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DIAGNOSIS OF POWER TRANSFORMER THROUGH SWEEP FREQUENCY RESPONSE ANALYSIS AND COMPARISON METHODS

Sandeep Kumar, Mukesh Kumar and Sushil Chauhan Abstract-- Sweep Frequency Response Analysis (SFRA) is an effective diagnostic tool used for finding out any possible winding displacement or any other mechanical deterioration inside the Transformer, due to large electromechanical forces occurring from the fault currents, winding shrinkage due to loosening of clamping pressure or during Transformer transportation and relocation. These may leads to the core deformations, bulk winding movement relative to each other, open circuits and short circuit turns, residual magnetism, deformation within the Transformer winding, axially or radially. Through this test we get the different signatures for the different winding and by comparing these signatures using different methods the information about the health of transformer is obtained. This paper presents case studies related to the SFRA testing, & their result interpretation through the statistical indicator and the standard interpretations available.

Index Terms— SFRA, Power Transformer, mechanical integrity, interpretations, and comparison methods, statistical indicator.

I. INTRODUCTION

ower Transformers play a very critical role in the efficient and effective transmission of Power. They are the vital link between the generating station and the point of utilization. As demand is increasing rapidly and many electricity utilities are now facing uphill task of bridging the gap between the supply and demand, transformer are required to carry higher loads as compared to past. Any failure in a Transformer or its associated peripherals will result in loss of power transmission adding up to huge losses. Therefore, the present Competitive era demands, uninterrupted power to the end user[1].Hence the emerging technology of Condition Monitoring/diagnostics testing of power transformers and other major equipment has come to the fore and is increasingly being considered to predict the condition of the equipment before it forces outages and/or major breakdowns. Conventional condition monitoring techniques are: dissolved gas analysis, winding resistance measurement, capacitance and Tan delta measurement etc [2].But none of these tests conclusively detect winding movement due to short circuit force or any changes in the winding inductance or capacitance during the relocation of Power Transformers, which alter the impedance network due to various forces acting on the Transformer during the transportation, and in turn, which alter the transfer function. The other issue related to this change is the abnormality occurring on the working Transformer i.e. short circuit etc, which produces enough axial and radial stresses, and these may lead to disturb the mechanical integrity of the Transformers. So to pre identifies the pre damaged in the Power Transformer due to changes in the winding geometry, it is desirable to monitor the mechanical condition of Transformer periodically during their service life to provide an early

warning of such failure. For this purpose SFRA is tool, which can be use to access the mechanical integrity of the Power Transformers [4].

II. FUNDAMENTALS

The primary objective of SFRA is to determine how the impedance of a test specimen (i.e. winding in case of Transformers) behaves over a specified range of applied frequencies. The impedance is a distributed network of active and reactive electrical components. The components are passive in nature, and can be modeled as resistors, inductors, and capacitors. The reactive properties of a given test specimen are dependent upon and sensitive to changes in frequency. The change in impedance versus frequency can be dramatic in many cases. The result is a transfer function representation of the RLC network in the frequency domain. The transfer function of a RLC network is the ratio of the output and input frequency responses, when the initial conditions of the network are zero. Both magnitude and the phase relationships can be extracted from the transfer function. The transfer function helps us better understand the input/output relationship of a linear Two Port Network [5].

Fig 1.Two Port Network

The transfer function also represents the fundamental characteristics of a network, and is a useful tool in modeling such a system. The Transfer Function is represented in the frequency domain and is denoted by the Fourier variable H(jω), where (jω) denotes the presence of a frequency dependent function, and ω = 2πf. The Fourier relationship for the input/output Transfer function is given by Equation below.

H (jw) =)()(

jwVinputjwVoutput

The goal of SFRA is to measure the impedance model of the test specimen. When, we measure the transfer function H (jω), it does not isolate the true specimen

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XXXII NATIONAL SYSTEMS CONFERENCE, NSC 2008, December 17-19, 2008

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impedance Z (jω). The true specimen i.e. Transformer winding impedance Z(jω) is the RLC network, which is positioned between the instrument leads, and it does not include any impedance supplied by the test instrument. It must be noted that, when using the voltage relationship, H (jω) is not always directly related to Z (jω). For Z (jω) to be directly related to H (jω), a current must be substituted for the output voltage and then ‘Ohms Law’ can be realized. However, SFRA uses the voltage ratio relationship for determining H (jω) A (dB) = 20log 10 (H (jw))

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Response in dB=20log 10 )()(

jwVinputjwVoutput

A (θ) = tan-1 (H (jw))

These two equation can be use to plot the Bode plot, which will give you Magnitude plot and Phase plot. So H(s) can be plot on large span by varying the frequency to wide ranges. Corresponding to this we will get a signature for each phase, which can be compared between phases and previous results the measured frequency range is usually rather large (10 Hz to 2 MHz in the tests reported here) and so the results are usually presented on a graph of amplitude or phase against frequency. In this SFRA Method, Signals are applied and measured with respect to ground. This test also include voltage transfers between windings i.e. applying a signal to one winding of a Transformer and measuring the response at another winding to determine the amplitude change and phase shift of the signal.

