Application of Dielectric Spectroscopy to

Monitor Insulating Materials

Presented By

Furkan AhmadElectrical Engineering

DepartmentZHCET

AMU Aligarh

Content

Introduction

Polarization

Type of dielectric spectroscopy

Time domain (PDC)

Frequency domain

PDC OR FDS?

Measurements on Transformer Oil-paper Insulation

Analysis of influence factors on FDS

Discussion

Introduction

Insulation indispensable components for power transmission and

distribution systems and large industrial plants. the weak link in

the chain still remains the insulation system It is therefore

essential that they function properly for many years

There are a large variety of spectroscopic methods, with multiple

practical and scientific applications. This variety includes

dielectric spectroscopy (DS), sometimes called impedance

spectroscopy or electrochemical impedance spectroscopy (EIS)

Inherent to all dielectric spectroscopy measurements in either

time or frequency domains is their “off-line” character

3

Polarization

As soon as a material is exposed to an electric field the

positive and negative charges become oriented thus forming

different kinds of dipoles even on atomic scales

The equal amounts of positive and negative charges,(+ -)q,

become separated by a small distance d, thus creating a dipole

with a dipole moment, p=qd, the dipole moment can also be

written as p = E

As the distance d will be different for different species as well

as their number of dipoles per unit volume, their polarizability

will also different

The dielectric polarization is the result of a relative shift of

positive and negative charges in a material. During all of these

processes, the electric field is not able to force the charges to

escape from the material, which would cause inherent electric

conduction

Electronic Polarization is effective in every atom or molecule as

the center of gravity of the electrons surrounding the positive

atomic cores will be displaced by the electric field E. This effect is

extremely fast and thus effective up to optical frequencies.

Ionic (or atomic/molecular) Polarization refers to material

containing molecules forming ions that are not separated by low

electric fields or low working temperatures. It can be polarized up

to infra-red frequencies.

Dipolar (or orientational ) Polarization refers to materials

containing molecules with permanent dipole moments with

orientations statistically distributed due to the action of thermal

energy

Dielectric spectroscopy

Polarization P and the electric field E are of equal direction and

related by

P = χε0 E (1)

χ is the electric susceptibility of the material

ε0 is the permittivity of free space

From eq. (1), it follows that the polarization P will change or vanish

if the field E is changed or set to zero. In any dielectric (χ> 0), a

reduction in E will thus lead to a depolarization (or relaxation)

process, which will follow with some delay or retardation to the

reduction of E.

Dielectric properties thus become dynamic events that can be

quantified in the time- as well as in the frequency domain

Polarization and Depolarization Current

Measurement

Applying a dc charging voltage of magnitude Uc to the test object

for a long time

The voltage source should be free of any ripple and noise in order

to record the small polarization current with sufficient accuracy

ipol(t) = C0Uc[{σ0/ ε0} + ε∞ δ(t) + f (t)]

The voltage is then removed and

the object is short-circuited at

t = tC,enabling the measurement

of the depolarization current

idepol(t) = −C0UC[f (t) − f (t+ TC )]

Dielectric response function f(t) is proportional to the

depolarization current

The dc conductivity σo, of the test object can be estimated

from the PDC measurements currents

σo ≅ε0 / C0Uc [ipol (t)- idpol(t)]

Dielectric Response in Frequency Domain

An analytical transition from time to frequency domain can be

made using the Laplace- or Fourier transform

Where

tanδ (ω) is dielectric dissipation factor

real (ε’) and imaginary (ε”) parts of the dielectric permittivity

conductivity σo

PDC or FDS ??

PDC can provide the moisture content in the solid insulation

material and the conductivities of the oil and paper.

Other diagnostic quantities like tan δ, polarization index and

polarization spectra can be calculated from PDC measurements

directly

(FDS) enables measurements of the composite insulation

capacitance, permittivity, conductivity (and resistivity) and loss

factor in dependency of frequency also moisture content in solid

insulation

FDS has better noise performance and separates the behavior of

polarizability and losses of a dielectric medium, while the

dielectric response of an insulating system can be measured with

the PDC method in shorter times and with a good accuracy,

Both methods appear to have their own strengths and

weaknesses.

Measurements on Transformer Oil-paper

Insulation based on time damain

Consider A 220 V/35 kV, 100 kVA oil filled distribution

transformer, which was in service for thirty years, Perform

PDC Test

PDC measured on the transformer at three different temperatures. The water content of

the insulating oil measured at 20°C was 50 ppm.

Temperature (°C) σoil σpaper

20 0.77 0.5

25 1.2 0.55

50 26.5 5.7

Table 1. Oil and paper conductivity as function of temperature

PDC Measurement result on the distribution transformer at two different temperatures,

with a step charging voltage U0 = 200 V. The moisture content in oil measured at 20°C

was 20 ppm

Temperature (°C) σoil σpaper

20 0.43 0.22

45 12.15 2.3

Table 2. Oil and paper conductivity as function of temperature

PDC Measurement results at 20°C on the distribution transformer at two

different moisture content, with a step charging voltage U0 = 200 V

Condition Evaluation of Oil-paper Insulation

based on Frequency domain analysis

Instruments for the measurement of frequency response are

available on the market and IDAX-206 is utilized in this

project. By measuring the impedance at one point, i.e. at a

specific frequency and amplitude, parameters such as

resistance, capacitance and loss can be calculated.

By matching the modeling curve with the measured curve

using MODS (Modelling Software), the moisture content is

acquired, which is an eventful parameter for the diagnosis of

transformers

Connection diagram for measuring the insulation between high- and low-

voltage windings and insulation between high-voltage windings and

ground

Analysis of Influence Factors on FDS

Influence of Moisture

Tan δ-f curves of oil immersed papers with different moisture content

(thickness: 2mm).

Influence of Aging

Tan δ-f curves for transformers of different service time

Influence of Temperature

Tan δ-f curves at different temperatures

Conclusions

PDC measurements, it was found that polarization and

depolarization currents increase with temperature increase.

Also, the shape of polarization current changes as temperature

increases

Drying of the transformer shows a significant reduction of the

polarization/depolarization currents.

Moisture and aging have great effect on dielectric response of

oil-paper insulation in frequency domain both of them will

cause the increase of tan δ

Diagnostics of oil-paper insulation based on Frequency

Domain Spectroscopy has great advantage over traditional

techniques for its simple operation and non-destructivity

References W.S. Zaengl, "Dielectric Spectroscopy in Time and Frequency Domain for

HV Power Equipment, Part I: Theoretical Considerations", IEEE Elec.

Insul. Mag., Vol. 19, No. 5, pp. 5-19, 2003

W.S. Zaengl, "Application of Dielectric Spectroscopy in Time and

Frequency Domain for HV Power Equipment", IEEE Elec. Insul. Mag.,

Vol. 19, No. 6, pp. 9-22, 2003

A. Setayeshmehr et al.: “Dielectric Spectroscopic Measurements on

Transformer Oil-paper Insulationunder Controlled Laboratory Conditions”

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 4;

page 2-6, August 2008

Shuang-suo Yang “Condition Evaluation of Oil-paper Insulation based on

Dielectric Spectroscopy” IEEE Annual Report Conference on Electrical

Insulation and Dielectric Phenomena 2009, pp 2-4.

Doina Elena Gavrilă “Dielectric Spectroscopy, a Modern Method for

Microstructural Characterization of Materials” Journal of Materials Science

and Engineering, pp 1-2, Published: January 10, 2014.

Thank You