Frequency Response Analysis using the Impulse Frequency ... · PDF filemethods use very different measurement techniques to calculate the FRA test ... with the Impulse Frequency Response

Copyright Starlogic 2004 IEEE FRA Specification Starlogic IFRA Submission version 1.0

October 2003

Frequency Response Analysis using the Impulse Frequency Response Analysis

(IFRA) Method

Starlogic Instrument Development

South Africa

COPYRIGHT OF THIS MANUAL IS RESERVED. NO PUBLICATION OR DISSEMINATION OF ITS CONTENTS IS ALLOWED WITHOUT WRITTEN PERMISSION FROM AUTHOR.


October 2003

Winding Frequency Response Analysis using the Impulse Frequency Response Analysis (IFRA) Method Richard Breytenbach Starlogic October 2003 COPYRIGHT OF THIS PAPER IS RESERVED. NO PUBLICATION OR DISSEMINATION OF ITS CONTENTS IS ALLOWED WITHOUT WRITTEN PERMISSION FROM AUTHOR. 1 Scope ......................................................................................................................... 2 2 Introduction................................................................................................................ 2 3 Causes of Transformer Deformation ......................................................................... 2 4 Transformer Winding Modelling............................................................................... 3 5 Frequency Response Analysis (FRA)........................................................................ 4

5.1 IFRA Test Method .................................................................................................................6 5.2 SFRA Test Method ................................................................................................................7

6 FRAMIT Instrument .................................................................................................. 7 7 FRA Test Process .................................................................................................... 11

7.1 Performing the FRA test. .....................................................................................................11 7.2 Generating the FRA test results. ..........................................................................................12

8 FRA Test Equipment ............................................................................................... 14 8.1 IFRA.....................................................................................................................................14 8.2 Test data ...............................................................................................................................14

9 Application of FRA test technology ........................................................................ 15 10 Interpretation of FRA Test Results.......................................................................... 16

10.1 FRA example of shorted windings / partial discharge ........................................................16 10.2 FRA example of shorted & burnt windings ........................................................................19 10.3 FRA example of localized DISTORTED WINDINGS.......................................................20 10.4 FRA example of general DISTORTED WINDINGs ..........................................................21 10.5 Laboratory Experiments: Arizona State University. ............................................................21 10.6 Laboratory Experiments: ESKOM TRI – South Africa’s National Electricity

Supply Utility. ......................................................................................................................23 10.7 Laboratory Experiments: Institute of Energy Technology from the Aalborg

University in Denmark .........................................................................................................23 10.8 Substation Experiments: CLECO – Central Louisiana Electric Company..........................23 10.9 Substation Experiments: ESKOM TRI – South Africa’s National Electricity

Supply Utility .......................................................................................................................24 10.10 Substation Experiments: NGC – National Grid Company plc – UK...................................25 10.11 Substation Experiments: EPRI – Electric Power Research Institute –USA ........................26

11 Summary of Experiments and Field testing............................................................. 26 12 Conclusion ............................................................................................................... 27 13 References................................................................................................................ 28


October 2003

Page 2 of 24

1 SCOPE

This paper covers the background to Frequency Response Analysis (FRA) testing, and details the requirements and specifications needed to perform an FRA test and generate a practical FRA guideline. At present, FRA testing can be done using 3 different testing methods, namely; IFRA – Impulse Frequency Response Analysis, SFRA – Sweep Frequency Response Analysis, PRBS – Pseudo Random Binary Sequence (still a very new technique) It should be noted that although these 3 methods generate the same FRA test results, all 3 methods use very different measurement techniques to calculate the FRA test result. This means that if a general FRA guideline and specification is to be generated, it should be such that it encompasses all recognized techniques and remains open ended to allow for future FRA research and development. This paper deals specifically with the Impulse Frequency Response Analysis (IFRA) method.

2 INTRODUCTION

Power transformers constitute the core of electrical power transmission and distribution networks. They are also the most expensive equipment within the substation. Their performance will determine, to a large extent, the quality of power supply. It is therefore necessary to continuously monitor and assess the condition of transformers in order to ensure reliability and availability of power supply. Frequency Response Analysis (FRA) is becoming an increasingly popular technique used to externally monitor and assess the condition and mechanical integrity of transformer windings. This FRA technique calculates and computes frequency-dependent variables of the transformer’s windings, i.e. inductance and capacitance. It is these distributed winding parameters that will change when the windings are; short-circuited, open-circuited, deformed, or loose. The FRA test is performed by injecting a low voltage impulse waveform into one end of a transformers winding and measuring the voltage appearing at the other end of the same winding. The ratio of the transfer function of the input voltage to the output voltage is then plotted on a frequency domain graph. This paper demonstrates that FRA can be used and applied as a reliable transformer condition-monitoring tool to help assess winding condition without the need to open the transformer for inspection. The FRA technique can help maintenance personnel identify suspect transformers, enabling them to take those transformers out of service before failure.

3 CAUSES OF TRANSFORMER DEFORMATION

Power transformers are usually very reliable, but when faults occur, the transformer can be affected catastrophically. Transformers fail in service each year. Most of these failures are caused


October 2003

Page 3 of 24

by transformer winding faults and through faults generated by lightning and switching surges. As a transformer experiences a fault, it may suffer mechanical shock that gradually displaces and distorts the windings. In the process of winding movement, the insulation between the turns can be abraded, causing a short circuit and damage to the windings. Mechanical vibrations, initiated by short circuit forces, may cause the windings to loose their clamping pressure, eventually leading to collapse of the windings. The other cause of winding movement may be extensive vibration during transformer transportation. As the windings experience vibration, they may slacken and subsequently become unable to withstand mechanical forces exerted during faults. Ageing also contributes to winding looseness. In addition, harmonics generated under normal operating conditions may cause winding and core vibration. Short circuit faults are potentially very destructive because if the clamping pressure is not capable of restraining the forces involved, substantial permanent winding deformation or even collapse can occur almost instantaneously, often accompanied by shorted turns. A common cause of failure is a close-up phase to earth fault resulting from a lightning strike. It is expected that a transformer will experience and survive a number of short circuits during its service life, but sooner or later one such event will cause slight winding movement, and the ability of the transformer to survive short circuits in future will then be severely reduced. As the transformer ages, its components deteriorate and the likelihood of a failure increases.