III.DETECTIBILITY BY SFRA MEASUREMENT SFRA measurement can be used:-

After short circuit testing of Power Transformer. After Impulse testing of Power Transformer. For Quality assurance during manufacturing. To Assess Mechanical Condition of Transformers

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

currents. Winding Shrinkage causing release of clamping

pressure. Transformers Relocations or Shipping[18]

IV. DIFFERENT WINDING FREQUENCY RESPONSES

Results of SFRA measurements made on the LV winding of a 16 MVA is shown in Fig 2. The low-frequency response is typically characterized by decreasing amplitude reaching a minimum in a resonance at or below 1 kHz. This resonance is caused by the interaction of the shunt capacitance of the windings with the magnetizing Inductance. If there are two flux paths in the Core of different lengths, then it will be a double resonance. The first resonant frequencies can vary with the state of residual magnetization of the core. They will also be different on sister Transformers, where manufacturing differences in the core joints will give different reluctances. At medium frequency there is a group of resonances, corresponding to the interaction of the shunt capacitance and air-cored inductance of the windings. More significant differences may be found between windings on different phases of three-phase Transformers, owing to different lead configurations or different winding external clearances. At high frequency there is a more confused group of resonances, corresponding to the interaction of the shunt and series capacitances and air-cored inductances of parts of the windings [19].

Fig 2.Test on LV winding with HV open Circuited

Fig 3.Test on HV winding when LV open circuited

The high-frequency response is affected by manufacturing differences, lead configuration, and winding external clearances in much the same way as is the medium-frequency response. At the highest frequencies the influence of the measurement cables can become important, especially on large Transformers. Similarly in case of the HV winding frequency response is shown in Fig 3.

V. DIAGNOSING FAULTS SFRA consists of measuring the impedance of Transformer windings over a wide range of frequencies and comparing the results with a reference set. To detect a fault, it must change either the inductance or the capacitance of a significant part of the winding. Faults that do not cause such changes (partial discharge is probably the best example) are not detectable. Such faults may become detectable if, they become sufficiently severe to cause detectable secondary damage (short-circuited turns, severe local winding damage etc). Faults, such as short-circuit turns, which changes the magnetizing characteristics of the Transformer and hence 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). The high-frequency response is sensitive to faults that cause changes in the properties of parts of the winding. Localized

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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. The comparison is best made using measurements made earlier on the same winding, where appropriate, both sets of measurements should be made on the same tap position and with the same accessories, such as bushings, fitted. If the Transformer is oil filled, then the oil should have the same relative permittivity as previously. If it is suspected that the oil has been changed since the baseline measurement was made or that there has been a significant change in the relative humidity, then caution is necessary and it may be advisable to make inter-phase comparison to supplement the comparisons with the baseline measurement. Inter-phase comparison is possible with three-phase Transformers. Owing to differences in the magnetizing inductance between the three phases, there will be differences between the SFRA results at low frequencies. At medium and high frequencies, the results usually agree quite well. For some designs the agreement is not as good, owing either to differences in the lead configurations or in the winding external clearances. There may be quite large differences in the low-frequency results, but at higher frequencies the results tend to agree quite well. The faults causing changes to the low-frequency response can all be reliably detected using other means i.e. Short-circuited turns by magnetizing current or turns ratio measurements, circulating currents by DGA or a thermal scan, the faults causing changes at medium and at high frequency are difficult to detect using other means of turns ratio, capacitance measurement, and leakage impedance measurement. These are effective in some cases but not all. For a detailed evaluation of SFRA for fault diagnosis see [19].The rest of this article, and the practical examples presented, will concentrate on the detection of various faults

VI. COMPARISON METHODS The comparison of results is usually made by plotting a graph of the amplitude against frequency for both sets of measurements. An experienced observes then examines the two curves for any significant differences. Significant differences are usually understood to be:

Changes to the shape of the curve the creation of new resonant frequencies or the

elimination of existing resonant frequencies Large shifts in existing resonant frequencies [9].

