1

SWEPT FREQUENCY RESPONSE ANALYSIS TO DETECT POWER TRANSFORMERS SHIPPING DAMAGE

Michael Bocovich Xcel Energy

763-493-1560 / Fax 612-573-4043

9 September 2011

ABSTRACT

Xcel Energy specifies that new substation power transformers have a Swept Frequency Response Analysis (SFRA) test performed in the as shipped condition. If winding movement during shipping is suspected, the SFRA test can be performed again with the transformer on site in the as shipped condition. The results of these two SFRA tests can then be compared with the expectation that minor winding movement that occurred as a result of shipping would be detected. Xcel Energy performed several SFRA tests on a transformer in a controlled environment to determine if this expectation is realistic. The tests consisted of deforming the transformer winding and performing SFRA tests during various stages of damage. This paper will present that process and results. A different view for frequency response analysis was discovered to explain test observations; this view will also be presented and explained.

INTRODUCTION

This paper and material were first presented at the 78th International Conference of Doble Clients 27 March 1 April 2011 under the title Swept Frequency Response Analysis, Realistic Expectations.[1] Parts of the paper have been modified and expanded upon.

Power transformers consist of conductors (coils and leads), an iron core, supporting structures, insulation, and a tank. Transformers can be modeled as a complex circuit network of capacitance, inductance and resistance. The conductors have resistance, inductance, mutual inductance between turns and mutual inductance between coils. Capacitance and resistance associated with the insulation exists between the turns, coils and grounded elements (core and tank). Every component within the transformer will affect the modeled circuit network of series / shunt resistance, inductance, and capacitance (RLC). As such, a unique frequency domain transfer function, H(j), can be used to represent this combination of resistive and reactive elements. The SFRA test set inputs a low level voltage signal at various frequencies at one end of a transformer winding. The output signal is then measured at the other end of the winding. A transfer function (output/input) is represented as magnitude 20log10 |H(j)| and angle tan1(H(j)) with respect to frequency.[2,3,4]

Frequency Response Analysis (FRA) testing on power transformers is relatively new within the power industry. SFRA is one type of FRA testing. Several excellent case studies exist regarding SFRA testing.[2] Many of these case studies use SFRA testing to confirm failures within a transformer that had previously been identified through other testing methods. Supporters of FRA testing claim that the primary benefit of this type of testing is the potential for detection of minor deviations that might be related to the mechanical or electrical integrity of the transformer, that are not apparent with other electrical tests.[3] Although the power transformer is tested as a two port network of inductive, capacitive and resistive elements, the modeled network is more than a few simple passive elements. It is a very complex, infinitely large network of infinitely small elements.[4] The claim that minor winding movement (i.e. movement that may affect only a small portion of those infinitely small network elements) can be detected using FRA. In an attempt to validate this claim, extensive SFRA tests were performed on a power transformer winding that was deformed with various stages of damage to simulate minor winding movement.

2

THE TEST SUBJECT

A used transformer (in reserve stock) was evaluated for re-deployment. An internal inspection uncovered severe coking associated with the no-load tap switch and internal cross connection straps. The coking contaminated the whole transformer making it un-fit for re-deployment. It was determined the transformer is not a good candidate for rewind due to the particular construction (wrapped core design) and size, so it was made available to validate expectations. The nameplate reads as follows: Kuhlman, S/N 2-57715, 34.5kV X 68.8kV (Delta) 4.36kV X 13.90kV (Wye), 2.5 // 2.8 / 3.5 MVA, OA / FA, 550C // 650C. Figure 1 shows a photo of the transformer and nameplate. The transformer was strapped at 68.8kV (Delta) 13.9kV (Wye) nominal.

FIGURE 1 Photo and Nameplate of Test Transformer

3

THE TESTS

System operations took precedence over this particular project. The SFRA testing was conducted over an extended period of time as resources became available. This resulted in some time lag between initial tests. All winding damage tests were conducted on the same day. Nine SFRA tests as recommended by the Doble SFRA Users Guide for a transformer of this type were conducted at each damage stage of the transformer winding.[2] These nine tests are listed below in Table 1. Some tests were duplicated to verify if minor shifts of the traces could be detected with no physical change to the transformer.

Source Sensing Configuration H2 H1 All Other Terminals Floating H3 H2 All Other Terminals Floating H1 H3 All Other Terminals Floating L1 X0 All Other Terminals Floating L2 X0 All Other Terminals Floating L3 X0 All Other Terminals Floating H2 H1 Short [X1 X2 X3] H3 H2 Short [X1 X2 X3] H1 H3 Short [X1 X2 X3]

TABLE 1 Two Winding 3 Phase Delta-Wye Transformer Tests

The windings of the transformer were tested both inside the tank and outside the tank. In-tank tests were conducted under the following conditions: with oil; a change to the no-load tap; and without oil. The transformer was then un-tanked, coking was cleaned off the no-load tap, and the transformer was configured for 72.4kV (Delta) 13.09kV (Gnd Wye) to assure all windings were included in the tests. An outer set of coils-the H3 winding-was moved and distorted to produce the different stages of winding damage. SFRA tests were then conducted at the different stages of winding damage.

