IEC 60076-18:2012

IEC 60076-18 ®
Edition 1.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Power transformers –
Part 18: Measurement of frequency response

Transformateurs de puissance –
Partie 18: Mesure de la réponse en fréquence

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IEC 60076-18 ®
Edition 1.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Power transformers –
Part 18: Measurement of frequency response

Transformateurs de puissance –

Partie 18: Mesure de la réponse en fréquence

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 29.180 ISBN 978-2-83220-222-7

– 2 – 60076-18 © IEC:2012
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Terms and definitions . 7
3 Purpose of frequency response measurements . 8
4 Measurement method . 9
4.1 General . 9
4.2 Condition of the test object during measurement . 10
4.3 Measurement connection and checks . 11
4.3.1 Measurement connection and earthing . 11
4.3.2 Zero-check measurement . 11
4.3.3 Repeatability check . 11
4.3.4 Instrument performance check . 11
4.4 Measurement configuration . 12
4.4.1 General . 12
4.4.2 Principles for choosing the measurement configuration . 12
4.4.3 Star- and auto-connected windings with a neutral terminal . 13
4.4.4 Delta windings and other windings without an accessible neutral . 13
4.4.5 Zig-zag connected windings. 14
4.4.6 Two-winding three-phase transformers . 14
4.4.7 Three-phase auto-transformers . 14
4.4.8 Phase shifting transformers . 14
4.4.9 Reactors . 14
4.4.10 Method for specifying additional measurements . 14
4.5 Frequency range and measurement points for the measurement . 15
5 Measuring equipment . 15
5.1 Measuring instrument . 15
5.1.1 Dynamic range . 15
5.1.2 Amplitude measurement accuracy . 16
5.1.3 Phase measurement accuracy . 16
5.1.4 Frequency range . 16
5.1.5 Frequency accuracy . 16
5.1.6 Measurement resolution bandwidth . 16
5.1.7 Operating temperature range . 16
5.1.8 Smoothing of recorded data . 16
5.1.9 Calibration . 16
5.2 Measurement leads . 16
5.3 Impedance . 17
6 Measurement records . 17
6.1 Data to be recorded for each measurement . 17
6.2 Additional information to be recorded for each set of measurements . 18
Annex A (normative) Measurement lead connections . 20
Annex B (informative) Frequency response and factors that influence the
measurement . 23
Annex C (informative) Applications of frequency response measurements . 37

60076-18 © IEC:2012 – 3 –
Annex D (informative) Examples of measurement configurations . 39
Annex E (informative) XML data format . 43
Bibliography . 44

Figure 1 – Example schematic of the frequency response measurement circuit. 10
Figure A.1 – Method 1 connection . 21
Figure A.2 – Method 3 connection . 22
Figure B.1 – Presentation of frequency response measurements . 23
Figure B.2 – Comparison with a baseline measurement . 24
Figure B.3 – Comparison of the frequency responses of twin transformers . 24
Figure B.4 – Comparison of the frequency responses from sister transformers . 25
Figure B.5 – Comparison of the frequency responses of three phases of a winding. 25
Figure B.6 – General relationships between frequency response and transformer
structure and measurement set-up for HV windings of large auto-transformer . 27
Figure B.7 – Effect of tertiary delta connection on the frequency response of a series
winding . 28
Figure B.8 – Effect of star neutral connection on the tertiary winding response . 29
Figure B.9 – Effect of star neutral termination on series winding response . 29
Figure B.10 – Measurement results showing the effect of differences between phases
in internal leads connecting the tap winding and OLTC . 30
Figure B.11 – Effect of measurement direction on frequency response . 30
Figure B.12 – Effect of different types of insulating fluid on frequency response . 31
Figure B.13 – Effect of oil filling on frequency response . 31
Figure B.14 – Effect of a DC injection test on the frequency response . 32
Figure B.15 – Effect of bushings on frequency response . 32
Figure B.16 – Effect of temperature on frequency response . 33
Figure B.17 – Examples of bad measurement practice . 34
Figure B.18 – Frequency response of a tap winding before and after partial axial
collapse and localised inter-turn short-circuit with a photograph of the damage . 34
Figure B.19 – Frequency response of an LV winding before and after axial collapse
due to clamping failure with a photograph of the damage [8] . 35
Figure B.20 – Frequency response of a tap winding with conductor tilting with a
photograph of the damage [1] . 36
Figure D.1 – Winding diagram of an auto-transformer with a line-end tap changer . 40
Figure D.2 – Connection diagram of an inductive inter-winding measurement on a
three-phase YNd1 transformer . 41
Figure D.3 – Connection diagram for a capacitive inter-winding measurement on a
three-phase YNd1 transformer . 42
Figure D.4 – Connection diagram for an end-to-end short-circuit measurement on a
three-phase YNd1 transformer . 42