4 TRANSFORMER WINDING MODELLING

For a clearer understanding of what actually happens when a transformer’s winding structure undergoes deformation, we need to analyze the equivalent circuit model of a transformer’s windings as shown in the figure below. Specifically, we are examining the effects of the small parasitic capacitance and inductance of the windings of the transformer.

Transformer Winding ModelingTransformer Winding Modelingshowing parasitic inductance, capacitance and resistanceshowing parasitic inductance, capacitance and resistance

CrossCross--section of a transformersection of a transformer

CC

CCCC

RR

LL

LL

Tank WallTank Wall

WindingWindingRR

CCCC

CC

WindingWinding

CoreCore

Cross-section of a transformer showing equivalent circuit model


October 2003

Page 4 of 24

Malewski, Gockenbach, Maier, Fellmann & Claudi1 explain that at the first approximation a transformer winding can be represented by a ladder network with series inductance and capacitance as well as the parallel capacitance to ground. The transfer function of such a network calculated by FRA shows a number of poles at the resonance frequency of the local L and C circuits. A breakdown between the turns or coils of the transformer under test corresponds to a short circuit of one or more of these local LC networks. This will result in shifting the resonant pole to another frequency or the creation of a new pole. A partial discharge condition with the transformer will not significantly affect the resonant frequency of the affected pole, but the pole height will be reduced. Consequently, a change of the resonant pole frequency indicates a breakdown in the winding insulation, whereas a reduction in the pole height reveals partial discharge activity. Tlhatlhetji2 argues that the detection of a transformer winding condition that may lead to a fault and subsequent outage, and being able to take corrective action prior to failure (e.g. reclamping or insulation repair), could save a transformer rewind, estimated to cost in the region of $1-million for a large power transformer. The failure of a strategic transformer on the power supply network can be even more costly, considering consequential factors such as system destabilization, load shedding, outages and even complete system shutdown. These factors have led to the search for alternative methods of transformer winding condition monitoring and assessment such as Frequency Response Analysis (FRA). FRA is a sensitive technique that can be used non-intrusively to accurately predict the condition of transformer windings.

5 FREQUENCY RESPONSE ANALYSIS (FRA)

FRA technology was developed as a diagnostic tool for application to power transformers. The results of research showed that the FRA method of testing was reliable and repeatable for the detection of winding deformation. There are currently two popular techniques for performing Frequency Response Analysis tests. The first is the voltage impulse method (IFRA), and the second is the sweep frequency method (SFRA). The Department of Technology Research and Investigation (TRI) from South Africa’s national electricity supply utility – ESKOM undertook a research project to compare the IFRA frequency response analysis technique against the swept frequency method. Comparison tests were performed by Tlhatlhetji3 on a 20 MVA, 66/11 kV transformer. With both instrument test leads connected correctly to the transformer, the two sets of test results showed very close correlation. It was also evident from the results that the resonance’s lie at the same frequencies. Further comparison tests were performed on a small 16 kVA, 11kV/400V, single-phase transformer at the University of Stellenbosch in South Africa. These test results also

1 Malewski, Gockenbach, Maier, Fellmann & Claudi (1992, p1) 2 Tlhatlhetji (1999, p4) 3 Tlhatlhetji (1999, p29)


October 2003

Page 5 of 24

showed very close correlation. The same report also detailed comparative tests using IFRA, Pseudo-Random Binary Sequence, and frequency sweep. It was conclusively proven that frequency response measurement using these three techniques were almost identical. Below are some comparison FRA tests between IFRA and SFRA test techniques, performed on another transformer;

IFRA Test Result: Transformer SN: T2300

SFRA Test Results: Transformer SN: T2300


October 2003

Page 6 of 24

IFRA Test Results: Transformer SN: T2300

SFRA Test Results: Transformer SN: T2300

The following observations and comparisons can be made between the 2 different FRA testing techniques.

5.1 IFRA TEST METHOD

Description: The test equipment consists of a portable self-contained instrument. The 50Ω coaxial test leads are connected across a specific phase winding of the transformer. The IFRA equipment injects a voltage impulse of about 450 volts into this phase winding. A high-speed digital data capture unit samples the injected voltage signal, as well as the voltage signal appearing at the other end of the winding. These two recorded time domain signals are processed and the FRA trace is displayed. Because of the nature of the IFRA Impulse technique, the test technique is very quick, taking only a few minutes to generate the FRA traces. This makes this technique ideal for field application where there may be volatile conditions such as bad weather, intermittent high frequency noise pollution, and little time to perform the test due to traveling time.


October 2003

Page 7 of 24

5.2 SFRA TEST METHOD

Description: The test equipment consists of a portable self-contained instrument. The 50Ω coaxial test leads are connected across a specific phase winding of the transformer. The SFRA equipment injects a sinusoidal waveform of constant magnitude of 10 Vrms into the phase winding. The equipment measures the injected voltage signal, as well as the voltage signal appearing at the other end of the winding. Because of the nature of the SFRA technique, the test technique is very slow, taking about 21/2 hours to generate the FRA traces. This makes this technique more suited to laboratory application where there is a more controlled environment.

