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-destructive

1. Application of Dielectric Spectroscopy to
Monitor Insulating Materials
Presented By
Furkan Ahmad
Electrical Engineering
Department
ZHCET
AMU Aligarh

2. 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

3. 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

4. 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

5.  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

6. 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

7. 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 )]

8.  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)]

9. 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

10. 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.

11. 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.

12. 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

13. 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

14. PDC Measurement results at 20°C on the distribution transformer at two
different moisture content, with a step charging voltage U0 = 200 V

15. 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

16. Connection diagram for measuring the insulation between high- and low-
voltage windings and insulation between high-voltage windings and
ground

17. Analysis of Influence Factors on FDS
 Influence of Moisture
Tan δ-f curves of oil immersed papers with different moisture content
(thickness: 2mm).

18. Influence of Aging
Tan δ-f curves for transformers of different service time

19. Influence of Temperature
Tan δ-f curves at different temperatures

20. 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

21. 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.

22. Thank You