Advanced Research Workshop on Modern...

Advanced Research Workshop on Modern Transformers

28-30 October 2004, Vigo - Spain

Prof. dr hab. inż. Ryszard Malewski, LFIEEE, FCAE

Malewski ElectricWarszawa, Poland

[email protected]

Power Transformer Acceptance Tests

Modern TransformersARWtr 2004 28 -30 October. Vigo – Spain http://webs.uvigo.es/arwtr04

ARWtr 2004 Lecture: POWER TRANSFORMER ACCEPTANCE TEST by Prof. Dr. Hab. Inz.Ryszard Malewski

Acceptance tests are described in the technical specification prepared by the buyer. However, such specification often refers to international or national standards.

The American Standard is often used in Europe and other continents, either in parts or as the basic standard

Prof. dr hab. inz. Ryszard Malewski



In principle, the dieletric tests consist in an application of ahigher than rated voltage for a relatively short period of time.

It is assumed that such test shall reveal dielectric faults thatmay appear after a long time of service at the rated voltage.

In reality, a transformer failure in service may also be caused by overheating, excessive mechanical stress, insulating material contamination and aging, but all these factors result in the insulation breakdown.

The acceptance tests should also provide reference data for subsequent diagnostic procedures in service, since most of such procedures hinge on a comparison to initial records taken on a new transformer.



Prof. dr hab. inz. Ryszard Malewski Prof. dr hab. inz. Ryszard Malewski

The acceptance tests shall reveal flaws ofdesign and manufacturing

Impulse test Induced voltage test with partial discharge (PD) measurement

Heat run testLoad and no-loadloss measurement

These tests shall provideinitial reference data

for subsequent measurements

tg δ=ψ (f) characteristicDissolved gas in oil (DGA)

Short-circuit impedanceWinding frequency response (FRA)

Prof. dr hab. inz. Ryszard MalewskiModern TransformersARWtr 2004 28 -30 October. Vigo – Spain http://webs.uvigo.es/arwtr04


Impulse testChecking the dielectric stress distribution in the winding

Applied impulse

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Standard impulse formshave been defined to ensure reproducibility

of the impulse test carried out at different HV laboratoriesStandard lightning impulse:

T1= 1.2µs±30% T2=50µs ±20%T=T1/1.67

Chopped lightning impulse:on the tail on the frontTc=2 to 6µs Tc=0.5µs

Switching impulse:T1=1.67*T>100µsTd>200µsTz>500µs (better 1000µs) IECTz>1000µs IEEE




Impulse test of the regulation winding

IEC requires testing at two extereme tapping positions of the regulation winding, unless the regulation range is less than 5%. Then the principal tap only shall be tested

IEEE requires impulse application at the tap position that corresponds to the minimum number of turns.

Dangerous transient overvoltage may be induced in this part of the regulation winding that is not connected at the end. An internal surge protector is used to limit these overvoltages by some designers

Coarse and fine regulation winding

connections



Impulse test set-upis composed of the impulse generator, voltage divider, current

shunt, chopping gap, impulse recorder and an equipotential ground plane

Impulse generator

Chopping gap

Tested transformerVoltage divider

Impulse recorder

Current shunt

Equipotential ground plane

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Acceptance criterion: no significant differences between the impulse records taken at 100% and 50% test voltage level.

This means that the stressed winding insulation shall remain linear, i.e. do not change with voltage up to the Basic Insulation Level (BIL)

Superimposed and scaled 50% & 100% BIL impulses.

