Factory TestsFactory Testsdownload/files/Transformer... · 2019-08-23 · transformer total mass is more than 400 ton and it operates in power plant in Germany, owned by SWM Infrastruktur

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ContentsContents...............................................................................................5

Factory tests..........................................................................................7

No-load losses and currents.....................................................................8

No-Load Excitation Current....................................................................11

Measurement of impedance voltage and load loss......................................12

Purpose of the measurement...............................................................13

Apparatus and measuring circuit..........................................................13

Performance of the measurement........................................................14

Results............................................................................................15

Dielectric (Insulation) Test.....................................................................21

Insulation tests to be performed..........................................................21

Switching impulse test....................................................................21

Lightning impulse test.....................................................................22

Separate source AC withstand voltage test.........................................22

Induced AC voltage test (short duration ACSD and long duration ACLD). 22

Partial discharge measurement........................................................22

Repeating the dielectric tests..............................................................23

Switching Impulse Test..........................................................................25

Purpose of the Test............................................................................25

Lightning Impulse Test..........................................................................29

Partial Discharge Test............................................................................31

How Partial-Discharge occurs and measured magnitudes?.......................32

Measuring circuit and application.........................................................33

Application of the test........................................................................35

Voltage level.....................................................................................35





Evaluation........................................................................................36

Insulation Power Factor.........................................................................37

Insulation Resistance............................................................................38

Important Notes................................................................................39

Insulation Resistance Test Procedure....................................................40

Noise Measurement..............................................................................42

Where all this noise is coming from?....................................................44

Vibroacoustic energy sources in the power transformers..........................46

Matters of design...............................................................................46

Sound ways of seeing........................................................................47

On-site solutions...............................................................................48

Need for Research and Development....................................................49

Temperature Rise (Heat Run).................................................................50

Short-Circuit Test..................................................................................51





Factory testsThe remainder of the twelve factory tests are briefly summarized below. The details of

the test set connections and formulas of some of the listed tests are already described

in separatly published articles, and for the rest you are directed to ANSI/IEEE

Standard C57.12.90 for these details.

420 kV power transformer, rated power 400 MVA produced by KOLEKTOR ETRA;

transformer total mass is more than 400 ton and it operates in power plant in

Germany, owned by SWM Infrastruktur GmbH

This list is not complete, there are few tests missing, not mentioned here, like Turn

ratio test or Measurement of voltage ratio and check of phase displacement, but you

an find them also separatly published at EEP (use Search).





No-load losses and currents

The no-load losses of a transformer are grouped in three main topics:

1. Iron losses at the core of the transformer,

2. Dielectric losses at the insulating material and

3. The Copper losses due to no-load current.

The last two of them are very small in value and can be ignored.

So, only the iron losses are considered in determining the no-load losses.

Measuring circuit and performing the measurement

Connection diagram for measuring no-load losses





In general according to the standards, if there is less than 3% difference between the

effective (U) value and the average (U’) value of the supply voltage, the shape

of the wave is considered as appropriate for measurements.

If the supply voltage is different than sinusoid, the measured no-load losses have to

be corrected by a calculation. In this case, the effective (r.m.s.) value and the

average (mean) value of the voltage are different. If the readings of both

voltmeter are equal, there is no need for correction.

During measurements, the supply voltage U´ is supplied to the transformer by the

average value voltmeter. In this way, the foreseen induction is formed and as a result

of this, the hysteresis losses are measured correctly. The eddy-current losses should

be corrected according to equation below.

Pm = P0 · (P1 + k · P2)

Pm: Measured loss

P0: No-load losses where the voltage is sinusoidal

Here: P0 = Ph + PE = k1 · f + k2 · f2

k = [ U / U’ ]2

P1: The hysteresis loss ratio in total losses (Ph) = k1 · f

P2: The eddy-curent loss ratio in total losses (PE) = k2 · f2

At 50 Hz and 60 Hz, in cold oriented sheet steel, P1 = P2 = % 50. So, the P0

no-load loss becomes:

Po = Pm / (P1 + k · P2) where P1 = P2 = 0,5





According to IEC 60076-1: Pm = P0 · (1 + d) where d = [ (U’ – U) / U’ ]

During no-load loss measurement, the effective value of the no-load current of the

transformer is measured as well. In general, in three phase transformers, evaluation

is made according to the average of the three phase currents.

