CPC-100-ANP-10003-ENU

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OMICRON Page 1 of 18

Application Note

Power Transformer Short-Circuit Impedance / LeakageReactance

AuthorsLutz Hulka |[email protected] Ptter |[email protected]

DateJul 22, 2010

Related OMICRON ProductCPC 100

Application AreaPT Diagnosis

Versionv1.0

Document IDANP_10003_ENU
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

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OMICRON 2010 Page 2 of 18

Contents

1 Using this Document .............................................................................................................................31.1 Operator Qualifications and Safety Standards ...............................................................................31.2 Safety Measures .............................................................................................................................31.3 Conventions and Symbols Used ....................................................................................................31.4 Related Documents ........................................................................................................................4

2 Safety Instructions for Connecting the CPC 100 to Power Transformers .......................................52.1 Before Starting ................................................................................................................................52.2 General ...........................................................................................................................................52.3 Recommended Output Settings .....................................................................................................5

3 Short-Circuit Impedance Measurements on Power Transformers ...................................................63.1 Why Short-Circuit Impedance Measurements? ..............................................................................63.2 Impedance Test at Rated Frequency .............................................................................................6

3.2.1 Comparison of Power Losses ........................................................................................................... 63.2.2 Comparison of Leakage Reactances ................................................................ ................................ 63.2.3 Deviation Limits................................................................................................................................. 7

3.3

Frequency Response of Stray Losses (FRSL) ...............................................................................7

4 Performing Measurements ...................................................................................................................94.1 Preparing the Transformer Under Test ...........................................................................................94.2 Performing the Three-Phase Equivalent Test ................................................................................9

4.2.1 Connecting the Three-Phase Equivalent Test ................................................. ............................... 104.2.2 Connecting the Measurement Setup to the CPC 100..................................................................... 104.2.3 Preparing the CPC 100 Template ............................................................ ....................................... 114.2.4 Starting the Test.............................................................................................................................. 12

4.3 Performing the Per-Phase Test ................................................................................................... 134.3.1 Connecting the Per-Phase Test ............................................................... ....................................... 134.3.2 Starting the Test.............................................................................................................................. 14

5 Interpretation of Measurement Results ............................................................................................ 155.1 Loading the Test Result into Excel .............................................................................................. 155.2 Test Results ................................................................................................................................. 165.2.1 FRSL / AC Resistance Sheet: Rk(f) ......................................................... ....................................... 17

5.2.2 Short-Circuit Inductance Sheet: Lk(f) ................................................................ .............................. 17

Please use this note only in combination with the related product manual which contains several important safetyinstructions. The user is responsible for every application that makes use of an OMICRON product.

OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON.

OMICRON 2010. All rights reserved. This application note is a publication of OMICRON.

All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, opticalcharacter recognition and/or storage in electronic data processing systems, requires the explicit consent of OMICRON.Reprinting, wholly or in part, is not permitted.

The product information, specifications, and technical data embodied in this application note represent the technicalstatus at the time of writing and are subject to change without prior notice.

We have done our best to ensure that the information given in this application note is useful, accurate and entirelyreliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present.OMICRON translates this application note from the source language English into a number of other languages. Anytranslation of this document is done for local requirements, and in the event of a dispute between the English and a non-English version, the English version of this note shall govern.

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1 Using this Document

Using the Power Transformer Short-Circuit Impedance Template, you can quickly measure theshort-circuit impedance and hence also the leakage inductance or leakage reactance of powertransformers. Graphical display provides a well-arranged overview of the measured values. Thisdocument guides you through the whole testing process.

Reading the AN CP0901 Application Note alone does not release you from the duty of complyingwith all national and international safety regulations relevant to working with the CPC 100. Theregulation EN 50191 "The Erection and Operation of Electrical Test Equipment" as well as all theapplicable regulations for accident prevention in the country and at the site of operation have to befulfilled.

1.1 Operator Qualifications and Safety Standards

Working on power transformers is extremely dangerous. The short-circuit impedance measurementsdescribed in this Application Note must be carried out only by qualified, skilled and authorizedpersonnel. Before starting to work, clearly establish the responsibilities. Personnel receiving training,instructions, directions, or education on the measurement setup must be under constant supervisionof an experienced operator while working with the equipment. The measurements must comply withthe relevant national and international safety standards listed below:

EN 50191 (VDE 0104) "Erection and Operation of Electrical Equipment"

EN 50110-1 (VDE 0105 Part 100) "Operation of Electrical Installations"

IEEE 510 "IEEE Recommended Practices for Safety in High-Voltage and High-Power Testing"

LAPG 1710.6 NASA "Electrical Safety"

Moreover, additional relevant laws and internal safety standards have to be followed.