The main problem with these methods of comparison is that the expert’s opinion may lack both objectivity and transparency. One way of addressing both problems is to note down all of the resonant frequencies. This gives objective and transparent information on the number of resonances that have been created or eliminated, and how far any resonances may have shifted. However, it can only extract information from the results at the resonances and relies on there being sufficient resonances to give useful information but not so many as to cause confusion. An alternative, which the author has been involved in

promoting, is to calculate statistical indicators, which give the amount of agreement or disagreement between the two sets of measurements [9]. The information extracted from the Statistical Indicator for the whole of the repeatable range is easily applied by computers. Trials by the author indicate that the correlation coefficient is the most reliable statistical indicator. Full results of an evaluation may be found in [8]. The mathematical definition of the correlation coefficient is given below. Consider two sets of ‘n’ numbers, X (x1, x2... xn) and Y (y1, y2. . . yn). The correlation coefficient between these two sets of numbers is defined by the equation below.

ρ = /

VII. CASE STUDIES A small number of case studies are presented to show the application of SFRA to diagnosis the Power Transformers with the various comparison methods.

1. 16 MVA, 132/33KV Transformer, Apex Make (with oil and bushing) working at 132 KV substation. 1.1. Measurements Carried Out on the HV windings:-

The result of the measurements carried out on the HV windings are reported in Fig 4 and Fig 5 for the configurations with the LV windings open and floating and with the LV windings short-circuited respectively. As for this Transformer the previous SFRA signatures were not available. So to interpret the signatures, we have only the phase to phase comparison using the coefficient of correlation (numerical evaluation of the sharpen differences of the damaged phase with respect to the sound phases) as the comparing tool [12]. It can be seen in Fig.4 in which we carried out open circuit test on HV winding. The shape of the frequency responses corresponding to all the three phases deviate substantially in the low frequency as well as in the high frequency range. The response to mid frequencies is not substantially affected, so by applying the coefficient of correlation tool it can easily give the difference between all the three phases. As seen from the coefficient of correlation table we can easily conclude that in the range of low frequency i.e.100-1KHz there is wide variation in the coefficient of correlation of the ‘Y’ phase, so in the low frequency reason it may be due to the residual magnetism, because Transformer was shut down before 2 hours or it may also responsible for the core deformation due to some axial stress, because Transformer is working from the last 13 years. But also the variation in the high frequency is more as compared to the low frequency. In the high frequency i.e. in the last two band (100 kHz – 1 MHz). This corresponds to wide variation in the R phase. And this variation is responsible for the

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Fig 4. Test on HV Winding When LV is Open Circuited

Fig 5.Test on HV Winding When LV is short circuited

deformation in the winding also. By seeing the coefficient of correlation table the value of statistical indicator between R-Y, Y-B and B-R, the deviation is more for the R phase. And this deviation is more in high frequency region. It gives the deformation within the winding as per the standard interpretations given by the Doble So as deviation in the low frequency gives deformation in the core and for the high frequency gives indication of deformation in This may be due to axial as well as radially stresses. But the further verification depends upon other useful Diagnostics Techniques like turn ratio, DC resistance and leakage reactance tests. 1.2 Measurements carried out on the LV windings:-

The SFRA measurements were repeated on the LV windings keeping the HV windings open for all the three phases. The respective signatures obtained are shown in the Fig 6 containing for all the three phases on single plot. Wave shapes shows that there is wide variation in the middle phase of the Transformer, because its shape is somewhat different from the other two phases i.e. R phase and the B phase. But this variation is also in the same fashion i.e. in the low as well as in the high frequency region. As seen from the coefficient of the correlation table, there is wide variation in the limit of coefficient of correlation in the low and high frequency region. As these signature also posing the same kind of behavior. It also enlightens the same kind of problem. So the following conclusion, we can draw by inspecting the Transformer SFRA patterns. It confirms in the deformation between the winding i.e. specially in the middle phase i. e. ‘Y’ phase. And also in the deformation in the core of the Transformer, but Transformer is working properly in his life span.

