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Insulators for an overhead power line (OHL), in many substation applications and on the overhead electrification systems of railways, must, primarily, support the conductors. Also important, as already mentioned, is the need to avoid frequent flashover events from occurring.

Although the total mechanical failure of such an insulator is, fortunately, a rare event, its occurrence may be very serious. For example, should a vertical insulator of an OHL (often referred to as a suspension unit) break, then its conductor could be supported by the insulators of the neighboring support structures (often called towers) at either side. Then, it is possible that this conductor could be reenergized but with little ground clearance!

The consequences of a flashover vary from being annoying to being very costly. For example, the damage resulting from the external flashover of the insulating housing of a high power circuit breaker during a synchronizing operation, when the voltage across the polluted surface can increase to twice the normal value, could be extremely large.

TYPES OF INSULATORS

There are several types of insulators but the most commonly used are pin type, suspension type, strain insulator and shackle insulator.

PIN TYPE INSULATORS

The pin type insulator is secured to the cross-arm on the pole. There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.

Pin type insulators are used for transmission and distribution of electric power at voltages up to 33 kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence uneconomical.

SUSPENSION TYPE INSULATORS

For high voltages (>33 kV), it is a usual practice to use suspension type insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage, say 11 kV. The number of discs in series would obviously depend upon the working voltage. For instance, if the working voltage is 66 kV, then six discs in series will be provided on the string.

STRAIN INSULATORS

When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used. For low voltage lines (< 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators. The discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly high, as at long river spans, two or more strings are used in parallel.

SHACKLE INSULATORS

In early days, the shackle insulators were used as strain insulators. But now a days, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.

ABSOLUTE AND SECONDARY INSTRUMENTS

By Engr. Aneel Kumar September 27, 2014 No comments:

The various electrical instruments may, in a very broad sense, be divided into (i) Absolute Instruments (ii) Secondary Instruments.

Absolute Instruments

are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. The example of such an instrument is tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the horizontal component of earth’s field.

Secondary Instruments

are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, which are most generally used in everyday work; the use of the absolute instruments being merely confined within laboratories, as standardizing instruments.

BATTERIES FOR AIRCRAFT AND SUBMARINES

BATTERIES FOR AIRCRAFT

The on-board power requirements in aircraft have undergone many changes during the last three or four decades. The jet engines of the aircraft which require starting currents of about 1000A impose a heavy burden on the batteries. However, these days this load is provided by small Turbo-generator sets and since batteries are needed only to start them, the power required is much less. These batteries possess good high-rate capabilities in order to supply emergency power for up to 1 h in the event of the generator failure. However, their main service is as a standby power for miscellaneous on-board equipment. Usually, batteries having 12 cells (of a nominal voltage of 24 V) with capacities of 18 and 34 Ah at the 10 h rate are used. In order to reduce weight, only light-weight high impact polystyrene containers and covers are used and the cells are fitted with non-spill vent-plugs to ensure complete un-spill-ability in any aircraft position during aerobatics. Similarly, special plastic manifolds are molded into the covers to provide outlet for gases evolved during cycling.

BATTERIES FOR SUBMARINES

These batteries are the largest units in the traction service. In older types of submarines, the lead storage battery was the sole means of propulsion when the submarine was fully submerged and, additionally supplied the ‘hotel load’ power for lights, instruments and other electric equipment. When the introduction of the snorkel breathing tube made it possible to use diesel engines for propulsion, battery was kept in reserve for emergency use only. Even modern nuclear-powered submarines use storage batteries for this purpose. These lead-acid batteries may be flat, pasted plate or tubular positive plate type with 5 h capacities ranging from 10,000 to 12,000 Ah. One critical requirement for this service is that the rate of evolution of hydrogen gas on open-circuit should not exceed the specified low limit. Double plate separation with the help of felted glass fiber mats and micro-scoporous separators is used in order to ensure durability, high performance and low standing losses.

SECONDARY HYBRID CELLS

A hybrid cell may be defined as a galvanic electro-technical generator in which one of the active reagents is in the gaseous state i.e. the oxygen of the air. Such cells take advantage of both battery and fuel cell technology. Examples of such cells are:

1. Metal-air cells such as iron oxygen and zinc oxygen cells:

The Zn/O₂ cell has an open-circuit voltage of 1.65 V and a theoretical energy density of 1090 Wh/kg. The Fe/O₂ cell has an OCV of 1.27 V and energy density of 970 Wh/kg.

2. Metal-halogen cells such as zinc-chlorine and zinc-bromine cells:

The zinc-chlorine cell has an OCV of 2.12 V at 25°C and a theoretical energy density of 100 Wh/ kg. Such batteries are being developed for EV and load leveling applications. The zinc-bromine cell has an OCV of 1.83 V at 25°C and energy density of 400 Wh/kg.

3. Metal-hydrogen cells such as nickel-hydrogen cell:

Such cells have an OCV of 1.4 V and a specific energy of about 65 Wh/kg. Nickel-hydrogen batteries have captured large share of the space battery market in recent years and are rapidly replacing Nickel/cadmium batteries as the energy storage system of choice. They are acceptable for geosynchronous orbit applications where not many cycles are required over the life of the system (1000 cycles, 10 years).

The impetus for research and development of metal-air cells has arisen from possible EV applications where energy density is a critical parameter. An interesting application suggested for a secondary zinc-oxygen battery is for energy storage on-board space craft where the cell could be installed inside one of the oxygen tanks thereby eliminating need for gas supply pipes and valves etc. These cells could be recharged using solar converters.

Some of the likely future developments for nickel-hydrogen batteries are

(1) Increase in cycle life for low earth orbit applications up to 40,000 cycles (7 years)
(2) Increase in the specific energy up to 100 W/kg for geosynchronous orbit applications and
(3) Development of a bipolar nickel-hydrogen battery for high pulse power applications.

MAIN OPERATED BATTERY CHARGERS

A battery charger is an electrical device that is used for putting energy into a battery. The battery charger changes the AC from the power line into DC suitable for charger. However, DC generator and alternators are also used as charging sources for secondary batteries. In general, a mains-operated battery charger consists of the following elements:

1. A step-down transformer for reducing the high AC mains voltage to a low AC voltage.

2. A half-wave or full-wave rectifier for converting alternating current into direct current.

3. A charger-current limiting element for preventing the flow of excessive charging current into the battery under charge.

4. A device for preventing the reversal of current i.e. discharging of the battery through the charging source when the source voltage happens to fall below the battery voltage.

In addition to the above, a battery charger may also have circuitry to monitor the battery voltage and automatically adjust the charging current. It may also terminate the charging process when the battery becomes fully charged. However, in many cases, the charging process is not totally terminated but only the charging rate is reduced so as to keep the battery on trickle charging. These requirements have been illustrated in Figure.

Most of the modern battery chargers are fully protected against the following eventualities:

(a) They are able to operate into short-circuit.
(b) They are not damaged by a reverse-connected battery.
(c) They are operated into a totally flat battery.
(d) They can be regulated both for current and voltage.

