chapter 5 interpretation of sfra

Chapter 5 Interpretation of SFRA

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Chapter 5

Interpretation of SFRA

5.1 Introduction

The Sweep frequency Response Analysis (SFRA) is an emerging method for

investigation of transformer mechanical integrity after through fault in the system and

its relocation. There are cases found, where SFRA has been a key tool in the decision

making either to scrap, rewind or reenergize a transformer after an incident. Based on

the practical experience with SFRA analysis, the frequency range from 10Hz to

2MHz is sufficient for the analysis and can be divided into three frequency band.

These frequency bands are governed separately by the inductive effect of core, self

and mutual inductance of the winding, series and shunt capacitance of the overall

winding structures and the lead/tap connections.

Interpretation of SFRA responses is crucial in order to assess the integrity of

transformer windings. In order to achieve the correct interpretation of SFRA response,

the effect of various circuit parameters of transformer winding on SFRA plot is

studied in detail and discussed one of the major factors that influenced the SFRA

responses, the winding structure itself in low, medium and high frequency range. [4]

5.2 Modeling of Transformer Winding for Interpretation of SFRA

5.2.1 Basic Circuit of Transformer

SFRA normally measures the frequency response of a transformer from 10Hz to

2MHz. Circuit modeling thus needs to accurately represent the behavior of a

transformer across this wide range of frequency. But, no such universal circuit model

exists that can represent a transformer accurately over this entire range. Hence,

modeling techniques for SFRA have been developed in several frequency regions,

depending on the modeling accuracy required and the dominant components in each

frequency region. The different type of circuit model in each frequency band, which is

considered for the SFRA analysis are described below.


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5.2.2 Low frequency model

The equivalent circuit of transformer winding at low frequency from 10 Hz. to 1000

Hz. is shown in Fig. 5.1

Fig. 5.1 Equivalent circuit of Transformer winding at Low frequency

The dominant features of low frequency plots are the first minima at low frequency

normally below 1000 Hz in all windings. This is the general feature of any winding

and is due to the fact that at the lowest frequencies windings behave as simple

inductances. This result in increasing attenuation of a transmitted signal with

frequency, until a frequency is reached when core capacitance start to become

significant and allow a recovery in transmitted voltage. The low frequency minimum

is determined by self inductance of winding, inductance and capacitance of core. The

position of minimum will vary somewhat depending on the remnant magnetism of

relevant core flux circuits, which is prominent in this case due to different magnetic

state of the winding. In low frequencies, a transformer winding behaves as an

inductive element, and the SFRA response follows a increasing negative magnitude

trend across the frequency range with a linear slope and this may not be exact linear

also due to core non-linearity with frequencies. As the inductance is increased, the

magnitude is increased. Power transformers with higher voltage and larger power

rating usually have larger negative response magnitudes. Effectively there are two

parts of inductance affecting the SFRA response the core magnetizing inductance and

the self inductance of the windings. Each affects the response in different frequency

ranges. The leakage inductance affects the SFRA response in lower frequencies of no

more than 100 Hz while the core magnetizing inductance influences the SFRA

response at high frequencies up to 1 kHz.


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The magnetizing inductance of the core, Lm which decides the magnitude of Xm in

Fig. 5.1 is influenced by the winding number of turns, N and the reluctance, R which

is given by

Lm = N2 (5.1)

R

The magnetic path of the middle phase is different compared to the magnetic path of

the outer phases due to the symmetrical core construction of transformer in case of

middle phase and it is also affecting SFRA. This magnetic reluctance, R is analogous

to the resistance in the electrical circuit and thereby is influenced by the length of the

magnetic path, l and the area of the cross section of the core, A.

The inductance is divided in two groups self and mutual inductance of the winding as

shown in Fig. 5.2

Fig. 5.2 Winding self and mutual inductance

Because of such a coarse representation of the windings, localized winding movement

will not be reflected in this low frequency region unless the winding moves

significantly. The SFRA measurement in the low frequency region are primarily used

to detect problems related to the transformer core and major winding faults like

shorted turn, open circuit and high impedance fault in the early developing stage. [4]

5.2.3 Medium frequency model

The equivalent circuit of transformer winding at medium frequency from 1 kHz to

1000 kHz is shown in Fig. 5.3


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Figure 5.3: n-stages lumped ladder network

In medium frequency range, as the frequency increases, the effect of core will become

less significant as the flux penetration depth in the core is frequency - dependent and

it is worst effected by DC voltage creating the core saturation problem after the DC

test like resistance measurement. Hence, in medium frequency range around 10 kHz,

core will behave as an earth plate. The winding structure, especially the winding

under test, becomes dominant factor of the frequency responses.

