is also known as silicon steel.  It is an alloy of Iron with silicon. The silicon content can be upto 15%  The silicon increases the resistivity of steel minimizing eddy currents and the resulting heat loss.

Electrical steel also has a small hysteresis curve, reducing iron losses.  Heat treatment is done which increases the grain size of the steel and reduces the hysterisis loss.

In grain-oriented silicon steel, the orientation of the grain structure is made in a specific direction in order to increase the flux density and reduce the magnetic saturation.  This type of steel is used in the construction of transformer core where the direction of the magnetic field is predictable and in a specific direction.  The orientation of the grain structure ensures that all the molecules have poles are in the same direction.  This reduces the hysteresis loss.  This is usually more expensive.

In motors and generators though, the magnetic circuit is more complex, hence non-oriented silicon steel may be used.

In electric machines such as transformers, motors, etc., the steel is made in the form of thin sheets which are laminated on both sides. This is done in order to prevent eddy currents from circulating in the core. 

Electrical steel should be handled carefully.  Wrong bending or rough handling can adversely affect the magnetic properties of the steel.

Hot washing of insulators refers to cleaning the insulators in transmission lines when the lines are live. Transmission lines can afford very little downtime. Cleaning the hundreds of insulators has to be carried out when the lines are live with voltage.

Insulators get dirty due to dust, moisture, bird droppings and chemicals from smoke. These deposits will form a layer over the surface and can contribute to a flashover between the conductor and the grounded frame of the transmission tower. Thus, periodic cleaning of the insulators is essential.

Hot washing involves cleaning the insulator surfaces with de-mineralized water. De-mineralized has high resistivity (greater than 50000 ohm cm). The water is pressurized and sprayed in jets from special cleaning machines. These cleaning machines are stationed on the ground or in some cases fixed on helicopters which hover near the lines and clean the insulators.

The Hot washing is usually carried out from the bottom of the insulator. The whole insulator is not made wet at any given point of time. The bottom of the insulator is washed and then the washing proceeds to the middle sections and the then to the top of the insulator.

The water spray from cleaning one insulator should not fall on another insulator. This may cause a flashover. The wind direction should also be taken into account.

Paper being an insulating and dielectric medium finds wide application as an insulation for cables. Paper is made up of cellulose a very good dielectric.

Some of the common types of insulation paper are the kraft paper and the crepe paper.  kraft paper is obtained from the pulp of soft wood.  When this kraft paper is impregnated with liquids to enhance BDV and maintain pH, crepe paper is obtained.

Crepe paper is flexible and tough which enables it to be wound over sharp turns in the windings.  Semi-conducting insulation paper is used in some transformer winding to ground the leakage winding.  

Transformerboard is a kind of paper insulation which provides has a rigid structure.  It is widely used for transformer insulation.

Fish Paper is another kind of insulating paper which is vulcanized.  Fishpaper can even be machined to form a specific shape.  It has excellent insulating and mechanical properties

Hydrogen is a cooling medium used in high capacity generators which generate large quantities of heat. 

Hydrogen has a higher capacity to transfer heat, its specific heat is about 14 times that of air of similar weight.

The density of hydrogen is lesser than that of air.  This reduces the windage losses.  Thus the efficiency is improved.

Hydrogen also improves the life of the insulation as the hydrogen circulated is pure without dirt, moisture, etc.  This reduces the maintenance required.

Alternators which are to be hydrogen cooled have special circulating ducts in the windings to circulate hydrogen along with seals to prevent leakage and pumps.  The hydrogen used for cooling is locally produced using electrolyzers.  These equipments produce hydrogen by the electrolysis of water.  The hydrogen is used in a closed circuit at a pressure of around 6 bar(kg/cm2)

The major risk of using hydrogen is its flammability.  Hydrogen burns readily in the presence of oxygen(air).  To prevent this, the hydrogen is maintained at a purity of more than 70%.  This ensures that there is very little oxygen available so as to prevent combustion.    The high pressure in the hydrogen cooling circuit ensures that no air can enter the circuit.  A small quantity of hydrogen may leak into the atmosphere.  This is compensated by adding hydrogen.

Hydrogen is a odorless gas.  Hence, special gas detection equipment are required to detect its leakage.

Amortisseur windings are bars which are found in the rotor of synchronous motors.  These bars are short circuited similar to the rotor windings in a squirrel cage motor.  The function of these windings is to dampen the torsional oscillations in the rotor that may occur as a result of load fluctuations.  They are also known as damper windings.

The Amortisseur windings can also be used to start the synchronous motors.  The Synchronous motor is not self-starting.  Hence, motor starts initially as an induction motor through the action of the amortisseur windings.  When the sufficient speed has been attained, the excitation to the rotor of the synchronous machine is switched on.  The motor then runs at the synchronous speed as a synchronous machine. 

Amortisseur windings also play a role in preventing rotor overheating when a synchronous machine is exposed to negative sequence currents.  Negative sequence currents may be induced when a synchronous generator is subject to a short-circuit.  Under these conditions, a negative phase sequence currents is induced in the rotor of the generator.  The amortisseur winding provides a path for the rotor currents and prevents the from flowing through the rotor forging and cause overheating.

image courtesy : www.tecowestinghouse.com

Gas formation in Transformer oil is an indicator of problems inside the transformer.  Gas can be formed due to the decomposition of either the oil or the insulation of the windings.

The gas formation in transformers can be gradual or sudden.  Gradual  Oil decomposition can be caused due to minor leakage faults within the transformer, internal short circuits.  Loose connection of the windings can also be a reason for gas formation.   Overheating of insulation is another cause for the formation of gas.

The Gas thus formed tends to get dissolved in the transformer oil.  The dissolved gas can be detected by Dissolved Gas Analysis.  The dissolved gas can get released when the transformer experiences temperature fluctuations during its operation cycle.

Major faults such as flashovers lead to sudden gas formation in Transformers.

The Bucholz relay is the protection in transformer against gas formation.  Minor gas formation will accumulate inside the relay and activate the top most float.  This can be configured for alarm  Sudden discharge of gases due to major faults will cause the bottom float in the relay to operate and trip the transformer.

Three phase power transmission has become the standard for power distribution. Three phase power generation and distribution is advantageous over single phase power distribution. Three phase power distribution requires lesser amount of copper or aluminium for transferring the same amount of power as compared to single phase power.

The size of a three phase motor is smaller than that of a single phase motor of the same rating. Three phase motors are self starting as they can produce a rotating magnetic field. The single phase motor requires a special starting winding as it produces only a pulsating magnetic field. In single phase motors, the power transferred in motors is a function of the instantaneous current which is constantly varying.