MAINTENANCE OF LEAD ACID CELLS

The following important points should be kept in mind for keeping the battery in good condition:

1. Discharging should not be prolonged after the minimum value of the voltage for the particular rate of discharge is reached.

2. It should not be left in discharged condition for long.

3. The level of the electrolyte should always be 10 to 15 mm above the top of the plates which must not be left exposed to air. Evaporation of electrolyte should be made up by adding distilled water occasionally.

4. Since acid does not vaporize, none should be added.

5. Vent openings in the filling plug should be kept open to prevent gases formed within from building a high pressure.

6. The acid and corrosion on the battery top should be washed off with a cloth moistened with baking soda or ammonia and water.

7. The battery terminals and metal supports should be cleaned down to bare metal and covered with Vaseline or petroleum jelly.

STRUCTURE OF PLANTE PLATES

Since active material on a Plante plate consists of a thin layer of PbO₂ formed on and from the surface of the lead plate, it must be made of large superficial area in order to get an appreciable volume of it. An ordinary lead plate subjected to the forming process as discussed above will have very small capacity. Its superficial area and hence its capacity, can be increased by grooving or laminating. Figure a, shows a Plante positive plate which consists of a pure lead grid with finely laminated surfaces. The construction of these plates consists of a large number of thin vertical lamination which are strengthened at intervals by horizontal binding ribs. This results in an increase of the superficial area 10 to 12 times that possessed by a plain lead sheet of the same overall dimensions.

The above design makes possible the expansion of the plate structure to accommodate the increase in mass and the value of the active material (PbO₂) which takes place when the cell goes through a series of chemical changes during each cycle of charge or discharge. The expansions of the plate structure takes place downwards where there is room left for such purpose. Usually, a Plante positive plate expands by about 10% or so of its length during the course of its useful life.

Another type of Plante positive plate is the ‘rosette’ plate which consists of a perforated cast grid or framework of lead alloy with 5 to 12 per cent of antimony holding rosettes or spirals of corrugated pure lead tape. The rosettes (Figure b) provide the active material of the positive plate and, during formation; they expand in the holes of the grid which are countersunk on both sides of the grid. The advantages of such plates are that the lead-antimony grid is itself unaffected by the chemical action and the complete plate is exceptionally strong.

Other things being equal, the life of a Plante plate is in direct proportion to the weight of lead metal in it, because as the original layer of PbO₂ slowly crumbles away during the routing charging and discharging of the cell, fresh active material is formed out of the underlying lead metal. Hence, the capacity of such a plate lasts as long as the plate itself. In this respect, Plante plate is superior to the Faure or pasted plate.

STRUCTURE OF FAURE PLATES

Usually, the problem of Faure type grid is relatively simple as compared to the Plante type. In the case of Faure plates, the grid serves simply as a support for the active material and a conductor for the current and as a means for distributing the current evenly over the active material. Unlike Plante plates, it is not called upon to serve as a kind of reservoir from which fresh active material is continuously being formed for replacing that which is lost in the wear and tear of service. Hence, this makes possible the use of an alloy of lead and antimony which, as pointed out earlier, resists the attack of acid and ‘forming’ effect of current more effectively than pure lead and is additionally much harder and stiffer.

Because of the hardening effect of antimony, it is possible to construct very thin light plates which possess sufficient rigidity to withstand the expensive action of the positive active material. Simplest type of grid consists of a meshwork of vertical and horizontal ribs intersecting each other thereby forming a number of rectangular spaces in which the paste can be pressed and allowed to set. Such a thin grid has the disadvantage that there is not much to ‘key’ in the paste and due to a great shock or vibration the pellets are easily ‘started’ and so fall out.

A much better support to the active material can be given by the construction illustrated in Figure a, which is known as ‘basket’ type or screened grid. The paste instead of being is isolated pellets forms a continuous sheet contained and supported by the horizontal ribs of the gird. With this arrangement the material can be very effectively keyed in. Another type of grid structure used in pasted plates is shown in Figure b.

INTERNAL RESISTANCE AND CAPACITY OF A CELL

The secondary cell possesses internal resistance due to which some voltage is lost in the form of potential drop across it when current is flowing. Hence, the internal resistance of the cell has to be kept to the minimum. One obvious way to lessen internal resistance is to increase the size of the plates. However, there is a limit to this because the cell will become too big to handle. Hence, in practice, it is usual to multiply the number of plate inside the cell and to join all the negative plates together and all the positives ones together as shown in below Figure.

The effect is equivalent to joining many cells in parallel. At the same time, the length of the electrolyte between the electrodes is decreased with a consequent reduction in the internal resistance. The ‘capacity’ of a cell is given by the product of current in amperes and the time in hours during which the cell can supply current until its EMF falls to 1.8 volt. It is expressed in ampere-hour (Ah).

The interlacing of plates not only decreases the internal resistance but additionally increases the capacity of the cell also. There is always one more negative plate than the positive plates i.e. there is a negative plate at both ends. This gives not only more mechanical strength but also assures that both sides of a positive plate are used.

Since in this arrangement, the plates are quite close to each other, something must be done to make sure that a positive plate does not touch the negative plate otherwise an internal short-circuit will take place. The separation between the two plates is achieved by using separators which, in the case of small cells, are made of treated cedar wood, glass, wool mat, micro porous rubber and mocroporous plastic and in the case of large stationary cells; they are in the form of glass rods.

APPLICATIONS OF LEAD ACID BATTERIES

Storage batteries are these days used for a great variety and range of purposes, some of which are summarized below:

1. In Central Stations for supplying the whole load during light load periods, also to assist the generating plant during peak load periods, for providing reserve emergency supply during periods of plant breakdown and finally, to store energy at times when load is light for use at time when load is at its peak value.

2. In private generating plants both for industrial and domestic use, for much the same purpose as in Central Stations.

3. In sub-stations, they assist in maintaining the declared voltage by meeting a part of the demand and so reducing the load on and the voltage drop in, the feeder during peak-load periods.

4. As a power source for industrial and mining battery locomotives and for road vehicles like cars and trucks.

5. As a power source for submarines when submerged.

6. Marine applications include emergency or stand-by duties in case of failure of ship’s electric supply, normal operations where batteries are subjected to regular cycles of charge and discharge and for supplying low-voltage current to bells, telephones, indicators and warning systems etc.

7. For petrol motor-car starting and ignition etc.

8. As a low voltage supply for operating purposes in many different ways such as high-tension switchgear, automatic telephone exchange and repeater stations, broadcasting stations and for wireless receiving sets.

PARTS OF A LEAD ACID BATTERY

A battery consists of a number of cells and each cell of the battery-consists of (a) positive and negative plants (b) separators and (c) electrolyte, all contained in one of the many compartments of the battery container. Different parts of a lead-acid battery are as under:

(I) PLATES: A plate consists of a lattice type of grid of cast antimonial lead alloy which is covered with active material. The grid not only serves as a support for the fragile active material but also conducts electric current. Grids for the positive and negative plates are often of the same design although negative plate grids are made somewhat lighter.