Fig. 6. Test on LV Winding When HV open

Circuited

2. 16 MVA, 1-Ф, 3-winding Transformer, Apex Make, 1984, (with oil and bushing) at 132 KV Substation. Pre Analysis:- Transformer was out of working from the last six month due to damage in one of the bushing of the Transformer. The tank of the Transformer was partially filled with the oil and no previous SFRA patterns were available. So for this Transformer also the mode of comparison is phase to phase comparison of the same Transformer, using the coefficient of correlation tool. Post failure analysis:- As the Transformer was Y-Y connected and tertiary was in delta, so on the basis of connections, there were total eighteen test, i.e. six test for the each phase, So the tests corresponding to the tertiary winding were also be there. From the analysis of the signatures the interpretation of the signature corresponding to all the three winding are given below. 2.1. Measurements Carried Out on the HV Windings:-

1.Test On HV When All LV open Circuited Connections

H1-H0,H2-H0

H2-H0,H3-H0

H3-H0,H1-H0

100-1khz 0.7814 0.8016 0.9753 1-10khz .9981 .9965 .9996

10-100khz .9191 .902 .9764 100-500khz .8294 .6229 .3577 500-1000khz .701 .8789 .5229

2.Test On HV When All LV Shorted Circuited

Connections

H1-H0,H2-H0

H2-H0,H3-H0

H3-H0,H1-H0 100-1khz 1.0 1.0 1.0 1-10khz .9833 .9898 .9905

10-100khz .8925 .8805 .9511 100-500khz .8233 .6446 .3695 500-1000khz .7090 .8818 .5369

3.Test on LV When HV Open Circuited

Connections

H1-H0,H2-H0

H2-H0,H3-H0 H3-H0,H1-H0

100-1khz .7395 .722 .9993 1-10khz .9931 .9921 .9993

10-100khz .9114 .91 .9981 100-500khz .9853 .9892 .9821

5001000khz .841 .9813 .7762

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The result of the measurements carried out on the HV windings are reported in Fig 7, 8, & 9, for the configurations with the LV windings open and with the LV winding short-circuited & tertiary winding open circuited, and with tertiary short circuited respectively.. It can be analyze from the Fig 7, 8, & 9, that in the low frequency region the Transformer having excellent matching between the impedance of the three phase, i.e. all the three phases are overlapping on each other, same case also coming by seeing the middle frequency band that there is perfect matching of all three phases impedances. In case of open circuit test for HV winding the value of the indicator shows good agreement between all the three phases i.e. from 100 Hz-100 KHz .But there is slight variation between R and B. phases in 100-500 KHz, as the value of indicator is 0.871, but again the value of Indicators improve in the next higher frequency band i.e. from 500-1000 KHz. In the next test on HV winding i.e. short circuit test with Tertiary open circuited i.e. also and making LV short circuited and tertiary open circuited. For this test also the value of the statistical indicators have good agreement between all the three phases up to the quarter of the frequency range, but when we see in the higher frequency range specially in 100KHz-500KHz,

Fig 7.Test on HV Winding when LV & Tertiary Open

Circuited

there is slightly mismatch in the R and B phases, as the value of indicator is here 0.8518.In the next higher frequency band the value of indicator improves, means there is good agreement between all the three phases.

Fig 8. Test on HV Winding, LV shorted & Tertiary Open

Circuited

In the short circuit test on HV it shows good agreement up to the 100 kHz, but again in the 100-500 kHz frequency there is slight variation in the indicator values and the value of indicators improves again for the last high frequency band i.e. from 500-1000 KHz.

Fig 9.Test on HV Winding When LV is open Circuited & tertiary shorted

So from all the three test of the HV winding there is almost similar variation in all the three signatures, as the variation is in same fashion and is occurring in the high frequency band it signify that it may be the deformation within the HV winding. Or it may be deposition of the wining leads. As from the pre analysis of the Transformer that the bushing of the Transformer was damaged. Due to this the Transformer is out of working. But verification to this hypothesis can be proved by opening the tank of R and B phase for the further investigations. 2.2. Measurements Carried Out on the LV Windings Connections:- The SFRA measurements were repeated on the LV windings keeping the HV windings open and tertiary open in 1st case and then shorting the tertiary winding. i. e. open circuit test for LV winding and short circuit test for the LV by shorting the tertiary winding. The respective signatures obtained are shown in the Fig 10 &11.

