Transformer Testing - DV Power Products

Smart test devices for reliable electric power systems

Transformer Testing Using DV Power Devices Maid Dzambic, Application Engineer 2

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DV Power Devices – Transformer Group Winding resistance measurement and OLTC analysis – DC current

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RMO-T series

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TRT series

– RMO-TW

– TRT03

– RMO-TD

– TRT30

– RMO-TT

– TRT33

TWA series

– TRT4x

– TWA25A

– TRT63

– TWA40D

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Turns ratio test – AC voltage


Transformer Tests using DV Power Devices TYPES OF MEASUREMENTS •

Winding resistance measurement (IEC 60076-1)

RMO-T, TWA

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Analysis of On-Load Tap changers (IEC 60214-1)

RMO-T, TWA

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Heat run test (IEC 60076-2)

RMO-T

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Demagnetization process

RMO-T, TWA, DEM

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Turns ratio measurement of power transformers (IEC 60076-1) Turns ratio verification of instrument transformers (IEC 60076-1)

TRT TRT

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Excitation current measurement

TRT

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Phase angle measurement

TRT

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Vector group detection

TRT

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Demagnetization status verification

TRT

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Magnetic balance test

TRT

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RMO-T and TWA Family of Devices

Transformer Tests using DV Power Devices TYPES OF TESTS (RMO-T and TWA family) •

Winding resistance measurement

RMO-T, TWA

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Analysis of On-Load Tap changers

RMO-T, TWA

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Heat run test

RMO-T

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Demagnetization process

RMO-T, TWA, DEM

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Device Series Development Evolution RMO-TW series winding resistance measurement

The most advanced on the market TWA40D

RMO-TD series + tap changer analysis (DVtest) + OLTC motor current measurement + demagnetization process

+ connection of all 8 (or 6) bushings at once

+ OLTC synchronization measurement + simultaneous three-phase measurement + fast measurement

RMO-TT series + temperature measurement channels

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Winding Resistance Measurement Why is it important? •

Winding and connection damages

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OLTC contact malfunctions

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Defined by standard

How is it performed? •

Injecting DC current, measuring voltage drop, calculating resistance R = U / I

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Should be done for all phases and every tap position

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Compared to reference measurement, across phases

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Winding Resistance Measurement How difficult it can be? • Time to inject DC current to a winding (charging) • Stabilization time

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Winding Resistance Measurement Charging time • Volt-seconds needed • 100 kV winding at 50 Hz needs 450 Vs • 100 kV winding at 60 Hz needs 375 Vs

• Thus, 50 Hz winding needs 75 s at 6 V • Or, ≈8 s at 60 V • Residual flux makes time 0.25 – 1.7 times longer

All our devices have charging voltage up to 55 V DC!

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Winding Resistance Measurement Stabilization time • Depends on time constant L/R • Eliminate inductance influence • By saturating the core

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Winding Resistance Measurement How to saturate the core • Use high current • Low current may not saturate the core in a reasonable time

• High current may heat up the winding

Higher current => better saturation => lower L => shorter time

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Winding Resistance Measurement How to saturate the core • To get saturation 1.2 x Iex = 1.2 x 2-5% In • To avoid heating of the wire <10% In

• 10% of nominal current • Current cannot be reached? Winding resistance too high – Device power limit reached, lower the current value

Our instruments can generate up to 100 A DC!

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Winding Resistance Measurement Measuring at LV side may take very long time • LV side may have very low resistance • Therefore large time constant L/R

• On large units stabilization may take 15-60 minutes • Sometimes even 100 A might not be enough to significantly shorten the time

Use HV winding as an additional help for faster saturation!

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Winding Resistance Measurement Saturation by one or tw o windings?

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Winding Resistance Measurement Measuring at LV side by saturating core from HV side • Less current required • Need more cables on top of the transformer (if using single-phase

devices) • Need a little more thinking prior to the test (if using single-phase devices)

On a large GSU transformers this can speed up the measurement 10 to 20 times!

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Tap Changer DVtest Method Why is it i mportant? • New method (used and approved by MR and ABB) • Detecting problems within tap changer • No need to extract tap changer from the transformer

How it performed? • is Injecting and recording DC current through winding • Changes of current reflect changes of resistance/reactance

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Tap Changer DVtest Method Cooperation with MR tap changers Maschinenfabrik Reinhausen the one of the biggest tap changer manufecturer in the world Link for the brochure

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The Result is a Graph

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The Result is a Graph

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Why bother?

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“Give me one good reason to test!”

