Electrical Power and Energy Systems 58 (2014) 101–110

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Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

Experimental investigation on terminal connection and system functionpair during SFRA testing on three phase transformers q

0142-0615/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijepes.2014.01.003

q This manuscript is a part of an experimental work carried out by the authorduring his stay at HV Lab, Indian Institute of Science (IISc), Bangalore, India. Theauthor thanks Prof. L. Satish, (IISc) for his excellent research guidance and moralsupport in preparation of this manuscript.

E-mail address: saravanakumar.arumugam@gmail.com

Saravanakumar ArumugamHigh Voltage and High Current Technologies, University of Rostock, Tannenweg 22 (Speicher II Eingang West), 18059 Rostock, Germany

a r t i c l e i n f o

Article history:Received 3 October 2013Received in revised form 12 December 2013Accepted 1 January 2014

Keywords:Sweep frequency response analysisMechanical deformationTransformersTerminal connectionSystem function

a b s t r a c t

It is highly desirable to maximize the ability of detecting even a slightest movement in the winding-corearrangement of a power transformer while employing Sweep Frequency Response Analysis (SFRA) mea-surements. The terminal connection employed and system function considered significantly influencesthe detection sensitivity that can be achieved. Being an off-line, low-voltage method, SFRA tests permitany possible terminal connection and system function amongst the numerous possibilities. Currentlyavailable literatures and the IEEE guide on SFRA measurements (IEEE Std C57.149™-2012) list out mostof these possibilities for both single phase and three phase transformers. The terminal connection andsystem function listed in the IEEE guide on SFRA measurements is an outcome of past experience andknowledge. The IEEE guide provides much attention in detecting outer phase symmetry, mid phase asym-metry, changes in the inductance and other minute mechanical deviations. Despite all, an expert opinionis inevitable while making a meaningful interpretation from the measured SFRA data. In particular, moreexpertise is required while ascertaining the minor winding deviations which usually manifest in the midfrequency range. A possible way to simplify this task is by obtaining more information (distinguishableresonant peaks) in the measured amplitude frequency response function. It simply means enabling theSFRA method to reveal all the natural frequencies manifested by the winding-core assembly. The authorhas already addressed this issue for single phase, two-winding transformers. However, three phase multi-winding transformers are quite different in construction. So, the applicability of the same configuration(single phase, two-winding) on three phase transformers remains to be ascertained. Also, there exist sev-eral possibilities of alternate terminal connection and system function that might provide more insightinto the status of the winding-core assembly. In conclusion, it is believed that the proposed alternate ter-minal connection and system function pair can also be included during routine SFRA tests on three phasetransformers after adequate verification.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Monitoring using Sweep Frequency Response Analysis (SFRA)method is based on the principle of correlating physical deforma-tion/displacement in the winding-core assembly to the deviationsin its natural frequencies. So, it is most ideally suited to ascertainthe mechanical integrity of the windings of transformers. The SFRAmethod is essentially an off-line, low-voltage measurement andmany factors influence the achievable sensitivity. Quite naturally,there is a constant demand in achieving higher sensitivities, i.e.detect as small a winding displacement/deformation as possibleduring these measurements. The appropriate combination of

terminal connection and system function pair employed is consid-ered as an important factor that is fortunately under user control.Hence, it must be guaranteed that the right combination affordingthe highest sensitivity is identified and used.

At present, an IEEE Guide on SFRA measurements for oil im-mersed transformers (IEEE Std C57.149™-2012) is currently avail-able [1]. This document [1] provides an exhaustive list of terminalconnections, system functions and other relevant informationregarding the measurement considerations and interpretationstrategies. Additionally, there are literature that reports on cur-rently practiced methods and the choice of terminal conditionand system function [2–7]. It is surmised that these document havebeen prepared, mostly based on existing practices, knowledge fromlow-voltage impulse testing and past experiences. It has beenwidely accepted that the currently practiced methods can resolvethe mechanical status of the winding-core assembly. The mostcommonly occurring fault conditions such as bulk winding move-ment, disc collapse, loss of oil and other mechanical defects could

102 S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110

be distinctly identified [4]. However, resolving minor deviations,such as faults between inter-turn or inter windings, require moreinformation. In other words, obtaining more number of resonantpeaks in the amplitude frequency data might reveal minutemechanical deviations in the winding core assembly. In this regard,the author has published a paper [8] that identifies a particularcombination of terminal connection and system function for a sin-gle phase, two-winding transformer. The identified terminal con-nection (both neutral floating, secondary line end grounded) andthe system function (current flowing in the grounded terminal)pair has clearly shown to yield a higher sensitivity compared torest of them. In the present context, the term sensitivity is quanti-fied by the number of natural frequencies (i.e. peaks in the ampli-tude frequency response of the system function) that can bedistinctly observed. This fact was verified by experiments on asmall transformer and results proved that it was worthwhile tocontinue investigation of this matter further on three phasetransformers.