Therefore, it is necessary to use the multiple LC element equivalent network to model

the winding accurately in medium frequency range. However, in transformer winding,

the basic components are combined together and the transformer winding structure

becomes more complex than a simple LC element.

To represent a winding accurately in the medium frequency range, a detailed RLC

ladder network of the winding is required. Each winding is divided into cells. The cell

is represented as lumped - element unit, which consists of a series capacitance (Cs)

and a self inductance (L). The capacitive coupling between the cells and the tank wall

(Cg) for the outer winding cell and for the inner winding cell the shunt capacitance

are included between the cell and the core. This transformer model is considered to be

detailed enough to provide reasonably accurate SFRA results in the frequency range

governed by the main winding structure , which is normally from about 10 kHz to 500

kHz.


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A uniformly structured winding can be represented by an n-stage ladder network, as

shown in Figure 5.3. The winding total leakage inductance L, the winding total series

capacitance Cs and the total shunt capacitance Cg are evenly distributed between the

n stages. The effect of dielectric losses or resistances connected either in series with

the inductance or connected in parallel with the capacitance on the SFRA response, is

to attenuate the sharpness of the resonances and the anti-resonances. The effect on the

sharpness of resonance of series RLC circuit due to change in the resistance is shown

in Fig. 5.4 and Fig. 5.5

Fig. 5.4 Series resonance of the RLC circuit having low R value

Fig. 5.5 Series resonance of the RLC circuit having high R value


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The combination of winding inductance and winding series capacitance results in

parallel LC circuit and will produce parallel anti-resonance, consequently, blocking

the signal at that particular frequency. Also, the simplest representation of LC in

series is a T − connection where the shunt capacitance is connected in the middle of

the two halves of the winding inductance. The SFRA response of winding inductance

and shunt capacitance in the LC network shows a series resonance, amplifying the

signal at that particular frequency. In summary, the basic features of SFRA response

can be shown in Fig. 5.6

Fig. 5.6 Series and Parallel resonance of the winding

However, in transformer winding, these basic components are combined together and

the transformer winding structure becomes complex.

The general solution for voltage and current at any point x on the network shown in

Fig. 5.3 can be represented by Equation (5.2)

(5.2)


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Where,

A and B are constants, x is the number of stages along the winding, starting from the

injecting end, Z is the characteristic impedance and γ is the propagation constant of

the winding.

SFRA response oscillate between capacitive and inductive and when multiple local

resonances are produced at the frequencies as

(5.3)

In terms of the structure of single windings, these can be categorized into windings

with either high- or low- series capacitance in proportion to the shunt capacitance.

Correspondingly, the SFRA responses of transformer windings of high series

capacitance exhibited the increasing trend of magnitude in the frequency range

between 10 kHz and 500 kHz while the windings of low series capacitance displayed

the steady magnitude trend with the resonances and anti-resonances (camel humps)

features in the frequency range between 10 kHz and 2MHz.

Fig. 5.7 illustrates the effect of having high or low series capacitance, Cs in the 8-

stage lumped network obtained from simulation. With low Cs, the response begins

with flat magnitude trend and resonances at intervals of frequencies determined by

Equation (5.3) and then followed by a decreasing inductive trend. In Fig. 5.7, it is

illustrated that as Cs is increased, some of the resonances diminish and the anti-

resonance appears at lower frequency.


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Fig. 5.7 FRA response from 8-stage of lumped ladder network (L=800H) with

extreme cases of (a) Cs=0, Cg=480pF and (b) Cg=0, CsLOW=190pF, CsHIGH=3000

pF

The extreme cases of the 8-stage lumped network with negligible Cs or Cg are shown

in Figure 5.7. Fig. 5.7(a) depicts the features of winding with low Cs such as the

continuous disc while Fig. 5.7(b) depicts the features of winding with high Cs or

negligible Cg in comparison to Cs such as the interleaved winding.

Using the knowledge gained from the experimental studies during this research and

theoretical back-up, this factor is shown to dominate the SFRA responses of power

transformers in certain frequency ranges. In terms of the structure of single windings,

these can be categorized into windings with either high- or low- series capacitance in

proportion to the shunt capacitance. Correspondingly, the SFRA responses of

transformer windings of high series capacitance exhibited the increasing trend of

magnitude in the frequency range between 20kHz and 500kHz while the windings of

low series capacitance displayed the steady magnitude trend with the resonances and

anti-resonances (camel humps) features in the frequency range between 20kHz and

2MHz. [4]


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5.2.4 High frequency model

In high frequencies, a transformer winding behaves as a capacitive element, and

power transformers having both higher voltage and larger power rating usually have

smaller negative response magnitudes in high frequency range as the capacitance is

high. At very high frequencies, the network can be represented as a capacitive ladder

network as shown in Fig. 5.8

Fig. 5.8 n-stage capacitive ladder network at high frequencies

The general solution of this equivalent circuit can be represented by Equation (5.4).

