Engineering Diagnostic Tools EEEM 513 - (5) Transformer Advanced Electrical Diagnostic

8/13/2019 Engineering Diagnostic Tools EEEM 513 - (5) Transformer Advanced Electrical Diagnostic

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( 513)( 513)

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& 2007 & 2007


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Transformer Advanced Off-LineDiagnostic Testing


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Most of the techniques, whether chemical or electrical

methods, or destructive or non-destructive methods, onlyprovide partial information about the state of theinsulation condition of power transformers.

More advanced condition monitoring or condition

assessment techniques have been developed and arenow starting to come into more general use.

They have been developed in response to the need fornew materials assessment methods.

However, in some advanced diagnotics tools are still inthe developmental stage, either in the technicaldevelopment or, more likely, in the methods of analysisand interpretation of the test data.


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Recovery Voltage Measurement (RVM)

Polarization and Depolarization Current Measurement (PDC) Frequency Domain Dielectric Spectroscopy (FDS)

Frequency Response Analysis (FRA)

Partial Discharge (PD) Measurement

RVM, PDC & FDS are based on the used of the dielectricresponse of insulating materials to the application of electricfields Conductivity, Polarization & Dielectric Response


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When a dielectric material with polar molecular structure is

subjected to a DC voltage, the electric dipoles are oriented withinthe material in response to the applied electric field.

There is thus a polarization charge induced by the dipolemovement and realignment and this will effectively give a voltageacross the capacitance. When the dielectric is short circuited, thestored charge in the dielectric capacitance is dissipated by acurrent discharge with a time constant determined by theeffective intrinsic resistance and capacitance.

During the short circuit the voltage across the dielectric is zero,

but when the short circuit is removed before total charge toequilibrium occurs, then a voltage will appear across thedielectric. This measured voltage is known as the recoveryvoltage.

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A dielectric material becomes polarized when exposed to an electric field.Polarization is proportional to the intensity of the electric field and bymeasuring the current, polarization process can be observed. The currentdensity is the sum of the conduction current and the displacement current.

When the insulating material is exposed to a step voltage, polarizationcurrent is obtained. If the step voltage is removed, a reverse polarity currentknown as depolarization current is obtained. These two currents can beused to determine the response function and the conductivity of thedielectric material.

The PDC is a DC testing method which determining the polarizationspectrum in time constant domain between 10e-3 10e3 seconds in whichthe interface polarization phenomena of long time constant are active. Therange of polarization is strongly influenced by the absorbed moisture and

the deterioration by product content of the paper insulation. It applies a500V step of DC voltage to the high or low voltage winding insulations oftransformers. Time of voltage application is typically up to 10000 seconds.Both the polarization and depolarization times are performed for the sameperiod of time.

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The polarization current pulse has a peak magnitude, a final

steady state level and a time constant and duration that aredetermined by the quality of the oil including both the moisturelevel and the electrical conductivity. In genera the electricalconductivity affects the peak current in the first 100 seconds orso of the current pulse. The moisture in the insulation affects thelonger term polarization current level after about 1000 seconds.[Figure 8.6]

Polarization and depolarization current measurement methodgives general information about the state of insulation condition.This technique is proved to be a useful testing method inpredicting of moisture and development of ageing phenomena.

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Effect of moisture in oil and cellulose paper on the polarization

depolarization current measurement

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In the FDS technique, a known sinusoidal voltage is applied and

measured together with the current passing across the insulationmaterial.

The measurement is repeated for several frequency sweeps -from high frequency to low frequency for minimizing the memoryeffects.

Advantage - the complete diagnostic on the property change inthe material can be discerned

By dividing the current by the voltage and comparing the phasedifference, both the capacitance and the loss at the particular

frequency and amplitude can be calculated.

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The advantage of an analysis of the dissipation factor frequencyas compare at fixed frequency:

Behaviour of insulation caused by moisture affects can be evaluated.

At higher frequencies the pressboard and the oil volume determinethe dielectric loss, at medium frequencies the oil conductivity is thedominant factor and the lower frequency range is dominated by thepressboard dielectric loss.

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Measurement results of the insulation between primary andsecondary to tertiary windings on a power transformer.

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PROGRAMMA IDA 200

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How do you know whether you can energize A

TRANSFORMER after transportation to site or

after a protection trip?

Check Mechanical Integrity


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When does Mechanical Integrity matter?

Re-location Short Circuit

Lightning

Tap-changer fault

Transportation damage can occur if the clamping andrestraints are inadequate; such damage may lead to coreand winding movement.

Radial buckling or axial deformation may occur due toexcessive short circuit forces while in service.


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What you can identify by checking mechanical integrity?

Core Movement

Winding Deformation

Faulty Core Grounds

Partial Winding Collapse

Hoop Buckling

Broken or Loosened Clamping Structures

Shorted Turns and Open Windings


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What Test can be Done?

Frequency response analysis (FRA) using alow voltage AC wave of varying frequency to

identify changes in natural resonance


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Why FRA?

