FRA Standards .1 DL/T911-2004

The study conducted by Aljohani and Abu-Siada [112] proposed a consideration of both transfer function magnitude and phase plot in one combined 2-D image for successive detection and classification of transformer fault. After elimination of the noise and background from the pictorial data, it was further used for extraction of the features for oil and bushing fault classification.

Zhou et al. [156] proposed an innovative approach to determine transformer winding faults from FRA signature. Frequency response magnitude plot is transformed into a binary image and went through the data erosion in order to reduce some minor disturbances.

Afterwards, features extracted from the processed images were used to determine the deviation between reference (fingerprint) and new FRA measurement, which helped to diagnose any occurring faults in the winding structure. Furthermore, the SVM based classification model was utilized to classify the results of practical case studies among axial deformation, series capacitance deviation, and short circuit fault.

Zhau et al. [157] applied DIP technique to magnitude and phase plot of the transfer function and estimated similarity indicator for two dimensional images representing intact condition and mechanical deformation; and observations reported higher sensitivity of the proposed method compared to conventional statistical analysis engaged into standards.

The study by [158] introduced a new FRA results interpretation technique, where the phase response of the transformer winding is converted into 2D image with real and imaginary parts. Comparison of the FRA spectra is conducted via calculation of the Sum square max-min ratio error (ISSMMRE) and further used to define condition of the winding under the test. However, the practical results revealed that the proposed method is insufficient to define both type and severity of the applied mechanical fault.

3.4 FRA Standards

winding among severe, obvious, slight deformation and normal winding condition.

Classification criteria for relative factor is presented in Table 1 [19].

Table 1 – Relative factor and the deformation degree Degree of winding deformation Relative factor, R

Severe deformation R

LF

< 0.6

Obvious deformation 1.0 > R

LF

≥ 0.6 OR R

MF

< 0.6 Slight deformation 2.0 > R

LF

≥ 1.0 OR 0.6 ≤ R

MF

< 1.0 Normal winding R

LF

≥ 2.0, R

MF

≥ 1.0 AND R

MF

≥ 0.6 The relative factor is calculated using:

R

_XY

= { 10

- log

₁₀

(1-σ) 1-σ < 10

^-1

otherwise (10)

where R

XY

represents the relative factor R

LF

, R

MF

and R

HF

for low (1 kHz – 100 kHz), medium (100 kHz – 600 kHz) and high (600 kHz – 1000 kHz) frequency sub-bands, respectively, and σ is the normalization covariance factor estimated as:

𝜎(X,Y)= C

_XY

√D

_X

∙D

_Y

(11)

where the variables C

XY

, D

X

and D

Y

are evaluated using:

C

_XY

= 1

N ∑ [(X(i)- 1

N ∑ X(i)

N-1

i=0

)

2

× (Y(i)- 1

N ∑ Y(i)

N-1

i=0

)

2

]

N-1

i=0

(12)

D

_X

= 1

N ∑ (X(i)- 1

N ∑ X(i)

N-1

i=0

)

N-1 2

i=0

(13)

D

_Y

= 1

N ∑ (Y(i)- 1

N ∑ Y(i)

N-1

i=0

)

N-1 2

i=0

(14)

It should be noted, that this approach considers only the effect of the winding fault

on FRA spectrum, as it estimates relative factor for the range from 1 kHz to 1 MHz; hence,

the low- and high-frequency deviations are left uninvestigated [19].

3.4.2 CIGRE Working Group A2.26

The International Council on Large Electric Systems (CIGRE) standard undertaken by Working Group A2.26 [159] provided the recommendations on effective FRA measurement configurations, results interpretation and frequency response behavior affected by transformer winding movements. Along with those, the standard suggested three concepts to conduct comparative FRA data analysis depending on the availability of the fingerprint signature:

− time-based comparison: comparison of FRA spectra for exact similar measurement setup, when fingerprint is available;

− type-based comparison: comparison of FRA spectra of two identical (sister) transformer units, in case the fingerprint is not available;

− design-based comparison: comparison of FRA spectra of adjacent phases of the same transformer unit, if fingerprint is not available (only applicable to three-phase units) [159].

The technical brochure proposed a methodology to interpret FRA results via statistical analysis, namely, the estimation of CC. However, a successful classification of transformer working condition becomes challenging, since the guide do not provide enough information regarding definition of the critical CC values separating “normal” and

“abnormal” conditions.

3.4.3 IEC 60076-18:2012

The standard released by the International Electrotechnical Commission (IEC) [48]

has provided a comprehensive information on FRA measuring equipment and setup

configurations. It is capable to be applied to power and distribution transformers, reactors

and shifting transformer units. More than that, in the standard the frequency spectrum was

divided into four sub-bands and different affecting factors affecting each sub-band were

differentiated. In particular, low-frequency range (up to 2 kHz) is mostly influenced by

core structure, mid-frequency range (from 2 kHz to 20 kHz) represents the interaction

between windings, high-frequency range (from 20 kHz to 1 MHz) is affected by winding

structure, and very high-frequency range is influenced by deviations in setup configuration,

lead and earthing connections. The very high-frequency sub-band varies depending on the size of the transformer under the test. The upper limit is 1 MHz for units less than 72.5 kV and 2 MHz for units rated greater than 72.5 kV, respectively. However, these frequency sub-bands may vary depending on transformer type and construction. Beside major factors influencing FRA signature, the standard discusses other issues such as effect of the tertiary winding, insulation type, and temperature.

3.4.4 IEEE C57.149

The standard developed by the Institute of Electrical and Electronics Engineers (IEEE) of Power and Energy Society [50] is applicable to the oil-immersed power transformers. The guide provides information about FRA instrumentation, measurement setups, results interpretation guidelines, and data storage. More than that, the IEEE C57.149 guide has recommendations regarding the minimum and maximum number of FRA test configurations for six different transformer types as listed in Table 2 [46]. For instance, consider the three-phase two-winding transformer under the test. It is recommended to conduct six open-circuit end-to-end FRA tests on both LV and HV sides of each phase and three short-circuit end-to-end FRA tests on HV sides with shorted LV sides, adding up to nine minimum required FRA test configurations.

The practical part of the standard provided actual FRA signatures of the transformers undergoing frequently occurred faults. This gives on-site personnel a good perspective in terms of the FRA behavior towards different fault types.

Table 2 – The number of the recommended tests based on transformer types Transformer type Minimum Maximum

Two-winding transformer 9 15

Autotransformer without tertiary winding 9 12

Autotransformer with tertiary winding 18 33

Autotransformer with buried tertiary winding 9 18

Three-winding transformer Part 1 18 36

Three-winding transformer Part 2 18 36

3.4.5 NCEPRI

The North China Electric Power Research Institute (NCEPRI) [160] proposed the algorithm to define the short circuit fault of the transformer winding based on the numerical comparison of the reference and the new measurement of the frequency response transfer function based on estimated value of the effective deviation ED or assessment factor E

12

using the following expression [160]:

E

₁₂

= 1

N ∑(TF

_1i

-TF

_2i

)

²

N

i=1

(15)