(II) SEPARATORS: These are thin sheets of a porous material placed between the positive and negative plates for preventing contact between them and thus avoiding internal short-circuiting of the battery. A separator must, however, be sufficiently porous to allow diffusion or circulation of electrolyte between the plates. These are made of especially-treated cedar wood, glass wool mat, micro porous rubber (mipor), micro porous plastics (plastipore, miplast) and perforated PVC, as shown in Figure In addition to good porosity, a separator must possess high electrical resistance and mechanical strength.

(III) ELECTROLYTE: It is dilute sulphuric acid which fills the cell compartment to immerse the plates completely.

(IV) CONTAINER: It may be made of vulcanized rubber or molded hard rubber (ebonite), molded plastic, ceramics, glass or celluloid. The vulcanised rubber containers are used for car service, while glass containers are superior for lighting plants and wireless sets. Celluloid containers are mostly used for portbable wireless set batteries. A single mono-block type container with 6 compartments generally used for starting batteries is shown in Figure.

Full details of a Russian 12-CAM-28 lead-acid battery parts are shown in Fig. 9.3. Details of some of these parts are as follows:

(A) BOTTOM GROOVED SUPPORT BLOCKS: These are raised ribs, either fitted in the bottom of the container or made with the container itself. Their function is to support the plates and hold them in position and at the same time protect them from short-circuits that would otherwise occur as a result of fall of the active material from the plates onto the bottom of the container.

(B) CONNECTING BAR: It is the lead alloy link which joins the cells together in series connecting the positive pillar of one cell to the negative pillar of the next one.

(C) TERMINAL POST OR PILLAR: It is the upward extension from each connecting bar which passes through the cell cover for cable connections to the outside circuits. For easy identification, the negative terminal post is smaller in diameter than the positive terminal post.

(D) VENT PLUGS OR FILLER CAPS: These are made of polystyrene or rubber and are usually screwed in the cover. Their function is to prevent escape of electrolyte but allow the free exit of the gas. These can be easily removed for topping up or taking hydrometer readings.

(E) EXTERNAL CONNECTING STRAPS: These are the antimonial lead alloy flat bars which connect the positive terminal post of one cell to the negative of the next across the top of the cover. These are of very solid construction especially in starting batteries because they have to carry very heavy currents.

1. -ve plate
2. Separator
3. + ve plate.
4. + ve group
5. -ve group
6. -ve group grooved support block
7. Lug
8. Plate group
9. Guard screen
10. Guard plate
11. Cell cover
12. Plug washer
13. Vent plug
14. Mono-block jar
15. Supporting prisms of + ve group
16. Inter-cell connector

17. Terminal lug
18. Screw
19. Washer
20. Nut
21. Rubber packing
22. Sealing compound.

PRIMARY AND SECONDARY BATTERIES

An electric battery consists of a number of electrochemical cells, connected either in series or parallel. A cell, which is the basic unit of a battery, may be defined as a power generating device, which is capable of converting stored chemical energy into electrical energy. If the stored energy is inherently present in the chemical substances, it is called a primary cell or a non-rechargeable cell. Accordingly, the battery made of these cells is called primary battery. The examples of primary cells are Leclanche cell, zinc-chlorine cell, alkaline-manganese cell and metal air cells etc.

If, on the other hand, energy is induced in the chemical substances by applying an external source, it is called a secondary cell or rechargeable cell. A battery made out of these cells is called a secondary battery or storage battery or rechargeable battery. Examples of secondary cells are lead-acid cell, nickel-cadmium cell, nickel-iron cell, nickel-zinc cell, nickel-hydrogen cell, silver-zinc cell and high temperature cells like lithium-chlorine cell, lithium-sulphur cell, sodium-sulphur cell etc.

CLASSIFICATION OF SECONDARY BATTERIES

Various types of secondary batteries can be grouped in to the following categories as per their use:

1. AUTOMOTIVE BATTERIES OR SLI BATTERIES OR PORTABLE BATTERIES:

These are used for starting, lighting and ignition (SLI) in internal-combustion-engine vehicles.

Examples are; lead-acid batteries, nickel-cadmium batteries etc.

2. VEHICLE TRACTION BATTERIES OR MOTIVE POWER BATTERIES OR INDUSTRIAL BATTERIES:

These are used as a motive power source for a wide variety of vehicles. Lead-acid batteries, nickel-iron batteries, silver-zinc batteries have been used for this purpose. A number of advance batteries including high-temperature batteries are under development for electric vehicle (EV) use.

These high-temperature batteries like sodium-sulphur and lithium-iron sulphide have energy densities in the range of 100-120 Wh/kg.

3. STATIONARY BATTERIES:

These fall into two groups

(a) Standby power system which is used intermittently and
(b) Load leveling system which stores energy when demand is low and, later on, uses it to meet peak demand.

WEBER AND EWING MOLECULAR THEORY

This theory was first advanced by Weber in 1852 and was, later on, further developed by Ewing in 1890. The basic assumption of this theory is that molecules of all substances are inherently magnets in themselves, each having N and S pole. In an un-magnetized state, it is supposed that these small molecular magnets lie in all sorts of haphazard manner forming more or less closed loops. According to the laws of attraction and repulsion, these closed magnetic circuits are satisfied internally, hence there is no resultant external magnetism exhibited by the iron bar. But when such an iron bar is placed in a magnetic field or under the influence of a magnetizing force, then these molecular magnets start turning round their axes and orientate themselves more or less along straight lines parallel to the direction of the magnetizing force. This linear arrangement of the molecular magnets results in N polarity at one end of the bar and S polarity at the other (seen in figure). As the small magnets turn more nearly in the direction of the magnetizing force, it requires more and more of this force to produce a given turning moment, thus accounting for the magnetic saturation. On this theory, the hysteresis loss is supposed to be due to molecular friction of these turning magnets.

Because of the limited knowledge of molecular structure available at the time of Weber, it was not possible to explain firstly, as to why the molecules themselves are magnets and secondly, why it is impossible to magnetize certain substances like wood etc. The first objection was explained by Ampere who maintained that orbital movement of the electrons round the atom of a molecule constituted a flow of current which, due to its associated magnetic effect, made the molecule a magnet. Later on, it became difficult to explain the phenomenon of diamagnetism (shown by materials like water, quartz, silver and copper etc.) erratic behavior of ferromagnetic (intensely magnetisable) substances like iron, steel, cobalt, nickel and some of their alloys etc. and the paramagnetic (weakly magnetisable) substances like oxygen and aluminum etc. Moreover, it was asked: if molecules of all substances are magnets, then why does not wood or air etc. become magnetized?