(5.4)

Where,

To be accurate in higher frequencies, a transformer winding would need to be

represented in more detail. A distributed parameter model using Multiple

Transmission line theory is then needed. This modeling technique treats each turn of

the winding as one transmission line. The parameters of the winding are calculated as

distributed capacitance per unit length and the high frequency signal travels through

the winding as transverse electromagnetic waves.

This method of detailed winding modeling ensures sufficient accuracy for the higher

frequency range, where effects such as the arrangement of tapping lead connections

are regarded as significant. However, representing all of the phase windings down to

the details of individual turns will result in a massive matrix size. This modeling

technique is only suitable to model a part of winding or the lead connections, whilst

the rest of the transformer is modeled simply as a ladder network. [4]


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5.3 Basics of FRA Interpretation

5.3.1 Expected Resonance Frequency Range vs. Transformer Size and Winding

Type

Due to the typical range for natural frequencies, the interpretation methodology

should be adjusted according to the expected damage appearance. For investigating

winding displacements, FRA measurement and interpretation should then focus on the

natural frequency range of the respective windings.

For power transformer windings, there are various technical concepts. Even for

similar rated power, rated voltage and type of application, there could be very

different solutions. Details of technical solutions are defined by established design

concepts of manufacturers as well as technical boundaries in manufacturing and

transportation restrictions. Therefore, it is quite difficult to summarize general rules

for FRA patterns and the corresponding winding characteristics.

Natural frequencies are mainly defined by the absolute geometry of winding

assemblies. Based on the typical frequencies of large power transformers, smaller

transformers show similar frequency characteristics at systematically higher

frequencies.

Table 5.1 shows the expected resonance frequency range for windings of large power

transformer (above 100 MVA/limb) of different rated voltage. Table 5.2 shows

typical frequency ranges for windings of medium-power transformers (below 30

MVA/limb).

It is also to be noted that these tables represent calculation examples of the natural

resonance frequencies of separate windings. The interactions between components in

a real transformer could show different frequency characteristics. [15]


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Table 5.1 Frequency range for natural frequencies of large transformer windings. [15]

Table 5.2 Frequency range for natural frequencies of medium transformer windings.

[15]

5.3.2 Typical FRA Responses

It is observed that although the detailed form of a frequency response depends on the

winding design used, usually the basic overall form of the response is surprisingly

similar for the same type of winding, even for significantly different winding

arrangements, and therefore is presumably determined by some essential

distinguishing property of the type of winding involved rather than by the details of its

construction. The method of interconnection used, e.g. LV delta connection, also

results in very characteristic forms.

In view of the above, it is useful to be able to recognize typical features when

interpreting responses. In the following, ‘typical’ responses for various winding types

are described:

• HV windings of transmission and generator transformers (core-form)

• LV windings of generator transformers and double-wound network

transformers (core -form)


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• Shell-form transformers

Although specific examples are used to illustrate the typical responses, it is expected

that the general features described will be relevant to a wide range of transformers,

provided the windings involved are of an essentially similar type. [15]

5.3.2.1 HV windings of large power transformers

The essential features of the HV windings of large power transformers are that they

have large HV bushings, invariably have a large number of turns and for the highest

voltages are usually specially designed to spread out the distribution of high-

frequency impulses away from the line end terminals, usually by employing measures

to increase the series capacitance.

‘Typical’ HV responses exhibited by the series (HV to LV) windings of six three-

phase autotransformers (400/275 kV) with delta-connected tertiary windings are

shown in Fig. 5.9. Note that, although all six transformers are of the same type, they

feature a wide range of designs, built at various dates between 1967 and 2003 with

very different winding arrangements (multi-layer or inter-leaved disc): Despite the

large variation in age and design, there is a remarkable degree of similarity in the

general form of the responses.

Fig.5.9 HV winding response of large autotransformers

All show the usual open-circuit low-frequency (up to 2 kHz) response exhibited by all

transformer windings – an initially increasing attenuation with frequency (20 dB per


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decade) due to the basic core-influenced inductance of the winding becoming much

larger than the input impedance of the measuring equipment until the first minimum

(or two minima if an outer phase of a three phase transformer is involved), followed

by a voltage recovery to the first maximum, presumably due to the effects of series

capacitance becoming significant.

