Low Resistance Grounding

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

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

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

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

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

These are usually used in medium and high voltage Systems

Earthing in Three Phase Systems Part II

Resistance Grounding

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

• High Resistance Grounding and
• Low Resistance Grounding

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

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

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

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

See Also : 


Types of Earthing

Earthing in Three Phase Systems Part I

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

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

On the basis of the grounding used, Power Systems can be classified into

• Ungrounded Systems
• Solidly Grounded Systems
• Low Resistance Grounded Systems
• High Resistance Grounded Systems

 
Ungrounded Systems

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











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

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

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

What are the disadvantages of solidly grounded systems?

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

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

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

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

Related Posts:


Neutral Grounding Resistors

Grounding Resistances - An introduction

Protective Relaying in Resistance Grounded systems can be devised in two ways, measuring either the voltage or the current in the grounding in the event of a ground fault. When a ground fault occurs, the voltage between the neutral point increases. This can be measured to identify the earth fault. Current based protection use the current flowing through the grounding resistor to identify the presence of a fault.

Relays using the voltage measurement principle are advantageous as they can function even when the Grounding Resistor is open.

Grounding Resistances - An Introduction

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

Grounding resistances can be classified into high and low resistances.

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

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

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

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

Related Posts:

Disadvantages of Solid Grounded Systems

Grounding Resistances - An introduction

Shaft Currents in Motors and Generators

The magnetic field in a motor or a generator is ideally conducted along paths that are symmetrical. However, sometimes the magnetic fields within an electric machine such as a motor or a generator is asymmetrical. This asymmetry can be caused usually by variation in the quality of the steel used in the motor construction or in some cases to shaft eccentricity.

These asymmetrical magnetic fields which are varying over time can induce currents in the shaft of the motor or generator. These currents which are induced in the shaft tend to flow from one end of the shaft to the other through the bearings and then through the earth.

When these currents flow through the bearings, the tend to cause arcing and consequent pitting in the bearings. This can lead to failure of the bearings.

Shaft currents can be prevented by insulating one of the bearings. A Teflon layer is usually placed between the shell of the bearing and the bearing housing. This ensures that the shaft voltage induced does not have a return path. This prevents shaft currents from flowing.

Flame Proof Motors

Flame Proof motors are used in hazardous locations usually in chemical, petroleum and gas industries, where the environment may contain gases or inflammable vapours. Electric equipment are notorious for being the source of sparks during switching. This could be catastrophic in areas where inflammable gases are present.

Hazardous areas classified into zones depending on the possibility of the presence of gases and combustible dust-air mixtures.

The following are the classification for hazardous areas

Zone 0, An Explosive atmosphere is continuously

Zone 1, An Explosive atmosphere is present for 1000 hours in a year.

Zone 2, An Explosive atmosphere is present for 10 hours in a year.

Flameproof motors are specially designed so that any explosion which occurs within the motor is contained within the motor frame itself. This prevents any secondary explosion in the surrounding area. The flame-proof motor is specially enclosed within a protective shield which covers the entire motor including the bearings.

Flame proof motors are heavier that motors of similar rating due to the increased weight of the reinforced design of the end shields so as to withstand the impact of an explosion.

Flame proof motors have special arrangements for cooling. Since conventional air circulation is not possible with these motors, special arrangements such as using the internal air to carry heat to the surface of the motor where the heat is transferred to the external air by means of special tubes have been devised.

Another method of cooling is the use of a coolant such as water which circulate in the stator and rotor of the motor. This water is brought to the near the inside of surface of the motor where it is cooled by means of the atmospheric air.

The cooling system ensures the isolation of the internal cooling medium from the external atmospheric air.

Can Single Phase motors be reversed by changing the polarity?

Single phase motors have two windings, the main winding and the auxilliary winding. The auxiliary winding is used to start the motor and may be disconnected once the motor picks up sufficient speed.

Reversing a single phase motors cannot be done by reversing the polarity of the supply to the entire motor. To reverse the single phase motor, the polarity of the supply to only one of the windings needs to be changed.

This can be done by reconfiguring special links which may be provided in the terminal box of the motor.

Why are Power Cables Shielded?

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

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

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

The Energy Star Classification

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

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

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

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

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

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

What are Zero Watt Bulbs ?

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

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

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

Triplen Harmonics - An Overview

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

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

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

All Day Efficiency of a Distribution Transformer

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

Efficiency = output power in watts/input power in watts

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

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

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

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

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

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

Ferrite beads and their applications

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

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

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

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

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The Magnetic Center in Electric Machines.

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

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

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

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

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

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

Consequences of wrong Magnetic centering.

