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Novel method for detection of transformer winding faults using Sweep Frequency

Response Analysis Jashandeep Singh Yog Raj Sood Piush Verma Raj Kumar Jarial

Abstract-- Sweep Frequency Response Analysis (SFRA) is an

established tool for determining the core deformations, bulk winding movement relative to each other, open circuits and short circuit turns, residual magnetism, deformation within the main transformer winding, axial shift of winding etc. This test is carried out on the transformer without actual opening it and is an off line test. This paper explains the fundamental studies of SFRA measurement on basic electrical circuits, which can be extended for studying the mechanical integrity of a transformer after short circuit fault, transportation etc.

Index Terms— SFRA, Power Transformer, Winding

Deformation, Impulse response, Resonance frequency.

I. INTRODUCTION ower transformers are one of the most expensive elements in a power system and their failure is a very costly event

[4]. Power transformers are mainly involved in the energy transmission and distribution [1]. Unplanned power transformer outages have a considerable economics impact on the operation of electric power network. To have a reliable operation of transformer, it is necessary to identify problems at an early stage before a catastrophic occurs. In spite of corrective & predictive maintenance, the preventive maintenance of power transformer is gaining due importance in modern era and it must be taken into account to obtain the highest reliability of power apparatus like a power transformer. The well known preventive maintenance techniques such as DGA, thermal monitoring, oil analysis, partial discharge measurement, capacitance & tan delta measurements, sweep frequency response analysis, etc. are applied for transformer for a specific type of problem [1, 4]. In the FRA technique, a low amplifier swept frequency signal is applied at the end of one of the transformer windings and the response is measured at the other end of the winding with Jashandeep Singh is Research Scholar in the Department of Electrical Engg. (EED), NIT Hamirpur (H.P), India ([email protected]) Yog Raj Sood is Prof & Head, Electrical Engg. Department, NIT Hamirpur (H.P), India ([email protected]) Piuesh Verma is Prof & Head, Electrical & Electronics Engg. Dept, Lovely Institute of Engg.& Technology, Phagwara (Pb.), India ([email protected]) Raj Kumar Jarial is Lecturer (Selection Grade ), Electrical Engg. Department, NIT Hamirpur (H.P), India

one phase at a time. The method is based on the fact that every transformer winding has a unique signature of its transfer function which is sensitive to change in the parameters of the winding, namely resistance, inductance and capacitance. It consist of measuring the impedance of transformer winding over a wide range of frequencies and comparing the results of these measurements with a reference set taken either during installation or at any other point of time. Difference in signature of the responses may indicate damage to the transformer which can be investigated further using other techniques or by an internal examination [9].

II. WINDING DEFORMATION Winding deformation may be due to mechanical and

electrical faults. Mechanical faults occur in the form of displaced winding, hoop buckling, winding movement, deformations and damaged winding. They may be due to the loss of pressure, vibration during transportation and also excessive mechanical force during a close-up short circuit fault. Winding movements may also result from stresses induced by electrical faults such as an interturns short circuit as a result of lightning strikes [5, 10, 13]. It may also result in insulation damage. The deformation can also be due to ageing of paper. As a transformer ages the insulation shrink and the clamping pressure may be lost which reduces its voltage withstand strength. Winding deformations in transformers are difficult to establish by conventional methods of diagnostic tests like ratio, impedance/ inductance, magnetizing current etc. Deformation results in relative changes to the internal inductance and capacitance of the winding. These changes can be detected externally by low voltage impulse method or FRA method [4].

III. PURPOSE FOR SFRA MEASUREMENT SFRA measurement is required.

• After short circuit testing of Power Transformer. • After Impulse testing of power transformer. • Quality assurance during manufacturing. • Assess Mechanical Condition of Transformers

(mechanical distortions). • Detect Core and Winding Movement. • Due to large electromagnetic forces from fault

currents.

P

1-4244-1298-6/07/$25.00 ©2007 IEEE.

