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



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



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%



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



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.



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



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



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



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

2009 karl strom paper

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