1.2 Safety Measures

Before starting a measurement, read the safety rules in the CPC 100 User and Reference Manualcarefully and observe the application-specific safety instructions in this Application Note whenperforming measurements to protect yourself from high-voltage hazards.

1.3 Conventions and Symbols Used

In this document, the following symbols indicate paragraphs with special safety relevant meaning.

Symbol Description

Equipment damage or loss of data possible.

Personal injury or severe damage to objectspossible.

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1.4 Related Documents

The following documents complete the information covered in this Application Note:

Title Description

CPC 100User Manual Provides basic information on the CPC 100test system and relevant safety instructions.

CPC 100Reference Manual Provides detailed hardware and softwareinformation on the CPC 100 test systemincluding relevant safety instructions.

AN CP0711: Test Templates General information about working withtemplates for the CPC 100.

IEEE C57.12.90 Standard Test Code for Liquid-ImmersedPower Transformers.

IEEE 62 Guide for Diagnostic Field Testing of ElectricPower Apparatus

IEC 60076-1 International Standard for Power Transformers

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2 Safety Instructions for Connecting the CPC 100 to PowerTransformers

2.1 Before Starting

Warning: Make sure that both sides of the transformer are disconnected from themains supply.

Beware of nearby parallel systems which are not disconnected. They can inducehazardous loads in floating segments of a substation. Always touch the leads andterminals with a grounding hook first.

2.2 General

Always observe the five safety rules while working on high-voltage components:

Disconnect completely

Secure against re-connection

Verify that the installation is dead

Carry out grounding and short-circuiting

Provide protection against adjacent live parts

2.3 Recommended Output Settings

Warning: For the test current supply, use the 6 A / 130 V AC output in 3 A AC mode bydefault. However, in this mode the output voltage can rise up to 240 V. Consequently,be careful when you switch on the CPC 100.

The recommended test current is 2 A. If you feel the measured voltage is too low,higher current values can be set. Note that the measured impedance increases with thefrequency. If the current is too high, the output voltage exceeds the maximum outputvoltage, and the CPC 100 indicates an overload. However, the test is valid because theCPC 100 still measures the right impedances.

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3 Short-Circuit Impedance Measurements on Power Transformers

3.1 Why Short-Circuit Impedance Measurements?

There are typical problems inside a power transformer that can be detected with a short-circuitimpedance measurement, such as

winding movement caused by heavy-fault current,

mechanical damage during transportation,

broken fixation elements,

short-circuit between two or more parallel strands.

As the impedance values do not depend on the applied voltage, the measurement can be performedby using a low-power source.

3.2 Impedance Test at Rated Frequency

A three-phase transformer can be tested using a single-phase power source by applying the voltageto each two terminals of the not short-circuited windings. Three successive measurements have tobe conducted for all three phases at rated frequency.

Figure 1: Single-phase equivalent circuit of a transformer

With the test result the overall impedance (%Z) of the transformer can be calculated by taking theaverage of all three values, considering the vector group of the measured side. The results can becompared to the rated impedance of the transformer (three-phase equivalent test), or with formermeasurements. Additionally, the three phases can also be compared to each other (per-phase test).

3.2.1 Comparison of Power Losses

The factory test of a power transformer which determines the value on the nameplate is usuallycarried out by using three-phase excitation at 100% or 50% of the rated current. There are manydifferences to the on-site test setup that highly affect the measured ohmic losses (). Therefore, itis hard to compare factory losses with losses measured in the field.

3.2.2 Comparison of Leakage Reactances

The leakage reactance test method is based on the comparison with only the reactive part () ofthe impedances. Unlike the measured ohmic losses which can highly deviate from the factorylosses, the reactance can deliver a reliable statement on the transformer condition. However, inmost cases information about the leakage reactance (%X) is not given on the nameplate. You haveto draw upon former measurements, or you can compare the phases with each other. In somecases, %X can be identified with %Z when is very small in comparison to .