A majority of condition monitoring activity involves largecapacity, high voltage transformer units, which are invariably threephase and have multiple windings. Sometimes, a bank of singlephase transformers is also used. Since there is a difference in con-struction, it becomes important to verify suitability of the earlierwork [4] on three phase transformers. At first glance, it seems un-likely that the previous results would be equally applicable tothree phase, multi-winding transformers. Nevertheless, an investi-gation is required before an inference can be made. Additionally,the recent literature indicate the growing interest in correlatingthe SFRA data with the structural changes in winding-core assem-bly [9–11]. A possible way to achieve this is by employing synthe-sis procedures to recompute the values of inductance andcapacitance of the winding-core arrangement. Nevertheless, whileemploying such procedures, it is quite important to have all natu-ral frequencies observable [9]. It thus becomes trivial to identifyone or more terminal connections that could reveal all the naturalfrequencies of the winding-core assembly. Therefore, with this asprime motivation, the present study identifies the combination ofterminal connection and system function for use during SFRA mea-surements on three phase transformers.

2. Literature and motivation

Publications pertinent to the present study were gathered andanalyzed. In the present context, the literature could be broadlyclassified into following categories:

� Influence of terminal connection during estimation of tran-sient voltage and natural frequenciesEarlier investigations on transformer windings were mainlyfocused on identifying voltage distribution during transientvoltage conditions [12–14]. During these investigations, it wasobvious to retain the non-tested windings shorted in order toprevent a catastrophic failure [12]. However, the investigationson natural frequencies, forced and free oscillation on coils andwindings, virtually permitted any terminal connection. It wasidentified that there exist a correlation between certain naturalfrequencies of a coil or winding under different terminal con-nection [13].� Terminal connection pertaining to single phase two winding

transformersDick and Erven pioneered the current form of SFRA methodthrough network analyzers and sweep generators [15]. Subse-quently, many investigations were initiated to understand theinfluence of terminal conditions during SFRA measurements[16,17]. The ERA Technology report [2] specifies a preferred

terminal condition (non-tested windings are to be kept open-circuited) and quantity to be measured. In the post digital-era,there have been many efforts to correlate winding deformationto changes in amplitude response of transfer function, for differ-ent terminal connections [18–21]. Hence, a question that natu-rally arises is, which connection is the best to use? In thiscontext, a suitable terminal connection and system functionpair pertaining to a single phase two winding transformer hasbeen identified [8].� Currently practiced terminal connection during SFRA mea-

surements on three phase transformersIt has been identified that based on terminal conditions and thepoint of excitation, certain natural frequencies of the windingsare rendered non-excitable [22–24]. Obviously, such connec-tions are best avoided. Also, depending on the measured quan-tity, certain natural frequencies may remain non-excitable,either because they lie close to a zero, or to a dominant pole,or high damping [8,25]. Additionally, there has been manyefforts to identify terminal connections and their sensitivity totype of deformation [3,26,27]. The potential of SFRA methodin detecting inter-turn winding faults and identifying the mostappropriate measurement configuration has been investigated[27,28]. Although several proposals for the measurements,related considerations and interpretation exist, neither of themintegrate human expertise nor represent it by means of anexpert system [3].� IEEE standard on SFRA measurements [1]

At present, an IEEE guide (IEEE Std C57.149™-2012) thatdescribes about test procedure, test connections, test environ-ment, test setup preparation is currently available. This docu-ment has been prepared by the IEEE Working Group for useas a guide for the application and interpretation of frequencyresponse analysis for oil immersed transformers. An exhaustivelist of various test configurations for different types of trans-formers is also included in this IEEE guide. Although, certainconnections are currently being practiced, this guide (IEEE StdC57.149™-2012) does not recommend any particular terminalconnection as preferred. Finally, some hints have also been pro-vided for acquisition and interpretation of monitored data.

In summary, the following observations could be made fromliterature:

1. Terminal connection, system function and the point of excita-tion has significant influence over the sensitivity of SFRA mea-surements and natural frequency estimation.

2. Open and short circuited terminal conditions are popularlyadopted during field measurements.

3. Despite all improvements, an expert opinion becomes neces-sary to make a decision and meaningful interpretation of themeasured data.

