ENGINEERING ARTICLES

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...

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 desig...

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 ev...

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. Simi...

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/O 2  cell has an open-circuit voltage of 1.65 V and a theoretical energy density of 1090 Wh/kg. The Fe/O 2  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...

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. ...

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.

Since active material on a Plante plate consists of a thin layer of  PbO 2  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 ( PbO 2 ) which takes place when the c...

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 horiz...

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...

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 e...

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-treate...

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 ...

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 mag...

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. He...

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 ...

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.

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, th...

Factors Affecting Corona in Overhead Transmission Lines Author: Engr. Aneel Kumar Figure 1: Infographic illustrating the factors influencing corona discharge in transmission lines. Introduction The corona effect in overhead transmission lines is a phenomenon that occurs when the electric field intensity around conductors exceeds a critical value, causing ionization of the surrounding air. This ionization produces bluish light, hissing sound, power loss, and ozone gas. While corona may seem undesirable, it also has a few advantages such as reducing overvoltages by absorbing surges. Corona directly impacts power system efficiency, transmission losses, equipment life, and design cost . Therefore, engineers must understand the factors affecting corona in detail to ensure efficient and reliable design of high-voltage transmission systems. 1. Conductor Size (Diameter) ...

Advantages and Disadvantages of Corona Effect in Power Systems In high-voltage overhead transmission lines , the corona effect plays a critical role in system performance. Corona occurs when the air around a conductor becomes ionized due to high electric stress. While often seen as a drawback because of power losses and interference , it also provides certain engineering benefits . This article explains the advantages and disadvantages of corona effect in detail, with examples relevant to modern electrical power systems. ✅ Advantages of Corona Effect Increase in Virtual Conductor Diameter Due to corona formation, the surrounding air becomes partially conductive, increasing the virtual diameter of the conductor. This reduces electrostatic stress between conductors and minimizes insulation breakdown risks. Related Reading: Electrostatic Fields in High Voltage Engineering Reduction of Transient Surges Corona acts like a natural cushion for sudden ...

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 sup...

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...

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 fre...

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,...

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 A 1 , focusing anode A 2  and an accelerating anode A 3 . The sole function of the e...

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 tra...

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 ...