Table 1 – Standard measurements for a star connected winding with taps . 13
Table 2 – Standard measurements for delta connected winding without tap . 14
Table 3 – Format for specifying additional measurements . 15
Table D.1 – Standard end-to-end measurements on a three-phase auto-transformer . 39
Table D.2 – Tap-changer connections . 40

– 4 – 60076-18 © IEC:2012
Table D.3 – Inductive inter-winding measurements on a three-phase YNd1 transformer . 41
Table D.4 – Capacitive inter-winding measurements on a three-phase YNd1
transformer . 41
Table D.5 – End-to-end short-circuit measurements on a three-phase YNd1
transformer . 42

60076-18 © IEC:2012 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 18: Measurement of frequency response

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60076-18 has been prepared by IEC technical committee 14:
Power transformers.
The text of this standard is based on the following documents:
FDIS Report on voting
14/718/FDIS 14/728/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60076 series can be found, under the general title Power
transformers, on the IEC website.

– 6 – 60076-18 © IEC:2012
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
60076-18 © IEC:2012 – 7 –
POWER TRANSFORMERS –
Part 18: Measurement of frequency response

1 Scope
This part of the IEC 60076 series covers the measurement technique and measuring
equipment to be used when a frequency response measurement is required either on-site or
in the factory either when the test object is new or at a later stage. Interpretation of the result
is not part of the normative text but some guidance is given in Annex B. This standard is
applicable to power transformers, reactors, phase shifting transformers and similar
equipment.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
frequency response
amplitude ratio and phase difference between the voltages measured at two terminals of the
test object over a range of frequencies when one of the terminals is excited by a voltage
source
Note 1 to entry: The frequency response measurement result is a series of amplitude ratios and phase differences
at specific frequencies over a range of frequency.
Note 2 to entry: The measured voltage is the voltage developed across an impedance and so it is also related to
current.
2.2
frequency response analysis
FRA
technique used to detect damage by the use of frequency response measurements
Note 1 to entry: The terms SFRA and IFRA are commonly used and refer to the use of either a swept frequency
voltage source or an impulse voltage source. Provided the measuring equipment complies with the requirements of
Clause 5, this standard can be applied to both techniques.
2.3
source lead
lead connected to the voltage source of the measuring instrument used to supply an input
voltage to the test object
2.4
reference lead
V
in
lead connected to the reference channel of the measuring instrument used to measure the
input voltage to the test object
2.5
response lead
V
out
lead connected to the response channel of the measuring instrument used to measure the
output voltage of the test object