Hence, single phase motors are more prone to vibrations. In three phase motors, however, the power transferred is uniform through out the cycle and hence vibrations are greatly reduced. The ripple factor of rectified DC produced from three phase power is less than the DC produced from single phase supply.

 Three phase motors have better power factor regulation. Motors above 10HP are usually three phase. Three phase generators are smaller in size than single phase generators as winding phase can be more efficiently used.

Winding Resistance is an important measurement in electrical machines.  Winding resistance tells us about the condition of the winding.  Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value.  Besides, winding resistance is used to measure I2R losses in the winding.

The Winding Resistance is measured by winding measurement test kits.  In earlier times, winding resistance was measured using the Kelvin Bridge. The Kelvin Bridge is an arrangment of resistors which enables the measurement of very low resistances.  Winding Measurement Kits work by injecting known current through the winding and measuring the voltage drop across the winding.

The machine to be tested is disconnected from the lines and de-energized.  The measurement are usually taken phase-to-phase.  The three readings should be within 1% of the average value.

Winding resistance can change with temperature.  The measurement are usually taken at the cold temperature known as the cold resistance.  The transformer or the motor is allowed to cool for a few hours and the temperature taken.

Based on the measurement taken at a particular temperature, the resistance at any other temperature may be calculated from the following formula

Rs= Resistance value to be calculated at a specific temperature
Rm= Resistance valued measured
Tm= Temperature at which the resistance was measured
Ts= Temperature at which the resistance is to be calculated
Tk= Winding Material Constant ( 234.5 °C for copper or 225 °C for aluminum)

The windings can store a huge amount of electromagnetic energy when a current is passed through them during measurement. When the test current is stopped, there may be a voltage kickback from the winding.  The test equipment should be able to absorb the voltage kick and safely discharge it.

Winding Resistance is an important measurement in Electrical Machines.  Winding Resistance tells us about the condition of the winding.  Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value.

The Winding Resistance is measured by winding measurement test kits.  In earlier times, winding resistance was measured using the Kelvin Bridge .

Cooling is a vital aspect in the construction and operation of generators.  Cooling in generators can be broadly classified into two types
  • Open Circuit and
  • Closed Circuit
In Open Circuit cooling, air is drawn into the generator by means of fans and circulated inside.  The air is later released back into the atmosphere.  This is a simple method of cooling which does not require elaborate circulation equipment.  This kind of cooling is suitable for small alternators.

Closed Circuit cooling is used in large-sized alternators.  These alternators cannot be cooled by air as they generate a huge amount of heat.  

Here, hydrogen is usually used as the cooling medium.  The hydrogen is passed through the generators by means of pumps and then drawn back into a chamber.  Special circulation equipments such as radial and axial ducts, seals and pumps for this type of cooling.

Hydrogen can transfer heat better than air as it has a higher specific heat.  It has a low density which results in reduced windage losses for the alternator rotor.   The alternator frame can also be reduced.

Water can also be used as a cooling medium in closed circuit cooling systems.  Water has a better cooling capacity as compared to Hydrogen.  However, the circulation equipment for water are more expensive. Special systems for the purification of water are also required.

Some generators use both hydrogen and water cooling systems.

 www.SpecAmotor.com is a website that helps engineers quickly select a motor based on their requirement.  You just have to only key in the required speed and torque and specify the type of motor you are looking for, induction, dc, synchronous, etc.

The site launched in 2008, now claims to have a database of over 10000 configurations.  The site is brand independent and includes specifications from all leading brand. 

You can also test the characteristics of motors which fit your search online.

          Gang operated Switches or G.O Switches as they are commonly known are switching devices used in overhead power lines.

          They are called Gang Operated as they are operated in a Gang, all three switches together, using a single mechanism.  They are also called air-break switches as they use air as the breaking medium or G.O.D.(Gang operated Disconnector) switch.

          These switches do not have any load-breaking capacity.  They can only be operated when the transformer is on no-load and only the small magnetizing current flows through them.  A substation must be disconnected from the load it is feed and only then can these switches located on the incoming lines of the substation be disconnected.
          These switches are used in lines with voltages about 5 kV.  They can also be motorized and operated remotely.  Gang operated switches can be mounted vertically or horizontally.

          A thin film of non-oxide grease is usually applied to the contacts of the Gang operated switches.  The Gang operated switches should be checked periodically for proper alignment and rigidity.

Isolation Transformers are transformers which serve to isolate two parts of a circuit. This may be required to ensure that a fault in one part does not affect the other parts of a circuit.

Strictly speaking, all transformers are isolation transformers. The primary and secondary sides are connected through induction and not through conduction. An exception is the autotransformer which cannot provide isolation as its windings are shared.         

Isolation transformers have a transformation ratio of 1:1 as no voltage transformation is involved.         
A grounded shield is usually placed between the primary winding and the secondary winding to prevent any risk of capacitive coupling between the windings. Capacitive couplings can compromise the isolation and provide a linkage between the primary and the secondary circuits. Isolation Transformers are widely used in electronic circuits and in medical instruments.

Transformers are indispensable equipments of the electrical system. The transformer alters the voltage level at the same frequency. Here is an interesting video about the Transformer Construction

Bushings are insulating components which carry a conductor through a metallic component s for instance, bushings are found in Transformers as they carry the conductor from inside the transformer out to the terminals.  Bushings are usually made of ceramics.

          Bushings are also found in circuit breakers, alternators, motors and capacitors.  Some indoor substations use bushings to carry the conductors outside through the roof.  Electric locomotives also use bushings to support the overhead traction equipment.

          A bushing is usually a hollow ceramic tube through which the conductor passes.  Bushings also provide rigid support to the conductors inside.

          Bushings are used in both low voltage and high voltage applications.  In low voltage applications, the ceramic casing alone is sufficient to provide insulation to the conductor.  These bushings are known as non-condenser bushings.

           However, in high voltage application (>52 kV) the bushings are filled with insulating media such as oil, resin impregnated paper, oil impregnated paper, etc to provide greater dielectric strength.  Sulfur hexafluoride gas is also used in some high voltage bushings.   These bushings are also known as condenser bushings, as they form a capacitor between the live conductor and the equipment body which is at ground  potential

The Bending radius of a cable refers to the sharpest possible bend a cable can take.  Cables are flexible conductors and they needed to be bent at numerous places as they carry power.

Cable manufacturers usually specify the minimum possible radius that a bend can take.  This is known as the bending radius.  If the cable is bent at a radius below the minimum specified radius, the cable can get damaged.

