2009 Karl Strom Paper
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Moisture Assessment using DielectricFrequency Response and
Temperature Dependence of
Power Factor
ByMats Karlstrom, Peter Werelius
and Matz OhlenMegger
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Dielectr ic Frequency Response and Temperature Dependence of PowerFactor
Mats Karlstrom, Peter Wereliusand Matz Ohlen
Megger
Abstract
Modern technology and developments in signal acquisition and analysis techniques have provided new
tools for transformer diagnostics. Of particular interest are dielectric response measurements where
insulation properties of oil‐ paper systems can be investigated.
Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced
more than a decade ago and has been thoroughly evaluated in a number of research projects and field
tests with good results. Moisture assessment and bushing diagnostics in transformers as well as cable
testing and insulation testing in rotating machinery are important applications where DFR provides
accurate information for decisions on maintenance and/or replacement.
Measuring 60 Hz Power Factor at various temperatures is an established technique for identifying
aged/high moisture content in e.g. bushings. The testing takes substantial time since you have to wait for
the bushing to cool down, make a measurement, wait, take a new measurement etc. This paper presents a
new method (patent pending) where a DFR measurement is used to determine the temperature
dependence of the insulation and the result is presented as 60 Hz Power factor as a function of temperature. The technique saves a lot of time since the test is only one measurement sequence at the
actual ambient temperature.
Another well known challenge is how to do correct temperature compensation of Power Factor
measurements taken at high or low temperature to a reference temperature for comparison with
nameplate data. Traditionally, correction tables for “average” components are used giving poor accuracy,
especially when measuring old/aged components. The new technique calculates the correct temperature
correction for the actual unit based on the material used and condition of the insulation. This means that
power factor measurements on new or aged components can be performed at high or low temperatures
and individually converted to the reference temperature.
Introduction
With an aging power component population, today ’s electrical utility industry faces a tough challenge as
failures and consequent repair and revenue loss may inflict major costs. Transformers have become one of the most mission critical components in the electrical grid. The need for reliable diagnostic methods drives
the world’s leading experts to evaluate new technologies that improve reliability and optimize the use of the power network.
The condition of the insulation is an essential aspect for the operational reliability of electrical power
transformers, generators, cables and other high voltage equipment. Transformers with high moisture
content can not without risk sustain higher loads. Bushings and cables with high power factor at high
temperature can explode due to “thermal runaway”.
On the other hand it is also very important to identify “good” units in the aging population of equipment.
Adding just a few operating years to the expected end ‐ of ‐ life for a transformer or cable means substantial
cost savings for the power company.
Traditional Power Factor Measurements
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The first field instrument for DFR/FDS measurements of transformers, bushings and cables was introduced
1995 (2). Since then numerous evaluations of the technology has been performed and as an example,
several international projects/reports define dielectric response as the preferred method for measuring
moisture content in power transformers (3), (4), (5).
In DFR tests, capacitance and power factor is measured. The measurement principle and setup is basically
the same as for traditional 50/60 Hz testing with the difference that instead of measuring at line
frequency, insulation properties are measured from mHz to kHz. The results are normally presented as
capacitance and/or tan delta/power factor versus frequency. Measurement setup is shown in Fig 2, and
typical results in Fig 3.
Figure 2.
DFR/FDS test setup
Figure 3.
DFR/FDS power factor measurements on four different transformers with moisture content ranging from
0.3 to 3.4%
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Moisture Assessment
The capability of DFR to measure power factor as function of frequency, gives the user a powerful tool for
diagnostic testing. Moisture assessment is one example.
High moisture levels in transformers is a serious issue since it is limiting the maximum loading capacity
(IEEE Std C57.91 ‐ 1995) and the aging process is accelerated. Accurate knowledge about the actual
moisture content in the transformer is necessary in order to make decisions on corrective actions,
replacement/scrapping or relocation to a different site in the network with reduced loading.
The method of using DFR for determining moisture content in the oil‐ paper insulation inside an oil‐
immersed power transformer has been described in detail in several papers and articles elsewhere (3), (4),
and (5), and is only briefly summarized in this paper.
The power factor plotted against frequency shows a typical S‐ shaped curve. With increasing temperature
the curve shifts towards higher frequencies. Moisture influences mainly the low and the high frequency
areas. The middle section of the curve with the steep gradient reflects oil conductivity. Fig 4 describes
parameter influence on the master curve.
Figure 4.
Parameters that effects the power factor at various frequencies
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Figure 5 and 6.
PF values from 1kHz – 1mHz for Oil and Paper with different moisture content
Figure 7 and 8.
PF values for standard transformer insulation and influence frequencies for oil and paper
When measuring an oil sample in a complete frequency sweep, the losses are linear and increase at a one
to one ratio when lower frequencies are used and presented in a log scale (Fig 5). A paper sample has a
non ‐ linear response that varies with moisture content (Fig 6). The insulation between the high and low
winding in a typical transformer contains both oil and paper and the two material respond at different
frequencies providing an “S” shaped curve for a new transformer at 20 °C temperature (Fig. 7). Figure 8
illustrates that the paper contribute to the response in the highest and lowest frequencies while the oil
provides the dominating response in medium frequencies. The oil influence moves independently from
the paper if higher or lower conductivity is present. Likewise the paper response influences the curve
independently at different moisture content.
Using DFR for moisture determination is based on a comparison of the transformers dielectric response to
a modeled dielectric response (master curve). A matching algorithm rearranges the modeled dielectric
response and delivers a new master curve that reflects the measured transformer. The moisture content
along with the oil conductivity for the master curve is presented. Only the insulation temperature (top oil
temperature) needs to be entered as a fixed parameter.
