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Anna university HVE

Transcript of unit 1-HVE

  • OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS UNIT I

    Mr. S. VINODLECTUREREEE DEPARTMENT

  • Causes of over voltages and its effect on power system Lightningswitching surgestemporary over voltages protection against over voltages.

  • INTRODUCTIONThe examination of over voltages on the power system includes a study of theirMagnitudesShapesdurations frequency of occurrencestudy should be performed not only at the point where an over voltage originates but also at all other points along the transmission network to which the surges may travel.

    Lightning rapidly heats the air in its immediate vicinity to about 20,000 C (36,000 F), about three times the temperature of the surface of the Sun. This compresses the surrounding clear air and creates a supersonic shock wave which decays to an acoustic wave that is heard as thunder Lightning rapidly heats the air in its immediate vicinity to about 20,000 C (36,000 F) about three times the temperature of the surface of the Sun. This compresses the surrounding clear air and creates a supersonic shock wave which decays to an acoustic wave that is heard as thunder

  • TYPE OF OVER VOLTAGEThe voltage stresses on transmission network insulation are found to have a variety of origins. In normal operation AC (or DC) voltages do not stress the insulation severely, However, they remain the initial factor that determines its dimensions. Over voltages stressing a power system can generally be classified into two main types:1. External over voltages: generated by atmospheric disturbances. Of these disturbances, lightning is the most common and the most severe.2. Internal over voltages: generated by changes in the operating conditions of the network. Internal over voltages can be divided into (a) switching over voltages and (b) temporary over voltages.

  • LIGHTING PHENOMENONLightning phenomenon is a peak discharge in which charge accumulated in the clouds discharges into a neighboring cloud or in the ground, between clouds and cloud to ground which is very large.Some of the facts of clouds formation during thunder storms and lightning.Upper region of the cloud is positive charge and the lower region of the cloud are negatively charge except the local region near the base.Height of the cloud from ground is about 200 to 10,000m and the charge center at the distance of 300 to 20000m.Charge inside the cloud is 1 to 100C.Energy associate with the cloud is 250kWh

  • Theory of Charge formationSimpsons theory.Renold and Mason theory.

  • Below region A : Air current travel above 800cm/sec and no rain drops fall through.In region A: Air velocity is high enough to break the falling rain drops causing a +ve charge in the air. As the spray blown upwards, but as the velocity of air decreases. Region B : Above region A becomes negatively charged by air current.Region C : The temperature is low and only ice crystal exits. The impact of air on these crystals make them negatively charge and air is positively charge.

  • RENOLD AND MASON THEORYThe explanation presented by Simpson theory was not satisfactory.Thunder clouds are developed at heights of 1 to 2 kms above the ground level and may extend 12 to 14 km above the ground.For thunder clouds and charge formation air current, moisture and specific temperature are required.Air current controlled by the temperature gradient move upwards carrying moisture and water droplets.The temperature is 0 degree about 4km from ground.-50 about 12km from ground.Water droplets in the thunder clouds are blown up in the air currents and gets super cooled over a range of height and temperature.

  • CHARACTERISTICS OF LIGHTNINGStatic ElectricityUltra-High Voltage GenerationThe DischargeSurface DispersionBasic Laws of Electricity

  • LIGHTNING OVER VOLTAGEAccording to theories generally accepted, lightning is produced in an attempt by nature to maintain a dynamic balance between ionosphere and the earth

    During thunderstorms, positive and negative charges are separated by the movements of air currents forming ice crystals in the upper layer of a cloud and rain in the lower part. The cloud becomes negatively charged and has a larger layer of positive charge at its top. As the separation of charge proceeds in the cloud, the potential difference between the concentrations of charges increases and the vertical electric field along the cloud also increases.

    The total potential difference between the two main charge centers may vary from 1500V to 1000 MV.

    Only a part of the total charge-several hundred coulombs-is released to earth by lightning; the rest is consumed in inter cloud discharges. The height of the thundercloud dipole above earth may reach 5 km

  • ANATOMY OF A LIGHTNING STROKE

  • LIGHTNING VOLTAGE SURGESThe most severe lightning stroke is that which strikes a phase conductor on the transmission line as it produces the highest over voltage for a given stroke current. The lightning stroke injects its current into a termination impedance Z, which in this case is half the line surge impedance Zo since the current will flow in both directions. Therefore, the voltage surge magnitude at the striking point is

    The lightning current magnitude is rarely less than 10 kA and thus, for a typical overhead line surge impedance Zo of 300, the lightning surge voltage will probably have a magnitude in excess of 1500 kV.The shapes and magnitudes of lightning surge waves get modified by their reflections at points of discontinuity as they travel along transmission lines

  • CAUSES OF OVER VOLTAGES DUE TO LIGHTNING STRIKES A transient over voltage is a voltage peak with a maximum duration of less than one millisecond. Over voltages due to direct lightning strikesOver voltages due to the indirect effects of lightning strikes

  • OVER VOLTAGES DUE TO DIRECT LIGHTNING STRIKESThese can take two forms: When lightning strikes a lightning conductor or the roof of a building which is earthed, the lightning current is dissipated into the ground. The impedance of the ground and the current flowing through it create large difference of potential: this is the over voltage. This over voltage then propagates throughout the building via the cables, damaging equipment along the way. When lightning strikes an overhead low voltage line, the latter conducts high currents which penetrate into the building creating large overvoltages. The damage caused by this type of over voltage is usually spectacular (e.g. fire in the electrical switchboard causing the destruction of buildings and industrial equipment) and results in explosions.

  • Over Voltages Due To The Indirect Effects Of Lightning StrikesThe over voltages previously mentioned are also found when lightning strikes in the vicinity of a building, due to the increase in potential of the ground at the point of impact. The electromagnetic fields created by the lightning current generate inductive and capacitive coupling, leading to other over voltages.Within a radius up to several kilometers, the electromagnetic field caused by lightning in clouds can also create sudden increases in voltage.Although less spectacular than in the previous case, irreparable damage is also caused to so called sensitive equipment such as fax machines, computer power supplies and communication systems. Increase in ground potential, Electrostatic field, Magnetic field

  • Representation of the various disturbances on electrical networks

  • Diagram of an installation protected against lightning and its indirect effects

  • PRINCIPLES OF PROTECTIONLightning Protection SystemAir TerminalDischarge ConductorLightning Arresters1. Rod arrester 2.Horn gap arrester3. Multigap arrester 4. Expulsion type lightning arrester5. Valve type lightning arrester Counterpoise

  • AIR TERMINALAlso known as a Lightning RodTraditionally inch copper rodWith sharpened pointSix inches above object to be protected

  • Discharge ConductorNot less than #4 AWG, SWG, BWG- 0.23inUninsulated stranded copper wireStraight from Air Terminal to GroundNo sharp bends (bend radius of 1 foot)Should be run outside of hull

    Electrical wiring should be at right angles#4

  • Lightning ArrestersThe earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves.The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.surge diverter consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc.

  • ROD GAP ARRESTERIt is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between.One rod is connected to the line circuit and the other rod is connected to earth The distance between gap and insulator must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth

  • LIMITATIONS OF ROD GAP ARRESTER(i)After the surge is over, the arc in the gap is maintained by the normal supply voltage, leading to short-circuit on the system.(ii) The rods may melt or get damaged due to excessive heat produced by the ar