Before the no-load measurements, the transformer might have been magnetised by

direct current and it’s components (resistance measurement or impulse tests).

For this reason, the core has to be demagnetised. To do this, it has to be supplied by

a voltage value (increasing and decreasing between the maximum and minimum

voltage values for a few minutes) higher than the rated voltage for a certain time and

then the measurements can be made.

The no-load currents are neither symmetrical nor of equal amplitude in three phase

transformers. The phase angles between voltages and currents may be different for

each of three phases.

For this reason, the wattmeter readings on each of the three phases may not

be equal. Sometimes one of the wattmeter values can be 0 (zero) or negative (-).





No-Load Excitation Current

This current is measured in the winding used to excite the transformer with the

other windings open-circuited. It is generally expressed in percent of the rated

current of the winding. No-load excitation current is not sinusoidal and contains, as we

have seen, odd harmonics (predominantly third harmonic current).

The ammeter used to record the no-load excitation current is an RMS meter which

reads the square root of the sum of the squares of the harmonic currents.





Measurement of impedance

voltage and load loss

The transformer must be in a specific state before the load losses and impedance

voltage are measured. The temperature of the insulating liquid must be stabilized and

the difference between the top and bottom oil temperatures shall be less than 5°C.

The winding temperatures must be measured (using a resistance method) before and

after the test and the average taken as the true temperature. The difference in

the winding temperature before and after the test must not exceed 5°C.

The two test methods for measuring load losses and impedance voltage are:

1. Wattmeter-voltmeter-ammeter method and

2. Impedance bridge method.

Circuit for the impedance and load-loss measurement

These tests generally apply a reduced voltage to one set of windings with the other

set of windings short-circuited. For three-winding transformers, these tests are

repeated for each combination of windings taken two at a time.





Circuit for the impedance and load-loss measurement

Purpose of the measurement

The measurement is carried out to determine the load-losses of the transformer and

the impedanse voltage at rated frequency and rated current.

The measurements are made separetely for each winding pair (e.g., the pairs 1-2,

1-3 and 2-3 for a three-winding transformer), and furthermore on the principal and

extreme tappings.

Apparatus and measuring circuit

On figure above (Circuit for the impedance and load-loss measurement)

there are following figures:

• G1 – Supply generator

• T1 – Step-up transformer





• T2 – Transformer to be tested

• T3 – Current transformers

• T4 – Voltage transformers

• P1 – Wattmeters

• P2 – Ammeters (r.m.s. value)

• P3 – Voltmeters (r.m.s. value)

• C1 – Capacitor bank

The supply and measuring facilities are not described here. Current is generally

supplied to the h.v. winding and the l.v. winding is short-circuited.

Performance of the measurement

If the reactive power supplied by the generator G1 is not sufficient when measuring

large transformers, a capacitor bank C1 is used to compensate part of the inductive

reactive power taken by the transformer T2.The voltage of the supply generator is

raised until the current has attained the required value (25…100 % of the rated

current).

In order to increase the accuracy of readings will be taken at several current values

near the required level. If a winding in the pair to be measured is equipped with an

off-circuit or on-load tap-changer. the measurements are carried out on the principal

and extreme tappings.

The readings have to be taken as quickly as possible, because the windings tend to

warm up due to the current and the loss values obtained in the measurement are

accondingly too high.

It the transformer has more than two windings all winding pairs are

measured separately.





Results

Corrections caused by the instrument transformers are made to the measured

current, voltage and power values. The power value correction caused by the phase

displacement is calculated as follows:

Equation 1:

Where:

• Pc = corrected power

• Pe = power read from the meters

• δu = phase displacement of the voltage transformer in minutes

• δi = phase displacement of the current transformer in minutes

• ϕ = phase angle between current and voltage in the measurement ( is positiveϕ

at inductive load)

• K = correction

The correction K obtained from equation above is shown as a set of curves in below.





The correction caused by the phase displacement of instrument transformers

Phase displacement of istrument transformers

Where:

• K – correction in percent,

• δu – δi – phase displacement in minutes

• cosδ – power factor of the measurement.

The sign of K is the same as that of δu – δi.

The corrections caused by the instrument transformers are made separately for each

phase, because different phases may have different power factors and the phase

displacements of the instrument transformers are generally different.