For measurements across the series or common windings of auto-transformers, there

is a characteristic second maximum in the intermediate frequency range (2 to 20 kHz),

which is known to be dependent on the shunt capacitance of the winding and affected

by bulk movement of the winding or bushing capacitances, among other factors. It is

not known for certain what causes this feature, but the most likely explanation is some

resonance between series and common windings.

In the high-frequency range (20 kHz to 2 MHz), all these transformers exhibit

essentially the same response: a generally rising response (roughly 20dB per decade),

starting from about 50 dB at around 20 kHz, until a maximum at or slightly above 0

dB, which invariably occurs at about 1 MHz Within this high-frequency range, there

may be evidence of ‘ripples’ (part-winding resonances) superimposed on the overall

generic rising trend, more marked for some transformers than others.

Since essentially very similar responses have been obtained from very different

winding arrangements (multi-layer and interleaved disc), it would appear that the

general form of the response is determined by some basic global property of such

arrangements, probably high-series capacitance, rather than the detailed geometry.

Although the HV responses shown in Fig. 5.9 are entirely typical, they are also

somewhat ideal: not all auto-transformers exhibit such smooth responses.

The LV (common) windings of auto-transformers tend to show the same basic

response as in Fig. 5.10, particularly if they are also of a layer construction, but can

also show very marked resonances, particularly if plain disc windings are used.

HV winding responses of three generator transformers from different manufacturers

are shown in Fig. 5.10. The characteristics of the transformers are:


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TA → 430/23.5 kV, 800 MVA, single-phase unit

TB → 300/23 kV, 735 MVA, three-phase unit

TC → 290/16 kV, 190 MVA, three-phase unit

Up to 20 kHz, the response of these three differ because of different core and LV

winding arrangement : a single-phase core (A), a three-phase core with separate LV

phases (B) and a three-phase core with LV phases connected in delta (C).

Above 20 kHz, the responses of all three exhibit the same general form as those for

transmission

transformer series windings.

Fig. 5.10 HV winding responses of large generator transformers

To conclude, the ‘typical’ response for an HV winding is considered to be essentially

that shown in Fig. 5.9 and Fig. 5.10, although there may be more evidence of

resonances in the high frequency region, depending on the detailed winding

arrangement used. [15]

5.3.2.2 LV windings of large power transformers

The essential features of the LV windings of large power transformers are that they

have relatively small LV bushings and invariably have a relatively small number of

turns. LV windings are usually the innermost windings, and may be connected in star


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or delta arrangement. For the low-voltage/high-current LV windings of generator

transformers, relatively large conductors are used and therefore tend to be of a

particularly simple winding arrangement, often a simple spiral winding, although

sometimes with two (go and return) layers.

The simplest type of LV response is that shown in Fig. 5.11: this is obtained for large

generator transformers with single layer LV windings where the LV phases are

brought out separately to six LV bushings, the LV delta connection being made

externally by connecting together the relevant pairs of bushings (a2-c1, b2-a1 and c2-

b1 for the usual YNd1 vector grouping).

At low frequencies, there is the usual first core-related minimum at around 400 Hz,

although with significantly less attenuation than for HV windings, followed by a

maximum at about 8 kHz, with an intermediate maximum and minimum if the

transformer is three-phase rather than single phase.

Fig. 5.11 Generator transformer LV winding responses (3 phases of a transformer)

The characteristic high-frequency LV response is a sequence of four or five ‘U’

shaped resonances in the 2-MHz band starting with a first maximum at around 200

kHz, sometimes with occasional ‘notches’, as shown in Fig. 5.12 for three different

transformers from different manufacturers. This type of response is what is expected

from simple standing-wave resonances in a single-layer coil.


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Fig. 5. 12 Generator transformer LV winding responses (3 different transformers)

Some generator transformers have responses with significant resonances in the 200-

kHz band, as shown in Fig. 5.13. This is believed to be the characteristic form for LV

windings wound in a dual-layer arrangement.

Fig. 5.13 Generator transformer LV winding responses (dual-layer arrangement)

The result of connecting the three LV phase windings in delta is shown in Fig. 5.14,

which compares LV responses measured on the same 600-MVA three-phase

generator transformer. The two main effects of the delta connection are to remove the

intermediate resonance in the low frequency range and to produce a characteristic

‘dip’ just below about 100 kHz but, somewhat surprisingly; there are also significant

modifications to the responses at higher frequencies.