6 FRAMIT INSTRUMENT

South Africa’s national supply utility, ESKOM, though its research into FRA initiated the development of a specialized FRA measurement tool. Starlogic was commissioned by ESKOM to develop a complete, self-contained instrument that performs Frequency Response Analysis of power transformers. The Starlogic instrument called FRAMIT is the result of accumulated knowledge from the research and application of this test technology over the past 10 years. FRAMIT has been internationally marketed with great success for over the past 7 years. The instrument is housed in a rugged carry case in which the measurement electronics, connecting leads and manual are fitted to create a complete, portable instrument as shown in figure below. The software runs under Microsoft Windows 95/98/2000/ME/XP & NT and therefore inherits all of features of this operating system.

FRAMIT Instrument

UNITED STATES AEP – CANTON, OH AEP – CENTRAL POWER & LIGHT – CORPUS CHRISTI, TX ATLANTIC COAST TESTING – RALEIGH, NC BALTIMORE GAS & ELECTRIC – BALTIMORE, MD CENTER POINT ENERGY – HOUSTON, TX (6) CITY PUBLIC SERVICE – SAN ANTONIO, TX (2) DELTA STAR – LYNCHBURG, VA DENTON MUNICIPAL ELECTRIC – DENTON, TX EL PASO ELECTRIC – EL PASO, TX ENTERGY CORPORATION – NEW ORLEANS, LA (2) GARLAND POWER & LIGHT – GARLAND, TX KUHLMAN ELECTRIC CORP – CRYSTAL SPRINGS, MS LOUISIANA GENERATING,LLC – NEW ROADS, LA LOUISVILLE GAS & ELECTRIC – LOUISVILLE, KY MIRANT MID ATLANTIC - NEWBURG, MD NIPSCO – MERRILLVILLE, INDIANA OKLAHOMA GAS & ELECTRIC –OKLAHOMA CITY, OK ORLANDO UTILITIES COMMISSION – ORLANDO, FL PEPCO – WASHINGTON, D.C. PG&E – SAN FRANCISCO, CA (2) PORTLAND GENERAL ELECTRIC – PORTLAND, OR PSI – CINERGY – PLAINFIELD, IN (3) SALT RIVER PROJECT - PHOENIX, ARIZONA SOUTHERN CALIFORNIA EDISON – ROSEMEAD, CA (2) TXU ELECTRIC –ONCOR - FORT WORTH, TX (12) WESTERN FARMERS ELECTRIC COOPERATIVE – ANADARKO, OK (2) WAUKESHA ELECTRIC SYSTEMS – WAUKESHA, WI (3) SOUTH AFRICA: ESKOM—SOUTH AFRICAN ELECTRICITY SUPPLY AUTHORITY (12) TECHNOLOGY SERVICES INTERNATIONAL SPOORNET CHINA: DEXIN (8) MEXICO: CFE - COMISION FEDERAL DE ELECTRICIDAD (2) JAPAN: MITSUBISHI TRANSFORMER – JAPAN AUSTRIA: VA TECH ELIN - WEIZ, AUSTRIA VA TECH EBG – LINZ, AUSTRIA GERMANY: SGB STARTSTROM GERATEBAU GMBBH

FRAMIT INTERNATIONAL USERS LIST Date: April 2004

© 2004 Starlogic cc

Operating Conditions Operating main: 85 - 260 Vac (60 - 50Hz) Max Voltage Withstand: 275 Volts Impulse Voltage Withstand: 1 kV, 1.2/50µs Protection: Electrostatic Discharge Immune Operating temperature: 14°F to 140°F (-10°C to 60°C) Max relative humidity: 10% - 90% Weight: 28.67 lbs (13 Kg) Enclosure: Weatherproof Pelican carrycase Ensclosure Dimensions: 14.25” x 15.75” x 6.69” (362mm x 400mm x 170mm) Measurement System Calibration: Self Calibration Amplitude Range: 0 db to –90 db Measurement frequency band: 150 Hz - 1 MHz, 10% of active frequency Measurement accuracy: 0.1% of full scale Maximum sampling rate: 10 MHz Resolution: 12 bits Analog input ranges: (+-V) 0.1, 0.2, 0.5, 1.2, 5, 10, 20 Capture memory: 2 x 12 bits, 128K deep Measurement differentiation: 0.01dB Common-mode rejection ability: >50dB Disturbance rejection between channels: >120dB Measurement range: +/- 600VDC Output Impulse: 500V, 40µS Charge Time: 2 seconds Output Impedance: 50 Ohms Impulse repeatability: 50ppm Impulse stability: 100ppm

TECHNICAL SPECIFICATION


FRAMIT3

Electrostatic and Electromagnetic Protection Specification

FRAMIT Instrument Housing: The FRAMIT Instrument electronics module is enclosed in a mild steel housing, which is earthed. This pre-vents the unit from being electrostatically damaged by general handling. The housing is therefore electro-static discharge immune.

Power Supply: Electrostatic discharge immune. The power supply input 110/220V is protected against over voltage and volt-age spikes by a MOV (Metal Oxide Varistor) rated at 275V.

Impulse Generator: Electrostatic discharge immune. The FRAMIT Instrument uses an injected impulse signal in the order of 500V. This large voltage dramatically increases the signal to noise ratio, and because the impulse signal is fed from a low impedance source of 50 ohms, the effect of other interference signals on this impulse signal is virtually removed.

Input Channels: Electrostatic discharge immune. The 2 input channels are very low impedance (50 ohms). This will suppress any interference signal that may be present during the test. The FRAMIT Instrument works on the principal of monitoring both Input Channels during a test. Any interference signal whether this be EMC (Electromagnetic) or EI (Electrostatic) would be common to both measuring channels. This would be common mode noise and removed during the test.