If the measuring current Im deviates from the rated current IN, the power Pkm and

the voltage Ukm at rated current are obtained by applying corrections to the values Pc

and Uc relating to the measuring current.





The corrections are made as follows:

Equation 2:

Equation 3:

Mean values are calculated of the values corrected to the rated current and the mean

values are used in the following. According to the standards the measured value of

the losses shall be corrected to a winding temperature of 75° C (80° C, if the oil

circulation is forced and directed).

The transformer is at ambient temperature when the measurements are carried out.

and the loss values are corrected to the reference temperature 75° C according to

the standards as follows.

The d.c. losses POm at the measuring temperature ϑm are calculated using the

resistance values R1m and R2m obtained in the resistance measurement (for windings

1 and 2 between line terminals):

Equation 4:





The additional losses Pamat the measuring temperature are:

Equation 5:

Here Pkm is the measured power, to which the corrections caused by the instrument

transformer have been made, and which is corrected to the rated current according to

equation (2).

The short-circuit impedance Zkm and resistance Rkm at the measureing temperature

are:

Equation 6:

Equation 7:

• Ukm is the measured short-circuit voltage corrected according to Equation

(4.3);

• UN is the rated voltage and

• SN is the rated power.

The short circuit reactance Xk does not depend on the losses and Xk is the same at

the measuring temperature (ϑm) and the reference temperature (75 °C), hence:

Equation 8:





When the losses are corrected to 75° C, it is assumed that d.c. losses vary directly

with resistance and the additional losses inversely with resistance. The losses

corrected to 75° C are obtained as follows:

Equation 9:

Where:

s = 235° Cϑ for Copper

s = 225° Cϑ for Aluminium

Now the short circuit resistance Rkc and the short circuit impedance Zkc at the

reference temperature can be determined:

Equation 4.10:

Equation 11:

The report indicates for each winding pair the power SN and the following values

corrected to 75° C and relating to the principal and extreme tappings.

• D.C. losses POc (PDC)





• Additional losses Pac (PA)

• Load losses Pkc (PK)

• Short circuit resistance Rkc (RK)

• Short circuit reaactance Xkc (XK)

• Short circuit impedance Zkc (ZK)





Dielectric (Insulation) Test

These tests consist of applied-voltage tests and induced-voltage tests.

Applied-voltage tests apply a high voltage to all bushings of a winding, one winding

at a time, with the other windings grounded. A 60 Hz voltage is increased gradually

over 15 s and held for 40 s and reduced to zero over 5 s.

Induced-voltage tests apply a high voltage across a winding with the other

windings open-circuited in order to test the quality of the turn-to-turn insulation. In

order to prevent core saturation at the higher excitation voltage, the frequency of the

induced-voltage test is increased (typically around 120 Hz). The induced voltage is

applied for 7200 cycles or 60 s, whichever is shorter.

Insulation tests to be performed

The following insulation tests are performed in order to meet the transformer

insulation strength expectations. Unless otherwise requested by the customer, the

following test are performed in the following order (IEC 60076-3) :

Switching impulse test

To confirm the insulation of the transformer terminals and windings to the earthed

parts and other windings, and to confirm the insulation strength in the windings and

through the windings.





Lightning impulse test

to confirm the transformer insulation strength in case of a lightning hitting the

connection terminals.

Separate source AC withstand voltage test

To confirm the insulation strength of the transformer line and neutral connection

terminals and the connected windings to the earthed parts and other windings.

Induced AC voltage test (short duration ACSD and

long duration ACLD)

To confirm the insulation strength of the transformer connection terminals and the

connected windings to the earthed parts and other windings, both between the phases

and through the winding.

Partial discharge measurement

To confirm the “partial dicharge below a determined level” property of the

transformer insulationstructure under operating conditions.

According to standards, the transformer windings are made to meet the maximum operating voltage Um and the related insulation levels.

The transformer insulation levels and the insulation test to be applied according to

IEC 60076-3 is shown in the below table.