All this has been explained satisfactorily by the atom-domain theory which has superseded the molecular theory. It is beyond the scope of this book to go into the details of this theory. The interested reader is advised to refer to some standard book on magnetism. However, it may just be mentioned that this theory takes into account not only the planetary motion of an electron but its rotation about its own axis as well. This latter rotation is called ‘electron spin’. The gyroscopic behavior of an electron gives rise to a magnetic moment which may be either positive or negative. A substance is ferromagnetic or diamagnetic accordingly as there is an excess of unbalanced positive spins or negative spins. Substances like wood or air are non-magnetisable because in their case, the positive and negative electron spins are equal, hence they cancel each other out.

CAPACITOR

A capacitor essentially consists of two conducting surfaces separated by a layer of an insulating medium called dielectric. The conducting surfaces may be in the form of either circular (or rectangular) plates or be of spherical or cylindrical shape. The purpose of a capacitor is to store electrical energy by electrostatic stress in the dielectric (the word ‘condenser’ is a misnomer since a capacitor does not ‘condense’ electricity as such, it merely stores it). A parallel-plate capacitor is shown in Figure. One plate is joined to the positive end of the supply and the other to the negative end or is earthed.

It is experimentally found that in the presence of an earthed plate B, plate A is capable of withholding more charge than when B is not there. When such a capacitor is put across a battery, there is a momentary flow of electrons from A to B. As negatively-charged electrons are withdrawn from A, it becomes positive and as these electrons collect on B, it becomes negative. Hence, a PD is established between plates A and B. The transient flow of electrons gives rise to charging current. The strength of the charging current is maximum when the two plates are uncharged but it then decreases and finally ceases when PD across the plates becomes slowly and slowly equal and opposite to the battery EMF.

Figure: Capacitor

BREAKDOWN VOLTAGE AND DIELECTRIC STRENGTH

An insulator or dielectric is a substance within which there are no mobile electrons necessary for electric conduction. However, when the voltage applied to such an insulator exceeds a certain value, then it breaks down and allows a heavy electric current (much larger than the usual leakage current) to flow through it. If the insulator is a solid medium, it gets punctured or cracked. The disruptive or breakdown voltage of an insulator is the minimum voltage required to break it down.

Dielectric strength of an insulator or dielectric medium is given by the maximum potential difference which a unit thickness of the medium can withstand without breaking down. In other words, the dielectric strength is given by the potential gradient necessary to cause breakdown of an insulator. Its unit is volt/meter (V/m) although it is usually expressed in KV/mm.

For example, when we say that the dielectric strength of air is 3 KV/mm, then it means that the maximum PD which one mm thickness of air can withstand across it without breaking down is 3 kV or 3000 volts. If the PD exceeds this value, then air insulation breaks down allowing large electric current to pass through.

Dielectric strength of various insulating materials is very important factor in the design of high voltage generators, motors and transformers. Its value depends on the thickness of the insulator, temperature, moisture, content, shape and several other factors. For example doubling the thickness of insulation does not double the safe working voltage in a machine.

STATIC ELECTRICITY

Electrostatics is that branch of science which deals with the phenomena associated with electricity at rest. Generally an atom is electrically neutral i.e. in a normal atom the aggregate of positive charge of protons is exactly equal to the aggregate of negative charge of the electrons.

If, somehow, some electrons are removed from the atoms of a body, then it is left with a preponderance of positive charge. It is then said to be positively-charged. If, on the other hand, some electrons are added to it, negative charge out-balances the positive charge and the body is said to be negatively charged.

In brief, we can say that positive electrification of a body results from a deficiency of the electrons whereas negative electrification results from an excess of electrons. The total deficiency or excess of electrons in a body is known as its charge.

CORONA AND ITS FORMATION

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona.

The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and the radio noise. If the applied voltage is increased to breakdown value, a flash-over will occur between the conductors due to the breakdown of air insulation.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors; otherwise the rough points will appear brighter. With DC voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

CORONA FORMATION:

Some ionization is always present in air due to cosmic rays, ultraviolet radiations and radioactivity. Therefore, under normal conditions, the air around the conductors contains some ionized particles (i.e., free electrons and +ve ions) and neutral molecules. When pd is applied between the conductors, potential gradient is set up in the air which will have maximum value at the conductor surfaces. Under the influence of potential gradient, the existing free electrons acquire greater velocities. The greater the applied voltage, the greater the potential gradient and more is the velocity of free electrons.

When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it. This produces another ion and one or more free electrons, which in turn are accelerated until they collide with other neutral molecules, thus producing other ions. Thus, the process of ionization is cumulative. The result of this ionization is that either corona is formed or spark takes place between the conductors.

FACTORS AFFECTING CORONA

The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends:

(I) ATMOSPHERE: As corona is formed due to ionization of air surrounding the conductors, therefore, it is affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather.

(II) CONDUCTOR SIZE: The corona effect depends upon the shape and conditions of the conductors.

The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has irregular surface and hence gives rise to more corona that a solid conductor.

(III) SPACING BETWEEN CONDUCTORS: If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect. It is because larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

(IV) LINE VOLTAGE: The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

ADVANTAGES AND DISADVANTAGES OF CORONA

Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages.

ADVANTAGES

(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.

(ii) Corona reduces the effects of transients produced by surges.

DIS-ADVANTAGES
(i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.

(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.

(iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.

METHODS OF REDUCING CORONA EFFECT

It has been seen that intense corona effects are observed at a working voltage of 33 KV or above. Therefore, careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33 KV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment. The corona effects can be reduced by the following methods:

(I) BY INCREASING CONDUCTOR SIZE: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.

(II) BY INCREASING CONDUCTOR SPACING: By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated. However, spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger cross arms and supports) may increase to a considerable extent.

SUSPENSION TYPE INSULATORS AND ITS ADVANTAGES

For high voltages (>33 KV), it is a usual practice to use suspension type insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower. Each unit or disc is designed for low voltage, say 11 KV. The number of discs in series would obviously depend upon the working voltage. For instance, if the working voltage is 66 KV, then six discs in series will be provided on the string.

ADVANTAGES OF SUSPENSION TYPE INSULATORS

(i) Suspension type insulators are cheaper than pin type insulators for voltages beyond 33 kV.

(ii) Each unit or disc of suspension type insulator is designed for low voltage, usually 11 kV. Depending upon the working voltage, the desired number of discs can be connected in series.

(iii) If any one disc is damaged, the whole string does not become useless because the damaged disc can be replaced by the sound one.

(iv) The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

(v) In case of increased demand on the transmission line, it is found more satisfactory to supply the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs.

(vi) The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

LISSAJOUS FIGURES

Lissajous figures (or patterns) are named in honor of the French scientist who first obtained them geometrically and optically.

They illustrate one of the earliest uses to which the CRO was put. Lissajous patterns are formed when two sine waves are applied simultaneously to the vertical and horizontal deflecting plates of a CRO. The two sine waves may be obtained from two audio oscillators as shown in Figure. Obviously, in this case, a sine wave sweeps a sine-wave input signal. The shape of the Lissajous pattern depends on the frequency and phase relationship of the two sine waves.