4. It is equally important to investigate alternate combination ofterminal connection and system function that might providebetter insight into the mechanical condition of the winding-core assembly.

Therefore, it emerges that a test configuration that exhibitsmaximum sensitivity (that which reveals more number of resonantpeaks) has, so far, not been identified, and hence forms the subjectmatter of this paper.

3. Methodology

The correctness of simulation studies on a three phase multi-winding transformer entirely depends on the accurate representa-

Table 1Transformer specifications.

Unit Rating Remarks

Winding type kVA Volts Freq. Hz

1 D/y 15 11 kV/415 50 (Set-1) Neutral2 D/y 500 11 kV/440 50 present3 y/D 60 2.2 kV/220 25 (Set-2) Neutral4 y/D 70 2.2 kV/220 25 Absent

S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110 103

tion of electrostatic and electromagnetic couplings between all thewindings. Since, it is difficult, if not impossible, to accurately esti-mate and represent all the pertinent circuit parameters (particu-larly the couplings), it was considered best to avoid simulations.So, the present study was limited to experiments.

From the standpoint of performing SFRA measurements, it be-comes necessary to differentiate between three phase transform-ers, with and without the neutral terminal. This is becausecertain terminal connections involving the neutral terminal cannotbe implemented when the neutral is absent. Due to this reason,separate studies were conducted; first for transformers with neu-

G

ILs1

ILs2

ILs3

L1V

NV

IpVp L1

L3 L2

H1

H2 H3

N

G

IpVp

VL2

VL1

VH2

NV

V

V

V

H1

H2 H3 L2

L1

L3

L3H3

H1

N

G

IpVp

VL2

VL

VH2 VL

V

V

H1

H2 H3 L2

L1

L3H3

H1

G

ILs1

ILs2

ILs3

L1VNVIpVp L1

L3

H1

H2 H3 L2

G

IpVp

H2V H3V IL2

IL3

IL1

VL1

H2 H3

H1 L1

L2 L3

G

IpVp

IH1

IL3IL2VH2

N

V

V

L1

L2 L3H3

H1

H1

H3H2

G

IpVp

VH2IL2

IL1

V

V

L1

H3

H1

H1

H3H2 L2

N

L3

G

VL2

VL1

VH2

NV

(pfsg)neutral groundedPhase floating

line−endshorted

(LS)

opencircuited

(OC)

shortcircuited

grounded

(SG)

(SC)

line−end

neutralphase/

1) Input with respect to ground

V

V

V

H1

H2 H3 L2

L1

L3

L3H3

H1

N

Ip

VpExcitation Method:

floatingand neutralBoth phase

(bfloat)

Fig. 1. Terminal configurations and measurable quantitie

tral terminal and the other for transformers without neutral.Authors had access to four three phase units, out of which, twowere distribution transformers, while the remaining two were fur-nace transformers. These two sets were considered separately andare referred to as set-1 and set-2 respectively. Some important de-tails of the units are given in Table 1.

At this juncture, it is important to note that the IEEE guide [1] fol-lows a different approach, while listing out the various possible ter-minal connections. Most probably, the type of transformer appearsto be the basis which has been used for grouping the various possiblecircuit arrangements (cf. Tables I–VI in [1]). In contrast to the presentwork, neutral terminal has been assigned a fixed status during SFRAmeasurements. In fact, it is mentioned in each of the Tables I–VI inIEEE guide, that neutral terminal is not included for three phaseWye connections, but may be included for single phase test connec-tions. Lastly, it is indicated that neutral terminal be left ungrounded.

3.1. Three phase transformer with neutral terminal

The presence of neutral terminal in set-1 transformers permits(in most cases) a direct verification of the terminal configurations

1

3

IN

N

IN

N

ILs1

ILs2

L1V

NV

G

ILs3

IpVp L1

L3 L2

H1

H2

NH3

ILs1

ILs2

L1V

G

ILs3

G

IpVp

IH2 IH3

VL1

VL2 V

N

VH1

H2 H3

L1

L3

L2 L3

H1

G

IpVp

INIH2 IH3

VL1

VL2 V

VN

H1

H2 H3

H1

L1

L2L3

L3

G

IpVp

IH2 IH3

L1I

IL3

IL2

N

VH1

H1

H3H2

L1

L2L3

G

IpVp

INIH2 IH3

IL1

IL2

IL3

VN

H1

H1

H2 H3

L1

L2 L3IN

N

G

VL2

VL1

VH2

NV

neutral floating(pgsf)

Phase grounded

(bgnd)and neutral groundedBoth phase

IpVp L1

L3 L2

H1

H2

NH3

IN

2) Input between phases

V

V

V

H1

H3 L2

L1

L3

L3H3

H1

N

H2

Ip

Vp

s pertinent to 3/ transformer with neutral terminal.