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I will give you 4


Reason 1 •

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40% of problems in transformers


Reason 2

“About one in 20 OLTC failures lead to transformer main tank failure” - Presentation on OLTC diagnostics, Laurent Allard, ABB Secheron, Euro TechCon Chester UK 2010

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Reason 3

“HQ study showed 12% require maintenance before manufacturer’s suggested period.” - WECC Substation WG meeting minutes, Vancouver Washington, May 2006

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Reason 4

“About 33% of OLTC failures are caused by incorrect maintenance and bad reassembly.” - P.Kang, D. Birthwistle, et al. "Non invasive on line condition monitoring of OLTC", IEEE proceedings January 2000, Generation, transmission and distribution, Volume 3, pp.2223-2228

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Heat Run Test Why is it important? • Detecting winding hot spot temperature • Performed by transformer manufacturer

How is it performed? • Measuring winding resistance during transformer cooling • Measuring ambient temperature, top and bottom oil layer temperature • Extrapolating resistance curve

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Demagnetization Why is it important? Remanent magnetism can cause various problems: • High amplitude inrush current at start-up of power transformer,

• Incorrect operation of protective relays, • Erroneous results during electrical measurements on transformer (FRA test, excitation current measurement)

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Causes of Transformer Magnetization • When disconnecting a transformer from service • Consequence of high fault currents • Test using DC current

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Inrush Current • Comparison of maximum inrush current with and without residual flux

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Calculation

“It was shown that inrush currents calculated at the head of the feeder were reduced by about 60% when a power transformer was previously demagnetized.” - B. Kovan, F. de Leon, D. Czarkowski, Z. Zabar, L. Birnebaum, 2011: "Mitigation of Inrush Currents in Network Transformers by Reducing the Residual Flux With an Ultra-Low-Frequency Power Source", IEEE Transactions on Power Delivery, vol. 26 issue 3, 1563-1570.

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How? • DV Power has developed a simple and very effective demagnetization method: − Internally changing the polarity of a controlled DC current − Current is supplied at decreasing magnitude for each step − Safely discharges the energy and guarantees the current magnitude

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How? • Every next step, value of current will be 60% of the previous value • This will continue until 5 mA value is reached

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Theory • Alternate direction • Lower the magnitude • Bring flux to 0

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Why demagnetization • Before AC tests – to avoid bad results due to remnant magnetism • Before putting a transformer back in service – to avoid high inrush currents • Requires some additional time for tests, but improves safety

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Winding Ohmmeters RMO-TW series • RMO10TW • RMO20TW • RMO30TW

• RMO50TW

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Winding Ohmmeters RMO-TW series RMO100TD

RMO10TW

RMO20TW

RMO30TW

RMO50TW

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Max. current

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10 A DC

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20 A DC

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30 A DC

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50 A DC

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Max. charging voltage

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55 V DC

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55 V DC

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55 V DC

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55 V DC

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Typical accuracy

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0,1%

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0,1%

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0,1%

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0,1%

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Resolution

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0,1 μΩ

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0,1 μΩ

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0,1 μΩ

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0,1 μΩ

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Tap Changer Analyzers & Winding Ohmmeters RMO-TD series • RMO25TD • RMO40TD • RMO60TD

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Tap Changer Analyzers & Winding Ohmmeter RMO-TD series O00TD

RMO60TT RMO40TD RMO25TD 

RMO60TD

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Max. current

• 25 A DC

• 40 A DC

• 60 A DC

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• 55 V DC

• 55 V DC

• 55 V DC

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Typical accuracy

• 0,1%

• 0,1%

• 0,1%

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Resolution

• 0,1 μΩ

• 0,1 μΩ

• 0,1 μΩ

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Built-in TC control

• Optional

• Optional

• Included

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Tap Changer Analyzers & Winding Ohmmeters RMO-TT series • RMO40TT • RMO60TT • RMO100TT

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Tap Changer Analyzers & Winding Ohmmeter RMO-TT series RMO60TT RMO60TT RMO40TT 

RMO100TT

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Max. current

• 40 A DC

• 60 A DC

• 100 A DC

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• 55 V DC

• 55 V DC

• 55 V DC

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Typical accuracy

• 0,1%

• 0,1%

• 0,1%

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Resolution

• 0,1 μΩ

• 0,1 μΩ

• 0,1 μΩ

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Temperature channels

• 3

• 3

• 6

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Built-in TC control

• Included

• Included

• Included

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Tap Changer & Winding Analyzer TWA40D • Three-phase instrument • One-time connection of all 6-8 bushingss • All 6 resistances measured in one test • Max. 25 A per phase in step by step test • Max. 40 A, 13 A per phase in OLTC synchronization test

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Tap Changer & Winding Analyzer TWA series

TRT03C TWA40D 

TWA25A

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• Max. current

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25 A DC

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40 A DC

• Max. charging voltage

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55 V DC

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55 V DC

• Typical accuracy

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0,1%

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0,1%

• Resolution

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0,1 μΩ

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0,1 μΩ

• Motor current mes.