Two sine waves of the same frequency and amplitude may produce a straight line, an ellipse or a circle depending on their phase difference (below figure).

In general, the shape of Lissajous figures depends on

(i) Amplitude,
(ii) Phase difference and
(iii) Ratio of frequency of the two waves.

Lissajous figures are used for

(i) Determining an unknown frequency by comparing it with a known frequency
(ii) Checking audio oscillator with a known-frequency signal and
(iii) checking audio amplifiers and feedback networks for phase shift.

APPLICATIONS OF CATHODE RAY OSCILLOSCOPE

No other instrument in electronic industry is as versatile as a CRO. In fact, a modern oscilloscope is the most useful single piece of electronic equipment that not only removes guess work from technical troubleshooting but makes it possible to determine the trouble quickly. Some of its uses are as under:

(A) IN RADIO WORK

1. To trace and measure a signal throughout the RF, IF and AF channels of radio and television receivers.
2. It provides the only effective way of adjusting FM receivers, broadband high-frequency RF amplifiers and automatic frequency control circuits;
3. to test AF circuits for different types of distortions and other spurious oscillations;
4. To give visual display of wave-shapes such as sine waves, square waves and their many different combinations;
5. To trace transistor curves
6. To visually show the composite synchronized TV signal
7. To display the response of tuned circuits etc.

(B) SCIENTIFIC AND ENGINEERING APPLICATIONS

1. Measurement of ac/dc voltages,
2. Finding B/H curves for hysteresis loop,
3. for engine pressure analysis,
4. for study of stress, strain, torque, acceleration etc.
5. Frequency and phase determination by using Lissajous figures,
6. Radiation patterns of antenna,
7. Amplifier gain,
8. Modulation percentage,
9. Complex waveform as a short-cut for Fourier analysis,
10. Standing waves in transmission lines etc.

CATHODE RAY TUBE CRT

It is the heart of an oscilloscope and is very similar to the picture tube in a television set.

CONSTRUCTION

The cross-sectional view of a general purpose electrostatic deflection CRT is shown in below Figure. Its four major components are:

1. An electron gun for producing a stream of electrons,
2. Focusing and accelerating anodes- for producing a narrow and sharply-focused beam of electrons,
3. Horizontal and vertical deflecting plates-for controlling the path of the beam,
4. An evacuated glass envelope with a phosphorescent screen which produces bright spot when struck by a high-velocity electron beam.

As shown, a CRT is a self-contained unit like any electron tube with a base through which leads are brought out for different pins.

1. ELECTRON GUN ASSEMBLY

The electron gun assembly consists of an indirectly-heated cathode K, a control grid G, a pre-accelerator anode A1, focusing anode A2 and an accelerating anode A3. The sole function of the electrons gun assembly is to provide a focused beam of electrons which is accelerated towards the fluorescent screen. The electrons are given off by thermionic emission from the cathode. The control grid is a metallic cylinder with a small aperture in line with the cathode and kept at a negative potential with respect to K. The number of electrons allowed to pass through the grid aperture (and, hence, the beam current) depends on the amount of the control grid bias. Since the intensity (or brightness) of the spot S on the screen depends on the strength of beam current, the knob controlling the grid bias is called the intensity control.

The anodes A2 and A3, which are both at positive potential with respect to K, operate to accelerate the electron beam (below Figure). The cylindrical focusing anode A2, being at negative potential, repels electrons from all sides and compresses them into a fine beam. The knob controlling the potential of A2 provides the focus control.

2. DEFLECTING PLATES

Two sets of deflecting plates are used for deflecting the thin pencil-like electronic beam both in the vertical and horizontal directions. The first set marked Y (nearer to the gun) is for vertical deflection and X-set is for horizontal deflection. When no potential is applied across the plates, beam passes between both sets of plates un-deflected and produces a bright spot at the center of the screen.

If upper Y -plate is given a positive potential, the beam is deflected upwards depending on the value of the applied potential. Similarly, the beam (and hence the spot) deflects downwards when lower Y -plate is made positive.

However, if an alternating voltage is applied across the

Y -plates, the spot keeps moving up and down thereby producing a vertical luminous trace on the screen due to persistence of vision. The maximum displacement of the spot from its central position is equal to the amplitude of the applied voltage.

The screen spot is deflected horizontally if similar voltages are applied to the X-plates. The dc potentials on the Y -and X-plates are adjustable by means of centering controls.

It must be remembered that the signal to be displayed on the screen is always applied across the Y -plates. The voltage applied across X-plates is a ramp voltage i.e. a voltage which increases linearly with time. It has a saw tooth wave-form as shown in below Figure. It is also called horizontal time-base or sweep voltage. It has a sweep time of Tsw.

3. GLASS ENVELOPE

It is funnel-shaped having a phosphor-coated screen at its flared end. It is highly-evacuated in order to permit the electron beam to traverse the tube easily. The inside of the flared part of the tube is coated with a conducting graphite layer called Aquadag which is maintained at the same potential as A3. This layer performs two functions

(i) It accelerates the electron beam after it passes between the deflecting plates and

(ii) Collects the electrons produce by secondary emission when electron beam strikes the screen. Hence, it prevents the formation of negative charge on the screen.

The screen itself is coated with a thin layer of a fluorescent material called phosphor. When struck by high-energy electrons, it glows. In other words, it absorbs the kinetic energy of the electrons and converts it into light-the process being known as fluorescence. That is why the screen is called fluorescent screen. The color of the emitted light depends on the type of phosphor used.

CATHODE RAY OSCILLOSCOPE CRO

It is generally referred to as oscilloscope or scope and is the basic tool of an electronic engineer and technician as voltmeter; ammeter and watt meter are those of an electrical engineer or electrician. The CRO provides a two-dimensional visual display of the signal wave shape on a screen thereby allowing an electronic engineer to ‘see’ the signal in various parts of the circuit.

It, in effect, gives the electronic engineer an eye to ‘see’ what is happening inside the circuit itself. It is only by ‘seeing’ the signal wave forms that he/she can correct errors, understand mistakes in the circuit design and thus make suitable adjustments.

An oscilloscope can display and also measure many electrical quantities like ac/dc voltage, time, phase relationships, frequency and a wide range of waveform characteristics like rise-time, fall-time and overshoot etc. Non-electrical quantities like pressure, strain, temperature and acceleration etc. can also be measured by using different transducers to first convert them into an equivalent voltage.

figure 1

As seen from the block diagram of an oscilloscope (Figure 1), it consists of the following major sub-systems:

1. Cathode Ray Tube (CRT): it displays the quantity being measured.
2. VERTICAL AMPLIFIER: it amplifies the signal waveform to be viewed.
3. HORIZONTAL AMPLIFIER: it is fed with a saw-tooth voltage which is then applied to the X-plates.
4. SWEEP GENERATOR: produces saw-tooth voltage waveform used for horizontal deflection of the electron beam.
5. TRIGGER CIRCUIT: produces trigger pulses to start horizontal sweep.