FRA Technique: The technique covers the full dynamic range andmaintains the same energy level for each frequency, providing resultsthat are repeatable and accurate.

Impulse Technique: This technique requires high sampling rates andhigh resolution to obtain a valid measurement. The applied impulse doesnot produce constant energy across the specified frequency, which can

cause poor repeatability that is influenced by the non-linear properties ofthe test specimen.


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What is FRA ?

FRA is a tool that can give an indication of core or winding

movement in transformers. This is done by performing a measurement to look at how well

a transformer winding transmits a low voltage signal that variesin frequency.

Transformer does this in relation to its impedance, thecapacitive and inductive elements which are intimately relatedto the physical construction of the transformer.

Changes in frequency response as measured by FRAtechniques may indicate a physical change inside thetransformer, the cause of which then needs to be identified andinvestigated.


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Test Equipment


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What is the frequency range?

The measured frequency range is normally very large,

which can be from 5Hz up to 10MHz

This frequency range covers the most important

diagnostic areas:

Core and Magnetic Properties

Winding Movement and Deformation

Interconnections-Leads and Load Tap Changer


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The magnitude and the angle of the complex transfer functioncan be obtained using a network-analyzer

The resulting amplitude of the measurement can be expressedas,

H (dB) = 20 log10 [(ZS)/(ZS+ZT)]

The resulting phase is defined by

H () = tan-1 [(ZS)/(ZS+ZT)]


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What are the ANALYZING TECHNIQUES?

Signature

Difference

Transfer Function Statistical

FRA Signatures are analyzed based on 3 bandmethods


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What do the 3 Bands mean?

5Hz up to 10KHz defect in core and magnetic

circuit

10KHz up to 600KHz deformation in windinggeometry

600KHz up to 10MHz abnormalities in the

inter-connection and testsystem


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SIGNATURE TECHNIQUE


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SIGNATURE TECHNIQUE


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SIGNATURE TECHNIQUE


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DIFFERENCE TECHNIQUE

(Phase A before)


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DIFFERENCE TECHNIQUE

(Phase A after)


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DIFFERENCE TECHNIQUEThis technique can analyze the windings phase by phase, which is not

possible in the signature technique


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Historical data or Baseline Reference are, undoubtedly,the best reference to be used for FRA analysis

However, it is not practically easy to get historical data due

to constraints of outages

Criteria to choose reference FRA measurements in theabsence of historical data or baseline reference


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DifferentDifferentSameSamePeer

DifferentSameSameSameSister

SameSameSameSameTwin

S/S

LOCATION

MANU-

FACTURER

MVA

RATING

KV RATIOCATEGORY


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PD Classification


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Occurrence of PD Inception Voltage


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Occurrence & Recognition

Detection

Measurement

Location

Evaluation


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Evaluation

Amplitude in dB Energy or charge in pC

Duration in ms


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On-line acoustic PD Detection - Physical Acoustic DISP-24


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Why SFRA in a factory environment?

Quality assurance

Baseline reference

Relocation and commissioning preparation

Manufacturers are using SFRA as part of their quality program to ensure

transformer production is identical between units in a batch


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Why SFRA in a field environment?

Relocation and commissioning validation Post incident: lightning, fault, short circuit, seismic event

etc

Once a transformer arrives on site after relocation it must be tested

immediately, to gain confidence in the mechanical integrity of the

unit prior to commissioning


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( ) Electrical Tests

Perform insulation-resistance tests winding-to-winding

and each winding-to-ground

Perform turns ratio tests at the designated tap position

Perform power-factor or dissipation-factor tests

Measure the resistance of each winding at thedesignated tap position

Measure core insulation-resistance at 500 volts dc ifcore is insulated


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Inspection - look for cracks, dirt etc., tracking, copper wash,

mechanical damage Cleaning - Wash, dry wipe

Repairs - Usually replace except special cases

Testing - Megger & Power Factor test

Do not climb on or use for personal support!


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( )

Visual inspection

Inspect physical condition for evidence of moisture and corona

Verify operation of cooling fans

Verify operation of temperature and level indicators, pressurerelief device, and gas relay

Verify correct liquid level in all tanks and bushings

Verify correct equipment grounding

Verify the presence of transformer surge arresters

Test load tap-changer

Inspect all bolted electrical connections for high resistance usingone of the following methods:

1. Use of low-resistance ohmmeter

2. Perform thermographic survey


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( )

Electrical Tests

Perform turns ratio tests at all tap positions

Perform power-factor or dissipation-factor tests

Measure the resistance of each winding at all tap positions

Perform insulation-resistance tests winding-to-winding and each

winding-to-ground

If core ground strap is accessible, measure core insulationresistance at 500 volts dc

Remove a sample of insulating liquid in accordance with ASTMD923

Test for Oil Quality, DGA and Furan


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Engineering Diagnostic Tools EEEM 513 - (5) Transformer Advanced Electrical Diagnostic