104 S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110

evolved during the previous work on single phase transformers [8],or after a slight modification (if required), to harmonize it with D/ywindings. Instead of separately defining each arrangement consid-ered, Fig. 1 gives a pictorial representation of the terminal connec-tions, modifications if any, and the measured quantities. FromFig. 1, it can be seen that there are 13 possible terminal configura-tions and 7 measurable quantities pertinent to each arrangement.Amongst the 7 measurable quantities, two are pertinent to inputand remaining are responses. In each configuration, 7 system func-tions (involving one input and one output quantity) can be defined.In addition to this, there are two arrangements of connecting theexcitation, i.e. input fed to primary winding with respect to groundor with respect to another phase terminal of the same winding. Inthe present work, the delta winding is considered to be the primarywinding and neutral is treated as if it is just another additional ter-minal on the non-tested (i.e. secondary) winding. Hence, for thisreason, the option of feeding input with respect to neutral hasnot been included. Put together, there are 13 terminal configura-tions, 7 system functions and 2 ways of feeding the excitation,which implies that there are 13� 7� 2 ¼ 182 different possibili-ties to be examined.

3.2. Three phase transformer without neutral terminal

The approach adopted in the earlier work [8], and also for set-1,cannot be extended for set-2 transformers, because of the absenceof neutral terminal. However, if the earlier adopted approach is

G

VL2

VH1

VH2

1) Input with respect to ground

L1

L3L2

VL1

V

H1

H2 H3VH3

Ip

V

Testedwinding

Non−testedwinding

G

VH2 VH3 VL2 L3V

VH1VL1

G

VH1 VL1

VL3IL2

VH1

H3V

VH2

VL3

L1I

G

VH2

VH1VL2

VL2

G

VH2 VH2

VH1

G

L1I

VL3IL2

L3I

G

L1I

IL3VL2VH2

VH1

VH1

VH2IL2 IL3

Phase floating(pfloat)

aP

Vp

L3

gopen−circuited (OC)

Excitation Method

All terminalsfloating

L1

L3L2

H1

H2 H3

Ip

Vp

L1

L2 L3

H1

H2 H3

Vp

Ip

VH3

L1

L2 L3

H1

H2 H3

G

VL2

Vp

Ip

L1

L2

H1

H2 H3

VH3

Ip

Vp

L1

L2 L3

L1

L2 L3

H1

H2 H3

Vp

Ip

VH3

L1

L3L2

H1

H2 H3

VH3

Vp

Ip

VpH1

H2 H3

Ip

VH3H

Vp

Line end of

phasegrounded

(pfosc)

any one

(pftsg)

Line end ofany two

groundedphases

Fig. 2. Terminal configurations and measurable quantities

forcibly used, then many obvious combinations of terminal con-nection and system function will remain unexplored, and so, theexhaustiveness of the search becomes questionable. Hence, forset-2 transformers, a slightly different approach was planned. Thiswas made possible by a close examination of all the connectionalternatives listed in Tables I–VI of the IEEE guide [1]. From thisstudy, it emerged that any given circuit arrangement could becompletely described by defining the status/condition of four dis-tinct entities in the circuit, viz.

1. Non-tested winding and the condition imposed on it.2. Status of non-excited terminals of the excited winding.3. Arrangement of connecting excitation source.4. System function considered.

As shall be seen, this classification approach can include notonly all connections listed in the IEEE guide, but also permits aframework for examining other combinations of terminal connec-tion and system function, not considered by it. Hence, in this re-gard, a classification made based on the above factors is believedto be more general than the one based on the type of transformer.Following this principle, all circuit arrangements listed in the sixtables (i.e. Tables I–VI in [1]) can be compactly grouped into threemain categories as given below:

1. Non-tested winding open-circuited, driving-point admittancemeasured, excitation fed with respect to ground.

G

VL2

VH1

VH2

2) Input between phases

L1

L3L2

VL1

V

H1

H3VH3

Ip

H2

p

G

VL2VH2

VH1

3) Input between windings

L1

L3L2

VL1

V

H1

H2 H3VH3

VpIp

Line end of any one

phase connected(pfosc)

G

VH2IL2 IL3

L1IVH1

G

VH1

VH2 VH3 VL2 VL3

G

VH1

VH2 VH3 VL2 VL3

G

VL2

VH1

VH3

G

VH1

VH3VL3

G

VL3VH2 VH3

VH1

VH1

VL2

G

VH1 VL1

G

VH2

VH1

VH3

L1I

IL3IL2

nd groundedhase shorted

(pfsg)