6. High and low-voltage power supply.

The operating controls of a basic oscilloscope are shown in Figure 2. The different terminals provide.

1. Horizontal amplifier input,
2. Vertical amplifier input,
3. Sync. input,
4. Z-axis input,
5. External sweep input.

As seen, different controls permit adjustment of

1. INTENSITY: for correct brightness of the trace on the screen,
2. FOCUS: for sharp focus of the trace.
3. HORIZONTAL CENTERING: for moving the pattern right and left on the screen.
4. VERTICAL CENTERING: for moving the pattern up and down on the screen.
5. HORIZONTAL GAIN (also Time/div or Time/cm): for adjusting pattern width.
6. VERTICAL GAIN (also volt/div or volt/cm): for adjusting pattern height.
7. SWEEP FREQUENCY: for selecting number of cycles in the pattern.
8. SYNC. VOLTAGE AMPLITUDE: for locking the pattern.

The different switches permit selection of:

1. Sweep type,

2. Sweep range,

3. Sync. type

Figure 2

DC FET VOLT METER

The schematic diagram of a FET Volt Meter using difference amplifier is shown in Figure. The two FETs are identical so that increase in the current of one FET is offset by corresponding decrease in the source current of the other. The two FETs form the lower arms of the balanced bridge circuit whereas the two drain resistors RD form the upper arms. The meter movement is connected across the drain terminals of the FETs.

The circuit is balanced under zero-input-voltage condition provided the two FETs are identical. In that case, there would be no current through M. Zero-Adjust potentiometer is used to get null deflection in case there is a small current through M under zero-signal condition. Full-scale calibration is adjusted with the help of variable resistor R.

When positive voltage is applied to the gate of F1, some current flows through M. The magnitude of this current is found to be proportional to the voltage being measured. Hence, meter is calibrated in volts to indicate input voltage.

ESSENTIALS OF AN ELECTRONIC INSTRUMENT

As shown Figure, an electronic instrument is made up of the following three elements:

TRANSDUCER

It is the first sensing element and is required only when measuring a non-electrical quantity, say, temperature or pressure. Its function is to convert the non-electrical physical quantity into an electrical signal. Of course, a transducer is not required if the quantity being measured is already in the electrical form.

SIGNAL MODIFIER

It is the second element and its function is to make the incoming signal suitable for application to the indicating device. For example, the signal may need amplification before it can be properly displayed. Other types of signal modifiers are: voltage dividers for reducing the amount of signal applied to the indicating device or wave shaping circuits such as filters, rectifiers or chopper etc.

INDICATING DEVICE

For general purpose instruments like voltmeters, ammeters or ohm meters, the indicating device is usually a deflection type meter as shown in Figure. In digital readout instruments, the indicating device is of digital design.

ELECTRONIC VERSUS ELECTRICAL INSTRUMENTS

Both electrical and electronic instruments measure electrical quantities like voltage and current etc. Purely electrical instruments do not have any built-in amplifying device to increase the amplitude of the quantity being measured. The common dc voltmeter based on moving-coil meter movement is clearly an electrical instrument. The electronic instruments always include in their make-up some active electron device such as vacuum tube, semiconductor diode or an integrated circuit etc.

The main distinguishing factor between the two types of instruments is the presence of an electron device in the electronic instruments. Of course, movement of electrons is common to both types, their main difference being that control of electron movement is more effective in electronic instruments than in electrical instruments. Although electronic instruments are usually more expensive than their electrical counterparts, they offer following advantages for measurements purposes:

1. Since electronic instruments can amplify the input signal, they possess very high sensitivity i.e. they are capable of measuring extremely small (low-amplitude) signals,

2. Because of high sensitivity, their input impedance is increased which means less loading effect when making measurements,

3. They have greater speed i.e. faster response and flexibility,

4. They can monitor remote signals.

SELECTIVE RESONANCE DUE TO HARMONICS

When a complex voltage is applied across a circuit containing both inductance and capacitance, it may happen that the circuit resonates at one of the harmonic frequencies of the applied voltage. This phenomenon is known as selective resonance. If it is a series circuit, then large currents would be produced at resonance, even though the applied voltage due to this harmonic may be small. Consequently, it would result in large harmonic voltage appearing across both the capacitor and the inductance. If it is a parallel circuit, then at resonant frequency, the resultant current drawn from the supply would be minimum.

It is because of the possibility of such selective resonance happening that every effort is made to eliminate harmonics in supply voltage. However, the phenomenon of selective resonance has been usefully employed in some wave analyses for determining the harmonic content of alternating waveforms. For this purpose, a variable inductance, a variable capacitor, a variable non-inductive resistor and a fixed non-inductive resistance or shunt for an oscillo-graph are connected in series and connected to show the wave-form of the voltage across the fixed non-inductive resistance. The values of inductance and capacitance are adjusted successively to give resonance for the first, third, first and seventh harmonics and a record of the waveform is obtained by the oscillo-graph. A quick inspection of the shape of the waveform helps to detect the presence or absence of a particular harmonic.

NEUTRAL GROUNDING AND ITS ADVANTAGES

The process of connecting neutral point of 3-phase system to earth (i.e. soil) either directly or through some circuit element (e.g. resistance, reactance etc.) is called neutral grounding. Neutral grounding provides protection to personal and equipment. It is because during earth fault, the current path is completed through the earthed neutral and the protective devices (e.g. a fuse etc.) operate to isolate the faulty conductor from the rest of the system. This point is illustrated in Figure.

Figure shows a 3-phase, star-connected system with neutral earthed (i.e. neutral point is connected to soil). Suppose a single line to ground fault occurs in line R at point F. This will cause the current to flow through ground path as shown in Figure. Note that current flows from R phase to earth, then to neutral point N and back to R-phase. Since the impedance of the current path is low, a large current flows through this path. This large current will blow the fuse in R-phase and isolate the faulty line R. This will protect the system from the harmful effects (e.g. damage to equipment, electric shock to personnel etc.) of the fault. One important feature of grounded neutral is that the potential difference between the live conductor and ground will not exceed the phase voltage of the system i.e. it will remain nearly constant.

ADVANTAGES OF NEUTRAL GROUNDING

The following are the advantages of neutral grounding:

(I) Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain nearly constant.
(II) The high voltages due to arcing grounds are eliminated.
(III) The protective relays can be used to provide protection against earth faults. In case earth fault occurs on any line, the protective relay will operate to isolate the faulty line.
(IV) The over-voltages due to lightning are discharged to earth.
(V) It provides greater safety to personnel and equipment.
(VI) It provides improved service reliability.
(VII) Operating and maintenance expenditures are reduced.

Note: It is interesting to mention here that ungrounded neutral has the following advantages:

(I) In case of earth fault on one line, the two healthy phases will continue to supply load for a short period.
(II) Interference with communication lines is reduced because of the absence of zero sequence currents.

The advantages of ungrounded neutral system are of negligible importance as compared to the advantages of the grounded neutral system. Therefore, modern 3-phase systems operate with grounded neutral points.

