ITCE’15, Bagheri et al.: Transformer Core Fault Recognition Using Frequency Response Analysis
1
2
3
3
3
1. Department of Electrical and Computer Engineering, National University of Singapore, Singapore
2.School of EE&T, University of New South Wales, Sydney, Australia
3. Khouzestan Steel Company, Iran m.bagheri@nus.edu.sg

Abstract
— Frequency Response Analysis (FRA) has been used as a comparative diagnosis method to explore mechanical integrity of transformer. This technique is quite capable of providing worthy information about electromagnetic characteristics of the windings. This study has concentrated on core diagnosis of a new oil-filled transformer. Practical study is performed on a 230/33 kV, 73 MVA three-phase core type transformer to examine the manufacturing quality. Oscillations in different frequency bands are discussed and their fluctuations are interpreted. Low-frequency band oscillations of FRA spectrum influenced by center and lateral core legs are investigated, and transformer is suspected to have a core different reluctances in legs and limbs using FRA.
Keywords : Frequency Response Analysis (FRA), Transformer core defect, Transformer series capacitance.
I.
I NTRODUCTION
Condition monitoring applications in all aspects are becoming popular nowadays due to its useful impacts on
asset management [1]-[2]. High voltage plants and
equipment are very much exposed to this trend. In particular, life and condition assessment of transformers have long been
considered [3]-[4]. Nowadays, various advanced methods
are employed to recognize transformer condition. Among these, Transfer Function Measurement (TFM) is emerging as
an important diagnostic technique [5].
Transfer function measurement is generally utilized for black box system identification. It may be implemented in time domain as an impulse response or in frequency domain as a swept signal which is commonly called Frequency
Response Analysis (FRA) [6]-[7]. FRA has been employed
as a comparative diagnosis method over a number of years.
Initial FRA measurement during transformer factory tests is considered as the transformer winding fingerprint. In fact, distributed resistance, capacitance and inductance of the transformer winding makes a reference frequency response
trace [8]-[9]. Transformer geometric configuration changes
will lead to frequency response trace deviation, accordingly.
Mehdi Bagheri is with Department of Electrical and Computer Engineering, National
University of Singapore, Singapore 117583. Phone: +65 6516 5257; Fax: +65 6793 3318.
B. T. Phung is with School of Electrical Engineering and Telecommunications,
University of NSW, Australia 117583. Phone: +61 93855 407; Fax: +61 9385 5993.
Mehran Nashibi, Mohammad Khazabi, and Abdol-hossein Deris are with electrical distribution and maintenance department of Khouzestan Steel Company (KSC),
Khouzestan, Iran., m.nashibi@ksc.ir, m.khazabi@ksc.ir, a.deris@ksc.ir, Phone: +98
6112133100; Fax: +98 6112708095.
Deviated or measured frequency response trace can be compared with the reference trace to explore transformer winding deformation. This method is known as time based correlation. In practice, when the frequency response fingerprint is not available to be used as reference and compared with the recent measured trace, type based or design based correlation can be utilized. Type based correlation can be achieved through comparison between transformer measured trace and sister transformer corresponding trace. Comparison of frequency response spectrum between lateral phases of a transformer is called design based correlation
[10]-[13].
Indeed, FRA measurement has been industrialized and employed since the last decade while FRA interpretation is still under
development [14]. Nevertheless, there is no firm
interpretation on FRA trace variation in terms of the amplitude position of resonance peaks.
The long term goal of FRA researchers concentrates on exploring and developing an intelligent system to recognize and localize transformer winding deformation or active part displacement. In this regard, physical interpretation of FRA spectrum as well as resonance and anti-resonances cause and
effects can be considered as the fundamental steps [15]-[18].
To reach to an acceptable level of physical interpretation of frequency response trace, reference FRA spectrum should be analyzed precisely. Accurate understanding about reference
FRA will lead to explore appropriate solution in the direction of deviated frequency response interpretation.
Ryder [19] has studied and elaborated FRA spectrum oscillations. In [19], it is highlighted that FRA low frequency
response is typically characterized by transformer core influence. Mid frequencies are influenced by winding structure and high frequencies are affected by leads and connections.
This study is focused on FRA low-frequency band oscillations. Practical measurements are carried out to explore and interpret the transformer winding frequency response oscillations in the low-frequency range. Note that to differentiate between the two winding sides, lower-case symbols are used to denote low voltage parts (a, b, c, n).
Similarly, upper case symbols refer to high voltage parts (A,
B, C).
II.
F REQUENCY R ESPONSE A NALYSIS
FRA is considered as a high accuracy, fast, economical and non-destructive method in detecting winding defects.
