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Investigation of leakage Current on High Voltage Insulators Field
Measurements
Article in WSEAS Transactions on Circuits and Systems · July 2004
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Investigation of leakage Current on High Voltage Insulators
Field Measurements
KIRIAKOS SIDERAKIS
High Voltage Lab
University of Patras
37, Aristidou Str
713 07 Heraklion
GREECE
sidkir@ieee.org
DEMOSTHENIS AGORIS PYRGIOTI ELEFTHERIA
High Voltage Lab
High Voltage Lab
University of Patras
University of Patras
Rio
Rio
262 20 Patras
262 20 Patras
GREECE
GREECE
dagoris@ee.upatras.gr
epyrgioti@ee.upatras.gr
www.hvlab.upatras.gr
EMMANOUIL THALASSINAKIS
PPC
Kastorias St.
713 07 Heraklion
GREECE
mthalassinakis@deh-ptdmkr.her.forthnet.gr
Abstract: - Leakage current measurements can be used in order to investigate the performance of outdoor
insulation. The advantage of the method is that it can provide information regarding the development of the
surface activity, from the onset of current flow until the case of a flashover. This information is available from
continuous monitoring that can take place in a lab but even more in the field. In the last case the study of the
complete phenomenon is possible minimizing the influence of the lab simulation faults. In this paper the
characteristics of leakage current from field measurements are investigated. These measurements took place
on 150kV post porcelain insulators located at a high voltage substation of the Power System of Crete.
Key-Words: - Leakage current, field measurements, porcelain insulators, frequency analysis
1 Introduction
Surface activity on high voltage outdoor insulation,
is the result of the environmental influence, in
combination with the electric stress. The first step of
the phenomenon is the accumulation of
contaminants on the insulation surface. Then, in the
presence of a wetting mechanism, the contaminants
layer is transformed to a conductive film and the
flow of current on the surface is permitted [1, 3].
This current reflects the development of surface
activity and thus it provides plenty of information
about phenomena, that follow after the onset of the
current flow (dry band arcing) [1-4].
Therefore, leakage current monitoring is a method
that can be used in order to study the pollution
phenomenon. It can provide simultaneously and
comparable measurements on different insulating
materials (ceramic and composite). Additionally
these measurements may take place in a laboratory
or in field conditions.
The advantage of field measurements is that
monitoring of the phenomenon in real condition is
possible, avoiding the problems that occur during a
laboratory simulation. On the other hand long term
measurements are necessary for safe conclusions to
come up, since the appearance of surface activity
depends on the environment.
Field LC measurements may take place in specially
designed installations for this purpose (Test stations)
or on insulators that belong to a Transmission or
Distribution system. In this paper the evaluation of
field LC measurements on actual power system
insulators is presented. These measurements took
place at a High Voltage Substation in Crete, a Greek
Mediterranean island.
2 Measurements Setup
Crete is a Greek island located at the eastern basin of
the Mediterranean Sea. Due to the sea influence
intense pollution problems have been observed in
the Transmission and Distribution system of the
island. In order to investigate the pollution
phenomenon field measurements of LC take place.
For this purpose two specially designed LC
monitoring systems where installed at two High
Voltage substations and one of them is used to
monitor LC on porcelain insulators.
Each system is capable of simultaneous recording of
9 LC channels and 3 voltage channels. The
measurement of leakage current is possible by
inserting in the LC path a collection ring and a Hall
sensor (Figure 1). Due to the low input impedance,
the current is forced to go through the sensor on the
way to earth and thus the measurement is
accomplished. Then the data are transmitted to the
central unit where it is processed and stored. The
sampling frequency of each channel is 2 kHz and all
the channels are simultaneously sampled.
observed. In the first stage leakage current flows due
to the application of the voltage across the surface
conductive film (resistive behavior). During this
stage, equilibrium is observed between heating due
to the flow of current and wetting due to the
influence of the environment.
However when drying becomes the dominant
mechanism, the surface discharges that take place
change the LC waveform (stage 2). Higher peak
values are observed and non linear characteristics
can be observed. During this stage LC is the result
of the voltage application across a series
combination of a resistance and an arc [5]. Finally
the third stage is the flashover of the insulator where
the arc is the dominant component.
3.1
Stage 1: Drying period
In figure 2 the waveform of leakage current at the
end of the initial drying period is shown. As it can
be seen, the current at this stage has a sinusoidal
form, since surface conductivity is the dominant
factor for the surface behavior.
Figure 1: Measurements setup
The monitoring of the porcelain insulators takes
place at a Substation called Heraklion II. It is
Substation located at the east side of the city
Heraklion, in an urban area, in a distance less than
1km from the sea.
Figure 2: LC at the drying stage
Frequency analysis of the above waveform indicates
that the principal frequency component present is
the 50Hz fundamental. As it can be seen in figure 3
there is also a small 150Hz component.