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• Winding Shrinkage causing release of clamping pressure.

• Transformer Relocations or Shipping

IV. SFRA MEASUREMENT Frequency response analysis plots the ratio of the

transmitted voltage waveform to the applied voltage waveform in dB’s. The impedance attenuated the input voltage signal. The basic measurement circuit is shown in Figure 1. To remove the effect of test leads, a three lead system is used to measure both input and output voltages [7].

Figure 1: Measurement of voltages for SFRA

The test leads are made from low loss RG-58 RF coaxial

cable with the shield grounded to the instrument chassis through a standard connector. The M5200 SFRA Instrument requires a match impedance signal cable, & performs a single end measurement. The signal is measured w.r.t. the instrument ground. The shield of the signal cable must be connected to the chassis using 50 ohm impedance- matched RF BCN Connector. The length of the lead is 60 ft (This length is the shortest length useful to test the largest transformer from a location on the ground, adjacent to the test transformer). Nevertheless, it is the lead length that determines the max effective frequency [18]. The response in dB’s is calculated by the following equation:

Response in dB’s, )(log20 10 VinVoutdB = (1)

The output voltage, Vout is referenced via a 50 Ω co-axial cable to ground. This means we have:

VinVout

= Z+50

50 (2)

Where Z is frequency dependent impedance function for an Inductor or Capacitor or a combination of the two.

V. FREQUENCY RESPONSES OF INDUCTOR, CAPACITOR AND RESISTOR

Individual passive components- R,L,C – have identifiable and distinct frequency responses. In practice, however, there is no such thing as an ideal inductor, an ideal capacitor or an ideal resistor; each has elements of the other components. Consequently their responses contain elements of each component.

The expected responses for a short circuit, at any frequency, is that Vout = Vin, as Z=0 . This equals 0 dB across the frequency range. This is shown in Figure 2.

Ω

An ideal resistor reduces the output voltage across the frequency range e.g. a 50 Ω resistor would give an output voltage half the input voltage. This would be a straight line at -6dB, a 500 Ω resistor would give a response at -20.8 dB as shown in Figure 3. We have tested the 350 Ohm resistance using SFRA, its results appears to be around -18db, shown in Figure 4. It clearly indicates that, as the resistance of the system goes on increasing, the dB level of response decreases.

Figure 2: Response of a short circuit

Figure 3: Responses of a 50 Ω and a 500 Ω ideal resistor

An open circuit would provide infinite impedance, and an output voltage of zero. This is not calculated as a dB value, but equates to infinite dB down.

An ideal inductor at low frequency behaves as a short circuit, as frequency increases, the impedance increases, heading towards an open circuit. The response of an ideal inductor is shown in Figure 5. It starts at 0’dB and then shows a characteristics roll off, on the log scale, as the frequency increases.

Figure 4: Experimental response of 350 ohm Resistor

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Figure 5: Experimental response of inductor

A ideal capacitor behave like an open circuit at low frequencies but at high frequencies it behaves like a short circuit & its response climbs towards zero as frequency rises, as indicated in Figure 6 [7, 11].

Figure 6: Experimental response of capacitor

VI. THEORETICAL FRA MEASUREMENT ON BASIC ELECTRICAL CIRCUITS

A. Inductor only The connection diagram for measuring FRA of Inductor is

shown in Figure 7.

Figure 7: Circuit connection of L only

From the circuit,

jwLvvA

+==

5050

12

(3)

jwLdB

+=

5050log20 10 (4)

At high frequency,

fvv 112α (5)

ffv

v log201log2012log20 1010 −≅α (6)

Signal attenuation, SA is inversely proportional to the frequency and the rate of increment of the attenuation is -20 dB per decade.

B. Capacitor only

Figure 8: Circuit connection of C only

From the circuit,

1)*50()*50(

12

+==

jwCjwC

vvA (7)

1)*50()*50(log20 10 +

=jwC

jwCdB (8)

At low frequency,

fX C

1α (9)

fvv

fjwCvv

log2012log20

150

12

10 α

α

∴

≅∴ (10, 11)

Signal attenuation ‘SA’ increases as the frequency is increased and the capacitive reactance is inversely proportional to the frequency. ‘SA’ will increase at the rate of 20 dB per decade.

