uncertainty tranformer using konvensional testing methode.pdf

7/30/2019 uncertainty tranformer using konvensional testing methode.pdf

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THE UNCERTAINTY IN POWER TRANSFORMER FAULT DIAGNOSTICS USING

CONVENTIONAL TESTING METHODS

Yogendra Dev Vashishtha*, Paul Ascione* and Qi Su**

* Power and Water Authority

** Monash University

Abstract

The average age of power transformers in Australia is about 25-35 years. Conventional oil tests,

dissolved gas analysis (DGA) of oil, dielectric loss angle (DLA or tan) of winding and recoveryvoltage monitoring (RVM) are among many diagnostic techniques available. Power and Water

Authority (PAWA) of Northern Territory refurbished five 66 / 11 kV, 1030 MVA, 2433 yearsold transformers. From the test results it is observed that a transformer which has very good

results from these diagnostic tests and even has minimum moisture content in the winding

(from RVM) may still have very poor mechanical integrity. Winding cellulose paper may be

aged to such an extent (assessed from degree of polymerisation; DP) that any sudden force ortransient would cause failure. DP test is very difficult to perform regularly to ascertain

mechanical integrity of winding paper. Furan analysis is another alternative to DP. Most of the

times Furans are not detected and there is no universal correlation available for DP and Furans.Also, both ageing and moisture content influence Furans. Frequency response analysis (FRA)

gives information regarding winding movement but it does not give much information about

mechanical integrity of a winding which is essential for transformer reliability. This paper also

compares moisture contents in the winding assessed from RVM and estimated from oil PPM(using Norris diagram) and estimated from winding DLA (based on Gussenbauers relationship).

It appears that moisture content from RVM needs further validation and research to

separate out ageing and moisture effects.

1. INTRODUCTIONTransformers represent a high capital investment in a

transmission substation at the same time as being a

key element determining the loading capability of thestation within the network. With appropriate

maintenance, including insulation reconditioning at

the appropriate time, The technical life of atransformer can be in excess of 60 years. The end of

life, however, can be strategic or economic.

Quantitative, risk based approach can be used to aid

costly investment decisions involving transformer life,

otherwise made from a subjective viewpoint.

Diagnostic systems have been listed under three

categories as portable / periodic, continuous on-lineand specialist support [1]. There are various on-line

monitors for load, operating conditions, gas-in-oil and

moisture-in-oil are available these days [5]. Dissolvedgas analysis (DGA) is the main condition monitoring

technique used at Power and Water Authority(PAWA) in the Northern Territory. Recovery Voltage

Monitoring (RVM) has been found as a better

technique to estimate moisture content in the winding

paper insulation and its ageing. However, RVM does

not distinguish between moisture content and

degradation of paper. Some experiments have been

reported correlating charging current with paperageing. Due to very limited work, these are not very

conclusive. There is a possibility that charging current

may also correlate with insulation resistance (IR) as

we found RVM following polarisation index (PI).

2. BACKGROUND

2.1 The major factors involved in transformer

insulation life are:

Insulation moisture content

Extent of oil oxidation

Extent of cellulose degradationwater, CO,

CO2 and furans

Insulation Structure compression

MOISTURE, OXYGEN and HEAT are the three

ageing accelerators.


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2.2 Transformer Life Assessment Indicators:

Insulation moisture content in oil provides a simple

method of estimating residual life. The following

guide may be useful (based on equilibrium water

content of paper as a function of water content of

impregnating transformer oil):

3 % Maximum acceptable

4 % Entering at risk zone

5 %--6 % Considerable risk of failure

7 % Failure imminent

For life assessment, emission associated with the

insulation ageing process is of interest, principally the

levels of CO, CO2 and furans in the oil. Cellulose

degradation can be assessed by furan (HPLC) analysisand preferably supported by degree of polymerisation

(DP) for a more definitive assessment. DP testinvolves the removal of samples of paper from the

winding insulation. Due to the need for internal access

and expensive analysis, the test is only recommended

on suspect transformer on the basis of DGA and other

tests. DP is correlated to the tensile strength. The

paper is considered to have no mechanical strength at

a DP of 150 to 200. New transformers have DP above

900.