 As a rule of thumb, the bending radius is 10 to 15 times the diameter of the cable.  However, the bending radius value provided by the manufacturer should be consulted before laying any cable.

In a transformer, the primary and the secondary windings are linked magnetically.  The magnetic flux created by the exciting current of the primary winding induces a voltage in the secondary.

In an ideal transformer, all of the flux generated in the primary winding will be linked to the secondary winding. 

In such a transformer, the efficiency will be maximum.  Modern Transformer cores use high silicon steel with high permeability and winding which are placed close to each other to achieve a high coefficient of coupling.

However, in actual transformers, some of the flux built in the primary winding is not linked to the secondary.  This flux which is not linked to the secondary is known as the leakage flux.  The leakage flux causes a drop in secondary voltage. 

Leakage inductance.

For the purpose of calculation, the leakage flux is assumed to be an inductance connected in series with the primary winding which causes a drop in the applied voltage in the primary.  This is known as the leakage inductance.

Turns Ratio of a transformer is defined as the ratio of the number of turns in the Primary to the number of turns in the Secondary.

The voltage ratio is defined as the ratio of the Primary voltage of the transformer to the secondary voltage.

In transformers with small transformation ratios, the Turns ratio is usually equal to the Voltage ratio. However, for transformers with higher transformation ratio, the voltage ratio may be different from the Turns Ratio. This is known as Ratio Error.

Ratio error can be caused by a number of factors such as leakage inductance, copper losses within the transformer and inter winding capacitance.

The Ratio Error is detected by a ratio test.  A simple ratio test involves applying a fixed voltage on the primary and measuring the voltage induced in the secondary.

The induction motor is at the very heart of industry.  This simple device efficiently converts electrical energy into rotating motion.  The principle of operation of the induction motor is described in this video.

How do you ensure a steady supply of electricity from fickle renewable sources such as solar, wind and biomass? 'Smart grids' that manage and distribute flows of electricity could be the answer.

     The EU's ambitious goals of a 20 percent increase in energy efficiency, a 20 percent increase in renewables and a 20 percent reduction of carbon dioxide emissions - all by the year 2020 - are a huge undertaking. Experts agree that none of the "20:20:20" goals are achievable without a functional smart grid that ultimately optimizes energy generation.

     That's because simply building more wind plants and solar collectors won't be enough, as renewable energy is hampered by the fact that it's not constantly available – after all, if the wind doesn't blow or the sun doesn't shine, wind turbines or solar cells don't generate energy.
     'Smart grids' on the other hand can manage and distribute intermittent energy supply from small power plants, wind mills or solar systems without a hitch, leading to a steady supply of electricity.   If there's too much electricity in the grid, it can be stored in batteries, for example when an electric car rolls up to a recharging point.

Smart energy draws big players
     Smart grids provide utilities with the information and flexibility required to manage intermittent electricity supply from renewable and micro-generation sources, allowing them to balance this with more traditional, consistent supply.

     Large companies such as Switzerland's ABB, a rival of Germany's engineering giant Siemens, say smart grids are a huge trend in the energy sector.

     "There are lots of solutions that are already available. We could begin with them right away," Peter Smits, the head of ABB Europe said. But Smits says incentives, like feed in tariffs, are still needed to encourage the switch to a bigger pool of renewable energy sources.

     "The more renewable energy we have to feed in the network, the more the utilities and electricity distributors are going to need this solution," Smits said. The International Energy Agency estimates that by 2030, worldwide investments worth several trillion dollars will be needed for modern energy generation and new electricity networks.

     Indeed, the booming market, which promises lucrative returns, has attracted new players to the utilities sector, including IT giants such as Google, IBM, Cisco, Microsoft or telecommunications firms such as Deutsche Telekom.

Various smart approaches
     From the use of smart meters in households in Italy or France, to government-backed pilot-projects in the US, there's a growing momentum for groundbreaking smart energy schemes.  In Germany, too, the government is trialing smart grids in a few hand-picked regions.
     The impression that Germany is lagging behind on smart energy projects is not true, says Hartmut Schmeck from the Institute for Technology in Karlsruhe.  He says selected regions in Germany are testing the entire supply chain - from electricity generation, and distribution to supplying the end consumer.  "Other countries are trying out projects where individual processes are monitored. But comprehensive, holistic approaches like the ones in Germany - they don't exist elsewhere."

Customers needed
     It's expected that in two years, Germany's model regions will have developed their smart energy concepts to the point where they'll be ready for the market and everyday use.  This summer, project organizers set up a model house complete with washing machine, refrigerator and an electric car. The latter plays a key role in the project - both to store electricity as well as consume it.

     Still missing are the thousands of electricity consumers needed to test the whole thing.  "The most important thing is that the customers play along because you can't have a smart grid without customers," said Joern Kroeplin from the energy utility firm EnBW, which is involved in the project.  Kroeplin says it is important to find tariffs and models that will create the necessary incentives.  "We also need the necessary appliances that customers accept and use. Without all that, it won't work," he said.

Not all smooth sailing
     A first intelligent washing machine manufactured by German company Miele was on display at this year's consumer electronics fair IFA in Berlin. It switches on when the electricity price falls below a certain level. But it also needs a smart plug which recognizes what the electricity currently costs.  For smart grids to work, a number of players will need to come together, including industrial and private consumers, to help share the introductory costs.

     Another obstacle remains. Aside from their upfront costs, smart meters also use a lot of energy to maintain, due to all the data flows operating in real time.  The required internet connection alone, which runs 24/7, uses over 100 kilowatt hours of electricity in a year – almost exactly as much as a modern refrigerator.

courtesy : http://www.dw.com

High Voltage probes are used to measure high voltages which are beyond the range of common measuring instruments.  For instance, ordinary multimeters may not be able to measure the high voltages generated in the Television set.  In these conditions, High voltage probes may be used.  High Voltage probes usually contain resistors in series.
High power probes can be used with multimeters, oscilloscopes, synchroscopes and a wide variety of industrial measuring instruments. 

The instruments which are connected to the High voltage probes usually have a high internal impedance to limit the current.

High voltage probes have a voltage ratio similar to a transformer.  High voltage probes are also rated on the amount of power they can withstand.  Some High voltage probes are designed only for low power application.

Liquid Rheostats are variable resistors which use a liquid electrolyte.  The electrolyte, usually common salt, is used as the resistor whose resistance value is varied by changing the level.

The construction consists of two electrolyte placed in a container.  The container is filled with a solution of common salt.  There is a provision to vary the level of the electrolyte in the container.

This changes the conductivity and thus the resistance is varied.  At high levels of the electrolyte, the resistanc is very low and increases as the level decreases.