The idea of using FRA method to monitor the condition of
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ITCE’15, Bagheri et al.: Transformer Core Fault Recognition Using Frequency Response Analysis critical power transformers running in the transmission and distribution networks is not a new one. The knowledge that the
AC impedance or admittance of any RLC network is a function of frequency is almost as old as the discipline of electrical power engineering itself. FRA has been regarded as a major advance in transformer condition monitoring, providing virtual visualization of transformer active part without costly transformer de-tanking. It can be employed for an individual winding from end to end or through injecting a predetermined signal at one end of primary side winding and measuring from corresponding secondary side winding. The ratio of the output signal to the input signal reveals frequency response. As for transformer, f min
and f max
could be determined for swept signal as 20 Hz and 2 MHz, respectively. The frequency response can be referred to as a kind of trace which is demonstrated in a Bode diagram. The horizontal axis of the trace would be frequency while the vertical axis shows frequency response magnitude. Initial FRA measurements during factory testing are considered as the winding fingerprint. Changes in winding configuration would almost certainly cause changes in the frequency response trace. The initial measurements of the distributed resistance, capacitance and inductance of a winding may also be usefully compared with the same measurements following transformer maintenance, repair or transport to recognize core defect, winding deformation, or displacement.
III.
S IGNATURE M EASUREMENT R ESULTS
The measurement was performed at (40 °C) to explore the winding frequency response signatures of 73 MVA, 230 kV transformer. Transformer specification as well as upper side schematic is provided in Table and Fig. 1, respectively. This was the first time as the transformer was examined for FRA measurement after the shipment. However, factory results on this case were not unfortunately available in detail. Two rough graphs on FRA trace provided at the end of factory report without detail information. It might not be requested by owner or not even taken by manufacturer. Hence, whatever was recorded as the winding signature spectrum in this study is just for the first time and in fact individually recorded and interpreted.
Table I. Tested transformer specifications.
Power Rate 55/73 MVA,
Voltage 230/33 kV
Freq.
V.G.
Cooling
50 Hz
YNyn0(d1)
ONAN/ONAF
Lim. No.
Winding No.
Phase No.
Max. Insult.
3
3
3
1050 KV
Max. Amb. Temp. 60 °C
Technically speaking and also based on standards [20],
[21], it has been tried to examine 4 different setup
connections to find the transformer winding frequency response. End to end, inductive inter-winding, capacitive inter-winding and end to end short-circuited measurements.
Each and every setup has its own advantages. In addition, some extra connections have been tried to reach maximum information from active part condition of this transformer.
The connections which came through different parts short circuit and found very useful in this examination. All measurements were resulted in two significant understandings on this transformer. The spectra along with interpretation, findings and conclusion are provided herein after.
IV.
R ESULTS A NALYSIS
According to IEC and IEEE standards, end to end measurement was conducted on this transformer for LV and
HV windings individually when other windings were left open circuit. Figure 2 shows the LV winding traces for phases a, b and c independently, while phase a is represented by X1, phase b by X2 and phase c via X3. X0 also represents the neutral terminal in LV side. Figures 3 and 4 are just providing the close view of LV side frequency response for the frequency bands of 20 Hz – 10 kHz and 10 kHz – 2 MHz, respectively. Consequently, in Figure 5 the frequency band which in fact all the trace are well-matched is highlighted by dash-line. Figure 6 then clearly demonstrates the anti-resonance frequency for low frequency parts. The phase graph information of these frequency responses are not provided as it does not convey important message in this case.
The frequency response data may be conveniently divided into three bands, namely low-, medium- and high- frequency
bands. In [22] it is stated that the data are dominated by the
transformer core at low frequencies, by the winding structure at medium frequencies, and by the connection leads at high frequencies. However, the boundaries between the bands are not widely agreed.
In addition, relative factor R
XY
is recommended as one of the interpretation method for FRA spectrum in [1, 2 and 3].
Low-, medium- and high-frequency bands for R
XY
, and deformation levels related to the R
XY
values, have been
defined in the Chinese standard [23] and by some other workers [24] (see Table 2). These definitions have been
widely used for FRA trace evaluation, and have been adopted in the present work.
Table II. Deformation levels and the corresponding RXY values at low
(LF), medium (MF) and high (HF) frequencies [23], [24]
Deformation level
Severe
Moderate
Slight
Normal winding
Limits for Rxy
RLF < 0.6
1.0 > RLF
2.0 > RLF