Isc

rounded (SG) Line−end shorted (LS)

L1

L2 L3

H1

2 H3

VH3

H1

H2 H3

L1

L2 L3

Ip Ip

Vp

Isc

H1

H2 H3

L1

L2 L3

Ip

Vp

Isc

H1

H2 H3 L2 L3

L1IpVp

Isc

H1

H2 H3 L2 L3

L1Ip

Vp

H2 H3

H1

L1

L2 L3Isc2

VpIsc1Ip

L2 L3

L1Isc1H1

H2 H3

Ip

Vp

Isc2

H1

H2 H3

L1

L2 L3Isc1

Isc2

Ip

Vp

L1

L2 L3

H1

H2 H3

Vp

Ip

short−circuited (SC)

(pftsc)

Line end of any two

phases connectedPhase shorted

(pfsc)

and floating

pertinent to 3/ transformer without neutral terminal.

Fig. 3. Measured amplitude frequency response corresponding to unit-2 trans-former without neutral terminal.

S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110 105

2. Non-tested winding short-circuited, driving-point admittancemeasured, excitation fed with respect to ground.

3. Non-tested winding open-circuited and transferred voltagemeasured, excitation fed between primary and secondary (i.e.inter-winding input).

Fig. 2 is a schematic representation of the proposed way of clas-sifying the various terminal connections for transformers withoutneutral terminal. In this figure, columns 1–3 correspond to open-circuited condition, columns 4 and 7 corresponds to short-circuitedand grounded/floating, and column 5 and 6 corresponds to inter-winding connection. All possible measurable quantities pertinentto each terminal connection are indicated. As can be observed,

0

0.5

1(a)

Mag

0

0.5

1(b)

Mag

0 1 20

0.5

1(c)

Mag

freque

12

3 45

1 23

Fig. 4. Measured amplitude frequency response pertinent to transformer with neut

any terminal connection mentioned in IEEE guide can have a fewmore other possibilities, and these are included in 2. In all, 16 dif-ferent configurations result. It is important to note that any of theterminal connections mentioned in Tables I–VI can be found inFig. 2. Thus, it shows that the proposed framework is more general.

For each configuration in Fig. 2, seven measurable quantitieswere selected. Amongst them, two pertain to input (i.e. input volt-age and current), while the remaining five are responses. Hence, foreach configuration, seven system functions (involving an input andresponse) can be measured. In addition to these possibilities, themethod of connecting source to network is yet another factor tobe accounted. Namely, the source could be connected with respectto ground, between two phase terminals, and inter-winding. Thus,in all, there exist 16 configurations, 7 system functions and 3 waysof connecting the source, thus implying 16� 7� 3 ¼ 336 differentpossibilities.

4. Experimental setup

The experimental setup (Fig. 3) used for measuring SFRA con-sisted of an arbitrary waveform generator (producing 10 Vp-psinusoid), a 150 MSa/s, 8-bit digital oscilloscope, and a clamp-oncurrent probe (sensitivity: 2 mA/mV, bandwidth: 450 Hz to60 MHz).

All the signals were connected to instruments via 50 X co-axialcables having a maximum length of 1 m. Suitable arrangementswere made to connect cables to the transformer terminals. Thetransformer terminals were shorted and/or grounded (as required)using a flat 7 cm wide aluminum strip. Precautions were exercisedto keep these leads as short as practicable. The same set of con-necting leads and cables were used throughout the experiments.The oscilloscope was terminated with 50 X for all measurements.An automatic procedure was adopted for gathering SFRA data,and later transferred from oscilloscope memory to a PC. Duringmeasurements, counting peaks in magnitude plot can be some-what subjective when the peaks are either small, buried in noise,or very close to a dominant peak. To circumvent this ambiguitythe following procedure was used:

3 4 5ncy (MHz)

68

7

4

bfloat−oc,Ip(f)/Vp(f)

bgnd−oc,Isn(f)/Vp(f)

pfsg−ls,ILs(f)/Vp(f)

5

ral terminal – (a)–(c) corresponds to ‘Good’, ‘Medium’ and ‘Bad’ configurations.

Table 2SFRA measurements on transformers with neutral terminal.