C. Parallel connection of Capacitors and Inductors

Figure 9: Circuit connection of parallel L & C

−∞≅≅∴

=

+−−

==

0log2012log20

1)1(50

)1(5012

2

2

vv

LCw

jwLLCwLCw

vvA

(12, 13, 14)

D. Series connection of Inductors and Resistors

Figure 10: Circuit connection of series R & L

The Signal attenuation of the circuit,

4

)50(50

12

RjwLvvA

++== (15)

The above equation indicates that, at low frequency the value of inductance is small enough; the attenuation is dominated by the resistance. As the resistance increases, the attenuation decreases. However, this is not true if the value of the inductive reactance is larger compared to the value of the resistance. Therefore, at high frequencies, the attenuation is definitely dominated by the inductive reactance. The inductive reactance increases with the frequency and this will result in further decreasing of the signal attenuation. At the high frequency range, the FRA responses produced by the series connection of inductor and resistor are quite similar to the SA results for a single inductor. This shows that, the values of resistances are too small compared to the value of the inductive reactance to produce any changes on the responses in the high frequency region.

E. Parallel connection of Capacitors and Resistors

Figure 11: Circuit connection of parallel R & C

RjwCRjwCR

RjwCRjwCR

vvA

+++

=++

+==

50)**50(50)**50(

)1)*((50)1)*((50

12 (16)

At low frequency, the circuit behaves as a voltage divider

with the ratio depending on the value of R. The lower the resistance, the higher is the ‘SA’. As the frequency is increased, the attenuation increases and if the resistance is sufficiently low, the signal attenuation will be like a voltage divider. However, if the resistance is high, the capacitance will dominate the response [5].

VII. DIAGNOSING FAULTS The low frequency faults, such as short-circuited turns,

change the magnetizing characteristics of the transformer and hence affect the low-frequency response. Circulating currents loops, if they are sufficiently large, redirect leakage flux into the core and also change the low-frequency response. An ungrounded core changes the shunt capacitance of the winding closest to the core and also the low-frequency response.

The medium-frequency response is sensitive to faults that cause a change in the properties of the whole winding. A significant increase in the medium-frequency resonances normally indicates axial movement of a winding. A significant decrease normally indicates radial movement of the inner winding (hoop buckling). Slight differences are often accepted as being a result of “windings settling into place.”

The high-frequency response is sensitive to faults that cause changes in the properties of parts of the winding. Localized winding damage causes seemingly random changes

in the high-frequency response, often leading to the creation of new resonant frequencies. The high-frequency response may also be affected by the tank or cable grounding. Poor tank grounding is easy to spot, as it affects all windings, whereas damage is usually confined to one winding or at worst one phase. Poor cable grounds are more difficult to detect, as they may cause changes to just one winding, but are unlikely to lead to the creation of new resonant frequencies [13, 6].

VIII. MODELING TRANSFORMER AS A TWO PORT NETWORKS FOR SFRA MEASUREMENTS

Generally, any pair of terminals where a signal may enter or leave an electrical network is described as a port.

When performing SFRA we have an input signal, referenced to ground, and a measured signal, also referenced to ground. A transformer undergoing SFRA can thus be modeled by a two-port network. Figure 12, illustrates a basic two-port network.

Figure 12: SFRA Two Port Network

Z11, Z22, Z12, and Z21 are the open-circuit impedance parameters. It should be noted that the negative terminals are short circuited when transformer are tested. The transformer tank is common for both negative & lower terminals. The transformer tank & lead ground shields must be connected together to achieve a common-mode measurement. This assures that no external impedance is measured. It also reduces the effect of noise. It is very important to obtain zero impedance between the lower or negative terminals to assure repeatable measurements.

