transformars - frequency

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A vi sual inspection is time consuming,difficult, and does not always producedefinitive results. During a field inspection,the oi l has to be dra ined. Often, acomplete tear down is required to identifythe problem. An alternative method is

to implement diagnost ic techniquescapable of detecting damage. Frequencyresponse analysis (FRA) is one such toolfor diagnosis of t ransformer electro-mechanical integrity.

There are several occasions where there isa need to verify the integrity of transformers.Examples are:

l Quality check during manufacturing

l Before and after short-circuit tests atthe factory

l After transport or re-location of atransformer

l After a nearby short-circuit fault thatresulted in high currents (= high forces)in the transformer

l A f t e r c a t a s t r o p h i c e v e n t s l i k eearthquakes, lightning strikes, etc.

l As a diagnostics tool when promptedby gas analysis, vibration monitoring orother indicators of potential problems

l Before and after maintenance

Frequency response analysis (FRA)

measurements

FRA (frequency response analysis) or SFRA(swept frequency response analysis) is used

to measure and analyse characteristicresponse curves of a transformer (usuallyin the frequency range 20 Hz 2 MHz).

See Fig. 1

Analysis

FRA is a comparative method for assessingthe condition of power transformers. Toevaluate the FRA results, actual data arecompared with reference data either bydirect visual inspection of the curves or byusing processed FRA data.

There are three approaches for comparingmeasured curves to reference data:

l Compare new measurements tofingerprint measurements on the sameunit (time based comparison)

l Comparing measurements betweenidentical (twin/sister) transformers (typebased comparison)

Frequency response analysis ofpower transformers

Information from Megger

Power transformers are specified to withstand the mechanical forces arising from both shipping and subsequent in-service events. Mechanical

forces may exceed specified limits, and/or the insulation support structure is weakened due to aging. There is a need to effectively identify

such damage.

Fig. 1: FRA or SFRA is used to measure and analyse characteristic response curves of a transformer.

Fig. 2: Core problems will give deviation into low frequency range.

l Comparing measurements made on

symmetrical windings/limbs/phases on

the same transformer (design based

comparison)

When there is a reason to suspect

mechanical damage (transport, extensivemechanical forces due to e.g. short circuitcurrents), the user can identify mechanicalmovements by comparing the reference

curve with a curve obtained after theevent. If the curves are identical, nointernal displacements have occurred

and the transformer can safely be putback to service. Individual measurementsof every winding should be made whichallows for identification of the location ofthe problem.

For bes t resu l t s and most re l iab le


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analysis, a characteristic reference curve(fingerprint) of every winding should becaptured when the transformer is knownto be in good condition. This profile is aninvestment as a future reference when thetransformer has to be evaluated (time-based comparison).

Fig. 3: SFRA HV [open] measurements on a 40 MVA transformer.

If no reference measurement has beenmade of the actual transformer, youcan use measurements from a sistertransformer of the same design (type-based comparison) and/or use knowledgeabout symmetries between windings in atransformer (design-based comparison).

Sweep f requency response analys is

(SFRA)

The analysis of FRA measurements isperformed in the frequency domain, i.e.the response curves are plotted as functionof frequency. The measurements can bemade either in frequency domain or in

time domain.

Frequency domain measurements are

called sweep frequency response analysis

(SFRA) in order to distinguish them from

time domain measurements. The FRAX101

uses the SFRA method and applies a signal

with an amplitude of 10 V peak-to-peak at

all measured frequencies. A typical sweep

from 20 Hz to 2 MHz takes about 60 seconds

with default measurement sett ings. FRAX

SFRA units have the capability to optimize

sweep settings to shorten measurement

time while still keeping sufficient resolution

over the frequency range of interest.

In time domain, a pulse or several pulsesare applied and the response is measuredas function of time. These methods areusually called impulse or FFT (fast fouriertransform) methods. The measurementdata need to be transferred (usually usingFFT) to frequency domain to be analyzed.


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Fig. 6: Measurements on symmetrical phases show no deviation.

Fig. 5: Deviation due to disconnection of core grounding.

Fig. 4: Response above 10 kHz is identical but deviation at low frequency indicates a magnetically core.

As a consequence of FFT transformation,the resolution at lower frequencies islow and limits the users ability to noticechanges in the transformers magneticproperties.Time domain methods are alsoknown to have lower repeatability. MostFRA measurements done today use SFRA

and this is the method recommended ininternational standards.

