dielectrometry measurements of moisture diffusion and temperature dynamics in oil impregnated pilc...
TRANSCRIPT
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
Presentation Outline
• Motivation
• Dielectrometry Sensors
• Sample Materials and Setup
• Constant Temperature Measurements
• Transient Measurements
• Summary
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.
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
Sensor Excitation
Sensor Theory
Field Line Results
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.
Single Conductor Cable Constant Temp. Measurements
Arrhenius Temperature Dependence
Observe:Changes in temperature cause
a frequency shift of the permittivity.
• Dependence described by activation energy.
Transient Measurements
• We wish to observe moisture moving through test materials.
• Transient 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.
Maple Rod Measurements (130 F, 30% RH)
Tim
e, d
ays
Maple Rod Mapping at 1 Hz
Maple Rod Moisture Profiles at 1 Hz
Single Conductor Cable Measurement
Tim
e, d
ays
Single Conductor Cable Mapping at 1 Hz
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.