Windingstructure

Maximumoperating

voltage Um kV

Tests

Lightningimpulse

(LI)

Switchingimpulse

(SI)

Longduration

AC(ACLD)

Shortduration

AC(ACSD)

Appliedvoltage

test

uniforminsulated

Um ≤ 72,5type

(note 1)na

na(note 1)

routine routine

uniformand

graduallyinsulated

72,5 < Um ≤170

routine na special routine routine

170 < Um ≤ 300 routineroutine(note 2)

routinespecial

(note 2)routine

≥ 300 routine routine routine special routine

Note 1: In some countries, in transformers with Um ≤ 72,5 kV applied as routine testand the ACLD test is applied as routine or type test.

Note 2 : If the ACSD test is defined, the SI test is not applied.

In case of a transformer with one or more thanone gradual insulation, if foreseen by

the induced voltage test, the switching impulse test isdetermined according to the

maximum Um voltage winding.

The foreseen test voltage can not be reached in lower Um voltage windings. In this

case, the ratio between the tap changer’s optimum tap position and the windings shall

be such arranged that, the lowest Um voltage winding reaches the most appropriate

value. This is acceptable (IEC 60076-3).

If chopped wave is requested during ligthning impulse (LI) test, the peak value of

the chopped wave is 1.1 times the full wave value (10% higher). For transformers

with the high voltage winding Um> 72.5 kV, the lightning impulse (LI) test is a

routine test for all windings of the transformer.





Repeating the dielectric tests

If no modification is made in the internal insulation of a transformer, only maintenance

is made, or if insulation tests are required for a transformer which is in operation, and

if no agreement is made with the customer, test is performed with test voltages at

80% of the original test values. However, the long duration induced voltage test

(ACLD) is always repeated with 100% of the original value.





Switching Impulse Test

Purpose of the Test

The switching impulse test is applied to confirm the withstand of the transformer’s

insulation against excessive voltages occuring during switching. During switching

impulse voltage test, the insulation between windings and between winding and earth

and withstand between different terminals is checked.

The purpose of the switching impulse test as special test is to secure that the

insulations between windings, between windings and earth, between line terminals

and earth and between different terminals withstand the switching overvoltages,

which may occur in service.





The switching impulse voltage is generated in conventional impulse voltage generators

at the laboratories.

The polarity of the voltage is negative and the voltage waveform should normally be

T1/ Td/ T2 20/200/500 μS fiigure below according to IEC 60076-3.

Switching impulse voltage waveform

Due to over-saturation of the core during switching impulse test, a few low amplitude,

reverse polarity (e.g. positive) impulses are applied after each test impulse in order to

reset the transformer core to it’s starting condition (demagnetised). By this way,the

next impulse voltage waveform is applied. The tap position of the transformer during

test is determined according to test conditions.

The on-off impulse voltages are applied to each high voltage terminal sequentially.





Switching on-off impulse test connection diagram

Meanwhile, the neutral terminal is earthed. The windings which are not under test are

left open (earthed at one point). This connection is similar to the induced voltage test

connection. The voltage distribution on the winding is linear like the induced voltage

test and the voltage amplitudes at the un-impulsed windings are induced according to

the turn ratio.

Meanwhile, necessary arrangements should be made since the voltage between

phases will be 1,5 times the phase-neutral voltage.

The test circuit connections of three phase transformers depend on; structure of the

core (three or five legged), the voltage level between phases and the open or closed

state of the delta winding (if any). At first, a voltage with 50 % decresed value is used

at the tests,then impulse voltages at full values and at numbers given in standards

are used. The peak value of the voltage is measured.

The change of the voltage waveform and winding current are measured with a special

measuring instrument and recorded. The negativities in the transformer during the

test are determined by comptring the voltage and current oscillograms.





The sudden collapses of the voltage (surges) and abnormal sounds show deformation

of the insulation in the transfomer. The deformation of the voltage waveform and

increase in noise due to magnetic saturation of the core should not be considered as

fault.

The test voltage values, impulse shapes, and number of impulses at different voltage

levels must be stated in the report.

Switching Impulse Voltage Waveform :

Front : T1 ≥100 µS = 1,67 T

90% value : Td ≥ 200 µS

Time for cutting the axis : T2 ≥ 500 µS



https://electrical-engineering-portal.com/surge-protection-of-electronic-equipment



Lightning Impulse Test

The test sequence consists of one reduced full wave, two chopped waves, and

two full waves. Tap connections are made with the minimum effective turns in the

winding under tests and regulating transformers are set to the maximum buck

position. Oscillograms are taken of each wave.

