Presentation on theme: "Basic Electrical Characteristics"— Presentation transcript:
1 Basic Electrical Characteristics Carl LandingerHendrix Wire & Cable
2 When Electric Current Flows in a Path There is a voltage (electrical pressure) driving the currentAn electric field eminates from the current pathA magnetic field surrounds the currentExcept for superconductors, there is some resistance/impedance to the current flowThere is a loop path to-from the source
3 A Cable Carrying Current has a Magnetic Field Associated with the Current Flow CONDUCTORINSULATIONMAGNETIC FIELD FLUX LINES EXTEND OUT TO INFINITYNOTE THAT ANY COVERING OR INSULATION DOES NOTALTER THE MAGNETIC FIELD LINES
4 Two Cables Carrying Current Will Have Magnetic Fields Interacting With Each Other MAGNETIC FIELD (FLUX) FROM EACH CABLE LINKSTHE ADJACENT CABLETHIS CAUSES A FORCE TO EXIST BETWEEN THE CABLES.IF THE CURRENTS ARE TIME VARYING, A VOLTAGE IS INDUCEDINTO THE ADJACENT CABLE.
5 Force on Adjacent Current Carrying Conductors I d I2DC: F =lbs./ft.For RMS Symmetrical current Single Phase Symmetricallbs./ft.AC: F =
6 Force on Adjacent Current Carrying Conductors A B CIIRMS Symmetrical Current3F Asymmetrical FaultA or CFMaximumlbs./ft.F =lbs./ft.BFMaximumF =
7 Force on Adjacent Current Carrying Conductors RMS Symmetrical Current3F Asymmetrical FaultAssume: I = 10,000 Amps/Phase, d = 6in. (0.5 ft.)Maximum Force on A or C Phase is:= 689 lbs./ft.This is no small amount of force!
8 Resistivity Vs Conductivity Resistivity is a property of every materialResistivity is a measure of a material to resist the flow of DC currentResistivity is stated as per unit volume or weight at a specific temperatureConductivity is a measure of a material to conduct DC current and is the reciprocal of resistivityMaterials having a low resistivity make good conductors. Materials with high resistivities are insulators.
9 Percent ConductivityThe conductivity of conductor grade annealed copper was established as the standard and given as 100% (IACS)Other materials are stated as a percentage of being as conductive of this standardAluminum is approximately 61% as conductive as annealed copper on a volume basis. However, it is over twice as conductive on a weight basis.It is possible to exceed 100% i.e. silver is 104.6%Metal purity and temper effect conductivity
10 Relationship Between Resistance and Volume Resistivity height = hl = lengthcurrent flow w = width Area = w X hVolume Resistivity x LengthResistance =Area
11 Temperature Coefficient of Resistance RT2 = RT1[1 + a(T2-T1) + b(T2-T1)2]where:RT2 = DC resistance of conductor at desired orassumed temperatureRT1 = DC resistance of conductor at “base” temperatureT2 = Assumed temperature to which dc resistance isto be adjustedT1 = “Base” temperature at which resistance is knowna and b = Temperature coefficients of resistanceat the base temperature for the conductor
12 Temperature Coefficient of Resistance (Continued) For the range of temperatures in which most conductorsoperate the formula reduces toRT2 = RT1[1 + a(T2-T1)]values for a
13 Effective AC Resistance “Effective” ac resistance is required for voltage drop calculations“Effective” ac resistance includesSkin effectProximity effectHysteresis and Eddy current effectsRadiation lossShield/sheath lossConduit/pipe loss
14 Alternating Current Resistance For the general case when calculating impedance for voltage drop or system coordination;Rac = Rdc(1 + YCS + YCP) + DRWhere:YCS is the multiple increase due to skin effectYCP is the multiple increase due to proximity effectDR is the apparent increase due to shield loss, sheath loss, armor loss, ………..Note: The presence of enclosing metallic, magnetic and non-magnetic conduit or raceway will increase these factors as well
15 Alternating Current Resistance When Calculating for Ampacity Determination Rac = Rdc(1 + YCS + YCP)Where;YCS is the multiple increase due to skin effectYCP is the multiple increase due to proximity effectShield loss, sheath loss, armor loss, …are handled as separate heat sources introduced at their location in the thermal circuit.Note; The presence of enclosing metallic, magnetic and non-magnetic conduit or raceway will increase all of these factors.
