Diapositiva 1

TRANSMISSION LINESCHAPTER 2TRANSMISSION LINESFUNCTION - to transfer bulk of electrical energy from generating power plants to electrical substations located near demand centres.When interconnected with each other, they become transmission networks (Malaysia-National Grid).FOUR (4) basic parameters : resistance (R), inductance (L), capacitance (C), and conductance (G).Most transmission lines use HVAC.HVDC used for greater efficiency for long distances including submarine power cable.Electricity is transmitted at HIGH VOLTAGE to reduce energy lost in long-distance transmission.

Diagram of electrical power system INDUCTANCE (L)Inductance is the number of flux linkages produced per ampere of current flowing through the line.The greater the spacing between the phases of a transmission lines, the greater the inductance of the line.The greater the radius of the conductors in a transmission line, the lower the inductance of the line.

EXAMPLE 1Calculate the loop inductance per km of a single-phase transmission line consisting of 2 parallel conductors 1.5m apart and 1.5cm in diameter. If it is operating at 50Hz frequency, calculate the reactance of the transmission line.

D=distance (m) r=radius (m)

D=1.5mABr=1.5cmr=1.5cmSOLUTION 1

EXAMPLE 2A 3-phase transmission line 100km has its conductors of 0.6cm diameter spacing at the corners of an equilateral triangle of 100cm. Find the inductance per phase of the system.

100cm100cm100cmACBSOLUTION 2

CAPACITANCE (C)The charge deposited on the conductors is proportional to the applied voltage. The constant of proportionality is the capacitance.The greater the spacing between the phases of a transmission lines, the lower the capacitance of the line.The greater the radius of the conductors in a transmission line, the higher the inductance of the line.

EXAMPLE 3A single phase transmission line has two parallel conductors 5m apart, radius of each conductor is 1.5cm. Calculate the capacitance of the line per km. Given that 0=8.854 x 10-12 F/m.

EXAMPLE 4A 3-phase, 50Hz, 66kV transmission line are placed in horizontal plane. The conductor diameter is 1.25cm and distance between conductors is 2m. If the line length is 100km, calculate capacitance per phase.

2m2mABCSOLUTION 4

TYPES OF OVERHEAD TRANSMISSION LINESHORT TRANSMISSION LINEWhen the length of the transmission line is up to 80km and the line voltage is less than 20kV.Due to smaller length and lower voltage, the capacitance (C) effects are small and hence can be NEGLECTED.Only resistance (R) and inductance (L) are considered.

where, Z = series impedancer = per-phase resistanceL = per-phase inductancel = line length

SHORT TRANSMISSION LINECircuit diagram

VsVr LoadRXIsIrVs - sending end voltageVr - receiving end voltageI - load currentR - Loop resistance ()X - Loop inductance ()

+-MEDIUM TRANSMISSION LINEWhen the length of the line is about 80km to 250km and the line voltage is moderately high between 20kV to 100kV.Due to sufficient length and line voltage, capacitance (C) is considered.

MEDIUM TRANSMISSION LINECircuit diagramVsVr load RXISVs- sending end voltageVr- receiving end voltageIs- sending end currentIr- receiving end currentIc- capacitance current R- loop Capacitance ()X- loop Inductance ()C- capacitance (farad)

ClineNeutralIrICLONG TRANSMISSION LINEWhen the length of the line is more than 250km and line voltage is very high which is more than100kV.The line constants (R,L,C,G) are uniformly distributed over the whole length of the line.Resistance (R) and inductance (X) are serial elements of transmission line.Capacitance (C) and conductance (G) are shunt elements of transmission line. It caused the power losses and corona effects.LONG TRANSMISSION LINECircuit diagramVsIS LoadVrIrB/nG/nR/nX/nVs- sending end voltageVr- receiving end voltageIs- sending end currentIr- receiving end currentIc- capacitance current R- loop Capacitance ()X- loop Inductance ()C- capacitance (F)G loop conductance

SHORT TRANSMISSION LINE

Equivalent circuitPhase diagram for LAGGING power factorVOLTAGE REGULATIONVoltage regulation (VR) is the percentage change in voltage at the receiving-end of the line in going from no-load to full-load.

Lagging power factor %VR is positive Leading power factor - %VR is negative (capacitive load)

TRANSMISSION EFFICIENCYEfficiency is defined as the ratio of receiving-end power to the sending-end power.

where I2R is line losses.

EXAMPLE 1A single phase transmission line delivered 1,100 kW power to a factory at 11 kV in 0.8 p.f. lagging. This line have a resistance of 2 and inductance coil of 3. Calculate: i) Sending-end voltage [11,426V]ii) Regulation Percent [3.873%]iii)Transmission Line Efficiency [97.24%]

EXAMPLE 2An 11 kV,3-phase transmission line has resistance of 1.5 and inductance of 4 for each phase. Calculate regulation percent and efficiency if total end receiver load, 5000 kVA in 0.8 p.f. lagging. [I=262.4A ; Vs=7,295.8V ; Regulation= 14.88%]

POWER FLOW (3-phase)Real input power (watt)

Real output power (watt)

Apparent input power (VA)

Apparent output power (VA)

CORONACorona was electrical discharge emerge around overhead line conductor, due to air flow where would disturb radio waves and creating lost power. When a normal ac voltage is applied across two conductors with enough spacing between them, there is no change in the atmospheric conditions surrounding the conductors.But if the voltage exceeds a particular limiting value, then the air surrounding the conductors will gets ionized and luminous glow (weak purple color) will rise with hissing sound.This phenomena is called corona.