Terminal connection System function Number of peaks Remarks

Unit-1 Unit-2

bfloat-oca Ip/Vp 8 7 Goodbfloat-oca VL2/Vp 8 7pgsf-oc Vsn/Vp 8 6bfloat-oca VH2/Vp 7 7bfloat-sc VH2/Vp 7 5

bfloat-sc VH3/Vp 6 5 Mediumbfloat-sg Ig /Vp 6 4bgnd-oc Isn/Vp 5 5pfsg-oc Isn/Vp 6 5pgsf-sg IH2/Vp 6 2pgsf-sg IH1/Vp 4 2

pfsg-sg Isn/Vp 4 1 Badbgnd-ls Vp/Ip 4 2pfsg-ls Ils1/Vp 1 0

a Currently practiced during SFRA measurements.

H3

H3

H3

Y(f)=Ip(f)

Vp(f)

Vp(f)

Isn(f)H(f)=

Vp(f)H(f)=

Ils(f)

(c) Bad

Ip

Vp

Vp

Isn

ILs

Vp

(b) Medium

(a) Good H1

H2

L1

L2 L3

N

H1

H2 L2 L3

N

H1

L1

H2

L1

L2 L3

N

Fig. 6. Terminal connection and system function identified as ‘good’, ‘medium’ and‘bad’ for transformer with neutral terminal.

106 S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110

� First, an automatic sweep frequency measurement was doneand in the normalized amplitude versus frequency plot, alldominant peaks were counted and labeled.� In the amplitude plot, frequency intervals pertaining to small

peaks or peak-like kinks are noted, and for this frequency inter-val, a manual sweep frequency measurement is done. Duringwhich, vertical scale of oscilloscope is dynamically changedeach time to ensure full-scale-deflection. (This feature couldnot be incorporated in the automated measurement procedure.)� Additionally, to make identification of peaks foolproof, occur-

rence of zero-crossing of phase angle (i.e. angle between inputand output) was ensured. Thus, it was guaranteed that all peakscounted and labeled (however small in magnitude), was indeeda pole.

5. Experimental results

Table 1 provides certain important details of the transformerschosen in the present experimental study. As the chosen trans-

0

0.5

1

Mag

0

0.5

1

Mag

0 1 20

0.5

1

freque

Mag

(a)

(b)

(c)

1 2

3 4

1

2

34 5

1

X X

Fig. 5. Measured amplitude frequency response corresponding to unit-2 transformer with

formers are of smaller rating, SFRA measurements are done up to10 MHz. Since there are no dominant peaks observable above5 MHz, the results are presented only up to this frequency. Allthe amplitude frequency responses have been normalized with re-spect to its own maxima and all the natural frequencies (i.e. peaks)are numbered as per procedure described in previous section.

5.1. Three phase transformers with neutral terminal

SFRA experiments were conducted on two distribution trans-formers rated D/y-13,200/415 V, 15 kVA and D/y-11,000/440 V,

3 4 5ncy (MHz)

56 7

bfloat−oc,Ip(f)/Vp(f)

bgnd−oc,Isn(f)/Vp(f)

pfsg−ls,ILs(f)/Vp(f)

neutral terminal – (a)–(c) corresponds to ‘Good’, ‘Medium’ and ‘Bad’ configurations.

Table 3SFRA measurements on transformers without neutral terminal.

Terminal connection System function Number of peaks

Unit-3 Unit-4

VH3/Vp 8 11pfosg VH2/Vp 8 11L2-grounded VL1/Vp 8 9

VL3/Vp 7 9

VH2/Vp 7 11VL2/Vp 6 11

afloat VL1/Vp 8 8VH3/Vp 8 9VL3/Vp 7 5

VL1/Vp 8 8pfosg VH3/Vp 8 8L3-grounded IL3/Vp 7 6

VL2/Vp 6 8

VH3/Vp 5 7pftsg IL3/Vp 6 6H2L3-connected IgL3/Vp 5 6

Ig/Vp 5 6IL2/Vp 5 6

pftsc Ip/Vp 5 5H1L1-connected VL3/Vp 5 6H2L2-connected IH3/Vp 5 6

VL3/Vp 4 5pfosc VL2/Vp 3 5H1L1-connected VH3/Vp 3 5

VH2/Vp 3 6

S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110 107

500 kVA respectively, for each of the possibility shown in Fig. 1. Allthe amplitude frequency responses were measured on unit-1, andarranged in the descending order, based on the number of naturalfrequencies observable in each of them. From amongst this list, theamplitude frequency responses corresponding to the ‘good’, ‘med-ium’ and ‘bad’ configuration are shown in Fig. 4, along with the sys-tem function. A small part of the actual list is presented in Table 2.In Fig. 4(a), it can be seen that there are 8 distinct peaks in itsamplitude spectrum. Next, consider, Fig. 4(c), which correspondsto the ‘bad’ configuration in Table 2, and none of the natural fre-quencies detected in Fig. 4(a) are observable here. Such connec-

