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Showing posts from November, 2014

GENERATION OF AC HIGH VOLTAGE BY CASCADED TRANSFORMERS

For voltages higher than about 300 to 500 kV, the cascading of transformers is a big advantage, as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier. Also, with this, the transformer cost for a given voltage may be reduced, since cascaded units need not individually possess the expensive and heavy insulation required in single stage transformers for high voltages exceeding 345 kV.It is found that the cost of insulation for such voltages for a single unit becomes proportional to square of operating voltage. The low voltage. supply is connected to the primary winding ‘l’ of transformer I, designed for an high voltage output of V as are the other two transformers. The exciting winding ‘3’ supplies the primary of the second transformer unit II; both windings are dimensioned for the same low voltage, and the potential is fixed to the high potential V. The high voltage or secondary windings ‘2’ of both units are se...

CASCADED TRANSFORMERS METHOD FOR GENERATING AC HIGH VOLTAGE

For voltages higher than 400 KV, it is desired to cascade two or more transformers depending upon the voltage requirements. With this, the weight of the whole unit is subdivided into single units and, therefore, transport and erection becomes easier. Also, with this, the transformer cost for a given voltage may be reduced, since cascaded units need not individually possess the expensive and heavy insulation required in single stage transformers for high voltages exceeding 345 kV. It is found that the cost of insulation for such voltages for a single unit becomes proportional to square of operating voltage. Figure shows a basic scheme for cascading three transformers. The primary of the first stage transformer is connected to a low voltage supply. A voltage is available across the secondary of this transformer. The tertiary winding (excitation winding) of first stage has the same number of turns as the primary winding, and feeds the primary of the second stage transformer. The pote...

EFFECTS OF VOLTAGE SAGS ON LIGHTING LOADS

Voltage sags may cause lamps to extinguish. Light bulbs will just twinkle; that will likely not be considered to be a serious effect. High pressure lamps may extinguish; it takes several minutes for them to re-ignite. All lamps, except incandescent lamps, require high voltage across the lamp electrodes during starting. This voltage is essential to initiate the arc. Traditionally, a choke coil is employed across the electrodes to produce high voltage pulses. The lamp starting voltage is affected to a large extent by the ambient temperature and humidity levels as well as the supply voltage. Fluorescent lamps reach their full emission level immediately after ignition. High-pressure lamps need a few minutes to reach their full light output, while low-pressure lamps take up to 15 minutes for the same. The types of industrial lights are described below. INCANDESCENT LAMPS This is the oldest and therefore, the most basic technology used in lighting systems. Current passed through a filament...

SURGE PROTECTION OF ROTATING MACHINE

A rotating machine is less exposed to lightning surge as compared to transformers. Because of the limited space available, the insulation on the windings of rotating machines is kept to a minimum. The main difference between the winding of rotating machine and transformer is that in case of rotating machines the turns are fewer but longer and are deeply buried in the stator slots. Surge impedance of rotating machines in approx. 1000 Ω and since the inductance and capacitance of the windings are large as compared to the overhead lines the velocity of propagation is lower than on the lines. For a typical machine it is 15 to 20 m/ µ sec. This means that in case of surges with steep fronts, the voltage will be distributed or concentrated at the first few turns. Since the insulation is not immersed in oil, its impulse ratio is approx. unity whereas that of the transformer is more than 2.0. The rotating machine should be protected against major and minor insulations. By major insulation is m...

EFFECT OF PRESSURE ON BREAKDOWN VOLTAGE IN VACUUM

It has been observed that in case of very small gaps of less than a mm and the gas pressure between the gap lies in the range  10 –9  to  10 –2   torr, there is no change in the breakdown voltage i.e, if the gap length is small a variation of gas pressure in the range given above doesn't affect the breakdown voltage. However, if the gap length is large say about 20 cm, the variation of gas pressure between the gap adversely affects the withstand voltage and the withstand voltage lowers drastically.

CLUMP MECHANISM

The vacuum breakdown mechanism based on this theory makes following assumption: (i) A loosely bound particle known as clump exists on one of the electrode surfaces. (ii) When a high voltage is applied between the two electrodes, this clump gets charged and subsequently gets detached from the mother electrode and is attracted by the other electrode. (iii) The breakdown occurs due to a discharge in the vapor or gas released by the impact to the particle at the opposite electrode. It has been observed that for a certain vacuum gap if frequent recurrent electric breakdowns are carried out, the withstand voltage of the gap increases and after certain number of breakdown, it reaches an optimum maximum value. This is known as conditioning of electrodes and is of paramount importance from practical reasons. In this electrode conditioning, the micro-emission sites are supposed to have been destroyed. Various methods for conditioning the electrodes have been suggested. Some of these are (i) To t...