Communication Port: Electrostatic discharge immune. The FRAMIT Instrument’s communication port uses opto-coupled devices (electrical isolation devices). This allows the parallel port cable to be inserted and removed while the instru-ment and laptop are powered up, without causing any damage to either unit. The Parallel port cable is also shielded against electrostatic and electromagnetic interference.

Test Leads: Electrostatic discharge immune. The test leads are 50 ohm co-axial cables with co-axial shielding wire. The shielding runs from the BNC Connectors all the way to the cable junction box. The shielding is earthed along with the Instrument Casing, and this protects the test leads from induced electromagnetic signals.

TECHNICAL SPECIFICATION ADDENDUM


FRAMIT3


October 2003

Page 11 of 24

7 FRA TEST PROCESS

7.1 PERFORMING THE FRA TEST.

IFRA Test Method

Test preparation Peforming a FRA test must be done in a safe and controlled manner. The transformer under test must be removed and isolated from the live system and securely grounded. The local safety regulations and guidelines must be adhered to. The transformer must be tested in as basic a state as possible. This means that any external circuitry such as bushing connections etc etc must be removed where physical practical. Transformer under test Any size transformer can be tested using IFRA. The transformer can be tested in any state, i.e. with or without oil, with or without bushings, and on any tap setting. It is however important to realize that any change to any of these factors will change the equivalent circuitry of the transformer under test, and will therefore affect the FRA test results in some way. The transformer must be tested with the tap setting set to that tap which exposes all the windings to the test process. Any unused bushings must be left disconnected (floating). Test set The FRA equipment should be grounded and the appropriate calibration performed before commencing with a test. Test leads The test leads must consist of a 3-cable system of the same length. I.e. one for injecting the input signal into the transformer winding, one to receive back this input signal from the winding, and


October 2003

Page 12 of 24

one to receive back the signal from the other side of the winding. The ground test lead should be applied firmly to the metal surface of the transformer under test. This will ensure good grounding of the test signal. The test leads must be visibly inspected to ensure that there is continuity and shielding down the length of lead. Test procedure The test-leads are connected across a specific phase winding of the transformer, as shown above. The FRA Equipment injects a voltage impulse of about 450 volts into this phase winding. A high-speed digital data capture unit samples the injected voltage signal, as well as the voltage signal appearing at the other end of the phase winding. These two recorded time domain signals are processed and the FRA trace is displayed.

7.2 GENERATING THE FRA TEST RESULTS.

The FRA equipment takes these two time domain signals and calculates the frequency spectrum. It does this by applying a Fast Fourier Transform Algorithm (FFT) to the time domain signals. This yields the frequency spectrum. The frequency response is then determined by dividing the frequency spectrum of the output signal by the frequency spectrum of the input signal. This is shown in the figure below.

FRA MathematicsFRA Mathematics

300

INPUTINPUT

40

0

Time (uS)

Volts

200

x(t)

X(f)X(f)

FFTFFT

OUTPUTOUTPUT

y(t)y(t) Y(f)Y(f)FFTFFT

300

150

Volts

Time (uS)0 200

FREQUENCY RESPONCEFREQUENCY RESPONSEFINGERPRINT

H(f) = Y(f) / X(f) H(f) = Y(f) / X(f)

0.6

0.3

0 200

Frequency (Hz)Frequency (Hz)

MagMagH(f)

X(f)X(f)

IFRA Mathematics using Fast Fourier Transformer (FFT’s)

This test is repeated for each phase winding of the transformer (6 tests for a 3-phase transformer). This allows each winding of the transformer to be independently inspected and evaluated. The frequency response of all the tested phases together is called the ‘fingerprint of the transformer. This fingerprint is unique to every transformer, and remains unchanged for as long as the transformer’s winding structure remains unchanged. The FRA theory is as follows: The IFRA input data capture channel is connected directly to the IFRA on-board impulse generator. The input data capture channel will therefore record the signal delivered by the impulse generator. This voltage signal delivered by the impulse generator is injected into the selected phase winding of the transformer. The injected voltage creates a current to flow in the windings. The flow of current sets up a magnetic field. Once the impulse generator


October 2003

Page 13 of 24

has discharged, this magnetic field starts to decay. This induces an EMF voltage in the same direction to attempt keep the current flowing (Lenz's Law). The only path for the current to now flow is into the auxiliary capacitances of the windings themselves. This current charges these capacitances up until the potential on the capacitances is equal to the EMF voltage of the collapsing field. When no more current can flow then the capacitances begin to discharge back into the windings, setting resonance. The resonant frequency is 1/2.pi.f.c. Resonance decays as the energy is dissipated into the resistance of the windings. In essence this is what is measured by the IFRA and is defined as the "fingerprint" of the winding. Any physical change to the winding structure will cause the winding's inductive and capacitive properties to change, and these can be detected by Frequency Response Analysis. If there are any external windings near the winding being tested they will form part of the resonant circuit. Any built-in CT's etc. etc. will remain as part of the transformer setup allowing future IFRA tests to remain consistent.

Typical IFRA test results showing Input and Output voltages and calculated

Frequency Response


October 2003

Page 14 of 24

8 FRA TEST EQUIPMENT

FRA testing can be done using 3 different testing methods, namely; IFRA – Impulse Frequency Response Analysis, SFRA – Sweep Frequency Response Analysis, PRBS – Pseudo Random Binary Sequence (still a very new technique) It should be noted that although these 3 methods generate the same FRA test results, all 3 methods use very different measurement techniques to calculate the FRA test result. This means that if a general FRA guideline and specification is to be generated, it should be such that it encompasses all recognized techniques and remains open ended to allow for future FRA research and development.