MHz

µs

µs

Magnified (8x) difference between these impulses.Superimposed and scaled neutral terminal currents.Magnified (8x) difference between these currents.Winding transfer function derived from 50% and 100% BIL records.The minor differenceobserved between the current records shows up clearly as a winding fault at the transfer functiongraph


http://webs.uvigo.es/arwtr04


Analysis of the recorded HV impulseand the neutral terminal current

|U(f)||I(f)|T(f) =

A breakdown in the winding (say turn to turn) changes the local inductance and capacitance of the affected coil. This results in a change of the resonant frequency of this coil.A partial discharge is a flow of charge to ground that by passes the current measuring shunt. On the equivalent circuit it shows as ainserted resistor that dissipates energy. This resistor contributes to damping of the local resonant pole but does not affect its frequency

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A local insulation breakdown disqualifies transformer under test.A partial discharge is acceptable during the impulse test



Digital impulse recorder and analyserAn analog signal is converted to high resolution (>12 bit) digital record sampled at Fs>100 MHz. This conversion results in the quantization error that shows up on the frequency spectrum of recorded impulses at the higher frequency range. The winding transfer function is corrupted in this range.

A low value of coherence function reveals the frequency range where the transfer function is corrupted




Digital impulse recorder and analyser

Superimposed TF transfer functions derived from the full 100% and reduced 50% level impulse tests, and their coherence.

TF1

CoherenceTF2



Transient Electromagnetic Interferenceinduced in the impulse measuring circuit

by firing the generator and by impulse chopping

A transient current Ignd is induced in the ground plane by a discharge of the impulse genertor IGcapacitance to ground.. Moreover, the chopping gap CG discharges stage capacitors and injects a heavy current pulse in the ground plane. A transientpotential difference ∆U appears between thegrounding points of the divider VD, shunt SH and recorder DR.This imposes an oscillatory interference on the voltage and current records.

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Equipotential ground plane of the impulse test bayHigh frequency oscillatory current Ic induced in the coax. cable sheath results

in an interference Uc imposed on the impulse voltage and current records

Expanded copper mesh used as the ground plane

Expanded copper mesh

Impulse recorder

Measuring coaxial cable

Voltage divider




Induced voltage test with Partial Discharge MeasurementThis test verifies AC withstand of line terminal and winding insulation. It is performed at the test voltage Ut higher than the rated voltage U,

and applied for a time period tt.An absence of significant partial discharges (PD) recorded during this test demonstrates that the transformer will operate for its technical life (of t ≈ 30

years). This assumption is based on probability P of the PD onset depending on the applied test voltage Ut and the voltage application time tt.

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛∗⎟⎟

⎠

⎞⎜⎜⎝

⎛−−=

a

t

m

t tt

UU1t)P(U,

Weibull probability distribution of the uniform-field oil-gap breakdown voltage, with the voltage application-time as parameter. The distribution shape coefficient m≈8 was determined experimentally for uniform-field oil gaps of the spacing used in HV transformer insulation




The oil-gap breakdown probability was determined as a function of the test voltage application time t. The slope of this characteristic 1/n decreases for longer time t.

At the constant breakdown probability, e.g. P(U,t)=const. the breakdown voltage U will depend on the time t:

nma t

k

t

kU ==⎟⎠⎞

⎜⎝⎛

Where: and k depends on the oil-gap

60

nma 1=

Basis for determination ofthe induced voltage value



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IEC and IEEE induced voltage test procedureUm – highest voltage phase-to-phase for three-phase, and phase to ground for single-phase transformersD=60min for Um>

IEChttp://webs.uvigo.es/arwtr04

300kV, or 30min for Um<300kV;

Transformers with Um<170 kV have to pass AC Short Duration test that requires 1.3Um/√3 instead of 1.5Um/√3, induced for 5min rather than for 60min. The short duration (<60 s) voltage shall be close to 2Um, but no more than given in the Standard.

For higher voltage transformers (Group II) the IEEE requires 1.5Un to be induced first for a time needed to measure PD, then a higher „enhancement” voltage of 1.7Un is induced for 60 s (or less). Then 1.5Un is induced again for 60 min and PD’s are recorded. After that the test voltage is decreased and shut down.


Induced voltage level and duration specified by the IEC for transformers of a different rated voltage class.

Routine ACSD and ACLD tests are indicated with and without PD measurements.

The American standard requirements are indicated below.

IEC and IEEE induced voltage test procedure

IEEE allows PD and RIV measurements using a narrow-band instrument, whereas IEC prefers PD broadband measurements.