Liquid rheostats are silent and are supposed to have long life.  They are usually only used in AC circuits as DC may cause electrodeposition between the electrodes. 

Liquid rheostats can also be used as load banks to test generator output at testing facilities.  The output of the generator is connected to electrodes which are placed in a container filled with a salt solution.

Liquid rheostats are also used as resistances for starting induction motors.  Some large liquid rheostats have a heat exchanger to control the temperature of the electrolyte. 

Capacitors are components which are used in electric and electronic circuits to store charge, to filter dc, to improve power factor and so on.  There are many kinds of capacitors available.  Ceramic, Paper capacitors, electrolytic capacitors and so on.

Electrolytic capacitors are called as one of the plates of the capacitor is made of an ionic conducting liquid, an electrolyte.  These capacitors must be connected in a fixed polarity.  Hence, these capacitors cannot be used in AC circuits without a dc bias.  These capacitors have a high capacitance value.

Polarity is usually indicated in the capacitors with the positive lead longer than the negative lead.  Alternatively, the polarity markings are made in the capacitor body.

Connecting these capacitors in the wrong polarity will cause heating of the electrolyte and lead to an explosion, a catastrophic failure.  Most capacitors are provided with a vent to relieve pressure and prevent explosions.

Wound rotors are used in applications where high starting torque is required.  External resistances may be added to these rotors via slip rings shaft.  These resistances serve to increase the starting torque and ensure smooth starts.  

However, these rotors are more expensive than induction motors.  In the wound rotor, the rotor windings are insulated to the ground.  The slip rings and the brushes also require maintenance.

The starting current drawn by a wound rotor machine is lesser than that that of a squirrel cage motor.

The wound rotor is designed to have the same number of poles as the stator winding of the motor.  The windings are designed to with stand high mechanical forces as these motors are used for high-torque applications. 

Wound Rotors are used for applications which require soft-starts and adjustable speeds

Squirrel cage rotors are the most common type of rotors found in induction motors.  These rotors are simple to construct, robust and relatively inexpensive. 

They are particularly suited for low inertia loads.  Their easy construction enables lower rotor weight and lesser centirfugal force and windage losses.

Wound rotors are used in applications where high starting torque is required.  External resistances may be added to these rotors via slip rings shaft.  These resistances serve to increase the starting torque and ensure smooth starts.  

However, these rotors are more expensive than induction motors.  In the wound rotor, the rotor windings are insulated to the ground.  The slip rings and the brushes also require maintenance.

The starting current drawn by a wound rotor machine is lesser than that that of a squirrel cage motor.

The wound rotor is designed to have the same number of poles as the stator winding of the motor.  The windings are designed to with stand high mechanical forces as these motors are used for high-torque applications. 

Wound Rotors are used for applications which require soft-starts and adjustable speeds

Squirrel cage rotors are the most common type of rotors found in induction motors.  These rotors are simple to construct, robust and relatively inexpensive. 

They are particularly suited for low inertia loads.  Their easy construction enables lower rotor weight and lesser centirfugal force and windage losses.

A surge arrestor is an electric equipment used in substations and switch yards. The surge arrester is used to protect the substation equipment from surges caused by lightning or by sudden switching. The surge arrestor is an insulator which is a non-linear resistor.

This means that the surge arrestor has high resistance at the operating voltage and low resistance as the voltage increases. Thus when lightning strikes the overhead conductors in a substation, the arrestor acts like a conductor and discharges the surge to the ground.

Surge arrestors are usually constructed of MOV(Metal oxide varistor). Zinc oxide is a widely used non-linear resistor. The zinc oxide is the form of blocks which are stacked inside the arrestor.

Capacitors can fail due to a number of reasons.  The failure of capacitors can lead to short-circuit, damage to the circuit and sometimes even explosion. 

Let us look at some of the reasons for failure of capacitors.

Electrolytic capacitors fail due to leakage or vaporization of the electrolyte inside.  This can be caused due to heating in operation.  Heating can be caused by either wrong connection or the use of under-rated capacitors.

In electrolytic capacitors heating can cause the formation of gas inside which can explode through the vent provided.

Voltage surges can also cause capacitor failure.  Overtime, capacitors re-form themselves to a particular voltage.  When an unexpected surge occurs, a failure can take place. 

Ceramic capacitors crack during overvoltages.  This may create an open or short-circuit.

Tantalum capacitors are specially sensitive to voltage.

Electrolytic and Tantalum capacitors have polarity.  The leads are marked positive and negative.  Wrong polarity connections of these capacitors can cause explosion or failure. 

In addition to these causes, mechanical damage, heat and ageing can also cause capacitor failure.

          Parasitic loads are electric devices and appliances which draw power even when they are off.  For example, the television which has been switched off using the remote but, is still connected to the power socket continues to draw a small quantity of power.  Parasitic loads are other known as phantom loads or
vampire loads.

          It is estimated that around 5% to 10% of the total load consists of phantom loads.  Phantom loads can be avoided by disconnecting any device not in use from the mains supply.

           For example, a kitchen mixer should be disconnected from the mains or the switch of the power socket switched off when the device is not in use.  Another example would be a laptop that is switched off with a power adaptor which is still connected to the mains. 

What causes Parasitic loading

          Devices which have windings, such as motors, transformers draw a charging current.  The charging current is constant throughout the loading cycle of the machine.  The charging current causes Iron losses due to eddy current heating.  This power is wasted as heat and hence is a loss to the power system.   

          Many countries such as the US, Britain, and the EU have initiated laws which stipulate that appliances in the switched off mode should not draw power more than 1 watt.  This is known as the One Watt Initiative.  Better design of transformer with Torroidal cores which generate lesser losses is one way of reducing phantom power.

Power Factor can be defined as the cosine of the angle between the current and the voltage.  This power factor is also known as the Displacement power factor.

The conventional measurement of the power factor is relevant only for loads that are linear and the waveforms are purely sinusoidal.  With the increase in non-linear loads such as inverters, drives, etc this definition of the power factor is not adequate.  This is because the harmonics have an impact on the power factor.

Thus, the total harmonic distortion should also be considered while calculating power factor. 

How is True Power Factor different from Measured Power Factor ?

The true power factor refers to the measured power factor at the system frequency which is adjusted for the Harmonic distortion

Thus for loads which have high harmonic content, the True Power factor needs to be calculated.

   Cambridge University has developed a new Photo voltaic technology which could enable house owneres to produce electricity from sunlight without having to install expensive solar panels.
The technology is being developed by Eight19, a new venture built on the collaboration between the Cavendish Laboratory of Cambridge University and a British Environmental group called the Carbon Trust.  