– 8 – 60076-18 © IEC:2012
2.6
end-to-end measurement
frequency response measurement made on a single coil (phase winding) with the source and
reference (V ) leads connected to one end and the response (V ) lead connected to the
in out
other end
2.7
сapacitive inter-winding measurement
frequency response measurement made on two adjacent coils (windings of the same phase)
with the source and reference (V ) leads connected to one end of a winding, the response
in
(V ) lead connected to one end of another winding and with the other winding ends floating
out
Note 1 to entry: This type of measurement is not applicable to windings which have common part or connection
between them.
2.8
inductive inter-winding measurement
frequency response measurement made on two adjacent coils (windings of the same phase)
with the source and reference (V ) leads connected to one end of the higher voltage winding,
in
the response (V ) lead connected to one end of the other winding and with the other ends of
out
both windings grounded
2.9
end-to-end short circuit measurement
frequency response measurement made on a single coil (phase winding) with the source and
reference (V ) leads connected to one end, the response (V ) lead connected to the other
in out
end, and another winding of the same phase short-circuited
2.10
baseline measurement
frequency response measurement made on a test object to provide a basis for comparison
with a future measurement on the same test object in the same configuration
3 Purpose of frequency response measurements
Frequency response measurements are made so that Frequency Response Analysis (FRA)
can be carried out. FRA can be used to detect changes to the active part of the test object
(windings, leads and core).
NOTE FRA is generally used to detect geometrical changes and electrical short-circuits in the windings, see
Annex B.
Some examples of conditions that FRA can be used to assess are:
• damage following a through fault or other high current event (including short-circuit
testing),
• damage following a tap-changer fault,
• damage during transportation, and
• damage following a seismic event.
Further information on the application of frequency response measurements is given in
Annex C.
The detection of damage using FRA is most effective when frequency response measurement
data is available from the transformer when it is in a known good condition (baseline
measurement), so it is preferable to carry out the measurement on all large transformers
either in the factory or when the transformer is commissioned at site or both. If a baseline

60076-18 © IEC:2012 – 9 –
measurement is not available for a particular transformer, reference results may be obtained
from either a similar transformer or another phase of the same transformer (see Annex B).
Frequency response measurements can also be used for power system modelling including
transient overvoltage studies.
4 Measurement method
4.1 General
To make a frequency response measurement, a low voltage signal is applied to one terminal
of the test object with respect to the tank. The voltage measured at this input terminal is used
as the reference signal and a second voltage signal (the response signal) is measured at a
second terminal with reference to the tank. The frequency response amplitude is the scalar
ratio between the response signal (V ) and the reference voltage (V ) (presented in dB) as
out in
a function of the frequency. The phase of the frequency response is the phase difference
between V and V (presented in degrees).
in out
The response voltage measurement is made across an impedance of 50 Ω. Any coaxial lead
connected between the test object terminal and the voltage measuring instrument shall have a
matched impedance. To make an accurate ratio measurement, the technical parameters of
the reference and response channels of the measuring instrument and any measurement
leads shall be identical.
NOTE 1 The characteristic impedance of the coaxial measuring leads is chosen to match the measuring channel
input impedance to minimise signal reflections and reduce the influence of the coaxial lead on the measurement to
the point where it has little or no practical effect on the measurement within the measurement frequency range.
With a matched impedance lead, the measuring impedance is effectively applied at the test object terminal.
NOTE 2 As V /V varies over a wide range, it is expressed in decibels (dB). The relative voltage response in dB
out in
is calculated as 20 × log (V /V ), where (V /V ) is the scalar ratio.
10 out in out in
An example of the general layout of the measurement method using coaxial measuring leads
is shown in Figure 1.
– 10 – 60076-18 © IEC:2012
B
A
C
D
50 Ω
50 Ω
V V
out
in
IEC  1370/12
A source lead
B reference lead
C response lead
D earth connection
Figure 1 – Example schematic of the frequency response measurement circuit
4.2 Condition of the test object during measurement
For factory and site measurements, the test object shall be fully assembled as for service
complete with all bushings, but coolers and related auxiliaries do not need to be assembled.
Liquid or gas filled transformers and reactors shall be filled with liquid or gas of the same type
(similar relative permittivity) as that which is to be used in service. All busbars or other system
or test connections shall be removed and there shall be no connections to the test object
other than those being used for the specific measurement being performed. If internal current
transformers are installed but not connected to a protection or measurement system, the
secondary terminals shall be shorted and earthed. The core and frame to tank connections
shall be complete and the tank shall be connected to earth.
If the transformer is not assembled in the factory in the service condition, for example if oil/air
bushings are used in the factory and oil/SF bushings are to be used in service then the FRA
baseline measurement can only be performed at site. Transport configuration measurements
may still be possible see below.
If special connections have been specified by the purchaser and are provided on the test
object to enable a frequency response measurement to be made when it is arranged for
transport, then additional measurements shall be made in the transport configuration (drained
if required for transport) before transport and when delivered to site or as specified by the
purchaser.
For site measurements, the test object shall be disconnected from the associated electrical
system at all winding terminals and made safe for testing. Line, neutral and any tertiary line
connections shall be disconnected but tank earth, auxiliary equipment and current transformer
service connections shall remain connected. In the case where two connections to one corner
of a delta winding are brought out, the transformer shall be measured with the delta closed
(see also 4.4.4). In instances where it is impossible to connect directly to the terminal, then
the connection details shall be recorded with the measurement data since the additional bus
bars connected to the terminals may impact on the measurement results.