3 LC on Porcelain Insulators
The flow of leakage current on the surface of a
ceramic insulator results to the dissipation of energy
due to the Joule law. Therefore, drying of the
surface film should be expected and localized
changes in the surface conductivity are observed.
Where current density is increased, areas of higher
resistance are formed (dry bands) and consequently
the voltage distribution along the insulator is altered.
As a result increased stress along the dry bands is
observed and discharges appear (dry band
discharges). Finally these discharges may lead to a
complete flashover of the insulator.
Further, on the development to a complete flashover,
three stages regarding the leakage current form are
Figure 3: FFT Analysis of the LC in fig. 2
Due to this third harmonic the LC in figure 2 cannot
be considered as perfectly linear. However as it can
be seen in figure 4, in the I-V characteristic, the LC
in figure 2 is not far from the linear behavior (I-V
characteristic of a resistor).
It is worth mentioning that the fundamental and the
150Hz components are the dominant in this case.
Figure 5: FFT Analysis of the LC
waveform in fig. 4
Figure 4:I-V Characteristic for the LC in figure 2
3.2 Stage 2: Dry band activity
During dry band activity higher values of leakage
current can be observed. This is due to the
discharges that appear on the surface, that result to
higher surface conductivity. However the presence
of a discharge influences the leakage current
waveform, due to the non linear characteristics of
the arc [6].
Two typical waveforms of LC during dry band
activity are shown and analyzed in figures 4 and 9.
In figure 4 the waveform of LC during a dry band
discharge is shown. The duration of the discharge
exceeds the time of 12 voltage periods and the peak
current is 25mA.
The influence of the third harmonic can be evaluated
also from the I-V characteristic in figure 6. As it can
be seen, the characteristic in this case has obtained
the form of a hysteresis curve, in agreement to the
arc nature [6].
Figure 6: I-V Characteristic of the LC
waveform in fig. 4
Figure 4: LC in the case of a dry-band
discharge (early stage)
In figure 5 the frequency analysis of the leakage
current in figure 4 is shown. The non linear behavior
is indicated by the presence of a 150Hz component.
Under favorable for the phenomenon conditions
more intense activity may be observed. The
waveform in figure 7 is such a case. As it can be
seen, subsequent discharges of probably the same
dry band have been recorded. It is important to
notice that the duration of each discharge is about
seven voltage periods and the peak value of LC
reaches 100mA.
In figures 8 and 9 the frequency analysis and the I-V
characteristic are shown. The analysis of this
waveform verifies that the third harmonic is the
principal component that appears besides the 50Hz
fundamental. Additionally, in this case the
dissipation of energy is increased, something that is
reflected to the I-V characteristic in figure 9.
component, which however is too small comparable
to the resistive. Thus the current during the dry
period can be considered almost linear (figure 4) and
resistive. It must be noticed that due to the small size
of the 150Hz component in the first waveform, it
can be considered that this case is the limit between
stages 1 and 2.
Figure 9: I-V Characteristic of the LC
waveform in fig. 7
Figure 7: LC waveform in the case of intense activity (a.
LC, b. LC and Voltage)
Figure 8: FFT Analysis of the LC
waveform in fig. 7
4 Conclusion
The leakage current measurements shown above and
the analysis that followed verify the first two stages,
the drying period and the dry band activity period.
During the drying period the observed current is
sinusoidal, since the voltage is applied across a
resistance, which is the surface contaminants film. It
is important to notice that there is also a capacitive
The onset of surface activity (discharges) changes
the initial linear behavior. Due to the existence of a
non linear arc in series with the surface resistance, a
large third harmonic is present. The non linear
behavior is also verified in I-V diagrams.
It is important to notice that the transition from the
stage 1 to 2 is indicated by the presence of the
150Hz harmonic, which is the dominant non linear
component.
References:
[1] CIGRE, WG33-04,Taskforce 01, A Review of
current knowledge: Polluted Insulators , August
1998
[2] J.S.T. Looms, Insulators for High Voltage, IEE
Power Engineering, Series 7
[3] E.M. Sherif., Performance and aging of HVAC
and HVDC overhead line insulators, Techn.
Report No 169, Dept. of Electric Power
Engineering,
Chalmers
University
of
Technology, Goteborg 1987
[4] Torbjorn Sorqvist, Polymeric Outdoor
Insulators – A long term study, PhD Thesis,
Dept. of Electric Power Engineering, Chalmers
University of Technology, Goteborg 1998
[5] F.A.M. Rizk, Mathematical models for pollution
flashover, Electra No 78, CIGRE
[6] M. F. Hoyaux. Arc Physics, Applied Physics and
Engineering, Springer Verlag 1968
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