Organic Photovoltaic technology is different from conventional photo voltaic technology.  The technology mimics the process of photosynthesis in plants.  These solar panels are flexible and can be easily unrolled and installed in windows, walls and other flat surfaces.  This dramatically improves the scope of use for solar electricity.

More importantly, the low cost of this technology could enable wider installation of solar based power sources in remote places where there is no grid connectivity. 

For more information

image courtesy :  independent.co.uk

The Basic Impulse Insulation Levels refers to the ability of electric equipment such as transformers to withstand lightning surges.  When lightning strikes a transmission line, a traveling wave is created.  This traveling wave travels along the line and damages the transformer winding.

Surge arresters mounted on the line can mitigate the surge, however, they cannot totally eliminate it.  Voltage surges can also be created by the switching of circuit breakers and switches. 

The BIL or the Basic Impulse Insulation level indicates the ability of the transformer to withstand these heavy surges.  Transformers with a rating of 600 volts and below are designed to have a BIL of 10 kV.

LEDs have found wide application in the field of lighting.  Homes can also be illuminated with LED lamps.  LEDs come in a variety of shapes and can be chosen to suit the decor and application.

The advantages of LED lighting over conventional lighting are
  • Longer life - The life of a LED lamp is 100000 hours(11 years) as compared to 5000 hours for ordinary bulbs.
  • Cheaper - While LED lamps are costlier than filament lamps, their long life more than compensates for the high cost. 
  • Reduced Power Consumption - The power required for LED lamps is lesser than that required for conventional lighting. 
  • LED lamps have high efficiency and produce less heat.
  • LEDs are available in a wide choice of colors
  • The LED bulbs can be made in a wide variety of shapes and designs
  • LED bulbs are Eco-friendly as they do not contain mercury unlike conventional filament lamps.

LEDs are today connected in the form of clusters containing a number of LED lamps.  Handheld torches are also being produced with LED bulbs.

LEDs have also found application in industry, in traffic signals and in hospitals.  LEDs also help avoid downtime due to bulb failure which can be expensive and interruptive in applications such as traffic signals, in street lighting, etc.

With improved manufacturing techniques, the price of LEDs is expected to continue to drop further.

          Georg Simon Ohm is credited with developing the first empirical relationship between voltage, current and the resistance of a conductor.  This relationship popularly known as Ohm’s law is one of the basic rules of Electrical Engineering. It is also the first step in electric circuit analysis.

          Georg Simon Ohm (1789 – 1854) was born in Elagen, Bavaria in Germany. He had his early education at the the Elagen Gymnasium.  In 1805, he joined the University of Erlagen and later moved to Switzerland where he took up the job of a Mathematics teacher.  He obtained his doctorate from the University of Erlagen in 1811.  He then took up various teaching positions in Germany.

          Ohm, however, was not satisfied with his teaching career which was not particularly well paying.  He strived to make his mark in research and establish his credentials as a scholar.   Towards this end, he wrote a book on elementary geometry.  Ohm had sent a copy of the manuscript to to King Wilhelm III of Prussia.  The King was pleased and offered him a position at the Jesuit Gymnasium at Cologne. 

          The position suited Ohm well as the institution had extensive facilities for research in Physics.  Ohm soon involved himself with research in the field of current electricity.  The Italian Alessandro Volta had invented the battery and Ohm found himself drawn into studies into the flow of electricity in substances.

          He published his findings in 1827 in the  his famous book Die galvanische Kette mathematischbearbeitet (translated as The Galvanic Circuit Investigated Mathematically).  The publication, however brought him little recognition or appreciation. Disappointed, Ohm resigned his position at the Jesuit Gymnasium and joined the Polytechnic School at Nuremberg.

          While Ohm was able to give an empirical relationship for the relation between voltage, current and resistance, he was unable to give a convincing mathematical proof.  This was one reason for his not getting the recognition he rightly  deserved.    In those days, electrical technology was principle a science of the laboratory with little practical applications.  A sound mathematical proof was essential to convince the scientific fraternity. 

          Finally in 1841, the Royal Society in London recognized his work and honoured him with the Copley medal.  He was admitted to the Society the following year.  1849, Ohm was offered the position of Professor of Experimental Physics at the University of Munich.  He died in 1854 in Munich at the age of 65.

          Though largely ignored during his lifetime, the discovery of George Simon Ohm is fundamental to Electrical Engineering.  The relationship he discovered is one of the most used formulae by an Electrical Engineer.

          In 1893, the International Electrical Congress named the unit of electrical resistance, the ohm, in his honour.

          Power factor correction involves improving the power factor of a system by adding capacitors to reduce the parallel. Power factor correction is used widely nowadays as utilities increasingly levy penalties for low power factor. Low power factor causes loads to draw a higher current for the same power factor.

         The capacitors draw a leading kVAr to compensate for the lagging kVAr drawn by the load. Thus the total kVAr required for the load is reduced.

Calculation of capacitors required

          The total value of the capacitors to be connected can be calculated from

KVARneed for correction = kW X (tan φ uncorrected - tan φ target )

         Where φ refers to the phase angle of the target and the uncorrected power factor

 Excess Power Factor Correction and Self Excitation

The total kVAR to be connected should not exceed the kVAr required to bring the power factor to unity when the motor is running on no-load.

          This is to prevent a condition called self excitation which can cause high voltages and excessive torque. Self excitation is a condition that occurs in induction motors which have capacitors connected to them. Consider a motor coupled to a load running with capacitors connected to it. If the supply to this motor is cut off.

         The motor will continue to run for some time due to the inertia of the load coupled to it. During this time, the energy stored in the capacitor begins to excite the windings. The inductance in the windings and the capacitors together form a resonant circuit. The oscillations produced in this resonant circuit can produce high voltages which can cause damage.

          If the supply is again switched on during this period, the motor can experience sudden movements with high torques.

Extra low voltage refers to reduced voltages which are used in houses, parks, gardens, swimming pools to eliminate the risk of electric shock.

AC voltages below 50 volts and DC voltages below 120 volts are considered to be Extra low Voltage.

In many countries, Extra Low Voltage supplies are used to power traffic signals.  This has been facilitated with the advent of LED lighting technology.

Extra Low voltage systems can also be easily integrated with solar panels as the generating voltage is lower.

Reactive load refers to the load that is in consequence of the impedance or the capacitance of the load. Thus when a capacitive or inductive load is connected to a power source a current flows through the load which does not produce any active power consumption (kW). This charging current is called the reactive current (I .Sin φ).

The power caused by this current is called the reactive power. It is denoted by Q .