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3.2.3 Deviation Limits

According to IEEE Std. 62, there are two methods for performing impedance tests on three-phasepower transformers: the three-phase equivalent test, which compares the measured values with thenameplate, and the per-phase test, which compares the windings with each other. The three-phaseequivalent test should be within 3% of the nameplate value. The per-phase test should be within 3%of the average value of all phases.

Another interpretation possibility for the deviations is given by the following flow chart.

Figure 2: Interpreting the test results

3.3 Frequency Response of Stray Losses (FRSL)

With the CPC 100 the impedance measurement can be performed at several frequencies up to 400Hz. A frequency response analysis of the short-circuit impedance provides more information aboutthe condition of the windings.The short-circuit impedance of a transformer at different frequencies can be modeled with an

inductance , a DC resistance of the windings, and an additional resistance

connected in series.

The transformer windings are exposed to stray flux, which induces eddy currents in the windings.These additional losses can be modeled with an additional AC resistance, which is proportional tothe frequency.

Figure 3: Single-phase equivalent circuit at several frequencies

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Figure 4: Frequency sweep of the short-circuit measurement

Unlike the normal impedance test, the FRSL diagnosis is based on a comparison of the ACresistance curves. The comparison can be established with a similar transformer or with a previoustest performed on the same transformer. High deviations between the phases too can be anindication for faults in the winding, such as a short-circuit between two parallel strands.

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4 Performing Measurements

4.1 Preparing the Transformer Under Test

Warning: First, make sure that the transformer is not connected to the mains supply.Touch every single terminal once with a grounding hook.

Note: The tap changer has to be set on the right position in order to achieve a correctmeasurement.

During factory test, the current is normally applied to the high-voltage side of thetransformer by a three-phase power source, whereas the three line leads of the low-voltage windings are short-circuited. According to IEEE Std. 62, an on-site test can beperformed with a single-phase power source by feeding two of the three line leads at atime, and measuring the impedance. The test has to be repeated for every combination

of pairs of line leads.

It has to be considered that the winding resistance of the transformer can be very low. The contactbetween the leads and the terminals must have a very low resistance.

Figure 5: Overall resistance of a short-circuit connection between two terminals

4.2 Performing the Three-Phase Equivalent Test

The CPC 100 template provides two different tests, which can be performed independently fromeach other. While the three-phase equivalent test is the common test used for comparing the resultswith the nameplate values, the per-phase test provides more information about the condition of eachwinding.

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4.2.1 Connecting the Three-Phase Equivalent Test

To short-circuit the three terminals on the low-voltage side, three equal and massive leads have tobe used. Ideally, the leads are arranged radially from a common fixation. The fixation is notconnected to any terminal and this common point is not grounded. The neutral terminals, if any, arenot used.

Figure 6: Top view on a short-circuited power transformer

4.2.2 Connecting the Measurement Setup to the CPC 100

For the current supply, the 6 A / 130 V AC output is used. Connect the output with the I AC input asshown inFigure 7: Connecting the voltage inputs to the transformer terminals.Connect the currentleads with clamps to the high-voltage winding terminals. Connect the V1 AC input with the terminalsusing the four-wire measuring technique. Check the measuring inputs for correct polarity.

Figure 7: Connecting the voltage inputs to the transformer terminals

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4.2.3 Preparing the CPC 100 Template

First, you have to prepare the template "Transformer Short Circuit Impedance".

Load the template by clicking the File Operations button (1), choose the file "TR_Zk_50Hz.xmt" (2)for transformers with 50 Hz, or "TR_Zk_60Hz.xmt" for transformers with 60 Hz as rated frequency,

and click Open (3).

Figure 8: Opening the test template

The required blank test cards for the measurement will appear.

The transformer information has to be entered into the first test card TR Data. The information isneeded for the template to calculate the deviations from the nominal values.

Figure 9: Editing the transformer data

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Note: For a correct calculation of the impedances, the following data entries are mandatory:

S [kVA]: kVA rating in kVA

V HV [V]: voltage rating high-voltage side in V

V LV [V]: voltage rating low-voltage side in V

u_k [%]: (50 Hz template) / %Z [%]: (60 Hz template) impedance voltage / %Z in %

P_k [W]: power dissipation during short-circuit test with rated current. Often, this information isnot available.

