Electric machines, especially AC machines such as transformers and alternators are exposed to alternating magnetic fields during operation. 

This alternating magnetic field causes the induction of eddy currents in the core of transformers and the stator of motors.  The eddy current creates a loss of energy in the form of heat loss and hysteresis loss.

In order to avoid this, the core of transformers and the stator of motors and generators are made of a set of laminated steel sheets. Silicon Steel is used.  This steel is cold rolled and has special grain orientation.  Each steel sheet is around .3 mm thick. 

The sheets are insulated on both sides and laid of top of one another.  This arrangement ensures that the eddy current is reduced as it cannot flow over a wide area of cross section.  The laminated surfaces need to be very clean.  Presence of foreign particles can cause laminar faults which lead to core damage. 

Ferranti Effect

Ferranti Effect refers to a phenomenon in long transmission wherein the receiving end voltage is higher than the voltage at the sending end.    The line capacitance in long transmission lines cause the higher voltage at the receiving end.  The Ferranti Effect is more pronounced when the transmission line is lightly loaded eg. at night. 

A transmission line draws a charging current from the current source.  This charging current causes a voltage drop due to line inductance which is in series.  This voltage drop is in phase opposition to the voltage at the receiving end.  Thus, it causes the voltage at the sending end to drop lower than the receiving end.

The Ferranti effect can also be observed in cables based transmission systems.  The effect is more in cables even for shorter lengths as cables have higher capacitances.

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.

Water in Transformer oil is a serious issue which can reduce the insulating properties of the transformer oil.  Water also causes degradation of the paper based insulation.  This seriously and permanently damages the insulation of the winding.

Water enters the transformer through three ways

Through the Silica gel breather
If the silica gel is not monitored adequately, dry silica gel is always blue.  The silica gel should be replaced when it changes color.  If the silica gel is saturated, it may no longer filter the air and water can enter the transformer in the form of moisture.

Through improper sealing of accessories
Leaking gaskets, loose fitting components, cracked bushings can cause rain water to enter the transformer.

Water in Transformer oil exists in two forms as free water and as water in solution with the oil.  Pure oil and water do not mix.  However, over a period of time due to contamination and degradation the oil absorbs some water.

Disintegration of Cellulose
The insulation of the transformer winding is made of cellulose based paper.  At high temperatures, cellulose disintegrates and one of the byproducts is water.

Sampling the oil for Water content
The oil should be sampled for water content periodically.  It should be ensured that the sample collected is representative of the entire oil.  For instance, free water usually is at the bottom of the transformer tank.  Thus oil collected from the bottom valve may indicate a higher value of water content.  Thus samples should be collected from the upper and the lower valves of the transformer.

If the water content is found to be excessive.  The oil needs to be drained and dehydrated. 

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.

A bimetallic overload Relay
Thermal Protection is an important protection in motors.  Motors can get heated due to overloading, high ambient temperature, variations in power quality, etc.  Thermal overload can result in stator overheating, faulty operation and in some extreme cases even fire.  Hence, all motors need to be fitted with protection against thermal overload.

Thermal overload protections can be classified into three types  viz. Bimetallic, Magnetic and temperature sensing protection.

Bimetallic Protection
In bimetallic protections, a strip of two metals which are attached to one another is used.  The motor current is made to pass through the strips.  As current passes through the strips, the strips heat up and expand.  Since, the strip is made up of two different metals and these metals have different rates of expansion, the strip bends in one direction.  When the temperature of the strip reaches a particular value, it activates a mechanism which trips the motor.   This kind of protection is widely used and is simple in construction.    However, this method is not suitable in applications which require frequent starting and stopping of the motor.

The bimetallic protection gets reset faster than the motor cooling temperature and it may thus permit the motor to be started again when the motor has not sufficiently cooled from the thermal overload.  