The response shown for a delta-connected generator transformer LV winding is also

typical of responses measured on tertiary windings, which is not totally unexpected,

as the latter would usually be wound as a single spiral.


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Fig. 5.14 Comparison of LV responses with separated and delta-connected windings

The LV windings of many double-wound transmission and distribution transformers

with a star delta vector grouping also usually have very similar responses, as shown in

Fig. 5.15 and Fig. 5.16, for a range of voltages (400/66 kV to 132/33 kV),

manufacturers and ages.

In the intermediate frequency range, the response is essentially that observed for LV

windings connected in a delta arrangement. In the higher frequency range, the typical

response is a series of multiple resonances with a characteristic ‘M’ shape, as shown

in Fig. 5.16, probably because they will inevitably have a similar simple disc-type

winding arrangement. [15]

Fig.5.15 Double-wound transmission transformer LV response.


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Fig. 5.16 Double-wound transmission transformer LV responses.

5.3.2.3 Shell-form transformer winding responses

A selection of transformers from one manufacturer has been chosen to investigate

typical shell form FRA responses:

23-8324 → Single-phase transformer, 75 MVA, HV 400/ 3 kV, LV 15 kV

23-8325 → Single-phase autotransformer, 125 MVA, HV 400/ 3 kV, LV 230/ 3 kV,

TV 13.8 kV

23-8330 → Single-phase autotransformer, 125 MVA, HV 400/ 3 kV, LV 230/ 3 kV,

TV 34.5 kV


24-7218 → Three-phase autotransformer, 125 MVA, HV 230 kV, LV 115 kV, TV

13.8 kV, Wye-Wye- Delta


95-3005 → Three-phase transformer, 865 MVA, HV 345 kV, LV 24.8 kV, Wye-

Delta


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The HV winding responses fall mainly into two distinctly different types of typical

responses, as illustrated in Fig. 5.17

Family ‘A’ Family ‘B’

Fig. 5.17 Shell-form transformer end-to-end HV winding responses

Whereas the first type of response appears to be no different to the typical HV

responses for core form designs described previously, the second type appears very

different, particularly in the intermediate frequency band (2–20 kHz), with the

apparent absence of the usual first low frequency minima and maxima. In fact, this

second type of low frequency response is not unique to shell form transformers, being

occasionally seen for some core form units, e.g. delta connected HV windings. It is

believed that this alternative form of low frequency response arises for windings with

high inductance/low series capacitance, so that the first minima and maxima can no

longer be distinguished.

The LV and tertiary responses of shell-form transformers are similar to those of core-

form transformers, as demonstrated in Fig. 5.18.


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Fig. 5.18 Shell-form transformer end-to-end LV and tertiary winding responses

Fig. 5.19 illustrates the effect of shorting the LV winding when performing a HV end-

to-end test. In addition to the expected variation of the first low-frequency resonance

associated with the core, there are other variations, including new resonances up to

about 1 MHz [15].

Fig. 5.19 Shell-form transformer HV end-to-end ‘open’ and ‘short-circuit’ tests

5.3.2.4 Frequency Range for Interpretation

FRA testing and interpretation is affected by different physical boundaries. The total

signal measurement accuracy is usually limited by the test device (specifications) and

the test concept.


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Due to the large dimensions of some power transformers, there are special

requirements on the geometry of the test set-up to reach the transformer terminals.

Test set-ups including long signal cables and extension leads may be sensitive to

electromagnetic interference and could cause repeatability problems. A large test set-

up geometry can also show significant interactions with the transformer windings in a

typical FRA interpretation range. Additionally, residual magnetization of the core

may also affect the FRA results at low frequency (up to about 5 kHz). Following these

reasons, it is appropriate to define specific interpretation ranges based on the

requirements for the test set-up.

Fig. 5.20 shows the FRA interpretation range considering limitations due to test set-up

geometry and uncertain residual flux conditions. The classification is based on the

rated voltage, since this factor correlates well with the length of the bushing and,

consequently, the length of the test leads. It should be noted that the recommendations

shown in the figure should not be considered as clear-cut limits but as a general guide

intended only to show the general link between the relevant parameters. In general,

for FRA interpretation it is recommended to keep a definite gap between the

interpretation range and the range possibly affected by the test conditions. [15]

Fig.5. 20 Typical FRA interpretation range

Fault can be interpreted from SFRA plot from given zones of frequency. These

frequency bands indicate type and location of faults in transformer [39] as shown in

Fig. 5.21 and as mentioned in Table 5.3.


