SFRA Transformer Coil Manufacturing Quality

SFRA A TOOL TO ENSURE TRANSFORMER COIL MANUFACTURING 

QUALITY 

ABSTRACT 

Arun D Yargole 

Doble Engineering (India) 

Manufacturers of medium and large capacity transformers when under time / cost pressure tend to make some 

manufacturing / design variations. The variances are generally noticed after the manufacturing tests are conducted 

and only when compared to sister units. Recently, for a couple of cases, deviations were noticed in three sister 

single phase units having identical technical data. This change was seen only after comparing SFRA results. It is 

known that SFRA can be used as an effective tool by manufacturing to improve quality. The author suggests having 

variant (hardware / software) that can check the ‘wound coils’ during manufacturing stage itself. The idea is to 

allow the manufacturer to arrest the problem before the assembly is completed. The buyer then has the advantage of 

getting comparable sister units. 

INTRODUCTION 

Utility customers buy multiple units of transformers or reactors of same specifications. It is expected that they will 

be built with same design and manufacturing process hence are deemed to be sister units. Ideally they would be 

twin units. 

A few cases have shown that the above is not true always. Changes/deviations are made either in design or 

manufacturing. This was observed while analysis of the SFRA signatures of the sister units. Though we assume 

that the deviations taken are genuine and only for good, there is a concern as those deviations are generally not 

reported to the customer utilities. 

Probably it is because at present there is no test procedure to detect such deviations. Normally, all the units pass the 

tests stipulated in standards showing that the units that are manufactured will perform well on field. Questions then 

arise, why the deviations are taken and why they are not reported to the customers? 

The reality is even though the utilities come to know about this after proper analysis of frequency response, they 

have no choice but to commission these units as they are already installed on site and is too late. Utilities are 

observing such deviations frequently from some manufacturers. 

OBJECTIVE 

Since such deviations are taking place without the knowledge of the customer, it becomes a necessity that these 

should be arrested at manufacturing stage itself. Unless this practice becomes a part of a specification it would be 

difficult to arrest such anomalies. SFRA is well known tool to detect such anomalies in the transformer windings 

and core coil assemblies. 

This document suggests that purchasers of transformers may require the use of SFRA to ensure quality of their 

transformers. Clauses within their specifications can require SFRA comparisons for acceptance of their 

transformers. The SFRA tool may be used in the manufacturing process for comparison of signatures of windings, 

core coil assemblies and finished sister units. 

© 2011 Doble Engineering Company -78 th Annual International Doble Client Conference 

All Rights Reserved

DEVIATIONS ARE COMPROMISES 

Designers design the transformer for Guaranteed Technical Specifications {GTS} as specified by the utilities along 

with other features or customized solutions. Major guaranteed technical specifications that govern the material cost 

of the transformers are No-load losses, Full load Losses, %Impedance, Voltage factor, Voltage ratings, Number of 

taps, type of LTCs, in some cases height of the transformers, etc. 

In a competitive bidding situation, designers take a chance and commit all of above. This affects cost optimization 

on manufacturing. It then becomes a tough task for design and manufacturing teams. Cost and quality in such cases 

do not go hand in hand. If manufacturers have been awarded multiple units of same Guaranteed Technical 

specifications “GTS” the designers take a challenge to balance out cost in complete tender of multiple sister units 

while assuring the GTS. 

Assuring the GTS does not always mean that the product will work reliably on field. There are tests listed in various 

standards that cover the quality assurance from materials to final product. These tests generally ensure that the 

transformers are manufactured with good materials and in well defined processes that have been evolved over a 

period. However they are not able to detect the manufacturing variations if any between the multiple units of same 

specifications or we can call sister transformers. 

Such manufacturing deviations were noticed while analyzing the SFRA results of two utilities but when they were 

detected it was too late. Transformers were already installed and awaiting clearance for commissioning. Utilities 

were not having any choice but to accept for completing the project. 

SFRA AND TRANSFORMER 

A recap of the designer’s challanges in transformer design.- 

There is a definite and well established relationship between response signature, geometry of transformers and 

physical properties of transformers. 

Resonating impedances seen in response are governed by a complex L-C-R circuits that covers multiple small 

circuits of intrinsic capacitances and self/mutual inductances. While self/mutual inductances “L” are governed 

mainly by number of turns, inner and outer radius of windings, height of the winding, the capacitances “C” are 

governed by area of conductor, thickness of insulation over the conductor for capacitor elements within a disc in 

disc type windings. There are multiple capacitive localized circuits within discs, within multiple discs, between 

winding and ground and in between windings. It’s very complex. In short L-C-R components are decided by the 

dimensions and physical properties of insulation material. 