Magnetic Protection
This consists of a magnetic element whose field strength is a function of the motor current.  When the motor current exceeds a preset value, the electromagnet inside the relay operates and trips the motor.  The downside of this kind of protection is that it does not take into account ambient operating conditions such as temperature and ventilation which play an important role in the temperature rise of motors.  

Temperature based thermal overload protection
This is method of protection that is relatively new.  This method involves actually measuring the temperature of the motor and those of the winding hotspots using a temperature sensor such as the RTD (Resistance Temperature Detector).  This method uses direct temperature sensing and is the most reliable and accurate, though, it is more expensive.


Stalling of the motor refers to a condition, where the motor is unable to rotate.  This condition can be caused either due to any obstruction in the load or due to any problem with the motor such as bearing seizure, etc.

This condition is also known as locked rotor.  

When the speed of the rotor decreases to a very low value or stops completely due to stalling, the slip of the induction motor increases.  This causes higher voltage and consequently higher current to be induced in the rotor windings.  

The stator currents also increase.  The equivalent of the motor stalled condition is that of a transformer whose secondary is short circuited. 

The high current drawn will cause damage to the windings and cause the rotor to heat up.

Stall protection devices work by monitoring the motor current and the speed.  If the motor draws higher current at a preset low speed, the relay is activated. 

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.

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.

          Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing.  The making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

          When there is a short circuit in the line and the breaker is closed, the peak value of the first cycle is the most severe from an electrodynamic perspective.  This value is in kA.  The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into account.

          Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt.  This is also in the order of kA. 

          The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.

          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.

The Residual Current based earth Fault relay works by measuring the vector sum of the three phase currents.  

Under healthy conditions, the vector sum of the three currents is zero. In the event of an earth fault, however, the fault current flows through the ground and hence, the vector sum of the currents is not equal to zero.  This is known as the residual current.  This current can be used to operate the earth fault relay.

The connection of the earth fault relay consists of three current transformers connected in parallel to each other.  This kind of earth fault protection is also known as unrestricted earth fault protection.

The residual current protection is usually set to operate at around 10% of the nominal current.  For fault currents lower than this value, as may be the case in high resistance grounded generators and transformers, the sensitive earth fault relay is used.  This is because, the three current transformers used in the residual current protection may not be exactly identical in response, even if they are from the same manufacturer.  Thus for very low setting, there is the risk of false operation of the relay due to errors in the current transformers.  

Since the sensitive earth fault relay uses one Core Balance Current Transformer instead of three individual current transformers, it can be set to lower values of earth fault current. 

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

          The sensitive earth fault relay is a protective device that works by measuring the residual current across the three phases in a system. This is done using a Core Balance Current Transformer (CBCT).   In the ideal condition, the residual current will be zero as all the currents flow through the three wires and their magnetic fields cancel each other out. 

          In the event of a fault, the residual current over the three phases will not be equal to zero as the current from the faulted phase flows through the earth.
          The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding.  High resistance grounding restricts the earth fault current to less than 10A.  High resistance grounding enables electrical systems to continue running when one of the phases is faulted.  This prevents interruptions to the power supply.  This kind of earthing system provides time to identify and isolate the fault.

          Once an earth fault occurs in the high resistance grounding system, an alarm needs to be generated and the fault needs to be traced.  For this a reliable protection which detects earth faults even when the fault current is very low is necessary.  Undetected earth faults in this system are dangerous as a second earth fault in another phase may result in a short-circuit.  Conventional earth fault relays may not be accurate in detecting an earth fault at such low current values.

          The sensitive earth fault protection, as the name suggests, is a highly sensitive relay.  It can sense currents as low as 0.2% of the CT secondary current.

          The sensitive earth fault relay may be configured to either generate an alarm or a trip signal.

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.

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

ABB, the leading power and automation technology group will design, supply and install all six main transformers for the 500 kilovolt (kV) Tongyu substation, under construction in the Jilin province of north-eastern China.