0

0.5

1(a)

Mag

0

0.5

1(b)

Mag

0 1 20

0.5

1(c)

Mag

freque

14

5

32

1 34

2

1

2

Fig. 7. Measured amplitude frequency response on

tions are best to avoid. It is important to note that this particularterminal connection and system function is not mentioned in anyof the Tables I–VI of the IEEE guide. Also, this terminal connectionand system function pair was found to be ‘bad’ for single phase,two-winding transformers as well. Hence, this particular combina-tion has consistently yielded less number of natural frequencies.This clearly illustrates that any arbitrary combination of terminalconnection and system function should not be used during SFRAmeasurements. Incidentally, the terminal connection that emergedas ‘good’ in the present study is also currently practiced and re-mains listed in IEEE guide. The amplitude frequency response per-tinent to the configuration that has been ranked as ‘medium’ isshown in Fig. 4(b). The currently practiced terminal connection/system function during SFRA measurements and the one identifiedas ‘good’ terminal configuration is identical in all aspects. It pro-vides sufficient proof that currently practiced terminal connectionis ‘good’ compared to the remaining.

Next, experiments were carried out in a substation, on anotherdistribution transformer (unit-2) which had been taken out of ser-vice, for some repair. Thus, an opportunity became available toconduct SFRA measurements in a substation, and also incidentallycheck the repeatability of earlier findings on unit-1. Table 2 in-cludes these results, and Fig. 5 shows the amplitude response cor-responding ‘good’, ‘medium’ and ‘bad’ terminal connections andsystem functions respectively. It can be observed that, notwith-standing the minor discrepancies, unit-1 and unit-2 have yieldedreasonably comparable results. It also implies that the results arerepeatable.

No particular reason is attributed for the minor deviations ob-served in number of natural frequencies detected in unit-1 andunit-2. Moreover, both units seem to follow a similar descending or-der as well. In some amplitude frequency responses, a few instancesare encountered wherein peak-like kinks are observable. However,at the pertinent frequency (at which the ‘kink’ occurs), the phase an-gle criteria was not simultaneously satisfied. It is for this particularreason such peaks/kinks have been disregarded and not counted.One such case can be seen in Fig. 5, and is marked as ’x’. The sche-matic representation of terminal connections and system functionspertinent to results in Fig. 4 are shown in Fig. 6. In the table corre-sponding to results for set-1, the position occupied by the currently

3 4 5ncy (MHz)

6 7 8

5

3

pfosg−L2,VH3(f)/Vp(f)

pftsg−H2L3,VH3(f)/Vp(f)

pfosc−H1L1,VH2(f)/Vp(f)

unit-3 transformer without neutral terminal.

0

0.5

1

Mag

0

0.5

1M

ag

0 1 2 3 4 50

0.5

1

frequency (MHz)

Mag

pfosc−H1L1,VL2(f)/Vp(f)

pfosg−L2,VH3(f)/Vp(f)

pftsg−H2L3,VH3(f)/Vp(f)

(a)

111098

764

3

21

5

12

3

4

5 6

7

12

3

4 5

(b)

(c)

Fig. 8. Measured amplitude frequency response corresponding to unit-4 transformer without neutral terminal.

108 S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110

practiced terminal connection during SFRA measurements is also in-cluded and indicated by a ‘H’. This clearly indicates how good thecurrently practiced procedure is, when compared with the sugges-tions arising out of this investigation.

5.2. Three phase transformers without neutral terminal

Experiments were carried out on unit-3, for all configurations asmentioned in Fig. 2. A small and important portion of the experi-mental results is shown in Table 3. As in the previous case, theamplitude frequency response functions are normalized. Fig. 7(a)shows the normalized amplitude frequency response pertaining

L3

L3

VL1

VL1(f)H(f)=

Vp(f)

VH2

L2

Vp

Vp

Ip

Vp

H1

H2 H3

L1

L2

H1

L1

H1

H2 H3

L1

L2

H(f)=Vp(f)

V (f)

H(f)=Vp(f)

V (f)H2

H3 H2

H3

L3

(a) pfosg,L2 grounded

(b) pftsg,H2L3 connected

(c) pfosc,H1L1 connected

Fig. 9. Terminal connections and system functions corresponding to Figs. 7 and 8respectively for transformer without neutral.

to terminal connection that yielded the highest number of peaks.It can be seen that there are 8 distinct peaks in this amplitude fre-quency response. Fig. 7(c) shows the amplitude frequency re-sponse measured from a terminal connection and systemfunction pair that yielded the least number of peaks i.e. about 3alone. Meanwhile, Fig. 7(b) shows amplitude frequency responsepertaining to a configuration that yields a reasonable number ofpeaks (5 peaks) in its spectrum.

