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DIELECTRIC DIAGNOSTICS
MEASUREMENTS OF TRANSFORMERS
AND THEIR INTERPRETATION
Claudia Martorella
563408
Spring 2016
INTRODUCTION
The Dielectric Diagnostics Method
(DDM) and Measurements is a
family of methods used to assess
the ageing condition of
transformers, by mean of a
characterization of the dielectric
material(s) of the insulation
TIME-BASED
MEASUREMENTS
1) Return Voltage
Measurement (RVM)
2) Polarization and
Depolarization Current
Method (PDC)
The transformer insulation structure consists
mostly of mineral oil and paper. The ageing of
insulation material is due mainly to oxidative
reactions between the oil and the cellulose (both
organic substances) and the oxygen, which take to
the formation of water. The water contamination
causes a higher electric conductivity σ of the oil
and the hydrolytic degradation of the cellulose
molecules [9]. The moisture content of
transformers pressboards at the end of the
operation time can reach the 5% [10]
FREQUENCY-BASED
MEASUREMENTS
Frequency Domain
Spectroscopy (FDS)
When a dielectric is
subject to an electric
field, a shift of positive
and negative charges
take place within it giving
rise to polarization [7].
This phenomena adsorb
some part of the field
energy and then it
corresponds to a energy
loss, and it is stronger as
σ is higher
Main components of the transformer insulation:
- the insulation between the tank and the HV windings;
- the insulation between LV and HV windings;
- the inter-phase insulation [7]
RETURN (RECOVERY) VOLTAGE MEASUREMENT (RVM)
A step DC voltage is applied across the sample. The test object is
first charged for a certain time and then discharged for half of the
time. This is done creating a short-circuit either during the
charging and the discharging [2]: S1 is closed and S2 is open
during the charging and vice versa. Afterwards the return voltage
is measured under open circuit conditions, that is with both
switches open to allow the voltmeter to take the measure. The
recovery voltage is due to the remaining polarization of the
dielectric material after the charging/discharging cycle. The test is
carried forward applying the DC voltage for increasing charging
(and discharging) times and plotting the peak values Vr(t) of the
recovery voltage measured (the so called “polarization
spectrum”). Additional informations are shown in the spectrum:
- the initial slope of the recovery voltage dVr/dt: it is proportional
to the conductivity σ of the insulation material [1];
- the polarization and depolarization currents;
- the central time constant (CTC)
Figure 1: basic circuit for RVM measurements[1]
The polarization and depolarization currents are the currents flowing through the sample during the charging
and discharging times, due to the application of the electric field.
𝑖𝑝𝑜𝑙 𝑡 = 𝐶0 𝑈0
σ
+𝑓 𝑡
𝜀0
𝑖𝑑𝑒𝑝𝑜𝑙 𝑡 = 𝐶0 𝑈0 𝑓 𝑡 − 𝑓 𝑡 + 𝑡1
- U0 = step DC voltage applied
It is due to the charges
flowing to the initial
(neutral) configuration
when the step voltage is
no more applied
- σ = average conductivity of the insulation system
- Co = geometrical capacitance of the system
- f(t) = response function of the dielectric material
- t1 = time when the voltage is disconnected
- ε0 = electric permittivity of vacuum = 8,854*10-12 F/m
NOTE: The dielectric response function describes the behavior of dielectric materials to
electric fields which vary with time and space. It depends on the properties of the
dielectric and can be defined as follows [7]:
𝑡
𝑃 𝑡 = 𝜀0 𝜒∾ 𝐸 𝑡 + 𝜀0
𝑓 𝑡 − 𝜏 𝐸 𝜏 𝑑𝜏
0
P = polarization vector; χ∾ =ε∾ -1= electric susceptibility of the material; E = electric field
at the beginning the current is
only due to the conductivity of
the insulation material, then it
grows according to the
response function
The discharging current id is
negative, as it flows in the
opposite direction of ic
NOTE: THE CENTRAL
TIME CONSTANT (CTC)
IS THE TIME REQUIRED
TO REACH THE PEAK
VALUE OF THE
RECOVERY VOLTAGE [3]
Figure 3: peak value of the return voltage as a
function of the CTC [1]
Transformer
operation
time [years]
Figure 2: example of voltage and current spectrum [1].
Charging times: 0,5÷1024 s.
T4
3
T5
38
T6
33
REMARKS FOR THE INTERPRETATION OF THE RESULTS
Transformers with higher operation
times show lower values of the central
time constant. This can be explained
reminding that CTC is the time needed
to reach the peak value of the recovery
voltage. This recovery voltage is due to
the migration of the charges within the
dielectric
material
after
the
polarization. Such factors as the
presence of humidity (one of the
symptoms of ageing) can accelerate
this phenomenon, reducing the value
of CTC
Figure 4: example of results for transformer insulations
with different moisture content [1]
POLARIZATION AND DEPOLARIZATION CURRENT
METHOD (PDC)
A step DC voltage is applied through the sample for a
long period of time (e.g. 10000 s) keeping S1 closed
and S2 open, and the polarization current (charging
current) is measured by the ammeter. The
measurement is stopped when the current becomes
stable or very low. Then the test object is shortcircuited for a long time (opening S1 and closing S2)
and the depolarization current is measured by the
ammeter as well.
Figure 5: basic circuit for PDC
measurements [5]
REMARKS FOR THE INTERPRETATION OF THE RESULTS
The polarization processes have
different
time
constants
corresponding to different insulation
materials and different ageing
conditions.