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Figure 10.
MODS® moisture analysis
Two different transformers are shown in Fig 11. The two units have the same 0.7%, 60 Hz Power Factor,
characterized by IEEE 62 ‐ 1995 as “warning/alert” status calling for “investigation”. The investigation is
done as moisture analysis using MODS.
Figure 11.
MODS analysis for two transformers with different oil quality and moisture content
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With DFR and the technique for converting data to temperature dependence, it is possible to do accurate,
individual temperature compensation (patent pending). For a good component, the temperature
dependence is weak. When the component gets older/deteriorated, the temperature correction factor
becomes much larger, i.e. the temperature correction is a function of aging status. This is in line with
several projects and studies (8), (10).
Bushings
Examples of individual temperature correction for bushings are shown in Fig 16. Manufacturer’s table data
is only valid for as ‐ new bushings. As soon as the bushing starts to show deterioration, the temperature
dependence increases. “Bad” bushings have a very large temperature correction.
Figure 16.
Standard temperature
correction compared with
individual temperature
correction for samples of GE Type U bushings
Transformers
Individual temperature correction for transformers is more complex compared to “single ‐ material”
components e.g. bushings. The oil‐ paper insulation activation energy constant W a
in Arrhenius’ law,
κ = κ0
· exp( ‐ W a/k T ) with activation energy W
a and Boltzmann constant k, is typically 0.9 ‐ 1 eV, while for
transformer oil W a
is typically around 0.4 ‐ 0.5.
Individual temperature correction for transformers of various ages is shown in Figure 17. Transformer data
is summarized in Table 2.
Manufacturer Year Measured temp
Moisture Oil conductivity Power rating
Status
Hyundai 2008 27 Co 0.6 % 0.013% PF @ 60Hz 105
MVA
New, during
commission
Westinghouse 1987 27 Co 1.1 % 0.019% PF @ 60Hz 80 MVA Used, at utility
GE 1950 27 Co 2.1% 0.406% PF @ 60Hz 15 MVA Used, at utility
Yorkshire 1977 27 Co 4,5 % 0.467% PF @ 60Hz 10 MVA Used and scrapped
Table 2.
Transformer data
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Figure 17 Frequency Sweeps
from transformers listed in Table 2
Red = Hyundai
Black = Westinghouse
Green = GE
Blue = Yorkshire
Hyundai, new, 0.6% moisture Westinghouse, in service, 1.1 % moisture
GE, in service, 2.2% moisture Yorkshire, scrapped, 4.5% moisture
Figure 18.
Temperature correction for transformers in various conditions
As seen in figure 18, each transformer has its individual temperature correction. The above dry units have
a “positive” correction from 27Co to 20Co.
The wet transformers show a “negative” correction between the
same temperatures. Aged/wet transformers typically show higher oil conductivity which contributes to the
1.007 correction 1.055 correction
0.797 correction
0.581 correction
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dramatic temperature dependence on these samples. Using the average IEEE temperature correction as
indicated in Figure 1 would provide highly misleading data.
Summary and Conclusions
Dielectric Frequency Response (DFR/FDS) measurement is a technique/methodology for general insulation
testing and diagnostics. In comparison with standard 60 Hz power factor measurements, DFR
measurements provide the following advantages:
Capability of performing individual temperature correction of measured 60 Hz power factor.
Capability of estimating the moisture content of oil‐ immersed cellulose insulation in power
transformers and bushings
Capability of estimating power factor at operating temperature in order to assess risk of thermal
runaway catastrophic failure.
Capability of investigating increased power factor in power components
The insulation properties are very important for determining the condition of a power system component.
Knowing the condition of the power system component helps to avoid potential catastrophic failure.
Identifying “good” units and decide upon correct maintenance in aged populations of transformers and
other power systems approaching end ‐ of ‐ life, can save significant money due to postponed investment
costs.
References
1. IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus; Part 1: Oil Filled Power
Transformers, Regulators, and Reactors”, IEEE 62 ‐ 1995
2. P. Werelius et al, “Diagnosis of Medium Voltage XLPE Cables by High Voltage Dielectric Spectroscopy” ,
paper presented at ICSD 1998.
3. U. Gäfvert et al, “Dielectric Spectroscopy in Time and Frequency Domain Applied to Diagnostics of Power Transformers” , 6th International Conference on Properties and Applications of Dielectric Materials, June
21 ‐ 26, 2000, Xi'an, China.
4. S.M. Gubanski et al, "Dielectric Response Methods for Diagnostics of Power Transformers” , Electra, No.
202, June 2002, pp 23 ‐ 34¸also in CIGRE Technical Brochure, No. 254, Paris 2004
5. S.M. Gubanski et al, “Reliable Diagnostics of HV Transformer Insulation for Safety Assurance of Power
Transmission System.
REDIATOOL
‐a
European
Research
Project”,
paper
D1‐
207
CIGRE
2006
6. “Swedish Bushings Plant Sees Growth in RIP Designs”, INMR Quarterly, Issue 68, 2005
7. J.M Braun et al.” Accelerated Aging and Diagnostic Testing of 115 kV Type U Bushings” , paper presented
at IEEE Anaheim 2000.
8. C. Kane, “Bushing, PD and Winding Distortion Monitoring”, paper presented at ABB Seminar “Power
Transformer Health Monitoring and Maintenance” Johannesburg 2008
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