SOLID DIELECTRICS USED IN POWER APPARATUS

The main requirements of the insulating materials used for power apparatus are: 1. High insulation resistance 2. High dielectric strength 3. Good mechanical properties i.e tenacity and elasticity 4. It should not be affected by chemicals around it 5. It should be non-hygroscopic because the dielectric strength of any material goes very much down with moisture content VULCANIZED RUBBER : Rubber in its natural form is highly insulating but it absorbs moisture readily and gets oxidized into a resinous material; thereby it loses insulating properties. When it is mixed with sulphur along with other carefully chosen ingredients and is subjected to a particular temperature it changes into vulcanized rubber which does not absorb moisture and has better insulating properties than even the pure rubber. It is elastic and resilient. The electrical properties expected of rubber insulation are high breakdown strength and high insulation resistance. In fact the insulation strength of the v...

GROUNDING AND BONDING

GROUNDING Grounding is one of the most important aspects of an electrical distribution system but often the least understood. Your Electrical Code sets out the legal requirements in your jurisdiction for safety standards in electrical installations. For instance, the Code may specify requirements in the following areas: (a) The protection of life from the danger of electric shock, and property from damage by bonding to ground non-current carrying metal systems; (b) The limiting of voltage on a circuit when exposed to higher voltages than that for which it is designed; (c) The limiting of ac circuit voltage-to-ground to a fixed level on interior wiring systems; (d) Instructions for facilitating the operation of electrical apparatus (e) Limits to the voltage on a circuit that is exposed to lightning. In order to serve Code requirements, effective grounding that systematically connects the electrical system and its loads to earth is required. Connecting to earth provides p...

TOP 5 POWER QUALITY MYTHS

1) OLD GUIDELINES ARE NOT THE BEST GUIDELINES Guidelines like the Computer Business Equipment Manufacturers Association Curve (CBEMA, now called the ITIC Curve) and the Federal Information Processing Standards Pub94 (FIPS Pub94) are still frequently cited as being modern power quality guidelines. The ITIC curve is a generic guideline for characterizing how electronic loads typically respond to power disturbances, while FIPS Pub94 was a standard for powering large mainframe computers. Contrary to popular belief, the ITIC curve is not used by equipment or power supply designers, and was actually never intended for design purposes. As for the FIPS Pub94, it was last released in 1983, was never revised, and ultimately was withdrawn as a U.S. government standards publication in November 1997. While some of the information in FIPS Pub94 is still relevant, most of it is not and should therefore not be referenced without expert assistance. 2) POWER FACTOR CORRECTION DOES NOT SOLVE ALL POWER QU...

MAJOR FACTORS CONTRIBUTING TO POWER QUALITY ISSUES

The three major factors contributing to the problems associated with power quality are: USE OF SENSITIVE ELECTRONIC LOADS The electric utility system is designed to provide reliable, efficient, bulk power that is suitable for the very large majority of electrical equipment. However, devices like computers and digital controllers have been widely adopted by electrical end-users. Some of these devices can be susceptible to power line disturbances or interactions with other nearby equipment. THE PROXIMITY OF DISTURBANCE-PRODUCING EQUIPMENT Higher power loads that produce disturbances – equipment using solid state switching semiconductors, arc furnaces, welders and electric variable speed drives – may cause local power quality problems for sensitive loads. SOURCE OF SUPPLY Increasing energy costs, price volatility and electricity related reliability issues are expected to continue for the foreseeable future. Businesses, institutions and consumers are becoming more demanding and ...

POWER QUALITY

The Institute of Electrical and Electronic Engineers (IEEE) defines power quality as: “The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.” Making sure that power and equipment are suitable for each other also means that there must be compatibility between the electrical system and the equipment it powers. There should also be compatibility between devices that share the electrical distribution space. This concept is called Electromagnetic Compatibility (“EMC”) and is defined as: “The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable  electromagnetic disturbances to anything in that environment.” The best measure of power quality is the ability of electrical equipment to operate in a satisfactory manner, given proper care and maintenance and without adversel...

