VARIABLE LOAD ON POWER STATION

The load on a power station varies from time to time due to uncertain demands of the consumers and is known as variable load on the station. A power station is designed to meet the load requirements of the consumers. An ideal load on the station, from stand point of equipment needed and operating routine, would be one of constant magnitude and steady duration. However, such a steady load on the station is never realised in actual practice. The consumer require their small or large block of power in accordance with the the demands of their activities. Thus the load demand of one consumer at any time may be different from that of the other consumer. The result is that load on the Power station varies from time to time.

Need of additional equipment.the variable Load on power station necessitates to have additional equipment. By way of illustration, consider a stream 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 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. Increase in Production cost. the variable load on the plant increases the cost of the production of electrical energy. An alternator operation at maximum efficiency near its rated capacity. If a single alternator is used, it will have poor efficiency near its rated capacity. if a single alternator is used, it will have a poor efficiency during periods of light loads on the plant.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. the use of a number of generating unit 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..  

SWITCHGEAR

The great demand for electrical energy is a notable feature of modern civilisation. Most of this energy is need for lighting, heating, domestic appliance, Industrial electrical machinery and electric traction. The importance of electric supply in every day life has reached such a stage that it is desirable to protect the power system from harm during fault condition and to ensure maximum continuity of supply. For this purpose, means must be provided to switch on or off generators, transmission lines distribution and other equipment under both normal and apparatus called switchgear. A switchgear essentially consists of switching and protecting devices such as switches,fuse, circuit breakers, relays. during normal operation, switchgear permits to switch on or off generators, transmission lines, distributors and other electrical equipment. On the other hand,when a failure occurs on any part of power system, threatening damage to the equipment and interruption of service to the customer. However, the switchgear detects the fault and disconnects the unhealthy section from the system. in this way, switchgear protects the system from the damage and ensures continuity of supply.

the switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions. The tumbler switch with ordinary fuse is the simplest form of switchgear and is used to control and protect lights and other equipment in homes, offices, circuit of light rating, a high rupturing capacity fuse in conjunction with a cannot be used profitably on high voltage system. when a fuse blows, its takes some times to replace it and consequently there is interruption of service to the customers. the fuse cannot successfully interrupt large fault currents and that result from the faults on high voltage system.        

ELECTROMAGNETIC INDUCTION

In the beginning of nineteenth century, Oersted discovered that the magnetic field exists around a current carrying conductor. in the words, magnetism can be created by means of an electric current in a conductor in 1831, Michael Faraday, the famous English scientist discovered that this could be done. He demonstrated that when the magnetic flux linking a conductor changes, an e.m.f is induced in the conductor. The phenomenon is known as electromagnetic induction. the great discovery of electromagnetic induction by Faraday through a series of a brilliant experiments has brought a revolution in the Engineering world. Most of the electrical devices are based on this principle. in this chapter, we shall confine our attention to the various aspects of electromagnetic induction ."Magnetic lines of force from closed loop. Magnetic flux linking the conductor means that the magnetic flux embraces it encircles the conductor."

When the magnetic flux linking a conductor changes, an e.m.f is induced in the conductor. if the conductor forms a complete loop or circuit, a current flow in it. this phenomenon is know as electromagnetic induction. Two things are worth noting. first, the basic required for electromagnetic induction is the change in magnetic flux linking the conductor. secondly, the e.m.f and hence the current in this conductor will persist so long as this changes is taking place. To demonstrate the phenomenon of electromagnetic induction, consider a coil C of several turns connected to a centre zero galvanometer G. If a permanent magnet is moved towards the coil, it will be observed that the galvanometer in one direction. if the magnet is moved away from the coil, the galvanometer again show deflection but in the opposite direction. in either case, the deflection will persist so long as the magnet is in motion. the production of e.m.f and hence current in coil  C is due to the when the magnetic is in motion. the amount of magnetic-flux linking the coil changes the basic requirement for inducing e.m.f in the coil. if the movement of the is magnet is stopped, through the magnetic flux is linking the coil, there is no change in magnetic flux and hence no e.m.f is induced in the coil. the deflection of the galvanometer reduces to Zero. 

UNDERGROUND CABLES

An underground cable  essentially consists of one or more conductors converted with suitable insulation and surrounded by a protecting cover.Although several types of cables of available, the type of cable to be used will depend up on the working voltage and service requirements.In general, a cable must fulfil the following necessary requirements.The conductor used in cables should be stranded copper or aluminium of high conductivity.stranding is done so that conductor may become flexible and carry more current.

