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.