The general technique for interpreting the results is to look for differences in the

shapes of the reduced full wave and the two full waves, which indicate turn-to-

turn insulation failure.

Transformer impulse testing and fault detection connections





Additional test criteria are found in IEEE Std. C57.98-1993. The impulse tests

probably have the highest likelihood failures among all of the factory tests that are

typically performed.





Partial Discharge Test

This routine test aims to measure the partial discharges which may occur in the

transformer insulation structure during test.

Partial-discharges are electrical arks which form the surges between electrodes of any

area of the insulating media of a transformer between the conductors. These

discharges may occur in air bubbles left in the insulating media, gaps in the solid

materials or at the surfaces of two different insulators.

Although these discharges have small (weak) energy, the thermal energies due to

these discharges can cause aging, deformation and tear of the insulating

material.





The following conditions can be determined during partial-discharge

measurement:

1. To determine whether a partial-discharge above a certain value has occurred in

the transformer at a pre-defined voltage.

2. To define the voltage values where the partial-discharge starts by increasing the

applied voltage (partial-discharge start voltage) and the value where the

partial-discharge ceases by decreasing the applied voltage (partial-discharge

cease voltage).

3. To define the partial-discharge strength at a pre-defined voltage.

How Partial-Discharge occurs and

measured magnitudes?

The structure where a partial-discharge occurred in an insulating media is shown in

the simplified figure below. As seen on the simpliified diagram, the impulses forming

on the discharge point cause a ΔU voltage drop at the transformer line

terminals. This forms a measurable “q” load at the measuring impedance.

This load is called apparent load and given in pC (Pico-Coulomb) units.

During measurements: ΔU voltage drop, average value of apparent partial-

discharge current, partial- discharge power, impulse count within a time unit, partial-

discharge start and cease voltages can also be determined.





a) simple schematics of an insulator with gas gap b) equivalent circuit

Measuring circuit and application

Partial-discharge measurement structure of a transformer and related circuit in

accordance with IEC 60270 is explained below.

Partial discharge measuring connection circuit





Where:

1. Supply generator

2. Supply transformer

3. Test transformer

4. Voltage transformer and measuring circuit

5. Filter

6. Measuring impedance

7. Selective switch

8. Measuring instrument and ossiloscope

qo – calibration generator

The measurement circuit is formed according to Bushing-tap method stated in

standards.

Before starting to measure, complete measurement circuit should be calibrated.

For this, a calibrator (Calibration generator) is necessary. The calibrator produces

a q0 load with a predefined value. Calibrator is connected to the test material in

parallel. The q0 load produced in the calibrator is read at the measuring instrument.

These steps are repeated at all terminals of the transformer to be measured at no-

voltage.

K = q0 / q0m

Where:

K – correction factor

q0 – load at the calibrator

q0m – load read at the measuring instrument





Application of the test

After the calibration operations are completed, the calibration generator is taken away

from the measuring circuit. When the power system is connected (supply generator

switch is closed), the voltage level will be too low (remenance level).

This value which is considered as the base noise (interference) level of the

measuring system should be less than half of the guaranteed partial- discharge

level.

Voltage level

The voltage is substantially increased up to the level stated by the specifications and

in the meantime the partial-discharge values at the predefined voltage levels are

measured at each measuring terminal and recorded. The voltage application period,

level and measuring intervals are given in the induced voltage test section.

After the transformer is energised for measuring operations, the partial-discharge

value read at the measuring instrument is multiplied with the predefined K

correction factor, and real apparent partial-discharge value for each terminal is

found.

q = K · qm

Where:

• qm – load read at the measuring instrument m

• K – correction factor

• q – Real apparent load





Evaluation

The test is considered to be succesful if the partial-discharge value measured at the

transformer’s measuring terminals is lower than predefined values or values stated in

the standards and no increasing tendency is observed during test.

In addition to the measured partial-discharge level, the below conditions

should also be considered in transformers:

• Partial-discharge start and cease voltages are above the operating voltage.

• Depending on the test period, partial-discharge level stays approximately

stable.

• Increasing the test voltage causes almost no partial-discharge level change.





Insulation Power Factor

Insulation power factor is the ratio of the power dissipated in the insulation in

watts to the apparent power (volt-amperes) under a sinusoidal voltage. The applied

60 Hz voltage of this test is generally lower than the operating voltage of the trans-

former. The Doble Test Set is designed specifically to carry out this test.