Q= V x I x cos φ

The reactive power is measured by kVAr (Reactive Kilo Volt ampere). The higher the value of the reactive load, the lower the power factor. For example, as a motor consumes more reactive power its power factor decreases.  This is evident from the following diagram.

As the reactive power decreases, the power factor increases.

Alternatively, the power factor is the ratio between the Active power, P and the Apparent Power, S

Cos φ = P/S

An exploded battery
Battery explosions are serious accidents which can cause severe injuries and burns to operating personnel and damage to equipment.  Explosions in batteries are caused by accumulation of gas inside the battery.  The gas is formed when the electrolyte(usually acid) gets electrolysed resulting in the formation of gas, usually hydrogen.  Hydrogen is extremely combustible.

This acculumulated gas sometimes shows up in the form of a bulge in the batteries.  These batteries should be replaced immediately.  These explosions may be triggered by a short circuit inside the battery or a sudden load which results in high current, such as jumpstarting a car.

You can prevent battery explosions by making sure that the batteries are charged only by the correct chargers provided with them.

Ensure that the battery connections are properly tightened.  This can prevent sparks due to loose contact.  

Avoid sparks or naked flames near the battery bank.

Ensure that no overcharging takes place.

Do not short circuit batteries.  Cellphone batteries should not be carried in your shirt or trouser pockets where small coins can short them.

Do not dispose off batteries in fire.

Batteries can also explode if they are suddenly subject to mechanical impact or deforming forces.

image courtesy: www.tractorshed.com

The Rotor voltage drop test is used to identify the presence of shorted turns in the salient poles of the rotors of synchronous machines. The test works by measuring the impedance of each pole on the rotor.  If there are short-circuits in the rotor winding, the impedance of the rotor poles will be less.

The equipment required is relatively simple.  A fixed AC voltage is applied across each end of the rotor winding and the voltage drop across each pole of the rotor is measured. The voltage values are tabulated.  The average of the values is calculated.  Each individual value should be within a limit of 10% from the average value.  Any further variation would indicate shorted turns in that particular pole.

While the rotor voltage drop test is useful in identifying inter turn faults in the rotors, they are not considered comprehensive.  Since the rotor is a rotating object, there are shorts which may form only during running due the action of the centrifugal force on the rotor.

Bulging in batteries usually indicates gas formation. Gas formation can occur due to overcharging or due to an internal short circuit in the battery. Short circuit causes arcing which can cause gas formation. Overcharging can be caused by improper battery charger settings which apply current even the battery has become fully charged.

Batteries which have bulges need to be replaced. These batteries  may not charge fully or may not hold charge. If the reason for the battery bulge is found to be overcharging, the charger also needs to be checked and the charge settings changed.

The Di-electric Absorption Test is a test conducted on the insulation of the windings in electric equipment.  This test tells us about the cleanliness and the condition of the windings. 

The dielectric test is carried out using an insulation tester (Mega-ohm-meter).  The insulation test of the winding is conducted and the values noted after 1 minute.  The test is continued and the reading is noted after every minute.  Ten readings may be taken.

The readings thus taken are plotted on a graph with respect to time.  The curve obtained will look like curve A in the diagram.  This curve A indicates that the insulation resistance increases as time increases.  This is because the insulation  gets polarized as the test voltage is applied.

As the insulation gets polarized less and less charge carriers are available for the current.  This causes the resistance to increase.

If the curve is flat (Curve B) or drooping downwards, it indicates that the insulation is in bad condition and should be repaired.  

The Magnetic Balance test is conducted on Transformers to identify inter turn faults and magnetic imbalance.  The magnetic balance test is usually done on the star side of a transformer.  A two phase supply 440V is applied across two phases, say, 1U and 1V.  The phase W is kept open.  The voltage is then measured
between U-V and U-W.  The sum of these two voltages should give the applied voltage.  That is, 1U1W + 1V1W will be equal to 1U1V.

For instance, if the voltage applied is 440V between 1U1V, then the voltages obtained can be

1U1V = 1U1W  + 1V1W
440V =  260V +  180V

The voltages obtained  in the secondary will also be proportional to the voltages above.

This indicates that the transformer is magnetically balanced.  If there is any inter-turn short circuit that may result in the sum of the two voltages not being equal to the applied voltage.

The Magnetic balance test is only an indicative test for the transformer. Its results are not absolute.  It needs to be used in conjunction with other tests.

Underfrequency refers to a condition where the frequency of the AC supply drops to a value that is lower than its defined value such as 50 Hz or 60 Hz.  Underfrequency is usually caused by overloading a power source or problems with a prime mover such as engines or turbines.

The following are some of the effects of underfrequency

  • High flux density in electric machinery, thereby causing higher magnetizing currents.
  • High core loss and over-heating of the machines and possible failure.
  • Lower efficiency
  • Reduction in speed
  • The fault level increases due to reduced reactance

      Underfrequency situations can be avoided by the installation of underfrequency relays which isolate systems and machinery in the event of an underfrequency.

      The European Union is proposing an ambitious venture to install solar panels in the arid stretches of the Sahara, the world's largest desert.

      These solar panels will be of the concentrated solar thermal type. consisting of a parabolic mirror which will focus the light and heat to a central tower.  An engine in the tower( Stirling engine) will convert the heat to mechanical energy which will power a generator.  This method is superior to the photovoltaic type of solar power generation in that it enables the heat energy to be stored and enables power generation during night time as well using the accumulated heat.

      The power generated will be transported to Europe through a network of highly efficient underwater cables laid across the mediterranean.  In the next five years, a small test project is to be completed.  It is estimated that by 2050 almost all the power required for Europe will be generated from this solar network.

      According to estimates, if only 1% of the Sahara were covered in Solar Panels, there would be enough power to power the entire world.  Food for thought, indeed!

      Residual Current Devices are protective devices to safeguard against an electrical shock or any other consequence of a leakage current.  These devices find extensive use in homes, industries and other places.

      These devices functioning by measuring the current going into and coming out of an electric circuit.  In the ideal situation, both the currents are the same.   When a current leaks, either as an electric shock to a person or through an earth fault, the current which comes out of the device is lesser than the current entering the device (this difference is called the residual current).  The device senses this and trips the circuit.

      These devices are alternatively known as ground fault interruptors or appliance leakage current interruptor.

      Electrocution or death due to electric shock requires only small currents in the range of 5 to 50 mA.  Thus these devices are stipulated to operate at a leakage current of 5mA.

      Industrial versions of the RCCBs are sometimes used for situations other than electrocution.  These have a higher tripping threshold of the order of 500mA.

      RCCBs can also be used for three phase applications. 

