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Marcus O. Durham THEWAY Corp Robert A. Durham RADCo Consulting

A COST EFFECTIVE, NUMERIC TECHNIQUE FOR PROJECTING QUALITY OF INSULATION AND IMPENDING FAILURES. Marcus O. Durham THEWAY Corp Robert A. Durham RADCo Consulting. Abstract. Hi Pot Cable & Machines Numeric technique Predicts impending failure. DC Limitations.

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Marcus O. Durham THEWAY Corp Robert A. Durham RADCo Consulting

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

  2. Abstract • Hi Pot • Cable & Machines • Numeric technique • Predicts impending failure

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

  4. DC Limitations • Higher test V for DC than AC = Space Charge Etotal = Espace + Einsulation • Insulation = 5000 V, 90 mil • EAC = 55 volts / mil • EDC = 10 X • Result = Deterioration of insulation

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

  6. 120.01 VAC V DC Ohms OFF A AC A DC TestersDespite Limitations - DC Still Preferred • VOM (volt-ohmmeter) • IR (insulation resistance) • Hi Pot (high potential DC) • More info about quality than other • 60,000 V field • 200,000 V lab

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

  8. Resistance vs. Current Georg’s Law • Vtest = Rinsulation * Ileakage • Resistance decreases as length increases • Non-linear, parallel R • I exponential as length Increases • Conductivity often called leakage current

  9. D æ ö r =K log ç ÷ è ø d - EPDM 20,000 M W * k ft - Polyethylene 50,000 M W * k ft Leakage Conductance • One number is futile • Length, diameter, insulation, geometry, voltage • Bulk resistivity

  10. 20,000 M W * k ft K = G = 0 .05 m A / k V * k ft Leakage ConductanceEPDM • Derate for temp, moisture, oil • Lower values may still be good

  11. Suitability for Reuse • Visual & 5 minute DC withstand • Lack of Consensus • 75 mil EPDM • 11,000 - 25,000

  12. Comparison MethodsOverpotential Test • Specified leakage current • Leakage conductance • Leakage w/ ratio of 3 to 1 • No consistent guidelines

  13. Leakage Current ComponentsCapacitance Charging • Between conductor & ground • Starts extremely high, decrease exponentially • Drops to zero in few seconds

  14. Leakage Current ComponentsAbsorption • Dielectric insulation • Result of Polarization • Starts high, decreases slowly • Stabilize in 5 minutes • Reasonable in 2 minutes

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

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

  17. EvaluationMore 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

  18. A TechniqueNot a Number • Forecast failure levels • Normalize • p Megohm - k ft • I Microamps • V Kilovolt • L thousand feet

  19. 1000 l G = r Step 1Calculate Bulk Conductance Contrast with measured G = Ileak / Vtest

  20. Step 2Plot mA -vs- kV I = G V Becomes upper boundary for Itest

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

  22. ( ) d i ( i - i ) 2 1 m = = ( ) d v ( v - v ) 2 1 Step 4Calculate Slope Calculate derivative at each test point

  23. Step 5Compare Compare slope tobulk conductance M > G => impending failure

  24. av i = F ( 1 - e ) ln i - ln i + ln v - ln v ( ) ( ) 2 1 1 2 a = v - v ( ) 2 1 i F = av 1 - e Step 6Plot Forecast of Failure Point

  25. Step 7For Next Test Point Recalculate steps 3 - 6 to refine forecast current

  26. i = G v av i = F ( 1 - e ) 1 - G æ ö æ ö v = ln ç ÷ ç ÷ è ø è ø a aF Step 8Calculate Forecast Voltage Bulk Test Intersection

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

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

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

  30. Questions ?

  31. Questions ?

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