GENERATION OF ELECTRICAL ENERGY

By Engr. Aneel Kumar September 22, 2014

The conversion of energy available in different forms in nature into electrical energy is known as generation of electrical energy.

Electrical energy is a manufactured commodity like clothing, furniture or tools. Just as the manufacture of a commodity involves the conversion of raw materials available in nature into the desired form, similarly electrical energy is produced from the forms of energy available in nature.

However, electrical energy differs in one important respect. Whereas other commodities may be produced at will and consumed as needed, the electrical energy must be produced and transmitted to the point of use at the instant it is needed. The entire process takes only a fraction of a second. This instantaneous production of electrical energy introduces technical and economic considerations unique to the electrical power industry.

Energy is available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All these forms of energy can be converted into electrical energy by the use of suitable arrangements. The arrangement essentially employs (Fig. 1.1) an alternator coupled to a prime mover. The prime mover is driven by the energy obtained from various sources such as burning of fuel, pressure of water, force of wind etc. For example, chemical energy of a fuel (e.g., coal) can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by the alternator. Similarly, other forms of energy can be converted into electrical energy by employing suitable machinery and equipment.

IMPORTANCE OF ELECTRICAL ENERGY

By Engr. Aneel Kumar September 22, 2014

Energy may be needed as heat, as light, as motive power etc. The present-day advancement in science and technology has made it possible to convert electrical energy into any desired form. This has given electrical energy a place of pride in the modern world. The survival of industrial undertakings and our social structures depends primarily upon low cost and uninterrupted supply of electrical energy. In fact, the advancement of a country is measured in terms of per capita consumption of electrical energy.

Electrical energy is superior to all other forms of energy due to the following reasons :

(I) CONVENIENT FORM: Electrical energy is a very convenient form of energy. It can be easily converted into other forms of energy. For example, if we want to convert electrical energy into heat, the only thing to be done is to pass electrical current through a wire of high resistance e.g., a heater. Similarly, electrical energy can be converted into light (e.g. electric bulb), mechanical energy (e.g. electric motors) etc.

(II) EASY CONTROL: The electrically operated machines have simple and convenient starting, control and operation. For instance, an electric motor can be started or stopped by turning on or off a switch. Similarly, with simple arrangements, the speed of electric motors can be easily varied over the desired range.

(III) GREATER FLEXIBILITY: One important reason for preferring electrical energy is the flexibility that it offers. It can be easily transported from one place to another with the help of conductors.

(IV) CHEAPNESS: Electrical energy is much cheaper than other forms of energy. Thus it is overall economical to use this form of energy for domestic, commercial and industrial purposes.

(V) CLEANLINESS: Electrical energy is not associated with smoke, fumes or poisonous gases. Therefore, its use ensures cleanliness and healthy conditions.

(VI) HIGH TRANSMISSION EFFICIENCY: The consumers of electrical energy are generally situated quite away from the centres of its production. The electrical energy can be transmitted conveniently and efficiently from the centres of generation to the consumers with the help of overhead conductors known as transmission lines.

REQUIREMENTS OF SATISFACTORY ELECTRIC SUPPLY

By Engr. Aneel Kumar September 19, 2014

The electric power system in India is 3-phase AC operating at a frequency of 50 Hz. The power station delivers power to consumers through its transmission and distribution systems. The power delivered must be characterized by constant or nearly constant voltage, dependability of service, balanced voltage, and efficiency so as to give minimum annual cost, sinusoidal waveform and freedom from inductive interference with telephone lines.

(1) VOLTAGE REGULATION: A voltage variation has a large effect upon the operation of both power machinery and lights. A motor is designed to have its best characteristics at the rated voltage and consequently a voltage that is too high or too low will result in a decrease in efficiency. If the fluctuations in the voltage are sudden, these may cause the tripping of circuit breakers and consequent interruptions to service. Usually the voltage at the generator terminals, where this is done, in some cases the voltage variations at the load may be made sufficiently small by keeping the resistance and reactance of the lines and feeders low.

(2) DEPENDABILITY: One important requirement of electric supply is to furnish uninterrupted service. The losses which an industrial consumer sustains due to the failure of electric power supply are usually vastly greater than the actual value of the power that he would use during this period. It is on account of the expense of idle workmen and machines and other overhead charges. Interruptions to service cause irritation and are sometimes positively dangerous to life and property. For example, failure of power in hospitals, in crowded theatres and stores may lead to very grave consequences.

Therefore, it is the duty of electric supply company to keep the power system going and to furnish uninterrupted service.

(3) BALANCED VOLTAGE: It is very important that the poly-phase voltage should be balanced. If an unbalanced poly-phase voltage is supplied to a consumer operating synchronous or induction motors, it will result in a decrease in the efficiency of his machinery and also a decrease in its maximum power output. Motors called upon to deliver full load when their terminal voltages are unbalanced are liable to considerable damage due to overheating. One method of maintaining balance of voltage is by having balanced loads connected to the circuit.

(4) EFFICIENCY: The efficiency of a transmission system is not of much importance in itself. The important economic feature of the design being the layout of the system as a whole so as to perform the requisite function of generating and delivering power with a minimum overall annual cost. The annual cost can be minimized to a considerable extent by taking care of power factor of the system. It is because losses in the lines and machinery are largely determined by power factor. Therefore, it is important that consumers having loads of low power factor should be penalized by being charged at a higher rate per kWh than those who take power at high power factors. Loads of low power factor also require greater generator capacity than those of high power factor (for the same amount of power) and produce larger voltage drops in the lines and transformers.

(5) FREQUENCY: The frequency of the supply system must be maintained constant. It is because a change in frequency would change the motor speed, thus interfering with the manufacturing operations.

(6) SINUSOIDAL WAVEFORM: The alternating voltage supplied to the consumers should have a sine waveform. It is because any harmonics which might be present would have detrimental effect upon the efficiency and maximum power output of the connected machinery. Harmonics may be avoided by using generators of good design and by avoidance of high flux densities in transformers.

(7) FREEDOM FROM INDUCTIVE INTERFERENCE: Power lines running parallel to telephone lines produce electrostatic and electromagnetic field disturbances. These fields tend to cause objectionable noises and hums in the apparatus connected to communication circuits. Inductive interference with telephone lines may be avoided by limiting as much as possible the amount of zero-sequence and harmonic current and by the proper transposition of both power lines and telephone lines.

EFFECTS OF VARIABLE LOAD ON POWER STATION

By Engr. Aneel Kumar September 16, 2014

The variable load on a power station introduces many perplexities in its operation. Some of the important effects of variable load on a power station are

(i) NEED OF ADDITIONAL EQUIPMENT: The variable load on a power station necessitates to have additional equipment. By way of illustration, consider a steam power station. Air, coal and water are the raw materials for this plant. In order to produce variable power, the supply of these materials will be required to be varied correspondingly. For instance, if the power demand on the plant increases, it must be followed by the increased flow of coal, air and water to the boiler in order to meet the increased demand. Therefore, additional equipment has to be installed to accomplish this job. As a matter of fact, in a modern power plant, there is much equipment devoted entirely to adjust the rates of supply of raw materials in accordance with the power demand made on the plant.