The impedances, Z11, Z22, Z12, and Z21, are formed by the complex RLC network of the specimen. They include capacitances, inductances, mutual inductances and resistances and are related to the construction and materials of the transformer.

Voltage Transfer Function,

)()(

)(jwVjwV

jwHin

outV = (17)

The magnitude and phase is represented as follows [18].

))((tan)(

))((log20)(1

10

jwHAjwHdBA

−=

=

θ (18, 19)

In this diagram: • Z12 represents the impedance of the winding and any

other electrical paths between the input and output bushings; for a short circuit, Z12 would equal zero.

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• Z11 and Z22 represent the complex impedance paths to ground, through the bushing and the winding insulation.

• Z21 represents the impedance between the two reference grounds; this should be ~ 0

It should be clear that if we vary the values of Z11 and Z22, we change the network, and the results for SFRA may vary. This is the reason why good grounding is important, and why we try to be as consistent as possible in applying grounds to the base of the bushings.

In an ideal situation, there is no substantial impedance between the two bushing flange connections and Z21 approaches zero. Any stray impedance may affect the results.

Z12 represent not just the winding impedance but also any other impedance paths between the input and output signals. Due to the generally small size of Z12 compared to any other paths, it dominates and Z12 is a close approximation to the winding impedance under most circumstances [8, 16].

IX. ANALYSIS AND INTERPRETATION There is a hierarchy of analysis using SFRA. The best

method is to compare results to those obtained previously as a baseline. But sometime baseline results are not available then, we can rely on three further types of comparison over baseline comparisons:

• comparison with a sister unit of the same design • phase to phase comparison of short circuit test results • phase to phase comparison of open circuit test results

Comparison with a sister unit has clear benefits in that reference results may be determined for a number of transformers at one time as shown in Figure 13. Experience has shown, however, that sister units, even with successive serial numbers in the factory, may show variation: resonance shifts and form changes. Consequently we must use such results with caution.

Short circuit test results allow direct comparison between phases of a transformer. This is a very powerful test as, by shorting out the LV windings during a HV test, we remove the effect of the core. Hence we are looking at the response of the three winding arrangements as large inductors. These should show the classic shape of such a response: near zero dB down at low frequency as the DC resistance of each winding is small, and an inductive roll off as frequency increases. Any variation between phases should warrant an investigation. Figure 14 illustrate the impact of open circuit & short circuit connections in a transformer.

Open circuit phase-to-phase comparison is only possible in a few circumstances. Each winding has an expected shape, with some predictable variations. However, there are many causes of variation between phases which means that it is possible to have substantial differences with no problem in the transformer [15].

Figure 13: FRA comparison of identical sister unit transformers

Figure 14: FRA response under open circuit & short circuit connections

X. STANDARD INTERPRETATIONS Experience has shown that different frequency bands of the

SFRA trace relate to different elements within a transformer. A general overview is given in Table 1 for open circuit measurements.

TABLE I FREQUENCY BANDS AND POSSIBLE SOURCES OF VARIATION

Band Likely Causes of Variation <2kHz Core Deformation, Open Circuits,

Shorted Turns & Residual Magnetism

2kHz to 20kHz Bulk Winding Movement Relative to Each Other, clamping structure

20kHz to 400kHz Deformation Within the main and tap windings

400kHz to 2MHz Movement of main and tap winding Leads; axial shift

It is possible to use as the basis for an expert system,

however, caution must be used. The bands overlap and are not well defined; the band limits are not strictly set and vary both

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with manufacturer and transformer MVA and voltage. Hard and fast rules are difficult to generate as there are so many designs and manufacturers.

With short circuit measurements we are really only looking at low frequencies, below a couple of kHz. Variation between successive measurements means a change in impedance for the winding and needs to be investigated thoroughly. Variation between phases within a transformer may be the result of design and construction. Anything within about 0.2 dB is usually considered acceptable, but even here it may be an indication of a variation of significance [17].