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Fig. 5.21 Frequency bands for various types of faults (Courtesy: Doble Engineering)

[39]

Table 5.3 Frequency bands corresponding to different faults [39]

<2 kHz Core deformation, open circuits, shorted turns and

residual magnetism

2 kHz to 20 kHz Bulk winding movement relative to each other,

Clamping structure

20 kHz to 400 kHz Deformation within main and tap winding

400 kHz to 2 MHz Movement of tap winding leads, Axial shift of

winding

5.3.2.5 General Guidelines for accurate fault detection in SFRA

• The bands overlap and are not well defined, the band limits are not strictly set

and vary both with manufacturer and transformer MVA and voltage.

• Hard and fast rules are difficult to generate as there are so many designs and

manufacturers.

• If DC testing was performed the core must be demagnetized before SFRA

measurements


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• Measurements must be made at the tap position such that full winding take

under test.

• Results vary between units – depending on size and type of unit

• Results vary with magnetization & grounding

• Results vary with tap changer positions

• Results vary with oil level [39]

By comparing future traces with baseline traces, the following can be noted.

• In general, the traces will change shape and be distorted in the low

frequency range (under 5,000 Hz) if there is a core problem.

• The traces will be distorted and change shape in higher frequencies

(above 10,000 Hz) if there is a winding problem.

• Changes of less than 3 decibels (dB) compared to baseline traces are

normal and within tolerances.

• From 5 Hz to 2 kilohertz (kHz), changes of + or – 3 dB (or more) can

indicate shorted turns, open circuit, residual magnetism, or core

movement.

• From 50 Hz to 20 kHz +/- 3 dB (or more), change from baseline can

indicate bulk movement of windings relative to each other.

• From 500 Hz to 2 MHz, changes of +/- 3 dB (or more) can indicate

deformation within a winding.

• From 25 Hz to 10 MHz, changes of +/- 3 dB (or more) can indicate

problems with winding leads and/or test leads placement.

Note that there is a great deal of overlap in frequencies, which can mean more than

one diagnosis. [23]


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5.4 Interpretation Methodology

FRA is a comparative method for assessing the condition of power transformers. To

evaluate the FRA results, actual data are compared with reference data either by direct

visual inspection of the curves or by using processed FRA data.

There are three approaches for generating reference data:

Previous fingerprint measurements on the same unit;

Measurements on identical (twin) transformers;

Measurements on separately tested limbs or phases (Phase to Phase

Comparison)

5.4.1 Evaluation by Fingerprint Results

The fingerprint test data set is potentially the most reliable reference information for

evaluating FRA tests. Assuming a high repeatability of the test technique, it is

possible to obtain almost identical FRA results. An example would be two scans

collected from the same winding, such as H1-H3, on different test dates.

Data is collected before and after transformer relocation is expected to overlay well.

Any variance is such comparisons indicate a problem. One exception is caused by the

magnetic circuit and the state of the remnant magnetism occurs at low frequencies and

should be overlooked. Magnetization and temperature change can cause the beginning

of the trace to be slightly offset in certain cases. Fig. 5.22 illustrates a before and after

relocation response of a set of high-voltage windings. The results were not only

obtained on different test dates, but also were obtained with different test sets. Phase

to phase variations exist, but there are no differences before and after relocation.


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Fig. 5.22 Comparison to Baseline

It should be noted that the LTC and DETC position influences the results. If the test

results are obtained in different tap positions, expect variation. Fig. 5.23 shows two

traces collected in different tap positions; the difference is small, but noticeable at

frequencies greater than 500 kHz. The DETC was moved from position 3 to 5. [9]

Fig. 5.23 Different DETC Positions (3 & 5)

5.4.2 Comparison of Twin and Sister Transformers

Fingerprint results are not always available for FRA evaluation of FRA results.

Sometimes, customer orders include several transformers of identical specification so

that finally transformers of identical design are operated within one power grid.

Identically designed and identically assembled transformers (twins) typically show

almost identical FRA curves. Slight deviations between twin transformers are

generated exclusively by manufacturing tolerances and/or core magnetization effects

(Fig. 5.24). [15]


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Fig. 5.24 FRA of real twin units

(single-phase autotransformers, 370 MVA, HV 700/ 3 kV, LV 300/ 3 kV)

Sister unit results are also expected to compare well. Our database of sister units

shows very little difference between matched scans. All tests on sister units were

conducted with the LTC and DETC in the same position. If the results are

magnified small offsets can be noticed, but for the most part they are similar. Fig.