The corona discharge around a high voltage coil.Large corona discharge (white) around conductors energized by a 1.5 million volt transformer in a laboratory. CORONA EFFECTSPower lossThe 3rd harmonic components makes the current non-sinusoidal and this increase the corona loss. The ozone gas formed chemically reacts with the conductor and can cause corrosion.Light (faint violet glow).Audible noise (hissing or cracking).Insulation damage Radio, television and computer interference.METHODS TO REDUCE CORONA EFFECTSIncrease the diameter of the conductor -i.e. ACSR conductorsb.Increase the space between the conductorsc.Using bundled conductors-produced less resistances and reduce losses

INSULATORSFunctions:- to provide perfect insulation between the live conductors and the supports. - to prevent any leakage current from the live conductors to earth through the supports.

INSULATOR MATERIALSPorcelain (ceramic)- most commonly used material for the insulators- the dielectric strength is about 60 kV/cm- has a particular shape and covered with glazeGlass- cheaper but less stronger than the porcelain - the dielectric strength is about 140 kV/cmSynthetic resin- consist of the compounds of silicon, rubber, resin etc.- light weight and comparatively cheaper- high leakage current and short life

DESIGN PRINCIPLE & CONSTRUCTIONPhysical strength - able to withstand loads suitable with the weight of a conductor.Have high insulation resistance to prevent current leakage to earth.High resistance ratio of rupture due to surge voltage.The insulators material used must be water-proof and does not affected by changes in temperature.Construction must be free from any impurities and cracks as well as non-transparent to liquids and gases from materials from space.

TYPES OF INSULATORSPIN TYPE INSULATOR

Schematic designSmall, simple in construction and cheap.Used for transmission and distribution of electrical power up to 33kV.For lower voltage up to 11kV one piece is used.For higher voltage two or more pieces are used.It becomes more heavy and costly for higher voltages.Mounted on the cross-arm of the pole.The line conductor is placed in the groove at the top of insulator and is tied down with binding wire of the same material as the conductor.

PIN TYPE INSULATORSUSPENSION TYPE INSULATOR

Schematic designUsed for voltages above 33kV.Have no. of porcelain disc units which are connected to one another in series by using metal links to form a string of porcelain discs.The top of insulator is connected to the cross-arm of the tower while the lowest insulator holds the line conductor.Each unit is designed for the low voltage about 11kV.No. of units depend on the operating voltage i.e. if operating voltage is 132kV , the no. of units required is 12. SUSPENSION TYPE INSULATORTwo types of suspension type insulators:(1) Cemented cap type(2) Hewlett @ inter-linking type

In case of failure of any of the units, the replacement work done on that unit and entire string need not be replaced.Just add additional units to the string if the line voltage is required to be increased at some later stage.

SUSPENSION TYPE INSULATOR

Used for handling the mechanical stresses at angle positions of the line :- corner/ sharp curve- end of lines- intermediate anchor towers- long river-crossingsLow-tension (LT) line shackle insulators are usedHigh-tension (HT) line - assembly of the suspension insulators is used as strain insulator but are arranged on a horizontal plane.On extra long spans (river crossings) two or more strings of strain insulators are used in parallel.TENSION TYPE INSULATORADVANTAGES OF SUSPENSION INSULATORSFor higher voltages, these are cheaper than the pin insulator.Each unit is designed for low voltage (11kV) but by connecting such units in series to form a string, insulator for higher voltage level can be designed.In case of any failure, it is sufficient to replace the damaged disc and do not need to replace the entire string.Provide greater flexibility to the line. The string is suspended and is free to swing in any direction.The line conductors are less affected by lighting because the conductor is lower than the tower cross-arm and the string acts as lighting arrestor.

TYPE OF INSULATOR TESTSFLASHOVER TESTS

PERFORMANCE TESTSROUTINE TESTSA string of insulators or network insulator is the unit formed by connecting several discs in a series with help of metal links.The capacitance due to two metal fittings on either side of an insulator is known as mutual capacitance.The capacitance between the metal fittings of each unit and the earth/tower is known as shunt capacitance.The capacitance between the conductor and the metal link is neglected.

VOLTAGE DISTRIBUTION IN INSULATOR NETWORKDue to shunt capacitance, the charging current in all discs of a string is not equal.So, the voltage across each unit will be different.The discs nearer to the line conductor will have maximum voltage and minimum voltage across the top unit (near the cross-arm).

VOLTAGE DISTRIBUTION IN INSULATOR NETWORKVOLTAGE DISTRIBUTION IN INSULATOR NETWORK

The voltage across the unit nearer to the conductor is more than the voltage in the unit nearer to the tower.

100% efficiency means that the voltage across the disc will be exactly same.

NETWORK EFFICIENCY

Cross-arm-increase the length of cross-arms by increasing the distance between insulator and tower.- the ratio of shunt capacitance to mutual capacitance (k=C1/C) will reduce to 0.1.- the network efficiency increases and the voltage distribution is more uniform.- only suitable for high and large tower post to support long bar weight and network insulator.METHODS TO IMPROVE NETWORK EFFICIENCYMETHODS TO IMPROVE NETWORK EFFICIENCYDD = Bar lengthConductorTower BarFigure 2.15 Cross arm schematic

Guard ring-ring way obstruction can be done with use static shield.- this static shield assembled on end lower part insulator unit connected by using joining of metal in suspension insulator and then connects to line conductor. - reduce the earth capacitance and create capacitance between insulator line and cap.- higher capacitance in nearby unit with guard ring and this will reduce voltage fall in the insulator.- the same voltage in per unit is impossible to obtain practically.METHODS OF INREASING STRING EFFICIENCYMETHODS OF INREASING STRING EFFICIENCYCCCC1C1C1 CxI1I2I3i1i2i3IxCyIyV3V2V1(b) Equivalent circuitObstruction RingTower postConductorCzIz Obstruction RingTower Post(a) Construction Arc Horn