XI. CONNECTIONS OF SFRA Generally, an SFRA measurement is made from one

terminal on the transformer (e.g H1 or A) to another terminal (e.g. H2 or N). It is important to record all relevant information, which includes tap position, oil level and terminals grounded or shorted.

It is important to note that where previous test results exist, the best testing procedure is to repeat those tests: taking note of tap position, shorted or grounded bushings and any particular details for specific tests performed.

Doble make M5200 SFRA model is capable of doing all needed SFRA test which can be analyzed with SFRA software. Ensure the cables are connected to the test set following the color coded BNC connections. A test lead integrity check may be performed if required.

When running a test on a transformer winding, for example H1-H0, attach the Red Lead to the H1 bushing and the Black Lead to the H0 bushing, as shown in Figure 15.

When connecting leads to a transformer, ensure that you attach the lead grounds to a stud or bolt at the base of the bushing, and that good electrical contact is established and maintained. It is important to record nameplate and test arrangement data in the 'Nameplate' section of the software. Changing tap position or DETC position or removing core ground connections will give different SFRA results. This

Figure 15: Connecting Leads to the Transformer to Measure H1-H0 means that where previous results are available, measurements must be made in a manner consistent with those previous results. The following details are a minimum set required for each test. • Manufacturer, serial number

• LTC and DETC positions during test • Location • HV/LV/TV, MVA, Impedance • Red and black lead locations • Bushings shorted • LTC and DETC ranges and nominal position • Bushings grounded

XII. MEASUREMENT TYPES

A. Open Circuit An open circuit measurement is made from one end of a

winding to another with all other terminals floating. For a delta winding, connections would be H1 to H3, for example. For a star winding measurements are taken from HV terminals to neutral, such as X1 to X0.

B. Interwinding An Interwinding measurement is from one winding to

another with all other terminals floating. This would include, for example, H1 to X1 on a double wound transformer or H1 to Y1 on an autotransformer with a tertiary. Note that H1 to X1 on an autotransformer is not an Interwinding measurement but an open circuit measurement on the series winding. Interwinding measurements are usually considered as optional tests or tests for further investigation when open circuit and short circuit tests are inconclusive (Interwinding tests are marked with an asterisk to indicate their optional nature).

C. Short Circuit A short circuit measurement is made with the same SFRA

test lead connections as an open circuit measurement but with the difference that another winding is short circuited. To ensure repeatability, Doble recommends that the three voltage terminals on the shorted winding are all shorted together. This would mean, for example, shorting X1 to X2, X2 to X3 and X3 to X1. This ensures all three phases are similarly shorted to give consistent impedance. Any neutral connections available for the shorted winding should not be included in the shorting process.

XIII. TEST CONNECTIONS Make sure good electrical connections are made at bushing

terminals and at the base of bushings; clean, file or wire brush connection points if necessary. Test connections are given here for some common transformer designs; the red lead is first of the two named terminals.

Each table gives the recommended tests with position of the red lead and black lead clearly identified. Reversing these test leads may provide small variations in higher frequency response. Care must therefore be taken in attaching test leads in the appropriate manner.

Good grounds are key to good high frequency responses – make sure ground connections are not hampered by loose

connections, paint or dirt and grease [18]. Table 2 to 5, illustrate all possible combination of SFRA tests for deriving

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meaningful analysis related to mechanical integrity of power transformers.

XIV. CONCLUSION In this paper, various basic concepts related to Sweep

Frequency Response Analysis (SFRA) are presented in relation to SFRA of various R, L, C circuits. It has been explained that basic R, L, C elements responses are helpful for

modeling a power transformer which can be considered as a two port network. It has been shown that SFRA responses of different combination of transformer winding can highlight completely the mechanical integrity of power transformer by their careful comparison. In nutshell it has been demonstrated that FRA techniques are better than impulse response techniques.