0.6 or RMF < 0.6
1.0 or 0.6

RMF < 1
RLF

2.0, RMF

1.0 and RHF

0.6
LF:1 kHz-100 kHz, MF: 100 kHz-600 kHz, HF:600 kHz-1 MHz
The relative factor ( R
XY
) is defined as [22], [23] and [24]
(1)
R
X Y




1 0
 lo g (1
1 0

P
X Y
)
1

P
X Y

1 0

1 0
O th erw ise
Figure 1. Upper view of tested transformer terminals.
where P
XY
is given by
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ITCE’15, Bagheri et al.: Transformer Core Fault Recognition Using Frequency Response Analysis
P
X Y

1
N
 N i

1


X i

N
1 i
N


1
X i
 
 
Y i

1
N i
N


1
Y i


2
,
D D
X Y
-60 X0X1
X0X2
X0X3
-70
10
2
10
3
10
4
Frequency [Hz]
Figure 2. Frequency response spectra of LV side.
-10
-60 X0X1
X0X2
X0X3
-70
10
2
10
3
Frequency [Hz]
Figure 3. Frequency response spectra of LV side (20 Hz- 10 kHz).
-10
(2) where X i
and Y i are the i th elements of the fingerprint and measured FRA traces respectively, and N is the number of elements (or samples). CC is thus a number whose absolute value lies between 0 and 1. D
X
and D
Y
are given by
D
X
D
Y