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

Adjusting the Magnetic Center

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

What are the advantages of a three phase system over a single phase power system?

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

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

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

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

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

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

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

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

What does the meaning of the term "Ampere Turns" and "Ampere-turns per metre"

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

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

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

Can Permanent magnets lose their magnetism

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

They are

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

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

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

How are Permanent Magnets made?

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

Magnets can be broadly classified into two types

Electromagnets or temporary magnets and

Permanent Magnets

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

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

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

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

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

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

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

Schuko Plugs and Sockets

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

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

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

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

The Schuko sockets can also accept C type plugs.

What is the function of the Conservator in a Transformer

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

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

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

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

Proximity Effect

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

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

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

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

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

Noise in Transformers

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

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

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

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

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

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

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Substation, An introduction and types

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

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

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

Based on their functions, Substations can be classified into four types

• Distribution Switching Substations
• Switching Substations
• Transmission Substation
• Customer Substation

Distribution Substations

Distribution Substations are substations which reduce the voltage from the Transmission level, say, 132 kV to a lower level such as 66kV or 33kV. The power at this reduced voltage is then supplied for domestic consumption via distribution transformers which are located in each street. These transformers then reduce the voltage to 440V and so on.

Customer Substations

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

Switching Substations

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

Transmission Substations

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

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Solutions for Noise and Vibration in Transformers

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

Methods of Noise Attenuation:

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

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

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

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

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

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

Reducing Vibrations in Transformers.

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

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

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

See Also : Noise in Transformers

Checking Lightning Arrestors

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

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

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

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

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

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

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

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

What are Surge Capacitors?

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

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

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

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

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

What are Bleeder Resistors in Capacitors

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

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

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

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

What are the reasons for Bulging in Capacitors?

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

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

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

Transformer Classification based on the Cooling Medium

Dry - Type:

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

These Transformers can be further classified into

a)Natural Cooled:

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

b)Forced Air Cooling:

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

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

Oil Cooled Transformers:

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

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

a)Oil Natural Cooling:

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

b)Forced Air Cooled:

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

c)Forced Oil, Forced Air Cooling:

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

d)Water Cooled Transformers:

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

Comparison of Dry Type Transformers and Oil Filled Transformers

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

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

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

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

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

Ampacity - an introduction

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

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

The ampacity of any conductor depends on the following factors.

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

Overfluxing in Transformers

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

Magnetic Flux α Voltage/Frequency

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

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

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

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

What are opto-coupler relays?

Optocoupler relays are relays in which the changeover of contacts is effected by the switching on or off of a light source which is linked to a SCR or a TRIAC. The SCR or the TRIAC is switched on or off when the LED is switched unlike conventional relays, where it is done electromagnetically.

These relays are faster than electromagnetic relays. More importantly, the provide isolation between the control and the power circuits.

These relays do not have any moving parts which can deteriorate due to arcing or operational wear. However, they are expensive over conventional relays and hence still find limited application.

Another advantage of the Solid state relay is that it can open an AC circuit when the sinewave crosses zero. This ensures that the back-emf is minimum and this ensures that there are no voltage kicks in the opposite direction when the circuit is open due to inductance. This is because the Triac or the SCR used in the solid state relays conduct current till the waveform reaches the zero point even after the optocoupler LED has been switched off. This prevents premature failure of the contacts.

The Advantages of Optocoupler relays include

• Smaller Size
• Faster Response time
• Noiseless operations as there is no mechanical movement of the contacts.
• Optocoupler relays can withstand high vibration compared to conventional relays
• They do not generate Electromagnetic radiation as there is no coils to be energized.

However, they also have some disadvantages.

• They are more expensive
• They generate heat and require special heat-sinking fixtures.
• They cannot switch on very low currents
• When Solid state relays fail, they fail in the "closed position". In this situation, the machine which is connected will continue to be in the operating condition and there will be difficulty in isolating it. Electromagnetic relays usually fail in the "open" position.

Zero-Switching of Transformers

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

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

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

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

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

What is the difference between AC and DC relays?

AC and DC relays work on the same principle, that of, electromagnetic induction. However, there are some differences in construction. DC relays have something known as the freewheel diode which acts to discharge the emf built in the inductance when the coil is de-energized. (Click here to read about the phenomenon of freewheeling.) AC relays have cores which are laminated to prevent losses due to eddy current heating.

Another more conspicuous difference between a DC relay and an AC relay is presence of the Shading Coil. In AC relays, the alternating current supply changes direction about 100 times a second. At each instance, when the sine wave passes through zero, the current flowing through the coil becomes zero. This results in a loss of magnetism for a few milliseconds. When this happens about 100 times a second, the repeated drop and pickup of the coil produces a noise known as chattering. This also leads to the making and breaking of the relay contacts leading to disturbances in the connected electric circuits.