TABLE 2 TWO WINDING TRANSFORMER CONNECTIONS

Test Type Test 3 Phase Δ-Υ

3 Phase Υ-Δ

3 Phase Δ-Δ

3 Phase Υ-Υ

1 Phase

Test 1 H1-H3 H1-H0 H1-H3 H1-H0 Test 2 H2-H1 H2-H0 H2-H1 H2-H0

HV Open Circuit (OC) All other terminal floating

Test 3 H3-H2 H3-H0 H3-H2 H3-H0

H1-H2 or (H1-H0)

Test 4 X1-X0 X1-X3 X1-X3 X1-X0 Test 5 X2-X0 X2-X1 X2-X1 X2-X0

LV Open Circuit (OC) All other terminal floating

Test 6 X3-X0 X3-X2 X3-X2 X3-X0

X1-X2 Or (X1-X0)


Short Circuit (SC) High (H)to Low (L) Short [X1-X2-X3] Test 9 H3-H2 H3-H0 H3-H2 H3-H0

H1-H0 Short [X1-

X2 or X1-X0]

TABLE 3 THREE WINDING TRANSFORMER CONNECTIONS

Test Type Test 3 Phase Δ-Δ-Δ

3 Phase Δ-Δ-Υ

3 Phase Δ-Υ-Δ

3 Phase Δ-Υ-Υ

1 Phase



Test 3 H3-H2 H3-H2 H3-H2 H3-H2

H1-H2 or

(H1-H0) Test 4 X1-X3 X1-X3 X1-X0 X1-X0 Test 5 X2-X1 X2-X1 X2-X0 X2-X0


Test 6 X3-X2 X3-X2 X3-X0 X3-X0

X1-X2 Or

(X1-X0) Test 7 Y1-Y3 Y1-Y0 Y1-Y3 Y1-Y0 Test 8 Y2-Y1 Y2-Y0 Y2-Y1 Y2-Y0

Tertiary Open Circuit (OC) All other terminal floating

Test 9 Y3-Y2 Y3-Y0 Y3-Y2 Y3-Y0

Y1-Y2 Or (Y1-Y0)


Short Circuit (SC) High (H) to Low (L) Short [X1-X2-X3] Test 12 H3-H2 H3-H2 H3-H2 H3-H2

H1-H0 Short [X1-

2] Test 13 H1-H3 H1-H3 H1-H3 H1-H3 Test 14 H2-H1 H2-H1 H2-H1 H2-H1

Short Circuit (SC) High (H) to Tertiary (T)

Short [Y1-Y2-Y3] Test 15 H3-H2 H3-H2 H3-H2 H3-H2

H1-H0 Short [Y1-

2] Test 16 X1-X3 X1-X3 X1-X0 X1-X0 Test 17 X2-X1 X2-X1 X2-X0 X2-X0

Short Circuit (SC) Low (L) to Tertiary (T)

Short [Y1-Y2-Y3] Test 18 X3-X2 X3-X2 X3-X0 X3-X0

X1-X0 Short [Y1-

2]

TABLE 4 THREE WINDING TRANSFORMER CONNECTIONS

Test Type Test 3 Phase Υ-Υ-Υ

3 Phase Υ -Υ-Δ

3 Phase Υ-Δ-Υ

3 Phase Υ-Υ-Δ



Test 3 H3-H0 H3-H0 H3-H0 H3-H0 Test 4 X1-X0 X1-X0 X1-X3 X1-X3 Test 5 X2-X0 X2-X0 X2-X1 X2-X1


Test 6 X3-X0 X3-X0 X3-X2 X3-X2 Test 7 Y1-Y0 Y1-Y3 Y1-Y0 Y1-Y3 Test 8 Y2-Y0 Y2-Y1 Y2-Y0 Y2-Y1

Tertiary Open Circuit (OC) All other terminal floating

Test 9 Y3-Y0 Y3-Y2 Y3-Y0 Y3-Y2 Short Circuit (SC) Test 10 H1-H0 H1-H0 H1-H0 H1-H0

