1 Dielectrometry Measurements of Moisture Diffusion and Temperature Dynamics in Oil Impregnated PILC Cables Zachary M. Thomas Wolf, Greenfield & Sacks P.C. Markus Zahn Massachusetts Institute of Technology
Mar 30, 2015
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Dielectrometry Measurements of Moisture Diffusion and Temperature Dynamics
in Oil Impregnated PILC Cables
Zachary M. ThomasWolf, Greenfield & Sacks P.C.
Markus ZahnMassachusetts Institute of Technology
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Presentation Outline
• Motivation
• Dielectrometry Sensors
• Sample Materials and Setup
• Constant Temperature Measurements
• Transient Measurements
• Summary
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Motivation• Develop technology for cable health monitoring.• What can dielectrometry sensors tell us about the electrical
properties of PILC insulation?
Cable Aging Mechanisms• Temperature Fluctuations
– Temperature varies with loading conditions.• Moisture Ingress
– Cracks and corrosion provide sights.– Aging of cellulose releases water.
• Partial Discharge (PD)– Formed in gaps and voids formed during temperature cycling in the
cable insulation.– Regions of low oil content.
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Dielectrometry Sensors
• Capacitive sensing technique.• Requires access to one
surface of MUT (material under test).
• Sensor response determined by MUT “effective permittivity”
• Periodicity i.e. wavelength determines sensor’s “depth perception.”
• Frequency domain measurements taken from mHz to kHz.
3 λ Sensor
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Sensor Excitation
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Sensor Theory
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Field Line Results
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Sample Materials and Setup
• Sample Materials– PILC – Paper insulated
lead covered cables– Teflon– Wood (Birch & Oak)– Polycarbonate– Polyethylene– Acrylic
• Experiments conducted in a vacuum chamber.
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Single Conductor Cable Constant Temp. Measurements
Feedback capacitance is 5 nF on all channels.
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Arrhenius Temperature Dependence
Observe:Changes in temperature cause a
frequency shift of the permittivity.
• Dependence described by activation energy.
Material Wavelength Ea eV
Single Conductor Cable
1.0 mm 0.8981± 0.0137
2.5 mm 0.9083 ± 0.0136
5.0 mm 0.8909 ± 0.0138
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Transient Moisture Measurements
• We wish to observe moisture moving through test materials.
• Moisture prevented from entering everywhere except the exposed front surface.
• Transient moisture measurements are taken at a single temperature.
• Before time zero chamber is typically dried.
• At time zero moisture admitted into the chamber.
• Sensor is monitored at several frequencies during the diffusion process.
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Maple Rod Measurements (130 F, 30% RH)
Tim
e, d
ays
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Maple Rod Mapping at 1 Hz
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Maple Rod Moisture Profiles at 1 Hz
0.3
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Single Conductor Cable Measurement
Tim
e, d
ays
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Single Conductor Cable Mapping at 1 Hz
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Summary
• Theoretical solutions have been derived and tested for new geometries.
• Steady state measurements detail the permittivity’s dependence on temperature. Arrhenius temperature dependence is characterized.
• Transient moisture measurements provide insight into moisture dynamics in woods and cables.
• With the lead sheath in place, dielectrometry is not practical for manhole measurements.
• Dielectrometry sensors could be used as an inexpensive method for utilities to assess cable health.
• Future measurements should focus on comparing dielectric properties of failed and failing cables to healthy cables.