2.3 The following methods of moisturemeasurement are available:

a) Direct measurement of the water content in

paper on a layer by layer basis by means of

Karl Fisher moisture meter (KFMM) using

four bar probe.

b) Water content in winding established from

measurement of tan using Gussenbaeurscharacteristics [4].

c) Water content in winding established from

measurement of oil PPM using NorrisDiagram.

d) Water content in winding established from

RVM [6].

2.4 Condition Monitoring Methods:Method Suitable for

WINDING:

IR & PI Moisture &

ContaminationDDF & Capacitance Moisture &

Contamination

Winding Ratio Faulted turns

Winding Resistance Conductor damage

Impedance & Losses Winding movement

Applied HV Poor dielectric

Induced HV & PD Poor dielectric

PD ( acoustic &electrical)

Detect and locate PD

Frequency Response

Analysis

(Transfer Function)

Winding displacement,

possible loose winding,

and core faults

Voltage Recovery Moisture in paper

And ageing of paper

Off-Line Test

Vibration Analysis Slack winding &

Mechanical faults

DP & Gel Permeation Insulation degradation

OIL:

DGA & Ratio

analysis

Detecting incipient

faults

Furan Analysis Overheating and

ageing of paperWater content Dryness

Resistivity, Acidity,

IFT

and DDF

Ageing of oil

BUSHINGS:

Tan Delta (DLA) Moisture &

contamination

Tan or dielectric loss angle (DLA) is a measure ofoverall quality of insulation system in terms of

moisture and contamination.

2.5 Recovery Voltage Method:

If the DGA analysis is performed correctly (proper

sampling, storage and calibration), most of the

incipient faults in the oil may be detected. The paperinsulation is responsible for containing most of the

moisture due to ageing and thermal stress. The paper

insulation may fail under high electrical stress or may


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release moisture into the oil insulation. To detect

ageing or moisture content it is necessary to analyse

low frequency part of polarisation spectrum of

dissipation factor. A tan would have been sufficientbut finding a sinusoidal source voltage of 0.001 Hz is

very difficult [6]. The alternative is the recovery

voltage measurement.

It was found that IR & PI do not reflect complete

information on polarisation process. Cases were

reported where electrical motors having good PI werefound to have contaminated windings and also motors

having poor PI had no problems in the winding

insulation [2]. To resolve this, an one thousand

seconds charging and discharging test (dc absorption)was developed. Recovery Voltage Method for

transformer seems to be developed from this test.

In RVM, winding is charged for known time and then

shorted to ground for pre-decided time. The recovery

voltage is then measured and dominant time constant

is achieved which is essentially a polarisation time

constant.

Charging current is given as the sum of displacement

current, the polarisation current and the conduction

current. Polarising current is dependent on materialproperty and state of ageing. The polarisation of

dielectric can be expressed as sum of various slowpolarisation phenomena like ion migration, slow

relaxation and interfacial polarisation. Care must be

taken in the interpretation of results of RVM, in

particular the relative effects of moisture, genuine

ageing and temperature [3].

2.6 Condition Monitoring Model [1]

Insulation DP of Failure Expert

Ageing Insulation Rate Assessment

Products Paper DataIn Oil Sample RVM

INSULATION AGE FAILURE PROBABILITY

Remaining Reliable Life

Of Transformer

Data Base Of

Test Results

Statistical Analysis

RELIABILITY ANALYSIS


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3. REFURBISHMENT DATA

Five 66/11kV, 10-30MVA transformers of 24 --33

years old transformers were refurbished. Some of the

important parameters are given in table 1, 2, 3 & 4.