(ii) INCREASE IN PRODUCTION COST: The variable load on the plant increases the cost of the production of electrical energy. An alternator operates at maximum efficiency near its rated capacity. If a single alternator is used, it will have poor efficiency during periods of light loads on the plant. Therefore, in actual practice, a number of alternators of different capacities are installed so that most of the alternators can be operated at nearly full load capacity. However, the use of a number of generating units increases the initial cost per kW of the plant capacity as well as floor area required. This leads to the increase in production cost of energy.

EVALUATING SYSTEM HARMONICS

In order to prevent or correct harmonic problems that could occur within an industrial facility, an evaluation of system harmonics should be performed if the facility conditions meet one or more of the criteria below.

1. The application of capacitor banks in systems where 20% or more of the load includes other harmonic generating equipment.

2. The facility has a history of harmonic related problems, including excessive capacitor fuse operation.

3. During the design stage of a facility composed of capacitor banks and harmonic generating equipment.

4. In facilities where restrictive power company requirements limit the harmonic injection back into their system to very small magnitudes.

5. Plant expansions that add significant harmonic generating equipment operating in conjunction with capacitor banks.

6. When coordinating and planning to add an emergency standby generator as an alternate power source in an industrial facility.

Often, the vendor or supplier of non-linear load equipment, such as variable frequency drives, can evaluate the effects that the equipment may have on the distribution system. This usually involves details related to the distribution system design and impedances, similar to performing a short circuit study evaluation.

EFFECTS AND NEGATIVE CONSEQUENCES OF HARMONICS

The effects of three-phase harmonics on circuits are similar to the effects of stress and high blood pressure on the human body. High levels of stress or harmonic distortion can lead to problems for the utility's distribution system, plant distribution system and any other equipment serviced by that distribution system. Effects can range from spurious operation of equipment to a shutdown of important plant equipment, such as machines or assembly lines. Harmonics can lead to power system inefficiency. Some of the negative ways that harmonics may affect plant equipment are listed below:

1. CONDUCTOR OVERHEATING: a function of the square RMS current per unit volume of the conductor. Harmonic currents on undersized conductors or cables can cause a “skin effect”, which increases with frequency and is similar to a centrifugal force.

2. CAPACITORS: can be affected by heat rise increases due to power loss and reduced life on the capacitors. If a capacitor is tuned to one of the characteristic harmonics such as the 5th or 7th, over-voltage and resonance can cause dielectric failure or rupture the capacitor.

3. FUSES AND CIRCUIT BREAKERS: harmonics can cause false or spurious operations and trips, damaging or blowing components for no apparent reason.

4. TRANSFORMERS: have increased iron and copper losses or eddy currents due to stray flux losses. This causes excessive overheating in the transformer windings. Typically, the use of appropriate “K factor” rated units is recommended for non-linear loads.

5. GENERATORS: have similar problems to transformers. Sizing and coordination is critical to the operation of the voltage regulator and controls. Excessive harmonic voltage distortion will cause multiple zero crossings of the current waveform. Multiple zero crossings affect the timing of the voltage regulator, causing interference and operation instability.

6. UTILITY METERS: may record measurements incorrectly, resulting in higher billings to consumers.

7. DRIVES/POWER SUPPLIES: can be affected by mis-operation due to multiple zero crossings. Harmonics can cause failure of the commutation circuits, found in DC drives and AC drives with silicon controlled rectifiers (SCRs).

8. COMPUTERS/TELEPHONES: may experience interference or failures.

SOURCES OF HARMONICS

Conventional electromagnetic devices as well as semiconductor applications act as sources of harmonics. Conventional electromagnetic devices include stationary transformer as well as rotating machines. Harmonic generation in these machine depends on the properties of the materials used to construct them, different design constraints and considerations, operating principle and of course load environment. Beside these arcing devices produces considerable amount of harmonics. Other than conventional devices, semiconductor based power supplies, phase controllers, reactors, etc are used enormously in modern power system network and they are contributing huge amount of harmonics to the power system. In electric power system, main sources of harmonics may be classified as follows.

1. Arcing devices
2. Semiconductor based power supply system
3. Inverter fed A.C. drives
4. Thyristor controlled reactors
5. Phase controllers
6. A.C. regulators

1. DISTORTION CAUSED BY ARCING DEVICES

Arcing devices are very important source of power system harmonics. The voltage versus current characteristics of an electric arc in an arcing device is highly nonlinear. Arc ignition is equivalent to a short circuit current with decrease in voltage. The voltage-current is controlled by the power system impedance. In respect of harmonic generation, arcing devices are divided into three main categories:

1. Electric arc furnace
2. Discharge type lighting
3. Arc welders.

2. POWER SUPPLIES WITH SEMICONDUCTOR DEVICES

Semiconductor based power supply systems are the main sources of harmonics. Harmonics generated in power supply include integer harmonics, inter harmonics and sub harmonics. Frequencies and magnitudes of the harmonics depend on the type of semiconductor devices used in the power supplies, operating point, nature of load variation, etc.

3. INVERTER FED AC DRIVES

Application of AC drives has increased to a great extent, most of which are inverter fed AC drives. They use switching circuits using semiconductor devices like GTO, IGBT, etc. Pulse width modulation (PWM) has got very popularity in AC drive application. All these drives are sources of integer as well as fractional harmonics.

4. THYRISTOR CONTROLLED REACTORS

VAR compensators used in power system network are also source of harmonics. Different types of thyristor controlled reactors are used in power system like series controller, shunt controller, static VAR compensator (SVC), fixed capacitor thyristor controlled reactor (FCTCR), thyristor switched capacitor thyristor controlled reactor (TSCTCR). All these circuits are sources of harmonics in power system. Use of static synchronous generator (SSG), voltage source STATCOM, current source STATCOM, etc in power system are increasing rapidly. All these contribute harmonics of both integer and fractional type in power system. For example, SVC produces odd harmonics. Under perfectly symmetrical voltage conditions, triplen harmonics are kept out of the line by delta connection.

5. PHASE CONTROLLER

For supply of stable and balanced three phase electric power, phase controller plays important role in power system. Phase controllers used in power system act as source of harmonics. Modulated phase control method is used in cyclo-converter. It performs static power conversion from one frequency to another frequency. Most of the cyclo-converter wave-forms contain frequencies which are not integer multiples of the main output frequency.

6. AC REGULATORS

AC regulators used in power system apply both off line and on line control technique for voltage regulation which result in harmonic generation. On line regulation technique distorts wave-shape more than off line regulation along with other power system disturbances like transients, DC offset, flicker etc. Thyristor controlled single phase or polyphase regulators using half wave, full wave or integral cycle control technique produce sub-harmonics and inter-harmonics in power system.

ROTATING MACHINE AS A SOURCE OF HARMONICS