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Test 11 H2-H0 H2-H0 H2-H0 H2-H0 High (H) to Low (L) Short [X1-X2-X3] Test 12 H3-H0 H3-H0 H3-H0 H3-H0


Short Circuit (SC) High (H) to Tertiary (T)

Short [Y1-Y2-Y3] Test 15 H3-H0 H3-H0 H3-H0 H3-H0 Test 16 X1-X0 X1-X0 X1-X3 X1-X3 Test 17 X2-X0 X2-X0 X2-X1 X2-X1

Short Circuit (SC) Low (L) to Tertiary (T)

Short [Y1-Y2-Y3] Test 18 X3-X0 X3-X0 X3-X2 X3-X2

TABLE 5 AUTO TRANSFORMER CONNECTIONS

Test Type Test 3 Phase

1 Phase

Test 1 H1-X1 Test 2 H2-X2

Series Winding (OC) All other terminal floating

Test 3 H3-X3

H1-X1

Test 4 X1-H0X0 Test 5 X2-H0X0

Common Winding (OC) All other terminal floating

Test 6 X3-H0X0

X1-H0X0

Test 7 Y1-Y3 Test 8 Y2-Y1

Tertiary Winding (OC) All other terminal floating

Test 9 Y3-Y2

Y1-Y2 (Y1-Y0)

Test 10 H1-H0X0 Test 11 H2-H0X0

Short Circuit (SC), High (H) to Low (L) Short [X1-X2-X3]

Test 12 H3-H0X0

H1-H0X0 Short [X1-H0X0]

Test 13 H1-H0X0 Test 14 H2-H0X0

Short Circuit (SC), High (H) to Tertiary (T) Short [Y1-Y2-Y3]

Test 15 H3-H0X0

H1-H0X0 Short [Y1-Y2]

Test 16 X1-H0X0 Test 17 X2-H0X0

Short Circuit (SC), Low (L) to Tertiary (T) Short [Y1-Y2-Y3]

Test 18 X3-H0X0

X1-H0X0 Short [Y1-Y2]

XV. REFERENCES [1] Jorge Pleite, Carlos Gonzalez, Juan Vazquez, Antonio Lazaro, “

Power transformer core fault diagnosis using frequency response analysis”, IEEE MELECON 2006, May 16-19, Benalmadena, Spain, pp 1126-1129.

[2] Charles L. Sweetser , Patrick Picher, “ A Report on activities By IEEE WG Pc57.149 And CIGRE WG A2.26 On Frequency Response Analysis (FRA) Testing”, 2005 Doble Engineering Company.

[3] Ashok Kumar Yadav, Subash C. Taneja, “ Transformer diagnostics testing by SFRA”, The Journal of CPRI, vol.2, No.2, September 2005, pp 177-185.

[4] P.M.Nirgude, B. Gunasekaran, Channakeshava, A.D. Rajkumar, B.P. Singh, “Frequency response analysis approach for condition monitoring of transformer”, Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on 17-20 Oct. 2004, pp 186 – 189.

[5] D.M.Sofian, Z.D. Wang, S.B. Jayasinghe, “Frequency response analysis in diagnosing transformer winding movements - fundamental understandings”.; Universities Power Engineering Conference, 2004. 39th International Volume 1, 6-8 Sept. 2004 pp 138 - 142 Vol. 1

[6] S.A.Ryder, “Diagnosing transformer faults using frequency response analysis”, Electrical Insulation Magazine, IEEE Volume 19, Issue 2, March-April 2003, pp 16 – 22.

[7] Tony Mcgrail, “SFRA Basic Analysis Vol 1, Version 1.0”, 2003 Doble Engineering Co., pp 4-13.

[8] Tony Mcgrail, “SFRA Basic Analysis Vol 2, Version 1.0”, 2003 Doble Engineering Co., pp 3-5.

[9] Simon A. Ryder, “Methods for comparing frequency response analysis measurement”, IEEE International Symposium on Electrical Insulation, Boston, MA USA, April 7-10, 2002.