8.1 IFRA

IFRA has been extensively used within the USA and Europe for over 9 years. Constant feedback of FRA test results from customers has allowed developers of IFRA to develop and refine new and more sophisticated ways to interpret the test results. Test data and research obtained over the last 3 years, has shown that the frequency range of 0 – 500 kHz contains the essential indicators which identify the various modes of transformer failure. The FRA test result should be displayed on a linear frequency range displayed from 0 – 500 kHz. The linear frequency range is the more accepted way of displaying test results since it does not excessively emphasize the low frequency range while compressing the high frequency range, as is done by the logarithmic method. The linear frequency range allows one to clearly investigate the FRA test result at different frequency ranges while maintaining an easily understandable linear scale. The logarithmic scale does not allow one to easily recognize a particular waveform shape. This becomes increasingly complex if one zooms into part of a graph. The auto-logarithmic scaling will take the part of the graph under investigation and re-scale it under the logarithmic scale. This would make the shape of the area under investigation unrecognisable to the user.

8.2 TEST DATA

The test data should be recorded to include all of the transformer nameplate details, pertinent test information details, raw data before processing, and the calculated FRA test result. These should all be stored within a local database from where the user can easily access them. The test database must be such that the user can search and sort the transformer records into meaningful groups such as;

• Sort database according to transformer serial number,

• Sort database according to transformer manufacturer,

• Sort database according to transformer MVA,


October 2003

Page 15 of 24

• Sort database according to transformer high side/low side voltage,

• Sort database according to transformer location,

• Sort database according to transformer winding organization,

9 APPLICATION OF FRA TEST TECHNOLOGY

The international use of FRA test technology is growing steadily. Many companies in the USA and internationally are successfully implementing IFRA testing. These companies have authorized that IFRA tests be performed;

• on all their new transformers before they are purchased from the manufacturer,

• after they are transported to site,

• once they have been commissioned,

• with future routine tests being performed on a regular basis. This decision is enhancing their transformer preventative maintenance programs. Before a transformer is loaded onto a truck for delivery to the customer, a FRA test is performed at the manufacturers premises. Once the transformer has been delivered, it is tested again. If the two fingerprints match well, it means that there has been no movement of the winding structure during transport and loading. If a transformer suffers a high through fault condition, it can be tested to determine the extent of the distortion of the winding structure. The test results will indicate what further action (if any) needs to be taken. This can save a lot of time, as well as transformers. Transformers can also be tested periodically, to determine the cumulative effects of high through current faults. As has been discussed earlier, IFRA and SFRA produce the same test results. However, due to the nature of the IFRA Impulse technique, the test technique is very quick, taking only a few minutes to generate the FRA traces. This makes this technique ideal for field application where there may be volatile conditions such as bad weather, intermittent high frequency noise pollution, and little time to perform the test due to traveling time. Because of the nature of the SFRA technique, the test technique is very slow, taking about 21/2 hours to generate the FRA traces. This makes this technique more suited to laboratory application where there is a more controlled environment.


October 2003

Page 16 of 24

10 INTERPRETATION OF FRA TEST RESULTS

Over the years significant research has been conducted by companies such as Eskom, Starlogic, TRI, EPRI, Doble and NGC into the correlation between the mechanical winding deformation of transformers and FRA test results. This research will be examined in greater depth.

10.1 FRA EXAMPLE OF SHORTED WINDINGS / PARTIAL DISCHARGE

The transformer was transported to the repair facility in, Ohio and de-tanked. It was observed that water had gotten into the transformer through a failed gasket on the pressure relief device. This is located right above the phase C coils. These coils were covered with a sludge that resulted from the water mixing with the oil. After cleaning off of sludge, a second IFRA test was performed (with the transformer outside of the tank).

Sludge covering windings

Sludge at the bottom of the tank


October 2003

Page 17 of 24

Transformer: SN: 3081107

HIGH SIDE - BEFORE CLEANING OFF SLUDGE ON H2 WINDINGS (Yellow Trace)

HIGH SIDE - AFTER CLEANING OFF SLUDGE ON H2 WINDINGS (Yellow Trace)


October 2003

Page 18 of 24

Transformer: SN: 3081107

LOW SIDE - BEFORE CLEANING OFF SLUDGE ON X3 WINDINGS (Blue Trace)

LOW SIDE - AFTER CLEANING OFF SLUDGE ON X3 WINDINGS (Blue Trace)


October 2003

Page 19 of 24

10.2 FRA example of shorted & burnt windings

The transformer was experiencing a gassing problem, which tripped the protective relay. The transformer was de-tanked and inspected, and it was observed that the H3 tap lead was damaged. The tape around the lead was removed revealing large amounts of burning. The damage also appeared to extend up and into the H3 winding structure.

Transformer: SN: d-596272 HIGH SIDE - DAMAGE TO H3 TAP WINDING & H3 WINDING STRUCTURE (Blue Trace)

LOW SIDE


October 2003

Page 20 of 24

10.3 FRA example of localized DISTORTED WINDINGS

The oil sample for DGA analysis was sampled on date 8/30/2002 was taken as the results of a gas detector relay alarm. Being as it was Friday before a three day weekend, we de-energized the transformer. The following Tuesday the sample date 9/03/2002 was taken. There was an increased gas level, even though the transformer was only energized for an additional three (3) hours! The Doble tests showed the % power factor of the low voltage windings doubled from .39% to .75%. The transformer turns ratio and the bridge tests showed no changes. The core grounds tested good (> 100 Mohm) and an internal inspection did NOT reveal any problems. The other data available was the occurrence of 39 through faults in the previous month from the connected feeders. Tranformer loading was well below nameplate. The transformer was ‘torn down’ at the factory and visible inspected. There was winding distortion (petal failure partial axial collapse) of the X3 winding structure. Please see attached picture.