Tongyu will be the highest voltage substation in the country for the transfer of wind power. It is designed to integrate 2300 megawatts of electricity into the grid and plays an important part in the province’s wind power development plan.

“Renewable energy, especially wind power, is an important element of China’s focus on clean energy development,” said Tarak Mehta, global head of ABB’s Transformers business, part of the company’s Power Products division. ”The transformers used in this project will play a key role in integrating wind energy into the grid and ensure the efficient transmission of electricity to millions.”

The transformers are based on ABB’s patented TrafoStar technology and designed to minimize power loss and maintenance needs. Their ability to withstand short circuits will be integral to the reliable and safe operation of the substation, mitigating the occurrence of power outages. They will be supplied from ABB’s state-of-the-art transformer manufacturing unit in Chongqing, one of the largest of its kind in the world.

Transformers are integral components of any electrical grid and are critical elements in the efficient and safe conversion of energy across diverse voltage systems. ABB’s transformer portfolio includes power transformers, rated up to 1,000 kV, dry- and liquid-distribution transformers, as well as related services and components.

Courtesy: http://www.robotautomation.com.au

Slip-over CTs are used in applications where it is difficult to install conventional Current Transformers.

These current transformers can be "slipped over" bushings, terminals of Circuit breakers whenever any new upgradation or modification of the system needs to be carried out.

Slip over Current transformers are easy to replace with minimum downtime.

Kuhlman Electric Corporation (now acquired by ABB) has launched the Accuslip series of current transformers.  These current transformers can be used for both metering and protection.  ABB says that these current transformers can provide high accuracy measurement even at low ranges. The series is expected to deliver an accuracy of 0.15%.

According to ABB, these current transformers can be custom designed to meet any application.  The transformers can also be provided with an optional ground shield and mechanical supports.

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.

          Negative phase sequence in induction motors is caused due to unbalanced voltages in the supply voltage applied on the stator terminals or unbalanced windings.
          Negative phase sequence components create a rotating magnetic field in the stator which moves in the opposite direction.  This causes a decrease in the torque developed by the motor.  The motor will thus have to draw a higher current for the same mechanical load.

          The rotating magnetic field which rotates in the opposite direction induces voltages in the rotor.  These voltages have a frequency that is double the system frequency.  Since the frequency of this rotor voltage is higher, it flows on the surface of the rotor due to the skin effect and causes surface heating which can lead to motor damage.

          Negative phase sequence relays can identify negative phase sequence condition and trip the machine.  Negative phase sequence relays work by using a special filter which filters out the positive sequence and the zero sequence components.  The filtered negative phase sequence voltage alone is measured.  When the measured negative phase sequence voltage exceeds the set value, the relay trips the motor.

          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

In the earlier article, we saw what power factor is and how it is calculated. Now let us see why it is necessary to control the power factor. Power Factor Control refers to the reduction of the phase angle - the angle between the current and the voltage. As the power angle reduces, the power factor which is a cosine of the phase angle increases. It becomes closer to one. In the industry, around 80% of the power is inductive. This causes the current to lag behind the voltage resulting in a power factor that is less than one.

But how does this affect the power system? To understand this, we need to look at the the formula for power.

Active power = V x I x cos φ

If we are to increase the power factor, the current for a given value of kilowatts will be less resulting in a reduced loading of the system and reduced losses. This is the reason why the power factor is increased to a value closer to one.

Thus when the value of the power factor cos φ is less, more current is required to deliver the same amount of kilowatt. This increased loss will result in copper losses or I2R losses in the system. The conductors, cables will also be subjected to higher loading as they have to carry more current.

Let us take an example, say, a single phase motor with a rating of 100kW with a supply of 440V. Running this motor at a power factor of 0.5 will result in a current of 454.5A. However, running the motor at a power factor of 0.9 will result in a 252A only.

The reduction in current required is substantial.  This reduced current will also result in reduced loading of the source as power sources such as generators and transformers are rated in kVA

Power factor improvement is done by using capacitors, active power factor controllers and so on. We will discuss about them in the next article.