60076-18 © IEC:2012 – 11 –
NOTE There may be a difference in the connection of current transformers (CTs) between measurements made
on-site and those made in the factory, the change in frequency response between a transformer with shorted and
earthed CTs and one with the CTs connected to a low impedance protection system is normally negligible.
If the transformer is directly connected to SF insulated busbars then it may be possible to
make the measurement by connecting to the disconnected earth connection of an earth
switch. In this case, the measurement shall be made both directly on the terminals before the
SF busbar is assembled and using the earth switch.
When carried out in the factory, the measurement shall be conducted at approximately
ambient temperature (for example not immediately following a temperature rise test). The
temperature of the test object dielectric (normally top liquid temperature) during the
measurement shall be recorded. For measurements made on-site the temperature is not
controlled, and although extreme temperatures may have a minor effect this is normally not
significant. The effect of temperature on frequency response measurements is illustrated in
B.4.8.
It is recommended that if possible measurements on-site are not made whilst the test object
temperature is changing rapidly for example immediately following oil treatment.
4.3 Measurement connection and checks
4.3.1 Measurement connection and earthing
The methods of connection of the leads and lead earths to the test object are given in
Annex A.
Poor connections can cause significant measurement errors, attention shall be paid to the
continuity of the main and earth connections. The continuity of the main and earth
connections shall be checked at the instrument end of the coaxial cable before the
measurement is made. In particular, connections to bolts or flanges shall be verified to ensure
that there is a good connection to the winding or the test object tank.
4.3.2 Zero-check measurement
If specified, a zero-check measurement shall be carried out as an additional measurement.
Before measurements commence, all the measuring leads shall be connected to one of the
highest voltage terminals and earthed using the normal method. A measurement is then made
which will indicate the frequency response of the measurement circuit alone. The zero check
measurement shall also be repeated on other voltage terminals if specified.
The zero-check measurement can provide useful information as to the highest frequency that
can be relied upon for interpretation of the measurement. The zero-check measurement is not
a calibration check and no attempt should be made to remove any deviations seen in the
zero-check measurement from the measurement results.
4.3.3 Repeatability check
On completion of the standard measurements the measurement leads and earth connections
shall be disconnected and then the first measurement shall be repeated and recorded.
This check is necessary to evaluate the repeatability and useable diagnostic frequency range
under the specific conditions of the measurement.
4.3.4 Instrument performance check
To verify the performance of the instrument, one of the following three checks shall be made
whenever the performance of the instrument is in doubt.
a) Connect the source, reference and response channels of the instrument together
using suitable low loss leads, check that the measured amplitude ratio is 0 dB
± 0,3 dB across the whole frequency range.