5.25 demonstrates the similarities of sister units. Each plot consists of two high-

voltage winding traces and two low-voltage winding traces. [9]

Fig. 5.25 Comparison of Sister Units

The applicability of FRA interpretation based on a sister unit comparison therefore

has to be validated. It is quite difficult to discern real twin transformers from sister

units. Some parameters for identifying twin units are given by:

• Manufacturer

• Factory of production

• Original customer/technical specifications


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• No refurbishments or repairs

• Same year of production or +/-1 year for large units

• Re-order not later than 5 years after reference order

• Unit is part of a series order (follow-up of ID numbers)

• For multi-unit projects with new design: tested transformer is not first, second

or third unit.

The more indications are positive, the more certain is similarity of core-and-coil

assembly. [15]

5.4.3 Phase to Phase Comparison

Many times for old transformers when reference signature is not available the first

step is to compare the signatures of phases of the transformer. It means comparing the

signatures of phase U with phase V and phase W. It is assumed that for majority of

cases there would be good matching between phase U and phase W as they are

symmetrical being on extreme limbs.

Whereas phase V (Center phase) would not be matching with the other two phases

particularly in the region 10Hz to 2kHz as the magnetic path for the center phase is

different. In phase to phase comparison, the signatures obtained after short circuiting

other winding of the transformer on the same limb, compares well as the effect of core

is eliminated. Typical examples are given in Fig. 5.26 and Fig. 5.27 below.


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Fig. 5.26 Three Phase SFRA comparison for open circuit plot of normal transformer

Fig. 5.27 Three Phase SFRA comparison of Short circuit plot for normal transformer


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Open circuit responses measured after fault for the HV windings at highest tap are

shown in Fig. 5.26. The dominant features of these plots are the first minima at low

frequency near 200Hz. The position of minimum will vary somewhat depending on

the remnant magnetism of relevant core flux circuits. As there is no deviation in

SFRA plot after the fault among the three phases in Fig. 5.26, it gives indication of no

sign of any winding movement.

Winding having higher impedance will attenuate the signal more at beginning of the

plot. This is evident from the in general observation of the plot where starting dB

level of LV winding at 10Hz frequency is (around −40dB) always lower than the dB

level of HV winding at 10 Hz. (around −60dB).

Short circuit SFRA responses measured for the HV windings at highest tap is shown

in Fig. 5.27. The dominant features of these plots are that it starts from very low dB

due to shorting of the LV (2U − 2V − 2W). In this case, the low frequency minimum

is not determined by low frequency open circuit inductance of winding which involve

the core also. Hence it purely represents the status of winding, i.e. indication of fault

like Open circuit, Short circuit fault etc.

Short circuit virtually eliminates the effect of magnetic core due to opposite flux of

short circuit current and lowest impedance path of the shorted winding compared to

core as explained in Fig. 4.1 and Fig. 4.2. The response in band 10Hz to 2 kHz

matches well for all 3 windings U, V, and W which is clear in Fig. 5.27.

Comparison of Open and Short circuit responses measured for the same winding at

any specific tap position reveals that low frequency open circuit inductance of

winding involve the core which is clear from the first minima at open circuit plot.

This first minima is absent in short circuit plot due to shorting of LV winding and

after 10 kHz frequency both the response are identical as indicated in Fig. 5.28. At

higher frequencies a more complicated form of response is seen which is unique to the

detailed arrangement of winding involved. This represents the fingerprint or signature

of winding design involved. At these frequencies, winding inductance is dominated


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by leakage fluxes local to the winding conductors, and remnant magnetism of the core

has no influence.

Fig. 5.28 Open circuit and Short circuit SFRA plot comparison of same winding for

normal transformer

However it is not necessary that the good matching that is shown in Fig. 5.26 and Fig.

5.27 would be found always. Phase comparisons are the most difficult and are open to

subjective analysis. It overlays with reasonable similarity and can deviate in high

frequency region.

The center phase, especially in core type transformers, exhibits the most deviation

when comparing all three phases. Different flux paths seen by each phase contribute

to the observed differences. The affects of the core saturation and magnetic state of

the core are expected at the lower frequencies.