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No

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Yes

• Quick YN test

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No

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Yes

• TPC Analysis

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Verification (4 ms)

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DVtest (0.1 ms)


Summarized TWA40D Features

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Three tests in one device One time connection system Unique transformer saturation method Quick YN test (TWA40D)

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Synchronization test (TWA40D) Tap changer motor current measurement (TWA40D) Three-phase demagnetization Combined transformer testing with TWA and TRT 3 years warranty


Three-Phase Transformer Demagnetizer DEM60C

Basic technical data: • Demagnetization current : 5 mA - 60 A DC • Weight: 13,5 kg • Demagnetization graph recording

A , B , C , N connec tors

One- time cable setup

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TRT series

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Transformer Tests using DV Power Devices TYPES OF TESTS (TRT): •

Power transformers turns ratio measurement

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Excitation current measurement

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Phase angle measurement Vector group detection

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Instrument transformers turns ratio verification

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Instrument transformers polarity check

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Demagnetization status verification

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Development Evolution TRT03 series Single phase voltage TRT4x series + 125 V max. voltage TRT30 series + true three phase voltage

(Together with CVT40 it can output 5 kV AC for testing CVTs)

TRT33 series + test voltage 1 V (instrument transformers testing)

Most advanced TRT63 series + 250 V test voltage

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Turns Ratio Measurement Why is it important? • Detecting winding defects

How is it performed? • Voltage ratio between primary and secondary windings is measured for each transformer leg, applying voltage at HV side and measuring on LV side • Phase by phase with single-phase voltage • All phases at once with true threephase

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Single-phase vs. true three-phase voltage What the international standards say?

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•

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IEC doesn’t stipulate which of these to use ANSI stipulates single-phase voltage, but allows true threephase where single-phase is not applicable (“In the case of three-phase transformers, when each phase is independent and accessible, single-phase power should be used; although, when convenient, three-phase power may be used.”, ANSI C57.12.90-1999)


Single-phase vs. true three-phase voltage What is the right choice?

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The best is to have both options

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Exactly what we offer – both options in the same unit

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No additional modules, no additional cables needed

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User can perform single-phase only, true three-phase only, or both tests


Excitation Current Measurement Why is it important? • Detecting short-circuited turns • Core lamination shorts • Core de-laminations

• Verification of demagnetization process

How is it performed? • Excitation current is transformer primary no load current, or magnetizing current (with open secondary) tested at reduced voltage • Results are compared to values obtained under same test voltage values • Results should follow specific pattern based on transformer construction type

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Phase Angle Measurement Why is it important? • Can indicate problems in windings and/or magnetic core • Measuring displacement of transformers with specific vector groups (phase shifting, rectifier, arc furnace and traction transformers) • Verifying polarity of instrument transformers

How is it performed? • Comparing sine waves of AC voltage(s) on HV side and AC voltage(s) induced on LV side

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Vector Group Detection Why is it important? • Verification of transformer vector group • During/after manufacturing • During/after reparation

• Looking for incorrect connections

How is it performed? • Using proprietary algorithm & software solution loaded in TRT • Completely automatic

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Demagnetization Status Verification Why is it important? • Confirmation that transformer has been successfully demagnetized

How is it performed? • By comparing excitation currents before and after demagnetization

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Combined Measurement System

TRT

TWA

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One measuring system TRT & TWA •

Interchangeable test leads – only one cable set needed

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One time connection – saves time

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7 tests at once: – Turns ratio in all tap positions – Excitation current – Phase angle – Magnetic balance test – Winding resistances in all tap positions – On load tap changer dynamic resistance

– Demagnetization

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True three-phase transformer turns ratio testers •

TRT63 series – Additional 250 V test voltage – Important for comparing excitation currents obtained with mains voltage – Improved ratio accuracy, up to 0,03% – Higher ratio range, up to 50 000

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Summarized TRT63 Features • High test voltage 250 V

Better accuracy

• True three-phase test

Testing transformer with non-standard vector group (phase-shifting, rectifier, furnace...)

• Test plan

Preparation the test in office

• Automated test mode • Combined operation with TWA • 3 years warranty

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Automatic measurement and tap position changing Saving the time and money

Questions?

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Thank you for your attention!

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Transformer Testing - DV Power Products

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