– 12 – 60076-18 © IEC:2012
Connect the source and reference channels together and leave the response terminal
open circuit, check that the measured amplitude ratio is less than -90 dB across the
whole frequency range.
b) The performance of the instrument may be checked by measuring the response of a
known test object (test box) and checking that the measured amplitude ratio matches
the expected response of the test object to within the requirements given in 5.1.2 over
the whole frequency range. The test object shall have a frequency response that
covers the attenuation range –10 dB to –80 dB.
c) The correct operation of the instrument may be checked using a performance check
procedure provided by the instrument manufacturer. This performance check
procedure shall verify that the instrument is operating within the parameters given in
5.1.2 at least over an attenuation range of –10 dB to –80 dB over the whole frequency
range.
4.4 Measurement configuration
4.4.1 General
For common transformer and reactor winding configurations, a standard set of measurements
is given which is sufficient in the majority of cases to provide a baseline measurement. These
measurements shall be made in all cases. Additional measurements may be specified if
required either to provide some additional information under particular circumstances or to
match previous measurements. Standard measurements on other types of transformers and
reactors shall follow the following principles.
4.4.2 Principles for choosing the measurement configuration
4.4.2.1 Type of measurement
The standard measurements shall be end-to-end measurements of each phase of each
winding, with the phases and windings separated as far as possible and with all other
terminals left floating. Additional measurements, where specified, can include capacitive inter-
winding, inductive inter-winding, and end-to-end short circuit measurements.
4.4.2.2 Tap-position
For transformers and reactors with an on-load tap-changer (OLTC), the standard
measurement on the tapped winding shall be
a) on the tap-position with the highest number of effective turns in circuit, and
b) on the tap-position with the tap winding out of circuit.
Other windings with a fixed number of turns shall be measured on the tap-position for the
highest number of effective turns in the tap winding. Additional measurements may be
specified at other tap-positions.
For auto-transformers with a line-end tap-changer, the standard measurements shall be:
• on the series winding with the minimum number of actual turns of the tap-winding in circuit
(the tapping for the highest LV voltage for a linear potentiometer type tapping
arrangement or the change-over position for a reversing type tapping arrangement, or the
tapping for the lowest LV voltage in a linear separate winding tapping arrangement),
• on the common winding with the maximum number of effective turns of the tap-winding in
circuit (the tapping for the highest LV voltage), and
• on the common winding with the minimum number of actual turns of the tap-winding in
circuit (the tapping for the lowest LV voltage for a linear potentiometer or separate winding
type tapping arrangement or the change-over position for a reversing type tapping
arrangement).
60076-18 © IEC:2012 – 13 –
NOTE 1 The choice of tap-position is intended to provide at least one measurement with and one without the tap
winding in circuit so that any damage can be more easily identified as being in the tap-winding or the main winding.
For neutral or change-over positions, the direction of movement of the tap-changer shall be in
the lowering voltage direction unless otherwise specified. The direction of movement (raise or
lower) shall be recorded.
NOTE 2 The position of the change-over selector in reversing and coarse-fine arrangements has a profound effect
on the measured frequency response.
For transformers with both an OLTC and a de-energised tap-changer (DETC), the DETC shall
be in the service position if specified or otherwise the nominal position for the measurements
at the OLTC positions described in 4.4.2.2.
For transformers fitted with a DETC, baseline measurements shall also be made on each
position of the DETC with the OLTC (if fitted) on the position for maximum effective turns.
It is not recommended that the position of a DETC on a transformer that has been in service
is changed in order to make a frequency response measurement, the measurement should be
made on the ‘as found’ DETC tap position. It is therefore necessary to make sufficient
baseline measurements to ensure that baseline data is available for any likely service (‘as
found’) position of the DETC.
4.4.3 Star- and auto-connected windings with a neutral terminal
For the standard measurement, the signal shall be applied to the line connection, or for series
windings the higher voltage terminal. An additional measurement may be specified with the
signal applied to the neutral terminal if this is required for compatibility with previous
measurements. A star connected winding with the neutral not brought out shall be treated as
a delta winding. The list of standard measurements for a star connected winding with taps is
given in Table 1.
Table 1 – Standard measurements for a star connected winding with taps
Measurement Source and reference lead (V ) Response lead (V ) Tap position
in out
number connected to connected to
1 Line terminal phase 1 Neutral Max effective turns
2 Line terminal phase 2 Neutral Max effective turns
3 Line terminal phase 3 Neutral Max effective turns
4 Line terminal phase 1 Neutral Tap winding out of circuit
5 Line terminal phase 2 Neutral Tap winding out of circuit
6 Line terminal phase 3 Neutral Tap winding out of circuit

4.4.4 Delta windings and other windings without an accessible neutral
If delta windings can be split into individual phases (six bushings brought out) then the
standard measurement shall be made with the windings split.
For large generator transformers where it is inconvenient to remove the phase to phase
connections in service it is recommended that the baseline measurement in the factory and
during commissioning is performed both with the delta open and closed.
Standard measurements shall be made on each phase in turn with the signal applied to the
terminal with the lowest number or letter nearest the start of the alphabet first and the
response measured on the next numbered or lettered terminal, and continuing in a cyclic
rotation (see Table 2).
For delta tertiary or stabilising windings, the delta shall be closed.