Designers decide these dimensions for optimized design by using few main electrical parameters such as volts-perturn, 

flux density, current density, voltage stress (kV/mm) etc. These main parameters decide the performance 

parameters of the transformer such as winding losses, short circuit impedance, temperature rise, margin over high 

voltage withstand capability etc. Further they also control the dimension and weight of the transformer. Proper 

selection of above electrical parameters gives an optimized product. 

© 2011 Doble Engineering Company - 78 th Annual International Conference of Doble Clients 

All Rights Reserved 

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To illustrate this let us go through a typical small exercise matrix as mentioned in Table 1. 

Table 1 

Complexity in Design of Transformer to Match GTS 

Parameters 

Effects 

Volts / turn for 

a fixed flux 

density 

maximum 

allowed by 

GTS 

Current 

density 

Voltage 

stress 

Weight of 

copper 

Weight of 

core 

Impedance 

Losses and 

temperature 

rise 

- - 

- - 

- 

- 

- or 

or 

Actually it’s very complex and that’s why designers use software tools 

Table 1 is based on basic design principles and indicates clearly the challenges faced by the designer. This further 

becomes more complicated while designing for customized requirements. This technology is well known to all 

established manufacturers and there could be hardly any variations with different manufacturers while handling 

these complex parameter matrices. Competition makes manufacturers to build some innovative ideas or design 

compromises. 

With designs once frozen, it is expected that the multiple units of same specification (true sister transformers) shall 

be manufactured and processed identically. Transformers made this way can be called sister transformers. Typical 

responses of 13 sister transformers are given in Figure 1 below. 

Comparison of 13 Sister Transformers and Their Correlation Coefficients 

Rating-29MVA, 220 / 3 kV / 11kV 

Figure 1 



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SFRA A TOOL TO CONTROL QUALITY 

Figure 1 indicates the manufacturing quality of 13 transformers with regard to their mechanical structure. Minor 

variations in responses seen are due to manufacturing tolerances and values of Correlation Coefficients indicate the 

level of tolerances. Normally, for good quality transformers these will be close tolerances and any anomaly 

observed while comparison can easily be attributed to the manufacturing change or design change. 

Once designs are frozen for meeting GTS and other customized requirements the mechanical dimensions get frozen 

with tolerances for coils and core of transformer. Tolerances for other structural components of transformers such 

as, core frame, tap leads, winding conductors, LTC chamber/s, tie-rods, barriers, end insulation etc. are also 

finalized. Once they are frozen for all sister units, the dimensions are not expected to change beyond the limits 

specified by the tolerances. 

Here comes the role of Quality Assurance team to check and keep them in control but at times there are deficiencies 

and they are invisible to the customers. The manufacturing deviations listed in two cases below could have been 

arrested if the QA would have functioned efficiently. They could have been arrested if they would have been visible 

to the customer while in manufacturing stage. 

SFRA then can be used here to endorse this QA function if used for coils, core coil assembly and finished 

transformer at appropriate stages. 

Normally when coils get manufactured with prescribed tolerances it is expected that the response comparison 

between them shall be very close. To supplement visual comparison, we can decide standard correlation 

coefficients at different bands of frequencies by generating and studying data by using statistical analysis. 

CASES SUGGESTING PUTTING SFRA TOOL IN QUALITY PROCESS 

Case 1: Three Single Phase Sister Auto Transformers. {167MVA, 4400 kV/220 kV/33 kV ICT} 

This case was referred to Doble because the customer has seen some variations between the SFRA results of three 

sister transformers at factory and the site. These are three single phase auto transformers. When compared there 

was no deviation found between the factory signatures and site signatures of individual transformers. 

Additional Findings 

Though the signatures of individual transformers were matching well between the factory and the site, one of them 

was not matching with other two. This was not expected as they were supposed to be very identical being true sister 

transformers. Refer to Figure 2. 

#3 

#1 

#2 

Comparison of Partial Winding of Three Sister Transformers 

Figure 2 



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In Figure 2 above, they being sister transformers the matching of signatures between them was expected but 

transformer S/N #2 stood differently. 

Now there was deviation taken during manufacture of #2. Additional care was taken for impulse voltage 

distribution but probably this was not informed to the customer. After receiving our report, utility got wiser and 

investigated further. Manufacturer has accepted and disclosed the deviation in design. 

This change got disclosed very late when the transformers were ready in the field waiting to be commissioned. The 

utility had no choice but to investigate and accept. This could have affected their commissioning schedule. 

These are some hypotheses for what could have happened 

1. Manufacturer was cautious and selected a design for better impulse voltage distribution in the windings. 

This could have been the first unit. Knowing the better results and margin for optimization thereon, 

designer changed the design for remaining units. 

