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Resistance

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Resistance is defined as the property of a substance due to which it opposes the flow of Electricity i.e electrons through it. Metals, acids and salts are the good conductors of electricity. These free or loosely attached electrons (vagrant) while flowing pass through the molecules or the atoms of the conductor,collide and other atoms and electrons, thereby producing heat. The unit of Resistance The practical unit of resistance is ohm. A conductor is have a resistance of one ohm if it permits one ampere current to flow through it when one volt is applied through its terminals. The unit of resistance is ohm. Laws of Resistance  The resistance R offered by a conductor depends on the following factors: 1. It varies directly as its length l. 2. It varies inversely as the cross section A of the conductor. 3. It depends on the nature of the material. 4. It also depends on the temperature of the conductor. R= ρ.l/A where,   ρ is a constant depending o
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Electron drift velocity

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Let n be the number of free electron available per cubic metre of the conductor material. Let v be the axial drift velocity. In time dt, distance travelled would be vdt . If A is the cross section of the conductor, then the volume is vAdt . If e is the charge of each electron  then total charge which crosses the section in time dt is dq=nAev.dt. Since current is the rate of flow of charge, it is given as i=dq/dt=(neAv.dt)/dt i=venA Current density, J=i/A=vne Ampere/m2. Normal J for copper conductor= 1.55 x 10^6 A/m2, n=10^29, e=1.6x10^-19 coulomb, v=9.7 x 10^-5 m/s = 0.58 cm/min.
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Smart Grid

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The concept of Electrical grid was given by Nicole Tesla about 120 years ago. Since then the generation, transmission and distribution of Electrical energy has undergone vast changes. Giant size generators and EHV lines are very common now. The per capita consumption has increased manifold (and is still increasing) and consumers demand reliability and quality in Electric supply. The power industry is faced with challenges and opportunity of transforming the Electrical grid into smart grid to manage demand growth, conserve Energy, reduced losses, optimize efficiency, improve power quality and reliability. Smart grid also known as intelligent grid refers to combination of hardware, software and communication tools to meet the above objectives. The main features of a smart grid are: 1. It is self healing from power distuebances. 2. It ensures power quality and reliability. 3. It accommdates centralized giant size generators as well as distributed generation. 4. It uses conven
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Skin Effect in Conductors

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When a conductor is carrying dc current , it is uniformly distributed over the cross section of the conductor . However an alternting current flowing through the conductor does not distribute uniformly, rather it has the tendency to concetrate near the surface of the conductor. This is known as Skin effect.   Thus, skin effect is the tendency of an alternting current to concentrate near the surface of the conductor.     Due to skin effect the effective area of cross section of the conductor through which current flows is reduced consequently, the resistance of the conductor is slightly increased when carrying an alternating current. Cause of skin effect The cause of skin effect can be easily explained. A solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of current. The inductance of each strand will vary according to its position. Thus, the strands near the centre are surrounded by a greater magnetic flux and hence
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Black start in Hydro power Plant and Steam Plant

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A black start is the process of restoring an electric power station or a part of an electric grid to operation without relying on the external transmission network. Normally, the electric power used within the plant is provided from the station's own generators. If all of the plant's main generators are shut down, station service power is provided by drawing power from the grid through the plant's transmission line. However, during a wide-area outage, off-site power supply from the grid will not be available. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation. To provide a black start, some power stations have small diesel generators, normally called the black start diesel generator (BSDG), which can be used to start larger generators (of several megawatts capacity), which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service
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Proximity effect in conductors

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In a conductor carrying alternating current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions. The resulting current crowding is termed as the proximity effect. This crowding gives an increase in the effective resistance of the circuit, which increases with frequency. A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor, by electromagnetic induction. When an alternating current (AC) flows through an isolated conductor, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The result is that the current is concentrated in the areas of the conductor furthest away from nearby conductors carrying curre
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Core type and Shell type Transformers

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Constructionally, the transformers are of two general types, distinguished from each other merely by the manner in which the primary and secondary coils are placed around the laminated core. The two types are:- Core type   Shell type Another recent development is spiral core or wound core type, the trade name being spirakore transformer. Core type transformer:- In the core type transformer, the windings surround a considerable part of the core. In the simplified diagram of the core type transformer, the primary and secondary winding are shown located on the opposite limbs of the core, but in actual construction they are interleaved to reduce leakage flux. As in the figure, half the primary and half the secondary winding have been placed side by side or concentrically on each limb, not primary on one limb and the secondary on the other. The coil used are form-wound and are of the cylindrical type. In small size core type transformers, a simple rectangular core
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