Portable versions are used to measure the insulation power factor of transformers in

the field. This test usually must be done by a trained technician. The test results are

temperature-corrected to a reference temperature of 20°C.





Insulation Resistance

The insulation resistance test (meggering) is of value for future comparison and also

for determining if the transformer is to be subjected to the applied voltage test.The

winding insulation resistance test is a DC high voltage test used to determine the

dryness of winding insulation system. The test measures the insulation resistance

from individual windings to ground and/or between individual windings.

The measurement values are subject to wide variation in design, temperature,

dryness and cleanliness of the parts. This makes it difficult to set minimum acceptable

insulation resistance values that are realistic for wide variety of insulation systems

that are in use and performing satisfactorily. If a transformer is known to be wet or if

it has been subjected to unusually damp conditions, it should be dried before the

application of the applied voltage test.





Low readings can sometimes be brought up by cleaning or drying the apparatus. The

insulation resistance test should be performed at a transformer temperature as close

as possible or at 20 °C. Test conducted at other temperature should be corrected 20°C

with the use of temperature correcting factor.

The test equipment is calibrated to read in Megohm and commonly know as a HV

Megger. Typical maximum test set voltage values may be 1kV, 5kV or 15kV. The 30kV

Megger is also available.

Duration of the test voltage shall be 1 minute. In the absence of manufacture’s

recommended values, the following readings may be used. Refer to table below.

Table 1 – Transformer Insulation Resistance Acceptance Testing

Winding InsulationClass, kV

Insulation Resistance,MΩ*

1.2 600

2.5 1000

5.0 1500

8.7 2000

15 3000* Normally dried transformers may be expected to have readings 5 to 10 times the

above minimum values.

Important Notes

1. Table 1 was sourced from IEEE C57-94-1982 Recommended Practice for

Installation, Operation and Maintenance of Dry-type General Purpose

Distribution and Power Transformer. Table 6 differs from NETA Table 100.5

figures for transformer Insulation Resistance Acceptance Testing values. There

is no industry consensus for satisfactory values.

2. Other references noted a general rule of thumb for acceptable insulation values

at 1MΩ per 1kV of nameplate rating plus 1MΩ.

3. Under no condition should the test be made while the transformer is under





vacuum.

4. The significance of values of insulation resistance test requires some

interpretation depending on design, dryness and cleanliness of the insulation

involved. It is recommended that the insulation resistance values be measured

during periodic maintenance shutdown and trended. Large variation in the

trended values should be investigated for cause.

5. Insulation resistance may vary with applied voltage and any comparison should

be made with the same measurements at the same voltage and as close as

possible to the same equipment temperature and humidity as practically

possible.

Insulation Resistance Test Procedure

1. Isolate the equipment, apply working grounds to all incoming and outgoing

cables and disconnect all incoming and outgoing cables from the transformer

bushing terminals connections. Disconnected cables should have sufficient

clearance from the switchgear terminals greater that the phase spacing

distance. Use nylon rope to hold cable away from incoming and outgoing

terminals as required.

2. Ensure the transformer tank and core is grounded.

3. Disconnect all lightning arresters, fan system, meter or low voltage control

systems that are connected to the transformer winding.

4. Short circuit all winding terminals of the same voltage level together.

5. Perform a 1 minute resistance measurements between each winding group to

the other windings and ground.

6. Remove all shorting leads after completion of all test.





Table 2 – Insulation Resistance Test Connections for Two Winding Transformer

TestNo.

Single-phase transformer Three-phase transformer

1High voltage winding to low voltage winding and ground

High voltage winding to low voltage winding and ground

2 High voltage winding to low voltage winding

High voltage winding to low voltage winding

3 High Voltage winding to groundHigh voltage winding to ground with lowvoltage winding to guard

4 Low Voltage winding to high voltage winding and ground

Low voltage winding to high voltage winding and ground

5 Low voltage winding to groundLow voltage winding to ground and high voltage winding to guard





Noise Measurement

The noise measurement test is carried out while the transformer is energized at

rated voltage with all of the cooling equipment running. Room geometry can greatly

affect the measurements, so it is preferable that the transformer be inside an

anechoic chamber. However, if such a chamber is not available, no acoustically

reflecting surface may be within 3 m of the measuring microphone other than the

floor or ground.