A shading coil is a coil with high remanence. thus when the magnetism of the coil collapses when the current becomes zero. The shading coil still retains the magnetism. Thus, ensuring that the contacts do not drop off.

AutoTransformers

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

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

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

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

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

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

See Also:

Difference between Autotransformer, Variac and Dimmerstat

Scopemeters - An Overview

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

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

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

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

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

What is the difference between a Variac, variable autotransformer and Dimmerstat?

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

Freewheeling or Fly back diodes and their function

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

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

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

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

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

How does a dc clampmeter work?

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

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

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

images courtesy: commons.wikimedia.com
www.sensormag.com

What is a Rogowski Coil?

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

It is named after Walter Rogowski, a German Physicist.

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

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

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

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

What is the difference between Salient and Non-Salient Pole Rotors used in Synchronous Generators

Rotors used in Synchronous alternators can be classified into

 1)Salient and 2)Non-Salient Pole Rotors. 

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

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

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

These are sometimes known as "drum rotors". 

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

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

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

Corona and its Effects

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

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

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

Effects of Corona

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

Corona can also be used to identify problems. Corona in power transmission lines indicate the presence of dirt or other substances on power lines and may require cleaning.

Corona after commission an equipment may indicate improper installation.

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

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

Partial Discharge

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

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

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

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

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

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

Flash-Overs

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

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

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

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

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

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

Offshore Wind Farms to Power 17% of Europe’s Electrical Needs in 2030

STOCKHOLM, Sweden, Sept. 16 (UPI) — Offshore wind farms could meet up to 17 percent of Europe’s electricity needs in 2030, an industry group said.

“Offshore wind power is vital for Europe’s future,” the European Wind Energy Association said in a report published at an offshore wind industry conference in Stockholm.

Currently, 11 wind farms harness the stronger winds at sea, mostly in Britain and Scandinavia. Together, they account for only 0.2 percent of Europe’s electricity demand, but that figure could be boosted to 10 percent in 2020 and up to 17 percent in 2030, EWEA said.

Offshore wind farms with a total capacity of 100 GW are currently being developed or proposed in most countries that have an Atlantic, North Sea or Baltic Sea coastline.

“If realized, these projects would produce 10 percent of the EU’s electricity whilst avoiding 200 million tons of CO2 emissions each year,” EWEA said.

The European Union wants to boost the share of renewables in its energy mix to 20 percent, a target it aims to meet in part with the help of offshore wind.

Energy Commissioner Andris Piebalgs promised in Stockholm that Brussels is “committed to doing everything we can to support offshore wind developers and make sure their … projects come to fruition.”

The EWEA wants the EU and national governments to support the creation of a functioning North Sea grid. Wind turbines at sea produce more power because of stronger winds, but turbine maintenance and getting the electricity back on land — via seabed cables — makes the source much more expensive.

On Monday, the association presented what it called an “offshore network development plan” for the construction of a transnational offshore power grid.

“EWEA’s new offshore network plan will provide a truly pan-European electricity super highway,” Christian Kjaer, CEO of EWEA, said in a statement. “This will bring affordable electricity to consumers, reduce import dependence, cut CO2 emissions and allow Europe to access its largest domestic energy source — offshore wind.”

The EU is currently drafting its own plan for a transnational grid, and it is expected that Brussels will call on utilities to share a large part of the costs.

The industry nevertheless believes that offshore wind will see similarly high growth rates that the onshore industry did over the past decade.

“There is huge developer interest in offshore wind power,” Arthuros Zervos, president of EWEA, said in a statement. “The scale of planned projects is far greater than most people realize.”

The world’s largest offshore wind farm, the $1 billion Horns Rev 2, is due to go into operation Thursday. Developed and built by Danish utility Dong, it consists of 91 Siemens-made turbines placed around 20 miles off the Danish coast. With its total capacity of 209 MW, the farm will produce power for an estimated 200,000 households.

courtesy : ecoworld.com

Current Limiting Fuses

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

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

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

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

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

image courtesy :www.chfuses.com

Interharmonics

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

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

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

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

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

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

Why is Gravel used to pave substation surfaces?

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

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

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

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

Water Trees and Electrical Trees and their role in cable insulation failure

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

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

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

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

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


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

Earth Fault Monitoring in Ungrounded DC systems

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

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

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

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

Hipot Testing

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

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

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

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

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

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

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

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

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

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

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

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