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Measurement of PD’s during the induced voltage testPartial discharges (PD) develop in gas cavities inside stressed transformer insulation, at metallic protrusions, floating potential screens, inside cellulose with an excessive water content ...PD’s are measured by an impedance Zm attached to the transformer bushing tap. A PD detector displays their onset-voltage Uo, charge Q, phase φ of the test voltage and polarity.

Test transformer and test voltage source shall be free from PD’s, and the HV laboratory shall be protected by electromagnetic shield that attenuates external interferences from radio station, and other sources.






Electromagnetic shield of the HV laboratory enables measurement of PD with a wide-band instrument

Shielded HV laboratory prevents interference radiated by radio stations and other sources from inducing disturbing signals in the PD measuring circuit

Shielding efficiency map plotted on a HV laboratory floor plane



Measurement of PD’s with narrow and wide-band meter

fo=100kHz; ∆f=10kHzfo=100kHz; ∆f=50kHz

Wide-band

Narrow and wide-band

Narrow-band

fo=1.0MHz; ∆f=5kHz

fo=100kHz; ∆f=10kHz

fo=250kHz; ∆f=10kHz

100pC calibration pulse was injected at each disc of transformer coil, and measured by narrow, as well as wide-band instrument. The latter introduced a uniform attenuation of pulses coming from discs deep inside the winding.The narrow-band instrument introduced an irregular attenuation, and pulses coming from discs deep in the winding were completely eliminated when fo has reached 1MHz.





Instrument for simultaneous measurement of PD’s and RIVRadio Influence Voltage (RIV) meter has a low (50Ώ) input impedance and a narrow band, whereas PD detector needs a high (2.5kΏ) input impedance and frequency bandwidth from ~20 to 300 kHz. Some US Customers insist on RIV measurements, and a special instrument was developed to measure simultaneously PD’s and RIV.

PD monitor



Comparative measurement of PD’s and RIVPD’s (in pC) and RIV (in µV) have been simultaneously measured on a few hundred large power transformers. A ratio of pC to µV was calculated for these transformers and plotted on PD against RIV graph.

This is caused by a higher input capacitance of large transformers that was not accounted for by the voltage signal (100 µV ) calibration according to NEMA 107 Standard.

It was concluded that a lower reading µV was obtained for the same pC, on large transformers

The charge pulse calibration (100 pC) was independent of the transformer capacitance, and reflected the actual charge of PD’s inside the windings’ insulation.Apparently the National Electrical Manufacturers Association Standard 107 makes the RIV test easier to pass for large and expensive transformers500





Digital PD measuring instrumentDigital instruments can record each PD pulse, and store its charge, phase of appearance (point-on-the-wave) and polarity. The PD’s recorded during a fixed time period (i.e. 10 s) can be plotted on three-dimensional graph. Then a specific pattern reveals the origin of PD’s.A characteristic form of their distribution indicates whether they come from a metallic protrusion, gas cavity inside paper wrap or transformer-board layers, excessive amount of moisture in the cellulose insulation or a floating potential screen.




Distributions of identified origin PD’s has been stored in memory and can be compared to the test records. Probability of finding the same PD origin is evaluated.A computer-assisted interpretation of PD measurements indicates possible source of

PD’s and assists the test personnel in their diagnose of the problem.

Computer-assisted PD analysis

PD ORIGIN PROBABILITY %http://webs.uvigo.es/arwtr04



Acoustic location of PD’s inside the tankand UHF detection of PD’s in transformers in service

Acoustic detectors attached to the tank wall by magnets reveal the time of arrival of the sonic wave emitted by PD

The PS source can be located by triangulation between to position of acoustic sensors. To locate the PD source the test voltage shall be controlled in such a way to find the PD source onset voltage

110 kV transformer with three UHF sensors installed inside the tank near HV bushings

Simultaneous arrival of sonic signals from a few sensors indicates that they are equidistant to the PD source.

UHF signals




The transformer buyer penalizes the manufacturer for the loss that exceeds the guaranteed value, and the penalty for each kW of cumulative loss ranges between $3,000.- and $6,000.-

Measuring uncertainty of transformer loss is directly related to the loss of manufacturer revenue. However, a precise measurement of the load loss is difficult since large transformers with short-circuited winding represent almost purely reactive impedance.