The actual windings of a three phase transformer are almost identical, but the

connection scheme between phases is very different. As an example, the phases of a

wye winding are all at different distances from the neutral and also LTC connections

fall into the same category. Thus, since the windings are not equilaterally spaced, the

varying lead length entering and leaving the windings, influence the individual

transfer function of each winding. This would generally be found in two winding

three phase transformers. [4]


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5.5 Examples of FRA Interpretation

This section presents examples of FRA measurements and interpretation in the

following

conditions:

Hoop buckling of inner LV winding

Shorted core laminations

Effect of the oil (measurements with and without oil)

Effect of shorted turns

Effect of core residual magnetization

5.5.1 Hoop Buckling of LV Winding

Fig. 5.29 illustrates the LV winding responses of two identical single-phase generator

transformers (from a bank of single-phase units). The LV buckling (Fig. 5.30) of

phase B is clearly detected by comparing the FRA measurement with phase C. Note

that shorted turns and phase to earth fault were found on the phase-A LV winding, so

no FRA reference measurement on this phase was available for comparison. [15]

Fig. 5.29 FRA responses showing a hoop buckling failure of inner LV winding of

phase B


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Fig. 5.30 Buckling of inner LV winding

5.5.2 Shorted Core Laminations

The measurements were performed on a three-phase transformer rated 250 MVA, 212

kV / 110 kV / 10.5 kV, before and after the repair of the core is shown in Fig. 5.31.

The first core-related resonance is clearly modified by the fault: the shorted

laminations caused a decrease in the core magnetizing inductance (increase in

resonance frequency) and an increase in the eddy currents in the core (increased

damping). The core fault is shown in Fig. 5.32 [15].

Fig. 5.31 FRA responses with shorted core laminations (before and after repair)


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Fig. 5.32 Shorted core laminations.

5.5.3 Effect of the Oil

Fig. 5.33 shows the responses of the series (HV) phase-A winding of a 400-MVA

three-phase autotransformer, rated 230/120 kV, with and without oil. As expected, the

higher permittivity of the oil increases the capacitance, which in turn reduces the

resonance frequencies (whole curve is essentially shifted towards lower frequencies

since all the stray capacitances are increased by about the same factor). [15]

Fig. 5.33 Effect of oil on the FRA measurement

5.5.4 Effect of Shorted Turns

Fig. 5.34 shows the FRA responses of the series windings of a 140-MVA

autotransformer (220/69 kV with tertiary winding). The fault was located on phase C

of the tertiary winding. In this condition, the low-frequency measurement on the HV

winding of the same phase was influenced because of the lower inductance due to the


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shorted turns on a winding of the same phase (increased first resonance frequency).

This is analogous to what is observed for the end-to-end short-circuit test. [15]

Fig. 5.34 FRA response with shorted turns in a winding

5.5.5 Effect of Core Residual Magnetization

Core magnetization may affect FRA results due to different residual flux densities in

the transformer core. Generally, this effect has to be considered for FRA

interpretation below about 5 kHz. At higher frequencies, eddy currents prevent

magnetic-field penetration into the individual sheets of the core lamination.

Fig. 5.35 shows FRA results on a transformer before and after a dc winding resistance

test. Residual magnetization leads to lower magnetizing inductance and, hence, an

increase in the frequency of the first main resonance in the FRA curves. For higher

frequencies, FRA results are identical.

Fig. 5.35 Effect of magnetized core on FRA results


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In order to achieve the highest comparability of FRA results below 10 kHz, the

magnetic condition of the transformer should be identical. Either the data below 10

kHz can be disregarded, or the effect of non-identical core magnetization properties

for FRA results can be minimized by one of the following methods [15]:

• Perform end-to-end short-circuit test.

• Perform inductive inter-winding test.

5.6 Conclusion

There is a learning curve associated with interpretation of SFRA traces. The traces

need to be interpreted with experience, with reference to baseline results where

possible, with reference to manufacturer specific variations and with reference to

phase comparisons.

Where baseline data is available, traces may be interpreted to look for degrees of

difference. The main problem with this method is that small variations in one part for

an SFRA trace may be more meaningful than larger variations in another part of the

trace.

Baseline results may not always be available for a particular transformer. Here

reference may be made to sister units or to transformers from the same manufacturer.

Individual manufacturers may have variations that are specific to their transformers;

or to compare the signatures of phases of the transformer. Phase comparisons are the

most difficult and are open to subjective analysis. It overlays with reasonable

similarity and can deviate in high frequency region.

When interpreting a trace, it is important to make use of all the information present –

to look at the whole picture. Small variations or displacements across a large

frequency range may be much more important than a large variation in one part of the

frequency range.


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In analyzing traces, lower frequencies tend to relate to larger objects; higher

frequencies relate to smaller objects. In terms of size there is a general rule of thumb

that, while reviewing a trace from left to right, from 20 Hz to 2 MHz, this corresponds

to the core, clamping structure and yoke, main windings, tap leads and connecting

leads. The actual position of resonances in the trace depends on the size of the

transformer; lower MVA transformers tend to have their resonance shifted more to the

higher frequencies. However, there are always exceptions to this ‘rule of thumb’ and

individual traces should be inspected on their merits.

chapter 5 interpretation of sfra

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