Next, experiments were carried on unit-4 to verify the resultsobtained from unit-3. The results are found to be encouragingand included in Table 3. The normalized amplitude frequency re-sponse pertaining to unit-4 is shown in Fig. 8. It can be seen fromthe Table 3 and Fig. 8 that the order of the terminal connection andsystem function pair, more or less remains the same. The terminalconnection and system function pair that yielded more number ofpeaks occupied the same position, while those yielding the leastnumber of peaks also remained the same for both transformerunits 3 and 4. Fig. 9 shows pertinent terminal connections and sys-tem functions. Since there are deviations in the observed numberof peaks between unit-3 and unit-4, qualitative assignments suchgood, bad, etc. to individual configurations has been refrained. Un-less consistent results are obtained, such a declaration cannot bemade. Hence, in Table 3 for units 3 and 4 such a remark is missing.

Next goal was to check the position occupied by the currentlypracticed terminal connection with respect to the transformerwithout neutral terminal. So, separate experiments were carriedon unit-3 to ascertain. Fig. 10 shows the respective results. In thisfigure (Fig. 10), identification of the circuit arrangement as per the(Tables I–VI) IEEE guide is included for ready reference. It can beseen from this figure (Fig. 10) that the number of peaks yieldedby the currently practiced terminal connections varies from any-where between 2 and 7. Therefore, by adopting the combinationof terminal connection and system function shown in the first sec-tion of Table 3, it would be fair enough to expect some improve-ment in sensitivity.

6. Discussion

The results of the present study are more or less in consonancewith those obtained in the previous investigation [8]. The experi-

0

0.5

1

0

0.5

1

0

0.5

1

0

0.5

1

Mag

(p.u

)

0 1 2 3 4 50

0.5

1

frequency in MHz

Ip/Vp (HV−OC)

Ip/Vp (LV−OC)

Ip/Vp (LV−SC)

Ip/Vp (IW)

Transfered voltage (IW)

1 2

1 3 4

14 5 6

7

12

34

12

5

6

23

25

(a)

(b)

(c)

(d)

(e)

Fig. 10. Measured amplitude frequency response corresponding to currently practiced terminal connections – transformer without neutral terminal.

S. Arumugam / Electrical Power and Energy Systems 58 (2014) 101–110 109

mental results of this study once again reiterates the fact thatmerely identifying a terminal connection is alone not sufficient,but, an optimal combination of terminal connection and systemfunction has to be found. The task of finding this combination in-volved a vast search-space.

� Experiments reveal that for transformers with neutral terminal,the currently practiced terminal connections is ‘good’ and cancontinue to be adopted as it yielded more number of peaks inits spectra. However, for the case where neutral is absent, a dif-ferent terminal connection (primary phases floating, one of thesecondary phases grounded, input fed to primary with respectto ground) emerged to be ‘good’ in yielding more number ofpeaks. Fig. 9(a) describes the pertinent terminal connectionand system function and Table 3 gives the position occupiedby this pair.� It can also be observed that for transformers without neutral

terminal, one amongst the currently practiced terminal connec-tion (non-tested winding open-circuited and floating, voltage-gain measured) emerged to be almost as good as the identifiedterminal connection provided, the voltage gain is measuredinstead of driving-point admittance (Table 3).

The terminal configuration identified as ’best or ’good is basedon experiments on only a few transformers. Therefore, at this stageit is a bit premature to draw any final conclusions. So, only afterextensive verification on other types of transformers, such asauto-transformers, those with medium voltage, tertiary and tapwindings, and position of TAP it may be possible to make recom-mendation. Also, influences of type of windings (continuous-disc,partially/fully interleaved), winding arrangements, etc. has alsoto be ascertained. Proactive cooperation of power utilities is a pre-requisite to accomplish this task.

7. Conclusions

Experimental studies revealed that the ideal combination of ter-minal connection and system function to be employed for sweepfrequency response measurements on single phase and three

phase transformers is different. Results obtained for three phasetransformers with and without neutral illustrates that someimprovement in sensitivity can definitely be achieved, if a modifi-cation can be made in the terminal configuration (i.e. terminal con-nection and system function) employed during SFRAmeasurement. In this context, the contribution of this paper is no-vel, original and of practical relevance.

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