SFRA standards and recommendations

There are several international standardsand recommendations for SFRA testing ofpower transformers:

l Frequency response analysis on windingdeformation of power transformers,DL/T 911-2004, The electric powerindustry standard of Peoples Republicof China

l Mechanical-condition assessment oftransformer windings using frequencyresponse analysis (FRA), Cigr report

342, 2008

l IEEE PC57.149/D4 Draft trial-use guide

for the application and interpretation

of frequency response analysis for oil

immersed transformers, 2007 (Draft)

l Internal standards by t ransformermanufacturers

T h e s t a n d a r d s g i v e d e t a i l e d

recommendat ions on measurementprocedures and analysis (DL/T 911-2004).

Key issues to consider for good qualitySFRA testing are:

l T h e r e a r e s t a n d a r d s f o r S F R A measurements follow them!

l Select test equipment that fulfills allstandards

l Test the transformer as described in the

standards

To ensure high measurement quality and

repeatability the following is important;

l Use a high quality, high accuracyinstrument with inputs and output

impedance matched to the coaxialcables (50 )

l Use the same applied test voltage in

all SFRA measurements

l Make sure to get good connectionsand connect the shields of coaxial

cables to flange of bushing usingshortest braid technique

l Make su re the t rans fo rmer and

test setup is consistent f rom onemeasurement to another (tap settings,

external connections etc). Documentthe setup in detail.

Interpretation of results

A core problem, or an open or a shortwinding, will in general give deviat ion (i.e.the measured and the baseline curve will

be different) in the low frequency range.Changes can indicate shorted turns,open circuit, residual magnetism, or coremovement. In the example, in Fig. 2 thelow frequency deviation is caused by ashorted turn in the low-voltage winding


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A winding problem, except shor ted turns or openwinding that also wil l show up at lower frequencies,wi ll give dif ferences at mid-frequenc ies. Typ icalfaults can be bulk movement of windings relative toeach other or winding deformations. High frequencydifferences typically indicate internal connectionissues.

Measurement examples

Time-based comparisons

A single phase 105 MVA, generator step-up (GSU)transformer was measured after a short-circuit fault.Reference measurements for the same transformerwere available and a d irect comparison could bemade for the high and low voltage windings. As seenin the diagram in Fig. 3, the responses are identicalthus confirming that the short-circuit forces in thisexample did no damage to the transformer.

A one phase three winding step-up trans former(403kV / 16 kV / 16 kV, 103 MVA / 51,5 MVA /51,5 MVA) was measured before and after repair.

The transformer oil was drained, it was moved 10 m,the high voltage bushing was replaced and othermaintenance actions were taken and finally thetransformer was moved back and filled with oil.

The measurements shown Fig. 4 are the measurementof the high voltage winding (bushing A1 and bushingA2) wi th both low voltage windings open before themaintenance action (blue) and after the maintenanceaction (red). As can be seen the measurements areidentical at all frequencies above approximately10 kHz, a very good result.

At low frequencies, which are related to the core,there is a difference. This type of shift illustratesa typ ical d i f ference between normal and

magnetized core. This is not a fault; in this case awinding resi stance measurement (WRM) using h ighDC current has caused residual magnetization in thecore. (Note: SFRA standards recommend that WRMis performed after SFRA testing.)

Shows in Fig. 5 measurements on a sister unit to thetransformer in the previous example. The green curveshows the normal or baseline measurement and thered curve show a measurement after scheduledmaintenance where the core grounding wasdisconnected by mistake. A noticeable differenceis seen starting at around 300 Hz and continuing tojust above 100 kHz. This looks ver y different to themagnetized core issue in the previous example andindicates a problem.

Design-based comparison

Fig. 6 shows SFRA measurements taken on the HVside [open] on a 40 MVA transformer. The unit wastaken out of service but estimated to be in goodcondition and to be used as a spare. No comparisondata from previous tests were available.

SFRA analysis using DL/T 911-2004 standard,comparing measurements on symmetrical phaseson HV and LV, was performed. No deviation/d is tort ion was detected, ind icat ing that theelectromechanical properties are identical. Forthis transformer, the conclusion is that there are nodifference/changes between phases, thus indicating

that the mechanical condition is normal and theSFRA measurements can be used as reference/baseline for future measurements.

Contact Marius Pitzer, Megger,Tel 011 452-6287,[email protected] v

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