Transformer: SN: MLL 9315-3 LOW SIDE - LOCALIZED WINDING DISTORTION TO H3 WINDING STRUCTURE (Blue Trace)


October 2003

Page 21 of 24

10.4 FRA example of general DISTORTED WINDINGs

1975 Year old transformer with low clamping pressure with general winding distortion. Transformer: SN: 10552

HIGH SIDE

LOW SIDE - GENERAL WINDING DISTORTION TO X3 WINDING STRUCTURE (Blue Trace)

10.5 LABORATORY EXPERIMENTS: ARIZONA STATE UNIVERSITY.

The department of Electrical Engineering from the Arizona State University initiated a research project to investigate and evaluate the IFRA performance in transformer fault detection. A small laboratory 1kVA 800/250V transformer was repeatedly tested using a variety of different testing equipment. The small transformer was subjected to simulated electrical mode faults and was routinely tested after each different type of fault. The modes of failure that were simulated included within the laboratory were:

• Turn-to-turn short circuit


October 2003

Page 22 of 24

• Layer-layer shorts

• Coil-coil shorts

• Open-circuited windings

• Partial discharges etc. SENSITIVITY AND REPEATABILITY ANALYSIS. The research report conducted by the Department of Electrical Engineering from the Arizona State University by Dr. Karady, Dr. Reta & Amarg4 aimed to quantify the relative change in resonant frequency location (Df) and relative change in amplitude (DA), with respect to the values for normal operation, as a result of the simulated modes of failure. In table 2, frequency peak location (kHz) refers to the frequency at which the maximum value of the transfer function is observed. Peak magnitude is the magnitude of the transfer function at the peak frequency and peak range is the percentage change in peak magnitude for different experiments but same fault type. Relative changes will help distinguish the various types of failure and also provide an indication for test repeatability. Dr. Karady, Dr. Reta & Amarg5 documented the results of the sensitivity analysis in table 2. The relative changes are computed as

fand 100*A ni

n

ni ffDA

AAD −=

−=

(1) & (2) Where An and fn are respectively, the magnitude and resonant frequency location for fingerprint, Ai and fi are the magnitude and resonant frequency location for all other simulated conditions.

Fault Type Frequency Peak Location, f (kHz)

Peak Magnitude,A (units) x 10 exp 3

Peak Range (%)

DA (%) Df (kHz)

Normal 525 52.0 7.7 0.0 0.0

Coil Lifted 525 92.0 0.0 76.9 0.0

2 coils shorted 520 21.0 16.0 -59.6 -5.0

5 coils shorted 540 25.0 16.0 -51.9 15.0

10 coils shorted 540 26.4 4.5 -49.2 15.0

20 coils shorted 540 24.0 0.0 -53.8 15.0

2 primary coilsShorted

600 57.0 3.5 9.6 75.0

Sensitivity Analysis for FRAMIT Instrument

Department of Electrical Engineering

from Arizona State University

DA=Ai – Ar * 100 and Df = fi - frAr

Sensitivity Analysis for IFRA equipment

It was concluded that the FRA method of transformer testing using the IFRA equipment is reliable and repeatable for the detection of simulated transformer winding movement.

4 Dr. Karady, Dr. Reta & Amarg (1999, pg 12) 5 Dr. Karady, Dr. Reta & Amarg (1999, pg 13)


October 2003

Page 23 of 24

10.6 LABORATORY EXPERIMENTS: ESKOM TRI – SOUTH AFRICA’S NATIONAL ELECTRICITY SUPPLY UTILITY.

The Department of Technology Research and Investigation (TRI) from South Africa’s national electricity supply utility – ESKOM conducted laboratory experiments using IFRA on a small 25kVA; 11kv/400v; three-phase pole-mounted transformer. Tests were performed with the transformer un-tanked. At lower frequencies the test results show that the transformer is characterized by the magnetizing inductance and non-linear behavior of the iron core. Peak resonances were seen to occur in a few places. Tlhatlhetji6 explains that the major differences in the responses occur at the lower frequencies, up to 160kHz. Multiple resonances occur for the first short-circuit (yellow trace) between 0 and 40 kHz. It can be seen that the responses under short-circuit conditions, not only differ from the reference fingerprint, but also from each other. Frequency responses of short circuit faults at the minimum and nominal voltage taps differ both in magnitude and phase up to 56 kHz. However, from 160 kHz upward all three-frequency responses are very similar. It appears then that the distributed capacitance of the winding may be dominating at higher frequencies more that inductance. It can be concluded, that short circuit faults are easily detected in the lower frequency range.

10.7 LABORATORY EXPERIMENTS: INSTITUTE OF ENERGY TECHNOLOGY FROM THE AALBORG UNIVERSITY IN DENMARK

The Institute of Energy Technology from the Aalborg University in Demark carried out FRA tests on a two small 30VA; 20kv/110V; single phase transformer. The transformer was manipulated in a variety of different ways to simulate fault conditions. FRA tests were performed on the transformer after each set simulations. The object was to establish the correlation between different simulated fault conditions and actual changes in the frequency response. Different conditions were applied to the insulation oil/grease, the core, and the winding/coil insulations. These included a series of simulated accelerated ageing tests. Many different failure conditions were successfully detected using FRA testing.

10.8 SUBSTATION EXPERIMENTS: CLECO – CENTRAL LOUISIANA ELECTRIC COMPANY

In early May of 1999, the Central Louisiana Electric Company, CLECO, were having one of their generator step-up transformers re-blocked. Mr. Sandy Crochet of CLECO requested that IFRA tests be performed on their transformer prior to it being re-blocked. A re-test was also scheduled about 2 weeks later, after the re-blocking work was complete. As was expected, this particular transformer showed significant FRA result changes when it was re-tested after its re-block. The FRA differences between the reference test and the test that was performed after the transformer was re-blocked. It was seen that the response of the re-bock test was completely different from that of the reference test. The deviation is clearly notable above 153 kHz. This shows clearly that every winding structure has a unique FRA fingerprint. A re-blocking of a

6 Tlhatlhetji (1998, p11)


October 2003

Page 24 of 24

transformer will generate a new set of fingerprints since the newly tensioned winding structure will have its own unique ladder network of series inductance and capacitance as well as parallel capacitance to ground. This new FRA test that was performed after the re-block will now become the reference test against which future routine tests will be compared.