Power factor refers to the cosine of the angle between the voltage and the current. In AC circuits, the nature of the load determines the power factor. Power factor is a critical parameter in AC circuits as it determines the amount of current which goes into delivering a certain quantity of power.  Equipments which run at a lower power factor draw a high current for the same amount of load. What causes Power Factor All Electric loads can be categorized into three types, viz. Resistive, Inductive and Capacitive loads. Consider an AC voltage being applied across a simple resistor, the current drawn will be 'in phase' with the voltage.  That is, the current reaches maximum when the AC voltage reaches maximum and falls to the minimum when the current reaches minimum.
In a purely inductive circuit, the current lags behind the voltage by an angle of 90 degrees.  Thus the current is zero when the voltage is maximum and rises to the maximum when the voltage falls to zero.
In a capacitive circuit, the current leads the voltage by 90 degrees.
The angle between the current and the voltage is called the phase angle.  The cosine of the phase angle is called the power factor. In the next article we will see the relation between power factor and kW and why power factor control is necessary

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.

The Rotor Protection relay is used in synchronous motors and generators to identify the presence of an earth fault in the rotor winding.  While the winding in the rotor is insulated from the ground during normal operation, the Rotor is subjected to stresses due to vibration, heat, etc.  These stresses can cause the winding to give way in a particular place and the winding can get earthed.

While a single earthing in the winding is not immediately damaging.  It sets the stage for damage if a second failure should occur.  The second earthing can cause a short-circuit through the rotor causing extensive damage to the rotor and the winding.

The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic balance inside the alternator.  These forces can cause the rotor shaft to become eccentric and in extreme cases cause bearing failure. 

Hence, it is necessary that any earthing in the rotor is detected at the earliest.

In slip ring rotors, carbon deposits on the slip rings may compromise the insulation resistance of the rotor.  Hence, the slip rings need to be inspected for any deposits.

The Rotor Earth Fault Protection Device consists of a current injection device which applies an AC voltage to the rotor winding by means of a slip ring fitted on the rotor.  The current is applied to the rotor through a coupling capacitor.  In the normal condition, the system is floating and the current flowing through the device is zero as the resistance is high. 

When a fault occurs, the current increases causing the relay to operate.  The relay can be configured for alarm or trip depending on the criticality.

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.

      Sharp Corporation has signed an agreement with NED(2) to establish one of the world's largest(1) solar power generation plants with a power generation capacity of 73 MW, and to supply thin-film solar cell modules and surrounding systems for the plant. The construction of the solar power generation plant will start in July 2010 and the operation is planned to start by the end of 2011. NED is an independent power producer (IPP(3)) in the Kingdom of Thailand, with a 33.3% stake owned by DGA, a wholly owned subsidiary company of Mitsubishi Corporation. Together with ITD/ITE(4), the largest construction companies in Thailand, Sharp Corporation has signed an agreement for this project.

      The Kingdom of Thailand is renowned for its abundant solar radiation throughout the year. The government of Thailand targets an increase in the amount of renewable energy, up to 20%(5) of the total electricity demand by 2022. The use of solar power generation is expected to grow in the future.

      Thin-film solar cells are optimal for use in a high-temperature climate, compared to crystalline solar cells. Sharp's experience in the mass production of thin-film solar cells has enabled Sharp to take part in this project. The thin-film solar cell modules to be used in the power generation plant will be supplied partially from Sharp's solar cell plant at GREEN FRONT SAKAI in Sakai City, Osaka Prefecture, Japan, which started operation in March 2010.

      Sharp will collaborate with ITD/ITE on the design and construction of the plant. Sharp will take a step forward by not only supplying solar cell modules, but also by offering a business model that provides everything from related equipment procurement to system design and construction of the power generation plant, by collaborating with local companies both domestic and abroad.
      source: pressreleasenetwork.com