The recording microphones are positioned in 1 m intervals around the perimeter

of the transformer, with no fewer than four (4) microphone positions for small

transformers.





Noise measurement zone and layout of measuring points

of Power transformer 242 kV / 15.65 kV, 112 MVA

Sound power levels are measured over a specified band of frequencies. The sound

power levels are converted into decibels (dB).





Where all this noise is coming from?

Yes, we all know that transformers are never silent. This is actually quite impossible,

but in an environmentally aware, highly regulated world, the issue is not the level of

noise, but its nature – and it’s very important.

Transformers emit a low-frequency, tonal noise that people living in their vicinity

experience as an irritating “hum” and can hear even against a noisy background.

The power industry have a range of solutions to abate humming, which originates in

the transformer’s core and, when it is loaded, in the coil windings. Core noise is

generated by the magnetostriction (changes in shape) of the core’s laminations,

when a magnetic field passes through them. It is also known as no-load noise, as it

is dependent of the load passing through the transformer.

An effective and important noise source is the core of the transformer.

The noise of the core depends on the magnetic property of the core material (sheet

steel) and flux density. The sound frequency is low (twice the rated frequency). The

magnetic forces formed in the core cause vibration and noise. The load noise occurs

only on the loaded transforrmers and is added to the no-load (core noise). This noise

is caused by the electromagnetic forces due to leakage fields.

The source of the noise are tank walls, magnetic screenings and vibrations of

the windings.

The noises caused by the core and windings are mainly in the 100-600 Hz

frequency band. The frequency range of the noise (aerodynamic/air and

motor/bearing noise) caused by cooling fans is generally wide. The factors effecting

the total fan noise are; speed, blade structure, number of fans and arrangements

of the radiators.

The pump noise is not effective when the fans are working and it’s frequency is low.





Magnetostriction takes place at twice the frequency of the supply load: for a 50 Hz

supply frequency, a lamination vibrates at 100 c/s. What’s more, the higher the

density of the magnetic flux, the higher the frequency of the even-number harmonics.

Magnetostriction video (click on picture for play)

When core or tank resonance frequencies coincide with the exciting frequency, the

noise level further increases.

Hum also arises through the vibration caused when the load current passes

through the windings, interacting with the leakage flux it generates. This load noise

level is determined by the magnitude of the load current. It has always existed, but is

becoming proportionally more significant since there are efficient means of reducing

the core noise source.

In some situations, the load noise is the dominant noise and is raising increasing

concern among new transformer applications.

Note that the broadband noise generated by cooling fans contributes to overall noise




http://tesla-institut.de/index.php/electrical-engineering-articles/1876-magnetostriction


levels. But as cooling fans are widely used in the industry, solutions are not specific to

transmission and distribution and so are not discussed here.

Vibroacoustic energy sources in the

power transformers

Power transformer noise is mainly a low frequency narrow band noise, and the

noise spectrum includes the tonal components of the frequency being the multiple of

the power line frequency. The power transformers have many sources of vibroacoustic

energy.

The most important sources include:

1. The transformer core vibration as an effect of the magnetostriction

phenomena

2. The transformer winding vibration as an effect of the electrodynamic forces

3. The devices of the transformer cooling system, as fans, oil pumps.

Matters of design

Improvements in standard transformer design and materials are cutting the decibel

count.

High-permeability (Hi-B) steel, for example, restricts magnetostriction through a

surface coating with higher degrees of grain orientation.

Another increasingly popular method is high-precision stacking of the core’s

laminations in step-lap patterns, reducing the formation of air gaps in the core joints.

Focus on the linkages between the laminations to stop them striking each other

includes gluing their edges together, standardizing clamp pressure and removing

through-bolts.





In addition, robust, flexible mounts at all points of contact between core and tank

inhibit the structure- or oil-borne transmission of resonance from one to the other.

Sound ways of seeing

Areva T&D’s R&D department employs acoustic imaging, acoustic holography and

laser vibrometry to locate noise and vibrations. Acoustic maps noise rapidly and

comprehensively by differentiating sound levels to determine where it radiates from.