2. Manufacturer made the #2 design for type test 

3. Manufacturer optimized other two designs for cost savings. 

4. Under pressure of delivery the design deviations are taken. 

Case 2: Two three phase sister reactors {63MVAr 400 kV} 

This case was referred to Doble because the customer noticed the deviations in Y phase while phase-to-phase 

signature comparison of a three phase reactor. Refer to Figure 3. To justify the deviation a support of Y phase of 

sister unit was taken as explained in Figure 4. 

Additional Findings 

The above was not acceptable as while further investigations it was found that there were clear manufacturing 

variations seen in one of them which can be revealed from Figures 3 through 7. 

Unit #1 

Comparison of phases R, Y & B 

Y phase 

Y phase 

Deviation in Y phase (Center phase) beyond 100 kHz of Unit #1 

Figure 3 



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Comparison of phase Y of unit #1 & 

unit # 2 

Unit #2 Y 

phase 

Unit #1 Y phase 

Y Phases of Both Units Matching Reasonably Well 

Figure 4 

When further investigated it was found that 3 phase comparison of Unit #2 is matching well but the same was not 

matching well for Unit #1. Refer to Figures 5, 6 and 7. 

Unit #2 Phase to phase 

comparison of three phase 

R, Y & B 

Three Phases of unit #2 Matching Reasonably Well 

Figure 5 



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Unit #2 Phase to phase comparison of phases R, Y & B 

Three Phases of Unit #2 Matching Reasonably Well 

Figure 6 

Unit #1 Phase to phase comparison of phases R, Y & B 

Three Phases of Unit #1 Not Matching as Expected 

Figure 7 

The comparisons in Figures 6 and 7 indicate clear difference of about -5dB at 100 kHz to 200 kHz. Further there is 

no match between R to Y and B to Y. See area indicated by red lines. There is also a noticeable difference in range 

1 k to 10 kHz. 

If we work some hypotheses for case above, then what could have happened is as follows: 

1. Knowing the better results and margin the designer has changed the design for another unit. 

2. Manufacturer has optimized the cost of other design affecting manufacturability. 

3. Repeat orders if comes after some period then there is a chance that suitable components may have 

undergone the modifications. 

4. Under pressure of delivery the design deviations are taken 



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CONCLUSION 

Due to contractual obligations it is expected that manufacturer should keep the customer informed of a design or 

manufacturing change if any. But in both the cases it was not true. Utility was not aware of such manufacturing / 

design change until it got highlighted after comparison of sister transformers in SFRA. Further this got disclosed at 

the stage of commissioning where going back was difficult. 

At times design / manufacturing changes could be the result of spontaneous reactions to face unexpected or 

unforeseen challenges. Under these circumstances there are chances that quality of design review is not adhered to. 

This could lead to design weaknesses. Secondly could affect the use of transformer. 

The point of discussion here is, when such changes are made without prior information to the customer, should they 

be detected well in advance before dispatch clearance? And if yes there lies strength of SFRA. Next question 

would be how? 

This document suggests that purchasers of transformers may require the use of SFRA to ensure quality of their 

transformers. Clauses within their specifications can require SFRA comparisons for acceptance of their 

transformers. The SFRA tool may be used in the manufacturing process for comparison of signatures of windings, 

core coil assemblies and finished sister units. 

ACKNOWLEDGEMENTS 

The author would like to express a gratitude to the two utilities without mentioning their names for purpose of 

confidentiality. The author also expresses a deep gratitude to Mr. Sameer Gaikwad for the valuable critical review 

of this paper and guidance in the subject topic. 

REFERENCES: 

[1] A Study of Sweep Frequency Response Analysis on Sister Transformers - Arun D Yargole, Narendra 

Sharma, CMD-2006, Korea 

[2] “Experience with SFRA for Transformer Diagnostics” - Doble Engineering Company USA- Year 2003 

[3] “Experience with Sweep Frequency Response Analysis (SFRA) Measurements” - Doble Engineering 

Company USA- Year 2002 

BIOGRAPHY 

Arun D Yargole joined Doble Engineering in March 2010. Currently he is working as 

a Principal Client Engineer in INDIA. His prior experience includes 30 years with 

Global Research and Development Center of Crompton Greaves in Mumbai. During 

those years he worked in areas such as new product development in the field of power 

apparatus like dry transformers, RIP bushings, superconductor transformer, 

superconductor fault current limiter, etc. He developed high voltage test equipment 

such as inter-turn tester as per IEC 34-15 for high voltage machines, recurrent surge 

generators, impulse peak voltmeters, high voltage components and modules of solid 

state controller of impulse voltage generator. 

He is also an inventor of compact dry transformer that was granted a patent in US. He initiated a program on 

Condition Monitoring and Diagnostics, developed a team and worked for health check of about 100 power 

transformers. He published a paper titled “A Study of Sweep Frequency Analysis on Sister Transformers” in 

CMD2006, in Korea. 



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SFRA Transformer Coil Manufacturing Quality

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