With the values of the currents
obtained, the DC conductivity of the
sample can be estimated.
Higher σ => higher moisture content
in the solid part of the insulation
Figure 6: example of measured polarization currents
for samples with different moisture content [6]
FREQUENCY DOMAIN SPECTROSCOPY (FDS)
A sinusoidal voltage of varying frequency
is applied to the test sample. The voltages
and currents across the sample are
measured with an ammeter and a
voltmeter.
Unlike
time-domain
measurements, it is not necessary to
discharge the sample before changing the
frequency. Usually a frequency range of 1
mHz ÷ 1 kHz is used. The frequency is
reduced by discrete steps from the highest
to the lowest. The voltage may vary from 5
to 200 V (rms).
Figure 7: basic circuit for FDS measurements [7]
The aim of the FDS is to measure the dielectric dissipation (loss) factor (tanδ) and/or the complex capacitance and
permittivity as a function of frequency. This is made by calculating the sample impedance at each frequency step by
knowing the voltage and measuring the current [8, 7]:
𝑈 𝜔
′
′′
𝑍 𝜔 = 𝑍 𝜔 + 𝑗𝑍 𝜔 =
𝐼 𝜔
The real and imaginary part can be defined knowing the phase shift between the voltage and current waveforms.
Assuming the sample to be a complex capacitance => Z(ω)=1/jωC(ω):
𝐼 𝜔 = 𝑗𝜔𝐶(𝜔)𝑈(𝜔)
𝐶 𝜔 = 𝐶 ′ 𝜔 + 𝑗𝐶′′(𝜔)
the complex capacitance is related to the complex permittivity as follows:
𝐶 𝜔
𝜀 𝜔 =
= 𝜀 ′ 𝜔 + 𝑗𝜀′′(𝜔)
𝐶0
C0=geometrical capacitance of the test object. Then:
𝜀 ′′ 𝜔
𝐶 ′′ 𝜔
𝐼𝑅
tan 𝛿 = ′
= ′
=
𝜀 𝜔
𝐶 𝜔
𝐼𝐶
- IM = total current through the insulation
- IC = capacitive current through the insulation
- IR = resistive current through the insulation
- UA = voltage applied to the sample
Figure 8: Phasor diagram of measured
current and applied voltage [9]
Examples of curves (tanδ, f), (ε, f) according to
different conditions of the insulation
Different moisture content of the solid
part of the insulation
The oil of transformers T1 and T2 has a
higher conductivity than the one of T3
and T4.
Different conductivity
of the insulation oil
Figure 10: the effect of oil
conductivity on tanδ for four different
transformers [7]
Figure 9: Real and imaginary part of dielectric permittivity of pressboards
having different moisture content [7]
REMARKS FOR THE INTERPRETATION OF THE RESULTS
The change in the dielectric properties of the system as the
frequency change is due to the fact that different
frequencies are associated with different polarization
processes [7].
The value of tanδ changes according to different
conductivity and moisture content of the insulation. The
higher the tanδ, the higher the moisture content and the
higher the conductivity, that is, the more advanced the
ageing process is. Note that the reason why the diagrams
shown are sketched separately for the solid and liquid part
of the insulation is that the dielectric response in the low
frequencies (f<0,01 Hz) and high frequencies (>10 Hz) is
mainly influenced by the properties of the solid insulation,
while in the middle it is mainly influenced by the properties
of the insulating oil (check fig. 11). Therefore all the range
must be analyzed to get useful informations about the
conditions of both the parts.
Figure 11: loss factor for different
moisture contents of the cellulose
[6]
REFERENCES
[1] “Voltage response measurements for power transformer moisture and ageing condition assessment” - Zheng Tong Yao,
Tapan Kumar Saha
[2] “Return voltage measurements diagnostic interpretation based on the dielectric time constant” - R. Patsch , D. kamenka,
J. Menzel
[3] “A New Approach for determination of moisture in paper insulation of In-situ Power Transformers by Combining
Polarization-Depolarization Current and Return Voltage Measurement Results” - S. Sarkar, T. Sharma, A. Baral, B. Chatterjee,
D. Dey and S. Chakravorti
[4] “Investigation of Polarization and Depolarization Current for Evaluation of Moisture in Oil-Pressboard Insulation” - Amit
Kumar, J. Rattan, R.N Sharma, Sushil Chauhan
[5] “Polarization And Depolarization Current (PDC) Tests On Biodegradable And Mineral Transformer Oils At Different
Moisture Levels” - N.A. Muhamad, B.T. Phung, T.R. Blackburn, K.X. Lai
[6] “Dielectric diagnostics measurements of transformers” – Xiaolei Wang
[7] “Recent trends in the condition monitoring of transformers” - Sivaji Chakravorti, Debangshu Dey, Biswendu Chatterjee
[8] “Determination of water content in transformer solid insulation by frequency domain spectroscopy” - Belén García,
Baudilio Valecillos, Juan Carlos Burgos
[9] “Prediction of insulation degradation of distribution power cables based on chemical analysis and electrical
measurements” - Petri Hyvönen
[10] “Quantitative Analysis of Insulation Condition of Oil-paper Insulation Based on Frequency Domain Spectroscopy” Ruijin Liao, Jiefeng Liu, Lijun Yang, Ke Wang, Jian Hao, Zhiqin Ma, Jun Gao, Yandong Lv