10.9 SUBSTATION EXPERIMENTS: ESKOM TRI – SOUTH AFRICA’S NATIONAL ELECTRICITY SUPPLY UTILITY

The Department of Technology Research and Investigation (TRI) from South Africa’s national electricity supply utility – ESKOM performed IFRA tests on many of their large power transformers. One of the transformer had faulted in service. Tlhatlhetji7 documents that the fault was caused by a faulty tap-changer on the white phase. He goes on to say that IFRA tests were conducted with the transformer still inside the tank. There was no oil in the transformer and the bushings were removed. No reference FRA tests had been conducted on the transformer. The yellow phase was completely different from that of the red and blue phases. The deviation was clearly notable as from 160 kHz. Whereas the peak resonance occurs at 210 kHz for both the red and blue phases, the yellow phase is flat, which is similar to an open-circuit response. Although there is no reference fingerprint for this transformer, the response shown below for the red and blue phases compared very favorable with a similar “sister” transformer. This gives conclusive evidence that the yellow phase is faulty. The tap-changer was inspected and found to be defective of the yellow-phase.

Frequency response of a 220 MVA, 420/16.5 kV, generator step-up

transformer that faulted on the yellow tap-changer

7 Tlhatlhetji (1998, p15)


October 2003

Page 25 of 24

10.10 SUBSTATION EXPERIMENTS: NGC – NATIONAL GRID COMPANY PLC – UK

The Nation Grid Company plc from the United Kingdom have performed FRA tests on many large power transformers. An overview of their general FRA findings is included below: When examining FRA test results, Lapworth & McGrail8 commented that relatively simple responses are obtained at the lowest frequencies, irrespective of winding design. In this frequency range the inductance of the winding is influenced by the core. The different iron flux paths involved when outer and center phases of a three-phase transformer are excited explain why it is normal for the center phase to have a different response. Changes to the core limb remnant magnetizations can also affect these low frequency responses. An extreme case of core limb reluctance occurs when a shorted turn exists in one of the windings on that limb, and in such a case a very characteristic change in the low frequency response is obtained.

At higher frequencies, Lapworth & McGrail9 commented that eddy currents effectively screen the magnetic circuit so that the winding inductances are determined by local leakage fluxes. Being less dependent on magnetic characteristics, the winding responses of the 3 phases become much more similar and therefore more sensitive to winding movement. There is no typical form for the high frequency responses, which are characteristic of the winding design used and vary greatly from design to design. Interpretation of FRA test results is based upon a subjective comparison of equivalent responses. Measured responses are analyzed for:

Changes in the response of a winding Differences between the responses of the three phases of the same transformer, Differences between the responses of transformers of the same design.

The appearance of new features or major frequency shifts is cause for concern. When interpreting any differences observed between phases, Lapworth & McGrail10 mention that it should be borne in mind that for many windings there may be minor design differences concerning the disposition of internal connections between windings, bushings and tap-changers which could introduce small differences between phases in the frequency responses. Therefore, if minor differences between phases are observed and no reference results are available, it is not possible to make an unambiguous diagnosis of minor winding movement. Extensive testing by Lapworth & McGrail11 of NGC on a number of their transformers has resulted in the successful diagnostic of the following independent transformer conditions:

Collapse of end insulation structures Hoop buckling failure of inner windings Broken winding clamping structure / displacement of winding assemblies Tap-changer failure

8 Lapworth & McGrail (1999, p3) 9 Lapworth & McGrail (1999, p3) 10 Lapworth & McGrail (1999, p3) 11 Lapworth & McGrail (1999, p4-5)


October 2003

Page 26 of 24

NGC have concluded that the confidence in the interpretation of FRA results is continuing to grow as experience of detecting problems increases.

10.11 SUBSTATION EXPERIMENTS: EPRI – ELECTRIC POWER RESEARCH INSTITUTE –USA

The Electric Power Research Institute in the USA has performed FRA tests on many large power transformers. An overview of their general FRA findings is included below: From 1997 to 1999 EPRI tested 7 large power transformers of identical design. The rating of the transformers is 345 kV, 300 MVA, and 3 phase. They were built over 25 years ago with minor variations in internal design. Vandermaar, Wang, Stefanski & Ward12 remarked that FRA tests were performed on these seven transformers with the signatures compared to one of the transformers that had been reclamped relatively recently. Recommendations were made on which transformers to open. The transformer, which was ranked first for reclamping has now been opened and reclamped. When it was opened it was found to be loose.