– 14 – 60076-18 © IEC:2012
For delta tertiary or stabilising windings that are earthed at one corner in service, the earth
shall be removed if possible without removing liquid or gas.
Table 2 – Standard measurements for delta connected winding without tap
Measurement number Source and reference lead (V ) connected to Response lead (V ) connected to
in out
1 A, U, R or 1 B, V, S or 2
2 B, V, S or 2 C, W, T or 3
3 C, W, T or 3 A, U, R or 1
4.4.5 Zig-zag connected windings
Zig-zag connected windings shall be measured as star windings with a neutral connection.
NOTE The correspondence between the frequency responses of different phases of a zig-zag connected winding
is not expected to be as close as would typically be expected for a star connected winding.
4.4.6 Two-winding three-phase transformers
The standard measurements shall be one measurement of each phase of each winding, a
total of six measurements for a transformer without taps and nine for a transformer with an
on-load tap-changer.
4.4.7 Three-phase auto-transformers
The standard measurements shall be one measurement of each phase of the series winding
and the common winding separately with an additional measurement of the common winding
for transformers with an on-load tap changer, a total of six measurements for a transformer
without taps and nine for a transformer with an on-load tap-changer. If the transformer has a
tertiary winding brought out to three line (phase) terminals an additional three measurements
are required on this winding.
4.4.8 Phase shifting transformers
The standard measurement shall be from input terminal to output terminal on each phase and
from the neutral of the shunt winding to the output terminal on each phase, each on neutral
tap and on each extreme tap, a total of 18 measurements. If the phase shifting transformer is
of the two core type that has external interconnections that can be removed on site then it
shall be treated as two separate transformers.
4.4.9 Reactors
Series reactors shall be measured from input terminal to output terminal on each phase, a
total of three measurements for a three-phase reactor. Shunt reactors shall be treated as a
star winding on a transformer, a total of three measurements for a three-phase reactor without
taps and six for a reactor with taps.
4.4.10 Method for specifying additional measurements
Additional measurements, if required, shall be specified by giving the connection to each test
object terminal (signal and reference, response, earthed, floating or connected together), the
tap-position and the previous tap-position for each additional measurement. The format
presented in Table 3 shall be used.

60076-18 © IEC:2012 – 15 –
Table 3 – Format for specifying additional measurements
Measurement Tap Previous Source and Response Terminals Terminals Comments
tap reference (V ) earthed connected
out
(V ) together
in
.
.
.
The terminal identification entered in the table shall be those permanently marked on the test
object and shall be shown on a diagram included in the specification.
Examples of particular measurement configurations using this format are given in Annex D.
4.5 Frequency range and measurement points for the measurement
The lowest frequency measurement shall be at or below 20 Hz.
The minimum highest frequency measurement for test objects with highest voltage > 72,5 kV
shall be 1 MHz.
The minimum highest frequency measurement for test objects with highest voltage of
≤ 72,5 kV shall be 2 MHz.
It is recommended that a highest measurement frequency of at least 2 MHz is used for
compatibility and simplicity for all test objects.
NOTE Reproducibility of the measurement is better at frequencies higher than 1 MHz with the shorter earth
connections possible with smaller bushings and higher frequency information is more important for the diagnosis of
physically smaller windings (see B.3).
Below 100 Hz, measurements shall be made at intervals not exceeding 10 Hz; above 100 Hz,
a minimum of 200 measurements approximately evenly spaced on either a linear or
logarithmic scale shall be made in each decade of frequency.
If the transfo
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