A COST EFFECTIVE, NUMERIC TECHNIQUE FOR PROJECTING QUALITY OF INSULATION AND IMPENDING FAILURES Marcus O. Durham THEWAY Corp Robert A. Durham RADCo Consulting Marcus O. Durham THEWAY Corp Robert A. Durham RADCo Consulting

Abstract u Hi Pot –Cable & Machines –Numeric technique –Predicts impending failure

DC Limitations u No correlation between AC strength & DC test Experience is used u Aged insulation => AC Use => DC test Life 5 times longer if not tested u Flashover transient weakens insulation Reflected wave = 2 times peak DC

DC Limitations u Higher test V for DC than AC = Space Charge E total = E space + E insulation u Insulation = 5000 V, 90 mil u E AC = 55 volts / mil u E DC = 10 X u Result = Deterioration of insulation

To Tree or Not To Tree u Problems are recognized in polyethylene u May be in rubber as well u 5000 Volt insulation ???

Testers Despite Limitations - DC Still Preferred u VOM (volt-ohmmeter) u IR (insulation resistance) u Hi Pot (high potential DC) –More info about quality than other –60,000 V field –200,000 V lab

Elevated Voltage u Can cause any insulation to fail u Difficult to interpret w/o destruction u Most valuable w/ historical data u Experience, skill, knowledge of local conditions - major aids to determining suitability

Resistance vs. Current Georg’s Law u V test = R insulation * I leakage u Resistance decreases as length increases u Non-linear, parallel R u I exponential as length Increases u Conductivity often called leakage current

Leakage Conductance u One number is futile u Length, diameter, insulation, geometry, voltage u Bulk resistivity  K log       D d - EPDM 20,000 M  k ft - Polyethylene 50,000 M  k ft

Leakage Conductance EPDM u u Derate for temp, moisture, oil u Lower values may still be good K= 20,000 M  k ft G=.05 A/ k V* k ft0 

Suitability for Reuse u Visual & 5 minute DC withstand u Lack of Consensus u 75 mil EPDM u 11, ,000

Comparison Methods Overpotential Test u Specified leakage current u Leakage conductance u Leakage w/ ratio of 3 to 1 u No consistent guidelines

Leakage Current Components Capacitance Charging u Between conductor & ground u Starts extremely high, decrease exponentially u Drops to zero in few seconds

Leakage Current Components Absorption u Dielectric insulation u Result of Polarization u Starts high, decreases slowly u Stabilize in 5 minutes u Reasonable in 2 minutes

Leakage Current Components Conductance u Steady state value u Over, under, around, through insulation u Corona contributes u Low value is good

Leakage Current -vs- Time i = F + (I - F) e -t/RC F = constant, property of material I = initial 1/RC = time constant

Evaluation More Thorough 1 - Apply increasing voltage 2 - Plot leakage current -vs- applied voltage 3 - If R = ideal, V does not affect I 4 - Increasing I => insulation weakened

A Technique Not a Number u Forecast failure levels u Normalize –pMegohm - k ft –IMicroamps –VKilovolt –Lthousand feet

Step 1 Calculate Bulk Conductance Contrast with measured G = I leak / V test G  1000 

Step 2 Plot  A -vs- kV I = G V Becomes upper boundary for I test

Step 3 Hi-pot Test Perform hi pot test in steps

Step 4 Calculate Slope Calculate derivative at each test point     m di dv ii vv    () () 21 21

Step 5 Compare Compare slope to bulk conductance M > G => impending failure

Step 6 Plot Forecast of Failure Point iFe av  ()1    a iivv vv    ln F i av   1e

Step 7 For Next Test Point Recalculate steps to refine forecast current

Step 8 Calculate Forecast Voltage Bulk Test Intersection iGv  iFe av  ()1 v a G aF               1 ln

Step 9 Calculate Comparative Quality cq = forecast V / rated test V Changes between tests show deterioration Ratio < 40% indicates marginal quality

Summary Testing u Insulation for reuse - controversial u DC Hi Pot common - despite problems u Measure leakage I at test V u Math technique to compare quality

u Calculate bulk conductance (G) - limit u Calculate rate of change of leakage G u Compare derivatives (slope) u Calculate coefficients of leakage curve u Determine forecast V at intersection Summary Mathematical Technique

Questions ?