POWER QUALITY

The term power quality (PQ) is generally applied to a wide variety of electromagnetic phenomena occurring within a power system network. The ability of the power systems to deliver undistorted voltage, current and frequency signals is termed as quality of power supply. Unexpected variation of the voltage or current from normal characteristics can damage or shut down the critical electrical equipment designed for specific purpose. Such variations happen in electrical networks with a great frequency due to a competitive environment and continuous change of power supply. In a highly evolved electrical system PQ sensitive demands can be classified as Digital economy (such as banking, share market and railways), Continuous process manufacturing industries, and Fabrication and essential services. Cost incurred to operate all the above types of loads vary from 3 to120 per kVA per event. This is huge and greatly affects economic operation of power industries. To mitigate PQ issues; custo...

POWER QUALITY MEASURABLE QUANTITIES

VOLTAGE DIP is a reduction in the RMS voltage in the range of 0.1 to 0.9 pu (retained) for duration greater than hall a mains cycle and less than 1 minute, Often referred to as a ‘sag’, Caused by faults, increased load demand and transitional events such as large motor starting. VOLTAGE SWELL is an increase in the RMS voltage in the range of 1.1 to 1.8 pu for a duration greater than half a mains cycle and less than 1 minute, Caused by system faults, load switching and capacitor switching. TRANSIENT is an undesirable momentary deviation of the supply voltage or load current. Transients are generally classified into two categories: impulsive and oscillatory. HARMONICS are periodic sinusoidal distortions of the supply voltage or load current caused by non-linear loads. Harmonics are measured in integer multiples of the fundamental supply frequency. Using Fourier series analysis the individual frequency components of the distorted waveform can be described in terms of the ha...

BATTERY FAQ

1. What is the “end of useful life”? The IEEE defines “end of useful life” for a UPS battery as being the point when it can no longer supply 80 percent of its rated capacity in ampere-hours. When your battery reaches 80 percent of its rated capacity, the aging process accelerates and the battery should be replaced. 2. How can I ensure that my UPS batteries are maintained and serviced properly? With proper maintenance, battery life can be predicted and replacements scheduled without interrupting your operations. These are IEEE and OEM recommendations for general maintenance: • Comprehensive maintenance programs with regular inspections • Re-torque all connections, as required • Load testing • Cleaning the battery area, as required 3. Do I have to replace my UPS batteries with the same brand of batteries? Eaton recommends that if you use brand X and need to replace one or two batteries in the string, you should use the same brand because it will have the same characteristics. If you need...

MAINTENANCE OF BATTERIES FOR EXTENDING ITS LIFE

Quantifying the combined effect of the four factors that affect battery life discussed in the previous page is difficult. You need a way to determine when a battery is near the end of its useful life so you can replace it while it still works, before the critical load is left unprotected. The only sure way to determine battery capacity is to perform a battery run-down test. The module is taken off line, connected to a load bank and operated at rated power until the specified runtime elapses or the unit shuts down due to low battery voltage. If battery capacity is less than 80 percent of its rated capacity, the battery should be replaced. Thermal scanning of battery connections during the battery run-down test identifies loose connections. This test gives you the chance to see the battery during an extended, high-current discharge. Scanning should take place during discharge and recharge cycles. An effective UPS battery maintenance program must include regular inspections, adjustments a...

BATTERY ARRANGEMENT IN UPS

In most UPSs, you don’t use just one cell at a time. They’re normally grouped together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up. However, batteries are not quite as linear as the two graphics to the right depict. For example, all batteries have a maximum current they can produce; a 500 milliamp-hour battery can’t produce 30,000 milliamps for one second, because there’s no way for its chemical reactions to happen that quickly. It is also important to realize that at higher current levels, batteries can produce a lot of heat, which wastes some of their power. Like all batteries, UPS batteries are electrochemical devices. A UPS uses a lead-acid storage battery in which the electrodes are grids of lead containing lead oxides that change in composition during charging and discharging, and the electrolyte is dilute sulfuric acid. In other words, they contain component...

FACTORS AFFECTING BATTERY PERFORMANCE

Batteries have limited life, usually showing a slow degradation of capacity until they reach 80 percent of their initial rating, followed by a comparatively rapid failure. Regardless of how or where a UPS is deployed, and what size it is, there are four primary factors that affect battery life: ambient temperature, battery chemistry, cycling and service. 1) AMBIENT TEMPERATURE The rated capacity of a battery is based on an ambient temperature of 25°C (77°F). It’s important to realize that any variation from this operating temperature can alter the battery’s performance and shorten its expected life. To help determine battery life in relation to temperature, remember that for every 8.3°C (15°F) average annual temperature above 25°C (77°F), the life of the battery is reduced by 50 percent. 2) BATTERY CHEMISTRY UPS batteries are electro-chemical devices whose ability to store and deliver power slowly decreases over time. Even if you follow all the guidelines for proper storage, us...