[10] Simon Ryder, "Frequency Response Analysis for Diagnostic Testing of Power Transformers", Electricity Today Magazine, Issue June 2001.

[11] The Impedance Measurement Handbook”’ 2nd Edition, Agilent Technologies, 2000.

[12] J.A. Lapworth and T.J. Noonan, “Mechanical condition assessment of power transformers using frequency response analysis,” 1995 Conference of Doble clients, Boston, 1995.

[13] P. T. M. Vaessen and E. Hanique, " A New Frequency Response Analysis Method for Power Transformers", IEEE Trans. On Power Delivery, Vol. &No. 1, January 1992, pp 384-391.

[14] Lawrence P. Huelsman, 1984. Basic Circuit Theory, New Jersey, Prentice-Hall, Inc.

[15] E.P.Dick, C.C.Erven, "Transformer Diagnostic Testing by Frequency Response Analysis" IEEE Trans PAS-97, No. 6, pp 2144-2153, 1978.

[16] http://www.ece.pdx.edu/~ece2xx/ECE222/Slides/TwoPortsx4.pdf [17] Tony McGrail, Charles Sweetser, “Experience with SFRA for

Transformer Diagnostics”, Doble Engineering. [18] Manual M5200 SFRA, Version 1.1.

XVI. BIOGRAPHIES

Jashan deep Singh was born in Ludhiana (Punjab), India. He did his Diploma in Electrical Engg. From G.N.E. Ludhiana in 1999, B.Tech in Electrical Engg. and M.Tech in Instrumentation & Control in 2002 and 2004 respectively. He is doing his Ph.D from NIT, Hamirpur. He is working as lecturer in the Electrical Engineering department of NIT, Hamirpur. His interest researches are Energy management,

Transformer Diagnosis and Electrical machines.

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Dr. Yog Raj Sood obtained his B.Sc degree from P.U. Chandigarh in 1980. He received his B.E. degree in Electrical Engineering with “Honours” and M.E. in Power System from Punjab Engineering College Chandigarh (U.T.), in 1984 and 1987 respectively. He has obtained his Ph.D. from Indian Institute of Technology, Roorkee in 2003. He joined Regional

Engineering College Kurukshetra in 1986. Presently he is Professor & Head in the Electrical Engineering Department of National Institute of Technology, Hamirpur (H.P.), India.

He has published a number of research papers. He has been awarded “The Union Ministry of Energy- department of Power Prize” for publication of one of his research paper in the journal of the Institution of Engineers (India). His research interests are in the area of computer applications to power system, wheeling, deregulation, open access transmission system, power network optimization, high voltage engineering and non-conventional sources of energy.

Dr.Piush Verma graduated in 1991 with degree in Electrical Engineering from Institution of Engineers (India). He received his Master degree with Honors and Ph.D in Electrical engineering with from Thapar Institute of Engineering and Technology (Deemed University), Patiala (India) in 1995 and 2005. He has over 15 years of experience in research, industry and

academics. Presently he is Professor in the department of Electrical & Electronics Engineering and involved in research in the area of Condition Monitoring of Transformers.

Raj Kumar jarial, was born in India, and received his Degree(Electrical Engineering), Masters Degree(Power Systems Engineering) with Distinction and Ph.D. Degree(Power Systems Engineering) in the year 1990, 1993 and 1997 respectively in India. Then he has joined the Department of Electrical Engineering, Indian Institute of Technology (IIT), Roorkee, India, as a Lecturer, Assistant Professor and

Associate Professor during 1998, 2001, and 2005 respectively. He has worked as a Visiting Staff in the Department of Electronics and Electrical Engineering, University of Bath, UK under Boyscast Fellowship. Presently He is working as Associate Professor in IIT Roorkee. His field of interest is Power System Economics, Unit Commitment, Power System Privatization, Restructuring and Deregulation, Transmission and Distribution network charging, Artificial Intelligence Applications to Power System and FACTS.

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