      Residual Current circuit breakers usually have an inbuilt test function to ensure that the device is functioning.  This test button needs to be operated once a month to ensure a state of readiness.

      Residual current circuit breakers should not be used as a substitute for ordinary circuit breakers as they will not operate in the event of an overload or a short-circuit.  This is because the residual current does not rise in these events.  Therefore, an independent overcurrent protection needs to be given. 

      Fuses are used to interrupt electric circuit in the event of heavy flow of current to short-circuits or earth faults in a system.  In low voltage system, fuses are widely used as they are cheap, reliable and can be easily replaced.

      Low voltage fuses can be classified into
      • Cartridge Fuses where the fuse element or the fuselink is located in a cartridge and
      • Wirable Fuses where the fuselink is a wire that is wired over a ceramic fuse carriers
      In cartridge fuses, the fuselink consists of wires which have low resistivity.
        The material used for the fuselinks needs to have low melting and vaporising temperatures and low specific heat.  This ensures that the fuselink acts quickly, melts and interrupts the circuit.  The vast majority of fuselinks used today are alloys which use either copper or silver as the main ingredient in their construction. 

      The fuselink is enclosed in a filling material which is usually quartz.  The function of the filling material is to absorb the energy released during fuse operation and to extinguish the arc which may occur.

      Bolted Short Circuit refers to a worst case scenario of a short circuit where all the conductors in a system are imagined to be "bolted" together, thus causing the highest possible fault current.  This hypothetical scenario is used to calculate the fault level of an electrical power system.All components of a power system such as switchgear and conductor are designed to withstand this high current.    The impedance of the system is assumed to be zero in this kind of condition.

      Switches are used in low voltage for switching power on and off in a system.  These switches find application in industries, buildings and utilities, etc.  The switches are designed to be operated with a handle to interrupt the power supply.  They are sometimes enclosed within a plastic or metallic enclosure(aluminium or steel).

      Low voltage switches are available at different breaking capacities. 

      Low Voltage Switchgear refers to switchgear which is rated for a voltage of 600V and below.  Low voltage switchgear includes switches, fuse gear, enclosures, connection devices and cable distribution cabinets.

      The function of any switchgear is Electrical protection, electric isolation and local and remote switching. 

      The switchgear typically consists of a section in the front consisting of the circuit breakers, meters, protective relays.  Behind the front section, is the bus section consisting of the busbar and behind it is the cable entry section where the cables are routed.  

      Low voltage switchgear can be broadly classified into two types, outdoor types and indoor types.  

      Outdoor type switchgear differs from indoor type switchgear due to the presence of a weather-proofing enclosure.  Bus bars are usually made of copper or aluminum.  

      Modern low voltage switchgear products consist of integrated modules consisting of switchgear, metering and protective features. These integrated modules are driven by microprocessors.

      This article is the first in a series of articles focusing on Low Voltage Switchgear

      The exciting current of a transformer refers to the current drawn in order to build the voltage in the secondary.  The current is also known as the magnetizing current.  This current drawn lags behind the applied voltage by 90 degrees.  The exciting current is in phase with the magnetic flux.

      The exciting current of a transformer can be measured by measuring the no-load current of the primary when the secondary is kept open.  Any abnormality in the exciting current may indicate shorted turns or defects in the core.

      K rated Transformers are Transformers that are specially designed to supply Harmonics generating loads.  K rated Transformers are bigger in size that normal transformers of the same kVA rating.  They also have bigger conductors and are therefore more costly.

      Harmonics are generated by non-linear loads.  Most of the electric loads today such as Variable frequency drives, chopper circuits, induction heaters, etc would fall under that category. Harmonics in the power supply leads to heat generation in electric equipment such as transformers and consequently reduce the capacity. 

      Thus a 100kVA transformer cannot be loaded to its full capacity if the load it is supplying is generating harmonics.  Besides, high harmonics cause insulation ageing and reduce the life of the transformer.

      The K-factor refers to the level of harmonics in a particular system (See article). The k-factor exists from 1 to 50.  Thus, a transformer with no harmonics operating at the fundamental frequency is considered to be operating at k-factor of 1.

       As the level of harmonics increases, the k factor too increases.  The k-factor is calculated for loads which are connected to the transformer.

      Based on the k-factor of the loads the transformer is selected.  Thus a transformer with a k - rating of 4 will bigger in size over a transformer with k-rating of 1.  It will have its windings made of thicker copper gauge.  In k-rated transformers the thickness of the neutral conductor is double to prevent heating due to the flow of the triplen harmonics.  The k-rated transformer also occupies more space.

      Harmonics have become an integral part of modern electric systems.  Harmonics are caused by non-linear loads.  Most electronic devices will fall in this category.  Harmonics cause a wide range of adverse effects ranging from false operation of relays to overheating.  Excess heating is a principal effect of harmonics.  The high frequency of the harmonics cause increased heat losses in generators, conducting cables and motors. 

      Power sources such as transformers need to be operated at reduced capacity when connected to harmonics-causing loads.  For instance, a 100 kVA transformer may have to be operated at 60 kVA. 

      The K rating is a factor that is developed to indicate the amount of harmonics that the load can generate.  The K rating is extremely useful when designing electric systems and sizing components.

      The formula for K rating is

       Where Ih is the total harmonic current of particular harmonic.  Ih is expressed as p.u. basis

      The k rating of loads helps decide on the specifications of the power source such as transformers and also the size and capacity of the conductors.  The amount of harmonics drawn by a load determines its k rating.  Thus loads such as resistance heaters, transformers and motors with less harmonics have a k rating of 1.  Welders and induction heaters have a k rating of 4 as they generate a substantial amount of harmonics. 

      Loads which have higher k rating are connected to transformers having a corresponding k rating.  Thus a transformer of 100kVA with a k rating of 4 will deliver 100kVA as long as the k rating of the load is 4.  At higher k ratings, the transformer may have to be derated.

      Transformers with higher k ratings are made of thicker conductors and are designed to withstand higher eddy currents.  This leads to the transformers having bigger size.  These transformers also occupy higher space.  The neutral conductors in these transformers are thicker to permit the circulation of the triplen harmonics through them.

      Transformers can get heated due to a variety of reasons.  Excessive heating of the transformer increases transformer losses, weakens the insulation and may result in reduced transformer life.  The principal reasons for transformer overheating are


      Excessive loading can cause overheating.  Transformers are rated in kVA.  This kVA rating is dependent on temperature.  Some transformers have two kVA ratings.  One rating is for below a specific ambient temperature, say 45 degrees while another rating is above the ambient temperature.