HYDRO ELECTRIC POWER STATION

A generating station which utilizes the potential energy of water at a high level for the generation of electrical energy is known as a hydroelectric power station. Hydro-electric power stations are generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. In a hydro-electric power station, water head is created by constructing a dam across a river or lake. From the dam, water is led to a water turbine. The water turbine captures the energy in the falling water and changes the hydraulic energy (i.e., product of head and flow of water) into mechanical energy at the turbine shaft. The turbine drives the alternator which converts mechanical energy into electrical energy. Hydro-electric power stations are becoming very popular because the reserves of fuels (i.e., coal and oil) are depleting day by day. They have the added importance for flood control, storage of water for irrigation and water for drinking purposes. ADVANTAGES OF HYDRO...

EQUIPMENT OF STEAM POWER STATION

A modern steam power station is highly complex and has numerous equipment and auxiliaries. However, the most important constituents of a steam power station are: 1. Steam generating equipment 2. Condenser 3. Prime mover 4. Water treatment plant 5. Electrical equipment. 1. STEAM GENERATING EQUIPMENT: This is an important part of steam power station. It is concerned with the generation of superheated steam and includes such items as boiler, boiler furnace, super heater, economizer, air pre-heater and other heat reclaiming devices. (I) BOILER : A boiler is closed vessel in which water is converted into steam by utilizing the heat of coal combustion. Steam boilers are broadly classified into the following two types: (a) Water tube boilers (b) Fire tube boilers In a water tube boiler, water flows through the tubes and the hot gases of combustion flow over these tubes. On the other hand, in a fire tube boiler, the hot products of combustion pass through the tubes surrounded by water. Wate...

CHOICE OF SITE FOR STEAM POWER STATIONS

In order to achieve overall economy, the following points should be considered while selecting a site for a steam power station: (I) SUPPLY OF FUEL: The steam power station should be located near the coal mines so that transportation cost of fuel is minimum. However, if such a plant is to be installed at a place where coal is not available, then care should be taken that adequate facilities exist for the transportation of coal. (II) AVAILABILITY OF WATER: As huge amount of water is required for the condenser, therefore, such a plant should be located at the bank of a river or near a canal to ensure the continuous supply of water. (III) TRANSPORTATION FACILITIES: A modern steam power station often requires the transportation of material and machinery. Therefore, adequate transportation facilities must exist i.e., the plant should be well connected to other parts of the country by rail, road, etc. (IV) COST AND TYPE OF LAND: The steam power station should be located at a place wh...

SCHEMATIC ARRANGEMENT OF STEAM POWER STATION

Steam power station simply involves the conversion of heat of coal combustion into electrical energy, it embraces many arrangements for proper working and efficiency. The schematic arrangement of a modern steam power station is shown in Figure. The whole arrangement can be divided into the following stages for the sake of simplicity: 1. Coal and ash handling arrangement 2. Steam generating plant 3. Steam turbine 4. Alternator 5. Feed water 6. Cooling arrangement 1. COAL AND ASH HANDLING PLANT: The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air...

STEAM POWER STATION OR THERMAL POWER STATION

A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station. A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then expanded in the prime mover (i.e., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated. ADVANTAGES of STEAM POWER STATION (i) The fuel (i.e., coal) used is quite cheap. (ii) Less initial cost as compared to other generating stations. (iii) It can be installed at any place irrespective of the existence of coal. The coal can be transported to the site of the plant by rail or road. (iv) It requires less space as compared to the hydroelectr...

GENERATING STATIONS

Bulk electric power is produced by special plants known as generating stations or power plants. A generating station essentially employs a prime mover coupled to an alternator for the production of electric power. The prime mover (e.g. steam turbine, water turbine etc.) converts energy from some other form into mechanical energy. The alternator converts mechanical energy of the prime mover into electrical energy. The electrical energy produced by the generating station is transmitted and distributed with the help of conductors to various consumers. It may be emphasized here that apart from prime mover-alternator combination, a modern generating station employs several auxiliary equipment and instruments to ensure cheap, reliable and continuous service. Depending upon the form of energy converted into electrical energy, the generating stations are classified as under: (i) Steam power stations (ii) Hydroelectric power stations (iii) Diesel po...