11 SUMMARY OF EXPERIMENTS AND FIELD TESTING

International FRA research has proven that it is possible to detect a variety of different internal conditions. It can detect a change in the condition and quality of the transformer oil/grease, if this involves a change in the dielectric constant ε. It is possible to detect a change in the condition of the core, if this involves a change in the total reluctance or total iron losses of the core. It is possible to detect a change in the condition of the winding insulation and winding structure integrity, if this involves changes in the lumped/total winding capacitances of the transformer or short-circuit between turns. The international FRA research has shown that it is possible to detect the following internal mechanical deformation of transformers:

• Multiple core earths

• Loss of clamping pressure

• Shorted Turns (two) & Shorted Turns (Multiple)

• Magnetized Core

• Raised Winding & Loose Turns

• Core Movement & Collapse of end insulation structures

• Hoop buckling failure of inner windings

• Broken winding clamping structure / displacement of winding assemblies

• Tap-changer failure At lower frequencies the FRA test results (fingerprints) show that the transformer is characterized by the magnetizing inductance and non-linear behavior of the iron core. The distributed capacitance of the winding may be dominating at higher frequencies more that

12 Vandermaar, Wang, Stefanski & Ward (1999, p4)


October 2003

Page 27 of 24

inductance. It can be concluded, that short circuit faults are easily detected in the lower frequency range. At higher frequencies, eddy currents effectively screen the magnetic circuit so that the winding inductances are determined by local leakage fluxes. Being less dependent on magnetic characteristics, the winding fingerprints of the 3 phases become much more similar and therefore more sensitive to winding movement. Many years of testing experience with constant feedback of FRA test results from customers has allowed developers of IFRA to develop and refine new and more sophisticated ways to interpret the test results. Test data and research obtained over the last 3 years, has shown that the frequency range of 0 – 500 kHz contains the essential indicators which identify the various modes of transformer failure. Over this range one can determine the whole range of transformer failure modes including short circuits, localized winding distortion, and general winding distortion and loss of clamping pressure.

12 CONCLUSION

It is clear that the performance of power transformers will determine, to a large extent, the quality of power supply. It is therefore very important to continuously monitor and assess the condition of transformers to ensure reliability and availability of power supply. The detection of a transformer winding condition that might lead to a fault and subsequent outage, and being able to take corrective action (e.g. reclamping or insulation repair) prior to failure, can save a transformer rewind, estimated to cost in the region of $1-million for a large power transformer. The failure of a strategic transformer on the power supply network can be even more costly, considering consequential factors such as system destabilization, load shedding, outages and even complete system shutdown. Frequency Response Analysis (FRA) is becoming an increasingly popular technique used to externally monitor and assess the condition and mechanical integrity of transformer windings for short-circuits, open-circuits, deformation, winding insulation breakdown and lose of clamping pressure. The FRA technique can help maintenance personnel identify suspect transformers, enabling them to take those transformers out of service before failure. This paper has covered the background to Frequency Response Analysis (FRA) testing, and has detailed the requirements and specifications needed to perform an FRA test and generate a practical FRA guideline. FRA testing can be done using 3 different testing methods, namely; IFRA – Impulse Frequency Response Analysis, SFRA – Sweep Frequency Response Analysis, PRBS – Puesdo Random Binary Sequence (still a very new technique) It is important to note that although these 3 methods generate the same FRA test results, all 3 methods use very different measurement techniques to calculate the FRA test result. This means that if a general FRA guideline and specification is to be generated, it should be such that it


October 2003

Page 28 of 24

encompasses all recognized techniques and remains open ended to allow for future FRA research and development. IFRA has proven to be an extremely reliable FRA diagnostic tool for detecting transformer winding structure deformation. If a transformer is suspected of internal damage, the IFRA test results can assist in deciding whether or not to take the transformer out of service for further investigation. A test on a three-phase transformer can be useful in this instance even if no previous tests have been performed on the transformer. Since the winding structure is usually symmetrical from one phase to the next, and since it is unlikely that all three phases were affected similarly, the FRA test results (fingerprints) for each phase can be compared to one another, in order to check for significant differences. International FRA research has proven that it is possible to detect a variety of different internal conditions. As a preventative maintenance tool, IFRA is the ideal complement to the traditional tests of ratio, partial discharge and dissolved gas analysis. Combined with on-going international research into quantifying specific waveform patterns with corresponding winding faults the system is proving invaluable in providing base data on currently healthy transformers.

13 REFERENCES

1. Bak-Jensen J. B, Bak-Jensen B. B, Mikkelsen S. D, “Detection of faults and ageing phenomena in transformers by transfer functions”. IEEE Transactions on Power Delivery, Vol. 10, No. 1, (January 1995).

2. Dr. Karady G, Dr. Reta M and Mr. Amarg F.A, “SRP Transformer Fault Detection Project” – March 1999 3. Lapworth J.A, McGrail T.J – National Grid Company, “Transformer winding movement detection by frequency

response analysis (FRA)”, Paper No. 8O, presented at the Sixty-Sixth Annual International Conference of Doble Clients, Boston, Massachusetts, USA (April 1999)

4. Malewski R, Gockenbach E, Maier R, Fellmann K.H, Claudi A, “Five years of monitoring the impulse test of power transformers with digital recorders and the transfer function method” – September 1992

5. Minhas MSA, Reynders J.P, de Klerk P.J, “Failure in power system transformers and appropriate monitoring techniques” – October 1999

6. Noonan T. J - (ESB International), “Power Transformer condition assessment and renewal, frequency response analysis update”, Paper No. 8B, presented at the Sixty-Fourth Annual International Conference of Doble Clients, Boston, Massachusetts, USA (April 1997)

7. Tlhatlhetji N.P, “Frequency Response Analysis of transformers”, Report No. TRR/E/98/EL097 – October 1998 8. Tlhatlhetji N.P, “Frequency Response Analysis of transformers”, Report No. TRR/E/99/EL102 – October 1999 9. Vandermaar N.P and Wang M – Powertech labs inc. Canada, Stefanski P – Commonwealth Edison Company –

USA, Ward H – EPRI – USA, “Frequency response analysis using the impulse test method as a transformer diagnostic technique”, Paper No. 8C, presented at the Sixty-Sixth Annual International Conference of Doble Clients, Boston, Massachusetts, USA (April 1999)

Documents

Frequency Response Analysis using the Impulse Frequency ... · PDF filemethods use very different measurement techniques to calculate the FRA test ... with the Impulse Frequency Response