Areva T&D and AB Engineering used 110-microphone arrays 2 m from the

tank to measure noise in the 100 Hz to 500 Hz frequency bands.

For each band, an identically scaled map showed red hot spots on noise-free blue

backgrounds, making it easy to pinpoint noise sources. Acoustic holography which

analyzes near-field noise, was recently used to map transformer noise, arranging a

23-microphone antenna to scan a grid of 20 x 20 cm squares. Algorithm-based

software computed the pressure field and sourced the acoustic radiation, displayed as

spatially distributed 2-D maps for different frequencies up to 850 Hz.

Laser vibrometry is a no-contact technique for inaccessible or dangerous targets. It

uses the Doppler effect, measuring the frequency modulation in the laser beam that

rebounds from the vibrating target. Laser vibrometers can automatically scan large

numbers of consecutive points, delivering vibration measurements with high spatial

resolution.

When a transformer is loaded, vibration energy from the coil and any flux control

devices is transmitted to the tank and then to the air and local environment. It is

therefore important to design the tank so that it does not resonate at frequencies

close to the exciting frequency. Measures like resonance absorbers can gain 3 dB.





On-site solutions

A common on-site method of containing noise radiation is tank-supported wall panels.

They generally cover only the sides of the tank, bringing gains of between 4 dB and

10 dB depending on the wall area they cover. They may affect cooling, so acoustic

barriers are often used, mounted close to the transformer on one or more sides, or

enclosing it.

The simplest solution is a high acoustic screen, which must extend past each end of

the transformer by at least as much as it exceeds the height of the transformer. But

even single barriers can lower noise levels by 10 – 15 dB, depending on the

position of the observer.

Acoustic holography is used to map transformer noise

Complete top-bottom-and-side enclosure, of course, produces the most radical

results, up to 25 dB of abatement, or even 40 dB if the enclosure is a massive

structure made of concrete or steel and fully vibration-insulated. Care should always

be taken that the space between tank wall and the barrier is not an even multiple of

half of the wave length of the power frequency, e.g., 1,7 m, 3,4 m, etc. for 50 Hz

transformers.

The result is standing waves that will cause echoes and amplify sound levels.

Attenuation depends on how and how many of these methods are used. Combining

Hi-B step-lapped core lamination with core vibration isolators can gain 6 dB. Add tank-





mounted wall panels and that is 10 dB.

For greater improvement, a total contact-free enclosure is the answer.

Of course, designers can build low noise into transformers by lowering the core’s

induction level, or flux density. But the trade-off is a larger core, larger windings and

higher costs.

Need for Research and Development

Reasearch and development is addressing the need for reduced sound levels.

Some abatement techniques are well known, but others can be very innovative, such

as resonance absorbers or resilient internal lining absorbers. Most of the selected

solutions require a good knowledge of noise field and vibration mapping. New

techniques are available to identify this information and to better characterize noise

sources.

Benefits can be a reduction of measurement time, facilitated interpretation of

measurements, access to other information (as in source localization), and more.





Temperature Rise (Heat Run)

The transformer is energized at rated voltage in order to generate core losses. The

windings are connected to a loading transformer that simultaneously circulates rated

currents through all of the windings in order to develop load losses.

Naturally, the excitation voltage and the applied circulating currents are electrically

90° apart to minimize the KW requirements for this test. Nonetheless, a large power

transformer can consume up to 1 MW of total losses and the heat run test is an

expensive test to perform.

Therefore, in order to reduce the total expense, heat run tests are normally

performed on only one transformer on a purchase order for multiple transformers,

unless the customer chooses to pay for testing additional units.





Short-Circuit Test

The short-circuit test is generally reserved for a sample transformer to verify the

design of a core and coil assembly unless the customer specifies that a short-

circuit test be performed on transformers that are purchased.

The customer should be cognizant of the ever-present risk of damaging the

transformer during short-circuit tests.

A low-voltage impulse (LVI) current waveform is applied to the transformer before and

after the applications of short-circuit test. The ‘‘before’’ and ‘‘after’’ oscillograms of

the LVI currents are compared for significant changes in waveshape that could

indicate mechanical damage to the windings.








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Documents

Factory TestsFactory Testsdownload/files/Transformer... · 2019-08-23 · transformer total mass is more than 400 ton and it operates in power plant in Germany, owned by SWM Infrastruktur