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.