This impedance power factor cosφ equals approximately to the ratio of active loss power P and reactive power S drawn by transformer, and may be as low as 1%.

Measurement of transformer load loss


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Accuracy of the loss measuring system

ϕ∆

≅ϕ∆tg

P

The load loss measuring uncertainty is dominated by the angular uncertainty ∆φ of the voltage and current transducers, as well as of the wattmeter:

222 IU φ∆+∆+∆=ϕ∆

Assuming the short-circuited transformer power-factor cosφ=0.01, φ ≅ 1.560796 Rad, and tgφ ≅99.995.Then ∆φ ≅ ∆P / tgφ ≅ 100 µRad ≅ 0.343 min

and

1

1φ

It is rather difficult to achieve so small angular uncertainty in a loss measuring circuit composed of magnetic voltage and current transformers, and an electro-dynamic wattmeter. To comply to the requirement of the overall angular uncertainty smaller than 100 µRad the compressed-gas capacitor with an active low-voltage part, and zero-flux current transformer are employed to reduce the measured high-voltage and heavy-current to the wattmeter input level, and a time-division-multiplier wattmeter is used.

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Loss measuring system of high accuracyhttp://webs.uvigo.es/arwtr04

Three-phase loss measuring system contains a zero-flux current transformer with the angular uncertainty reduced by power-amplifier feeding the secondary winding. Voltage divider is composed of a virtually loss-less standard HV capacitor, and of the low-voltage capacitor with a solid dielectric. Its loss factor is compensated by a tuned phase-shifting network in order to reduce the divider angular uncertainty.


Measurement of transformer no-load loss

100%107%

Idle transformer supplied with power frequency voltage above the rated level is driven to saturation, and the magnetizing current shows conspicuous pulses around voltage zero-crossings. The regulated test voltage source of HV laboratory cannot deliver such current pulses and the voltage waveform is distorted. This does not reflect conditions prevailing in real power systemA large capacitor bank is needed to supply the

current pulses, and to maintain the sine voltage waveform.

Sub-harmonic resonance







Harmonic components of the transformer no-load lossIdeally, the test voltage waveform should contain the fundamental harmonic only.

However, with a relatively weak, regulated voltage source, a complete compensation of the non-linear load by the capacitor bank was not possible. A harmonic analysis reveals a small difference between the power frequency loss and the total measured loss.




Reference data for subsequent diagnostic procedures in service

Autotransformer 160 MVA, 230/115kV, Common open (Bp-N) and shorted (Ap-N, Cp-N)

HV winding

Hydrogen

Methane

Ethane

Ethylene

Acethylene


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Dissolved Gas in Oil (DGA)

Winding frequency response (FRA)



Transformer leakage impedance

LeakageImpedance,Ohm

Windingarrangement

Testcircuit

Phase

Nameplate

In-service

SNp−ε

%

A 35.39 0.25

B 35.08 -0.62

HV-LV

C

35.3

35.39 0.25

A 36.23 2.63

B 35.08 -0.62

HV-LV

C

35.3

35.33 0.08

HV2 HV 1

LV

In service measurements of leakage impedance reveal a change with respect to the original measurements and that

indicates a winding displacementModern TransformersARWtr 2004 28 -30 October. Vigo – Spain http://webs.uvigo.es/arwtr04

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Dielectric polarization methods to assess the transformer insulation condition

0.2 2 20 200 T(s)

10

100

Vr(V)

0.5%1%

2%3%4%

Temperatura=38ºC

Zawartość wody %

0.2 2 20 200 T(s)

10

100

Vr(V)

0.5%1%

2%3%4%

Temperatura=38ºC

Zawartość wody %Moisture content in cellulose

http://webs.uvigo.es/arwtr04http://webs.uvigo.es/arwtr04

Recovery voltage characteristic (RVM)

tg δ = ψ(f) dielectric loss-factor frequency characteristc


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