4:Test on HV When All Open Circuited Connections

H1-H0,H2-H0

H2-H0,H3-H0

H3-H0,H1-H0

100-1khz 0.9968 .9982 0.9967 1-10khz .9986 .9993 .9995

10-100khz .9753 .9909 .9636 100-500khz .9388 .9463 .87 500-1000khz .9707 .9767 .9572

5:Test on HV When LV Short Circuited

Connections

H1-H0,H2-H0

H2-H0,H3-H0

H3-H0,H1-H0 100-1khz 1 1 1 1-10khz .989 .9953 .9985

10-100khz .9786 .9852 .9682 100-500khz .9254 .9536 .8518 500-1000khz .9697 .9757 .9552

6:Test on HV when Tertiary Short Circuited

Connections

H1-H0,H2-H0

H2-H0,H3-H0

H3-H0,H1-H0 100-1khz 1 1 1 1-10khz .9836 .9999 .9857

10-100khz .9838 .9914 .9783 100-500khz .9303 .9468 .8432 500-1000khz .9707 .9765 .9568

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Fig 10 .Test on LV Winding When HV & Tertiary is

open Circuited

Wave shapes shows that there is good agreement between all the three phase in low frequency region, let’s take open circuit test for LV winding, the value of the statistical indicator is very near to the require limit starting from the low frequency to the high frequency (100Hz-500KHz), but beyond this 500khz the variation comes into picture i.e. the value of the statistical indicators stressed in the same reason. This also signifying that the issue is related to the deformation within the winding or it may be the deposition of the winding leads.

Fig 11. Test on LV Winding When HV open & tertiary is

short Circuited

2.3. Measurements Carried Out On the Tertiary Windings Connections:- For the tertiary winding we have only one test that is the open circuit test on the tertiary winding. In this test all the other windings i.e. the HV and the LV windings are open, the signature corresponding to this test is given below in Fig. 12 In this test also there is good agreement between the windings in the low and medium frequency region .i.e. the value of the statistical indicator is nearby to the range required .But as we see in the high frequency range the value of the statistical indicator have wide range of variation specially after the 500 KHz ,for which the value of the

indicator is 0.5668,0.8349& 0.7935 respectively .Amongst all the three tertiary winding, which are giving the same indication that the winding may have deformation or also it may have the deposition of the winding leads, by seeing all the six comparisons it shows that the impedance matching in the low frequency region is good, but in the higher frequency range after 500 kHz same type of variation is comes into picture which are giving the positive indication to make the Doble standard true deformation between the winding and it may be the i.e.

Fig 12. Test on Tertiary HV & LV is open Circuited

Deposition of the winding. But as from the pre analysis it shows that the Transformer is out of working condition due to damage in the B phase bushing, so this is making the hypothesis true, that due to bushing failure the winding lead of B phase get deposition, and so the Transformer is out of working .Further clarification can be obtained by opening the tank of the B phase by draining out the Transformer tank. From all the case studies, it shows that it is very usefulness of the statistical indicator to compare the phases of the Transformer and to interpret the results. But the verification to the hypothesis can be make true by using the results of the other analysis tools like winding DC resistance test ,turn ratio test ,and leakage inductance test.

VIII. CONCLUSIONS

SFRA is an effective tool, which considers that part of transformer for diagnostics, which cannot be detected by other methods. The test is easy to perform, data storage &

assessment of results on site can be semi automated and the results obtained are highly repeatable. In addition to the ability to detect winding & core faults, in practical applications it is also observed that SFRA method is also capable of providing an unambiguous indication, in cases where there has been no winding movement. It is nevertheless primarily a mechanical condition assessment test and must be used in conjunction with the other diagnostic tests, if a complete picture of the condition of the Transformer is to be obtained. It is recommended to carry out SFRA test in the factory, after receipt at site, after incidence by equipment of same make & with similar test set up.

7:Test On LV When All Open Circuited Connections X1-X0,X2-X0 X2-X0,X3-X0 X3-X0,X1-X0

100-1khz .9768 .9766 1 1-10khz .9993 .9963 .9987

10-100khz .988 .9777 .9663 100-500khz .9577 .9545 .9365 500-1000khz .6347 .8778 .2994

8:Test On LV when Tertiary short Circuited Connections X1-X0,X2-X0 X2-X0,X3-X0 X3-X0,X1-X0

100-1khz .9999 1 1 1-10khz .9869 .9841 .9769

10-100khz .982 .9841 .9793 100-500khz .8916 .9632 .945 500-1000khz .8156 .9382 .9272

9:Test On Tertiary All Open Circuited

Connections Y1-Y0,Y2-Y0 Y2-Y0,Y3-Y0 Y3-Y0,Y1-Y0 100-1khz .9979 9729 .9854 1-10khz .9996 .9969 .9945

10-100khz .9293 .9683 .8951 100-500khz .9054 .9604 .9685 500-1000khz .5668 .8349 .7935

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