      Excess current in the neutral of the transformer

      Excess current in the neutral of the transformer is usually caused by high zero sequence harmonic components.  This can be resolved by proper grounding usually through a zig zag transformer (See article).

      Problems in the Cooling system

      Malfunctioning of the cooling equipment such as blockages in the cooling oil circuit.  Failure of the cooling fans to operate in the case of Forced Air cooling systems.

      High Harmonic content in the power supply

      High harmonic loads can also cause heating in the transformer.  These loads need to be identified and suitable remedial measures such as harmonic filters can be implemented.

      Sustained Overvoltages

      Overvoltages which exist for a long period of time can overexcite the transformer and cause overheating.

      DC alone is used because we need to measure only the resistance of the winding insulation.  If we use AC to conduct the insulation test, we will be measuring impedance instead of the resistance as the insulation  has a substantial capacitive reactance.

      The health of the winding is best indicated by its resistance and not its impedance.  This is why only DC is used to measure the insulation resistance.

      When a short circuit occurs in an electric system, heavy current flows through all the sections of the system which are in the path between the power source and the equipment. The short circuit current is limited only by the impedance of the system.

      This heavy current can damage the components of the electric system if they are not properly rated. If circuit breakers are not able to interrupt the high short circuit currents in a system, arcing and explosions may occur
      The Rating of the components is done based on the maximum short circuit current.The short circuit current is calculated from the fault level KVA of the System.   The Fault Level in a distribution system is a very important parameter.  The kVA at the instant of a Fault should be correctly calculated and the components of the distribution system such as bus bars, circuit breakers, isolators, etc should be properly sized.

      To calculate the fault current in a system it is first necessary to calculate the MVA during a fault.

      The MVA during a fault is given by

      From this, the maximum current during the fault can be deduced as

      All the equipments should be rated to withstand this current.  The fault level should be calculated every five years and after any modification to the system such as the addition of any load or the installation of further sources of power such as transformers and alternators.

      This is a question that can be confusing to many engineers. This is because the terms "insulators" and "di-electrics" are often used interchangeably. Insulators are substances which permit very less current flow through them. Substance such as porcelain, wood are examples.

      Dielectrics are also insulators. But, more specifically, they are materials which can be polarized. In dielectric materials, the electrons are bound to the nucleus and have limited movement. When an external voltage is applied to the dielectric, the nucleus of the atoms is attracted to the negative side and the electros are attracted to the positive side. Hence, the material gets polarized. This is a key feature of a dielectric.

      Thus a dielectric can be defined as an insulator that can be polarized.  Thus all dielectrics are insulators, but all insulators are not dielectrics. A dielectric can thus store charge.  This characteristic makes it very useful in the form of capacitors.

      Dielectric substances conduct very little electricity but are good supporters of electric fields. They also dissipate very less energy, i.e. have low dielectric loss.

      Transmission and other electrical equipment can be exposed to overvoltages.  Overvoltages can be caused by a number of reasons such as lightning strikes, transient surges, sudden load fluctuation, etc.  In the event of an overvoltage, the insulating equipment such as the insulators on a transmission line or bushings in a transformer can be exposed to high voltages which may lead to their failure.

      Arcing horns are protective devices that are constructed in the form of projections in the conducting materials on both sides of an insulator. Arcing horns are fitted in pairs.  Thus in transmission lines they are found on the conducting line and the transmission tower across the insulators.  In transmission lines, in the event of a lightning strike on the tower, the tower potential rises to dangerous levels and can result in flashovers across the insulators causing their failure.  Arcing horns prevent this by conducting the arc across the air gap across them.

      Arcing horns function by bypassing the high voltage across the insulator using air as a conductive medium.  The small gap between the horns ensures that the air between them breaks down resulting in a flashover and conducts the voltage surge rather than cause damage to the insulator.The horns are constructed in pair so that one horn is on the line side and the other is on the ground side.

      Arcing Horns are also used along with air insulated switchgear equipment. Air insulated switchgear are vulnerable to damage due to arcing.  Arcing horns serve to divert the arc towards themselves thus protecting the switching equipment.  The arcing horns serve to move the arc away from the bushings or the insulators.  

      image courtesy : www.r-infinity.com/Hewlett

      When induction motors are started a very heavy current is drawn.  This current, which can be 5 to 6 times the rated current of the machine, occurs due to lack of back emf in the windings.  These currents  cause sudden loading on power sources such as generators or transformers.  Besides, such heavy currents also cause voltage dips and a momentary drop in frequency in generators which can trip other equipment connected to the power system.

      Softstarters are equipment which limit the initial start current of the motors by controlling the voltage.  This is done by means of thyristors.  This "soft" starting prevents the occurence of the high inrush currents.  The downside of using softstarters is that the reduced voltage applied on the stator windings can result in increased slip causing heating of the rotor. 

      Another advantage of the softstarter is that it can be used to gradually increase the torque.  In loads such as conveyor belts involving chains and other transmission gear, this avoids jerks which over time can cause wear on the machinery.  The voltage and the torque are gradually increased to the rated value.

      While stopping a motor, the softstarter usually provides the "softstop" function which involves doing the starting process in reverse.  The voltage is gradually reduced to zero this gradually stops the system avoiding sudden jerky motions due to system inertia.

      The Secondary of the potential Transformers are usually earthed.  There are various reasons for this.  The first reason is concerned with safety.  Since, the primary winding of the potential transformer is connected to High voltage, there are chances for the high voltage, in rare conditions, to get transferred to the secondary.  This can occur due to capacitance, insulation failure or any other damage.  Such a scenario would be extremely dangerous.  For this reason, the secondary is usually earthed at the star point for star connected transformers.

      Another reason for earthing the secondary would be functional.  For applications such as synchronization where two PT voltages need to be compared, the synchronizing equipment (dark lamp method and bright lamp methods) would function only if the star point of the PTs are grounded.

      Neutral Inversion is a phenomenon in which the neutral of an ungrounded three phase system falls out of the voltage triangle

      This usually occurs in a three phase system which is not earthed and where a single transformer is used in a voltage-based earth-fault sensing system.  The transformer may be connected to one of the phases and the ground.  This transformer needs to be provided without sufficient resistance loading.  The no-load or exciting impedance of the transformer is in parallel with the line capacitance of the phase to which the transformer is connected. 

      Hence, if the exciting impedance is more it the ratio of the capacitive impedance to the inductive impedance decreases causing a higher voltage in the neutral.  This high neutral can sometimes shift outside the triangle.

      This  situation can be avoided by providing sufficient resistance loading to the secondary of the voltage transformer.  This reduces the inductive impedance of the transformer and limits the shifting neutral voltage.