The conductor size should be such that the cable carries such that the desired load current without over heating and causes voltage drop within permissible limits.The cable must have proper thickness of insulation in order to have high thickness of safety and reliability at the voltage for which it is designed.The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.the material used in manufacture of cables should be such that there is complete chemical physical stability throughout.Construction of cables:Cores of conductor .A cable may have one or more than one core depending upon the type of service for which it is intended.The conductors are made of tinned copper or Al and are usually stranded in order to provide flexibility to the cable.
Insulation.Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending up on the voltage withstood by the cable.The commonly used materials for insulation are impregnated paper,varnished cambric or rubber mineral compound.Metallic Sheath.In order to protect the cable  from the moisture gases,or other damaging liquids in the soil and atmosphere,a metallic sheath of lead or aluminium is provided over the insulation.

NUCLEAR POWER STATION

A generating station in which nuclear energy is converted into electrical energy is known as a nuclear power station.In nuclear power station,heavy elements such as Uranium U 235,or Thorium TH 232,are subjected to nuclear fission in a special apparatus known as a reactor.The heat energy thus released is utilised in raising steam at high temperature and pressure.The stem runs the steam turbine Which converts steam energy in to mechanical energy.the turbine drives the alternator which converts in to electrical energy.

   The most important feature of nuclear station is that huge amount of electrical energy can be produced from a relatively small amount of nuclear fuel as compared to other convention types of power stations.It has been found that complete fission of 1 kg of Uranium U 235 can produce as much power as can be produced by the burning of 4,500 tons of high grade coal.Although the recovery of principal nuclear fuels is difficult and expensive, yet the total energy content of the estimated word reserved of these fuels are considerably higher than those of conventional fuels.coal,,oil,and gas.At present energy crisis is gripping us and, therefor, nuclear energy can be successfully employed for producing low cost electrical energy on a large scale to meet the growing commercial and industrial demands.
The amount of fuel required is quite small.Therefor, there is a considerable saving in the cost of fuel transportation.The nuclear power plant requires less space compared to any other  type of the same size.It has low running charges as a small amount of fuel is used for producing bulk electrical energy.This type of plant is very economical for producing bulk electrical power.It can be located near the load centres because it does not require large quantities of water and need not be near coal mines.Therefor the cost of primary distribution is reduced.The are large deposit of nuclear fuels available all over the world. Therefore,The cost of primary distribution is reduced.

CIRCUIT BREAKERS

An electrical power system is very complex. It needs some form of switch gear in order that it may be safely and efficiently controlled or regulated under both normal and abnormal operating conditions.A tumbler switch with a fuse serves to control the light and other electrical equipments and is,in a way a form of switch gear.On the other hand, a CB in Station serves exactly the same purpose but it has many added and much more complex features.
The basic construction of any CB Requires the separation of contacts in an insulation fluids which serves two functions.
1.It Extinguish the arc drawn between the contacts when the CB Opens.
2.It provides adequate insulation between the contacts and from each contact to earth.

Arc extinguishing ability: It should have a low dissociation temperature , short thermal time constant and should not produce conducting products such as carbon during arcing.
Commercial Availability at moderate cost:Of the simple gases air is the cheapest and most widely used for Circuit breaking.Hydrogen has better arc extinguishing property but it has lower dielectric strength as compared with air.Also if hydrogen is contaminated with air ,it forms explosive mixture. Nitrogen has similar as air , CO2 has almost the same dielectric strength as air but is a better arc extinguishing medium at moderate currents.Oxygen is a good extinguishing medium but is chemically active . SF6 has outstanding arc quenching properties and good dielectric strength.Of all these gases SF6 and air are used in commercial gas blast circuit breakers.

RELAY FOR PRETECTION

  The capital investment involved in a power system for the generation,transmission and distribution of electrical power is so great that proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiencies ,but  also that is protected from accidents.The protective Relaying system is to operate the correct circuit breakers so as to disconnect only the faulty equipment from the system as quickly as possible , thus minimising the trouble and damage caused by faults when they do occur.

The Modern power system is very complex and even though protective equipment's from 4 to 5% of the total cost involved in the system , they play a very important role in the system design for good quality of reliable supply.
The most severe electrical failures in a power system are shut faults which are characterised by increase in system current,reduction in voltage, power factor and frequency.The protective relays do not eliminate the possibility of faults on the system, rather their action starts only after the fault has occurred on the system.it would be ideal if protection could anticipate and prevent faults but this is impossible except where the original cause of a fault creates some effects which can operate a protective relay.So far only one type of relay falls within this category,this is the gas detector relay     ( Buchholz relay) used to protect transformers which operates when the oil level in the conservator pipe of transformer is lowered by the accumulation of gas caused by a poor connection or by an incipient breakdown of insulation(Slowly developing fault).