16 Insulation ThicknessCables are voltage rated phase to phase based on a grounded WYE three phase system unless statedThus, unless otherwise noted, the insulation thickness is designed for a voltage equal to the cable voltage rating divided by 1.732For a 15kV cable the insulation thickness is designed for; 15 kV/1.732 = 8.66 kVCables used on other systems must be selected accordingly
17 Insulation ThicknessFor an ungrounded 15 kV delta system the voltage to the neutral point varies from 15 kV/1.732 depending on load balance. For this case, it is common to select insulation thickness based on 1.33 x 15 kV or 20 kV as long as a fault to GRD. is cleared within 1 hour.This is the origin of the 133% insulation levelThe insulation thickness for a 20 kV cable is 215 mils/ICEA, 220 mils/AEIC
18 Insulation ThicknessWhen a phase to ground fault occurs on an ungrounded delta system, full phase to phase voltage appears across the insulationFor 15 kV this is equivalent to a 15 X = 26 kV cable.If such a fault is to be allowed to exist for more than 1 hour, it is common to select insulation thickness based on this voltage.This is the origin of the 173% levelthe 173% level is not common and the values are not widely published
19 Insulation Resistance No insulation is perfect. If the conductor is made intoone electrode, and the shield over the insulation, or madeshield such as water is used as the other electrode, and aDirect Current Voltage E, applied across the electrodes, acurrent I, will flow. Using Ohms Law, E = I/R, aninsulation resistance can be calculated..I.ER = insulation resistance (ohms) = E/I
20 Typical DC Leakage Current With Constant Voltage Applied IG = charging currentIA = absorbtion currentIL = leakage currentIT = total currentIL
21 Insulation Resistance Constant If one uses a 100 to 500 volt DC source to measure the resistance from conductor to shield, or a made shield such as water, of a 1,000 foot length of insulated cable at a temperature of 60°F, the following formula describes the relationship between the insulation thickness, the resistance reading obtained, and a constant which is peculiar to the insulation;R = (IRK) Log10(D/d)Where; R is the resistance in megohms-1,000 feetD is the diameter over the insulationd is the diameter under the insulationIRK is the insulation resistance constant
23 Insulation Resistance Constants Rubber Like Materials Ethylene Propylene Rubber Type I 20,000Ethylene Propylene Rubber Type II, 0-2kV 10,000Ethylene Propylene Rubber Type II, >2kV 20,000Code Grade Synthetic RubberPerformance Natural Rubber 10,560Performance Synthetic Rubber 2,000Heat Resistant Natural Rubber 10,560Heat Resistant Synthetic Rubber ,000Ozone Resistant Synthetic Rubber 2,000Ozone Resistant Butyl Rubber 10,000Kerite ,000
24 Insulation Resistance Constant Important Notes If the measurement is not made at 60° F but at a temperature not less than 50 or more than 85°F, correction factors must be used to correct to 60°If the measurement is made on a length other than 1,000 feet, correction to an equivalent 1,000 foot length is necessaryInsulation Resistance Constants (IRK) are published for different classes of insulations. These are minimums and actual values obtained from test measurements should exceed these values or there is an indication of a problem in the material or testUsing IRK to determine the condition of cables in the field is difficult and subject to error
25 Cable Average Electrical Stress G ave = Voltage to GroundInsulation thickness (mils)G ave = volts/milT
26 Cable Radial Electrical Stress at Any Point in the Insulation G x = Vgrd Volts/MilX Ln(R2/R1)XR1R2.Maximum Stress X = R1Minimum Stress X = R2
27 STRESS GRADIENT IN #2-7 STRAND 175 MIL CABLE AT 7.2 kV ac Maximum Stress = 60.7 V/milMinimum Stress = 29.2 V/mil
28 STRESS GRADIENT IN 1/0-19 STRAND 345 MIL CABLE AT 20.2 kV ac Maximum Stress = 105 V/milMinimum Stress = V/mil
29 The Formula for Calculating Per Foot Capacitance For Fully Shielded Cable Is: x 10-12where, e is the dielectric constant of the coveringDoc is the diameter over the conductor (or semi conducting shield, if used)Doi is the diameter over the covering (or insulation in the case of shielded cables)
30 Shunt Capacitive Reactance For single conductor shielded primary cables the shunt capacitance may be calculated bywhere:e = dielectric constant of the insulationDoi =diameter over insulationDui = diameter under insulationThe capacitive reactance may then be calculated as:µµfarad/1000 ftwhere:f = frequency in Hzj = a vector operator
31 i = 2pfce i = Charging current f = 60Hz e = Voltage Phase to grd The Formula for Calculating Charging Current, Per Foot, For A Fully Shielded Cable Is:i = 2pfcei = Charging currentf = 60Hze = Voltage Phase to grdc = Capacitance
32 Example of Charging Current, per Foot, For Fully Shielded Cable x x (14.4 x 103) = milliamps/fte = 2.3Doc = inchDoi = inche = 14.4 kV to ground
33 Power Factor Vs Dissipation Factor A Cable is Generally a Capacitor IcIc should be >>>IrabIrPower Factor == Cos (b) always < 1.0Dissipation Factor = Ir/Ic = Tan (a) ranging from 0 toFor the normal case where Ic>>>Ir;So, Power Factor and Dissipation Factor are often thought to bethe same, but they are very different.
34 Dielectric Power Dissipation (Dielectric Loss) Ic ItPower DissipationP = E (Ir)= E (It) cos q= E(Ic) tan dBUT;Ic = 2pfCEP = 2pfCE2(Tan d )dqIr E
35 Inductive Reactance henries to neut. per 1000 ft. Where: GMD = Geometric mean distance (equivalent conductorspacing) between the current carrying cables.GMR = Geometric mean radius of one conductor - inchesAt 60 Hz: 2p(frequency) = 377orXL = j Log10 GMD/GMR ohm to neut. per 1000 ft.j is a vector operator
36 Geometric Mean Distance BABBACUnequal triangleGMD =Equilateral TriangleGMD =A=B=CRight TriangleGMD = A
37 Geometric Mean Distance BABSymmetrical FlatGMD = 1.26 AUnsymmetrical FlatGMD =FlatGMD = A
38 Effective Cross Sectional Area of Sheath/shield (A) B-Tape Lap (mils) n-Number of wires/tapesb-Tape Thickness (mils) L-Tape overlap, %dis-Dia over Ins. Shield (mils)dm-Mean sheath/shield Dia. (mils)ds-Dia. of wires (mils)w-Tape width (mils)
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