1
N
1
N


N i

1


X i
N i

1


Y i


1
N
1
N i
N

 i
1
N


1
Y i
X i


2
,


2
(3)
-10
-20
-30
-40
-50
-20
-30
-40
-50
-20
-30
-40
-50
10
5
10
6
-60 X0X1
X0X2
X0X3
-70
10
4
10
5
Frequency [Hz]
10
6
Figure 4. Frequency response spectra of LV side (10 kHz – 2 MHz).
-10
-20
-30
-40
-50
10
4
-60 X0X1
X0X2
X0X3
-70
10
2
10
3
10
4
10
5
10
6
Frequency [Hz]
Figure 5. Frequency response spectra of LV side (the well-matched frequency band).
According to the Figures 1, 2 and 3, the trace for middle winding in LV side experiences a single anti-resonance in lower frequencies, while other spectra for lateral phases demonstrate double anti-resonance.
-10
-20
-30
-40
465.9 Hz, -5174 dB
-50
465.9 Hz, 63.04 dB 488.9 Hz, -59.43 dB
-60
X0X1
X0X2
X0X3
465.9 Hz, -67.5 dB 488.9 Hz, -64.72 dB
-70
10
2
10
3
10
4
Frequency [Hz]
Figure 6. Frequency response spectra of LV side (anti-resonance frequencies).
Technically speaking, for lower frequency band in FRA trace, the lateral windings (means phases a and c) regularly showing similar spectra in terms of decreasing and increasing trend as well as number of anti-resonance and its magnitude. While in these Figures, it is quite clear that the frequency response spectra for phases a and c show different magnitudes, however, the trends are similar. The phase a indeed demonstrates less inductance rather than other phases, and phase c shows very close inductance to the middle winding. It is obvious that dedicated reluctance due to magnetic core for phase a is almost larger than other phases. The frequency band of 2 kHz – 20 kHz for all spectra are well-matched and it means that shunt capacitance between HV and LV windings are almost similar and wellspaced. Indeed, the band (2 kHz – 20 kHz) shows the interaction between low and high voltage windings for each and every phase. Moving from 20 kHz to higher frequencies the discrepancy between the phases becomes obvious. This discrepancy is not common as at least phases a and c have to show similar oscillations or even identical in trends. Figure 4 clearly shows that they are experienced different resonances and anti-resonances. This indeed can be interpreted via winding structure discrepancy between windings specially series and shunt capacitance as well as mutual-inductance.
We have concluded earlier that shunt capacitance is almost similar for entire phases, also mutual-inductance initiating mostly from number of winding turns, height and width as well as diameter which has designed of course similar for all
LV side windings. Therefore, this deviation is interpreted via different series capacitances rather than other factors. The series capacitance itself influenced by many mechanical and insulation factors, though, mechanical factor is more visible in FRA trace. Hence, the windings of phases a and b and c are mechanically different from series capacitance point of view. One of those factors which can easily influence the series capacitance in transformer winding is indeed press pressure after winding manufacturing or throughout transformer winding assembling. When the winding interdisc distance experiences different values, then winding series capacitance are become easily different. Thus, different series capacitance is reflecting as a deviated and non-matched FRA traces in mid-frequency band for new transformer. It should be noted that interpretation scenario would be of course differed for aged transformers as compared to new ones. Therefore, winding traces for LV
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ITCE’15, Bagheri et al.: Transformer Core Fault Recognition Using Frequency Response Analysis windings show that transformer magnetic core reluctance for phase a is quite different with other phases specifically phase c. Furthermore, series capacitance due to inappropriate winding press during manufacturing or assembling was concluded on this case. It is worth highlighting that the relative factor R
XY
is also resulting to winding deformation when phases a and c are compared together.
To make sure on this pre-assessment and double-checking the results; FRA measurement on HV side is conducted.
Figure 7 shows the frequency response traces for each and every HV winding when other windings where left open circuit (end-to-end measurement).
According to Figure 7, the discrepancy in FRA traces of phases A, B and C are quite obvious for very low frequencies, while this deviation becomes less when moving to higher frequencies. This in turn emphasizes on last hypothesis about different magnetic reluctance for phases A and C. The trace of phase A demonstrates less inductance as compared to phase C, while this value should be similar for both windings. The transformer magnetic reluctance for these type of transformer should be similar for phases A and
C and winding turns are designed analogous, therefore, the inductance values should be almost identical or at least close together. While based on measured frequency responses, they are showing different values. The same was happened for LV windings. This discrepancy is around 2.93 dB
(means 40 % difference in magnetic reluctance) which is significant undoubtedly.
0
-20
-40
-60
52 Hz, -42.34 dB
52 Hz, -45.27 dB
-80
52 Hz, -47.55 dB H0H1
H0H2
H0H3
-100
10
2
10
3
10
4
10
5
10
6
Frequency [Hz]
Figure 7. Frequency response spectra of HV side (antiresonance frequencies)
V.
C ONCLUSION
The transformer with power rated 55/73 MVA was examined in Khouzestan Steel Company to find the frequency response signatures for transformer windings as the future evidence. Frequency response traces for each and every winding were measured and illustrated, however, just the end-to-end measurement results reported in this study.
Interpretation of FRA results on this specific transformer incidentally revealed that the transformer windings in LV side are suspected to have different series capacitances. This can be initiated via different press pressure during manufacturing time or assembling process, or even careless transportation. The transformer will work in this condition, even though; it might be vulnerable in course of unexpected incidents.
Furthermore, the discrepancy between LV winding frequency responses in very low frequency band revealed different magnetic core reluctances for phases a/A and c/C.
The HV winding signatures emphasized on this hypothesis where phases a/A and c/C were experienced around 2.93 dB difference in lower frequencies. This in turn showing 40 % inductance (and also reluctance) diverse for theses phases. In summary, the transformer seems to be vulnerable as compared to a well-assembled transformer in course of heavy faults, significant short circuit current as well as other considerable occurrences. Further monitoring and supervision is then recommended for this transformer in course of its operation.
VI.
A CKNOWLEDGEMENT
The authors would like to thank Mr. Amin Zahiri and Mr.
Sasan Ostad for their contribution and assistance in this project. Also, all other people supports in electrical distribution and maintenance department of Khouzestan
Steel Company (KSC) are greatly appreciated.
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