Caution: The information can also be filled in afterwards but make sure to have all thedata about the transformer you need. You must not change the abbreviations or theExcel template will not work properly.

The other entries are optional but they will appear on the test report sheet of the Exceltemplate.

4.2.4 Starting the Test

Three successive measurements have to be performed for the test. Therefore, the template has

prepared three test cards Zk AB, Zk BC and Zk AC. For each card, a combination of two windingterminals have to be chosen (for example: H1 and H2, H2 and H3, H1 and H3). The neutralterminals of the transformer are normally not involved.

Figure 10: Test card for the first measurement

After having connected everything properly, the measurement will take place at the rated frequency

and at several other frequencies for FRSL diagnostics.

By starting the measurement with the first measuring test card Zk AB the CPC 100 will perform the

test at 1 A by default and at several frequencies automatically. The test will last 30 seconds.

After the test card has finished, the next two terminal combinations have to be connected, and thesecond measuring test card Zk BC has to be chosen in the CPC 100. The same procedure has tobe applied to Zk AC.

Caution: Do not forget to save your data once you have completed all measurementsand before you switch off the CPC 100. You can also skip this test and continue withthe per-phase test if you only want to perform the per-phase test.

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4.3 Performing the Per-Phase Test

4.3.1 Connecting the Per-Phase Test

The per-phase test can be useful in further examining the individual windings. The test is performedby excitating and measuring each individual winding on the high-voltage side, whereas the

corresponding winding on the low-voltage side is short-circuited.

Figure 11: Short-circuit of one low-voltage winding

Note the differences to the three-phase equivalent test: only one winding at a time is short-circuitedon the low-voltage side and the neutral terminals are involved in the measurement.

Figure 12: Connecting the voltage and current inputs to the transformer terminals

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Figure 13: Connection example for a YNd transformer

4.3.2 Starting the Test

After the three-phase equivalent test cards, further test cards Zk A, Zk B and Zk C are prepared toperform the per-phase test. The measurement configurations for the CPC 100 remain the same as in

the three-phase equivalent test cards. Perform the measurement for each winding as described.

Caution: You can also skip the test if you only want to perform a three-phaseequivalent test.

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5 Interpretation of Measurement Results

5.1 Loading the Test Result into Excel

To evaluate the test, a special Excel Template File Loader is available in your OMICRON folder.Transfer your test file "xml" from the CPC 100 onto your computer and open it with the MicrosoftExcel template from the Start Page.

Figure 14: Locating the Microsoft Excel template in Start Page

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5.2 Test Results

Once you have loaded the test file into the Excel template, the results are shown on the first threeExcel sheets Calculation of Zk, Rk(f) and Lk(f). If the transformer data has not been entered yet, itcan be done in the Excel Test Report sheet Factory Test (seeFigure 15: Test result - comparisonwith the factory test)or on the TR Data sheet.

Figure 15: Test result - comparison with the factory test

The Calculation of Zk sheet summarizes both the three-phase equivalent and the per-phase testresults. First, you have to configure the calculations by stating the vector group of the measured sideat the top of the report. If you know the measurement temperature and the reference temperature,you can also state the values and the sheet considers a correction factor, according to IEC 76-1, forthe measured winding resistance.

The deviations of the measured impedances from the factory test are calculated. A change in theshort-circuit impedance of the transformer indicates a possible winding movement within thetransformer. Changes of 2% of the short-circuit impedance are usually not considered significant.Changes of more than 3% of the short-circuit impedance should be considered significant.

If you have performed a per-phase test too, the sheet shows you the results with the maximumdeviation from the average of the three phases. Also refer toFigure 2: Interpreting the test results onpage7 for assessment.

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5.2.1 FRSL / AC Resistance Sheet: Rk(f)

The Rk(f) sheet shows the frequency sweep of the AC resistance. The phases can be comparedwith each other. Significant changes between the curve progressions may indicate a short-circuit inparallel strands of the windings.

Figure 16: Example result for significant faults in winding C

The figure above shows the test result of a faulty transformer. At higher frequencies, differencesbetween the curve progressions can be seen quite clearly.

5.2.2 Short-Circuit Inductance Sheet: Lk(f)

The Lk(f) sheet shows the results of the inductance measurement. The value which must remain

with only a slight decrease in frequency may be an indication of whether the test has beenproceeding correctly.

Figure 17: Frequency sweep of the short-circuit inductance

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