Table 1

PI % Moisture

(from RVM)

% Mechanical

Strength (from

DP)

1.09 2.52 3555

1.79 2.10 3555

1.80 1.74 3555

2.34 1.56 3555

2.40 1.42 2535

Table 2

% Moisture

estimated from

Oil PPM

(Norris

Diagram)

% Moisture

estimated from

winding DLA

(Cigre)

% Moisture

estimated from

RVM

2 2 1.74

4 3 1.53

2.5 2 2.52

3.2 2.8 1.42

1.5 < 0.5 2.10

Table 3

% Moisture

(from RVM) DP

% Mechanical

Strength (from

DP)

2.09 250350 3555

1.53 250350 3555

1.21 200250 2535

2.38 250350 3555

2.18 250350 3555

Norris diagram gives estimated percentage of moisture

in paper based upon moisture content in oil (PPM) and

oil temperature.

Cigre paper has established a method to estimate

moisture in paper based upon winding tan (DLA)and temperature.

Table 4

2-Furfural

(PPM) DP

% Mechanical

Strength (from

DP)

1.10 200250 2535

0.49 250350 35550.39 200250 2535

0.54 250350 3555

2-Furfurylalcohol, 2-Acetylfuran, 5-Methyl-2-Furfuraland 5-Hydroxymethyl-2-Furfural were not detectedin all four cases.

4. DISCUSSION AND RESULTS It is observed from table 1 that RVM tends to

follow PI in this case. However, a conclusion

can not be drawn based on this limited

number of observations. Essentially RVM

does have DC absorption phenomenon and

may / may not follow IR / PI.

It is also observed that DP values (orpercentage retention of mechanical strength)

do not correlate with any of the electrical

parameters. DP reflects ageing and remaining

mechanical strength of paper but it is not a

diagnostic test as such.

From table 3, it is evident that moisture

percentage in the winding does not correlate

to DP or mechanical strength retention.

From table 2, it is evident that percentage ofmoisture estimated from Norris diagram(based upon oil PPM and temperature) and

percentage moisture estimated from Cigre

publication (based upon winding DLA) tend

to agree to some extent. However, these do

not correlate with moisture percentage

estimated from RVM. This is due to the fact

that RVM is a total reflection of ageing of

cellulose paper and moisture content in the

winding. There are no established methods

to separate these components.

It appears from table 4 that Furan analysis

may, in some cases, give indication about

paper ageing. However, most of the times,

Furans are not detected and there is no

universal correlation available for DP andFurans. Also, both ageing and moisture

content influence Furans.


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5. CONCLUSION

Mechanical integrity of paper is very important to

ensure reliability of transformers. However at this

stage this is not conclusively reflected in any of the

electrical diagnostic tests available. There is a need for

further research to validate RVM results and establish

more effective method of estimating moisture contentin the cellulose paper winding.

6.0 REFERENCES:

1. D.ALLAN, Condition Monitoring & Life

Assessment Of Aged Transmission / Sub-

Transmission Plant, Lecture notes of the short

course program on Insulation Condition

Monitoring And Reliability Centred Maintenance

Of Electrical Plant. February 1997, MonashUniversity.

2. CAJETAN PINTO, An Improved Method of

detecting Contamination of H.V. Stator Winding

in the Field", Proceedings of the 20th Electrical

Electronic Insulation Conference Boston, 1991,

pp. 55-59.

3. R.E.JAMES, Assessment Of Electrical

Insulation, Proceedings of the 1998 ResidentialSchool in Electrical Power Engineering,

Tasmania.

4. I.GUSSENBAUER, Examination of Humidity

Distribution in Transformer Models by Means of

Dielectric Measurements, Cigre paper 15-02,

International Conference on Large High Voltage

Electric System, 1980.

5. THOMAS LEIBFRIED, On-line Monitors Keep

Transformers in Service, IEEE Computer

Application in Power, July 1998, pp 36-42.

6. SYED MOFIZUL ISLAM, Power Transformer

Condition Monitoring Pacific Power

International and University of NewcastleDevelopments, Lecture notes of the short course

program on Insulation Condition Monitoring And

Reliability Centred Maintenance Of Electrical

Plant. February 1997, Monash University.

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uncertainty tranformer using konvensional testing methode.pdf