The high voltage bushing leads were cut short to minimize measurement error due to lead movement. The bushing flanges for the neutral lead connections were not available with the core and coils removed from the tank; neutral connections were specifically defined as being on the top core yoke. The ground connection was also specifically defined as being on the top core yoke. The core and coils were not returned to the main tank between winding deformations for ease of testing and minimization of test resources.

The traces were analyzed closely after all the tests were completed. The interference cancellation feature of the software produces smoother curve traces. Disabling the interference cancellation of the software produces sharp transitions on the curve traces. This allowed better viewing of the actual data for a more detailed analysis. This also allowed better observation of the test frequency resolution.

4

INITIAL TEST RESULTS

Figure 2 shows the initial tests on the transformer. Note that:

1. Core and coil assembly in the tank, full of oil and 65.2kV Tap (blue trace). A change of the no-load tap from 65.2kV to 72.4kV (red trace) results in a curve shift to the left.

2. Core and coil assembly in the tank with oil removed (green trace) resulted in a curve shift to the right. 3. Core and coil assembly removed from the tank (orange trace) resulted in a curve shift right.

FIGURE 2 H2 H3 SFRA Tests, Low Side Open Circuit, Magnitude to Frequency

Various Stages of Transformer Conditions

Blue In Tank, Oil, Tap E (65.2kV) Red In Tank, Oil, Tap A (72.4kV) Green In Tank, No Oil, Tap A Orange Un-Tanked, Tap A

5

VALIDATING EXPECTATIONS

Slight movement could be detected in the H3-H2 test results associated with having the H3 winding damaged. Open circuit tests showed more promise for analysis and are presented here. Figure 3 shows the results of eight test curves overlain on each other. Each curve represents a test conducted at a specific stage of winding damage. The legend colors are kept consistent in this paper for ease of analysis. Losses greater than 85db are beyond the limitations of the test set and therefore are not considered. Figure 3 illustrates that shifting or change between the traces is barely visible. Casual observation of the figure yields little, as the traces look the same with no movement of the windings detected. However, a more detailed observation limited to within the frequencies of 60kHz and 200kHz is more revealing. This area of interest is highlighted by the box shown in Figure 3.

FIGURE 3 H2 H3 SFRA Tests, Low Side Open Circuit, Magnitude to Frequency

Various Stages of Damage to H3 Winding - Eight Curves Overlaid on Each Other

Current industry practices consider the presence of additional or the loss of existing resonant peaks as an indicator of change in the winding. The shifting of existing resonant peaks is also considered to be a change indicator.[3] Current industry practices do not place significant value on analyzing magnitude valleys and phase angle shifts as an effective method to determine winding movement. However, for the analysis presented here, magnitude peaks and valleys will be focused on.

Area of more detailed analysis

6

Figures 4 through 7 illustrate a chronological view of the damage done to the windings during testing along with an analysis of the SFRA test results for each damage stage.

FIGURES 4

The first three tests (two baseline tests followed by clamping loosened) did not show significant shifting of the traces. A closer analysis of the test results shows shifting of a few peaks and valleys by about one test frequency iteration between the first two (baseline) tests. The first two base line tests were conducted twenty-one days apart. A possible explanation for the shift between the two base line traces could be due to additional oil leaching out of the paper insulation. The test trace produced by loosening of the clamping tracked close to the base line test of the same day. No visible movement of the windings was observed when the clamping was loosened. The test traces are shown in Figure 4.

The transformer is un-tanked The no load tap is set at position A High side leads are cut Test lead positions for the H3-H2 test Baseline Tests

Clamping was loosened, no noticeable movement of the windings were noted

7

FIGURES 5

One set of spacers was removed from the windings (Figure 5) and the two coil sets were pushed together. Two tests were conducted with a time interval in between to see if changes over time would occur. A comparison of the latest base line (red trace) with the two tests (1st test blue trace, 2nd test green trace) is done. Figure 5 illustrates that although the frequency peaks (resonant points) align fairly well, they do not match exactly. The frequency valleys appear to be shifted to the right by approximately one test frequency iteration.

One set of spacers removed Windings pushed together

8

FIGURES 6

Another set of spacers was removed and the coil sets were forced together (Figure 6). One test was performed (brown trace) and compared with the most recent baseline test (red trace). Comparing frequencies of peaks and valleys, some shifting to the right is noted, then the traces appear to align, then shift to the left, and finally align again. The shifting again appears to be by one test frequency iteration with the exception of the peak at about 163kHz; this appears to be shifted to left by two test frequency iterations.

Two sets of spacers removed Windings forced together

9

FIGURES 7

For the last set of tests, another set of spacers was removed. Then an outer coil turn was forced between two coil sets (Figure 7). A comparison of the latest base line (red trace) with the two tests (1st test brown trace, 2nd test orange trace) is done. The tests appear to align well with a few exceptions where the peaks and valleys appear to be shifted by one test frequency iteration.

Three sets of spacers removed One coil of winding pushed into space

10

The short circuit tests as listed in Table 1 were conducted for each stage of winding damage. The results of the short circuit tests on the same winding are shown in Figure 8. An analysis of the results indicates that short circuit tests appear to produce trace shifts similar to the trace shifts produced by open circuit tests.

FIGURE 8

CONCLUSION

For this project, a close analysis of SFRA test results was performed knowing the damage that had been done to the coil sets. Although significant damage was done to the coil sets, the shifting or movement of the test traces was very subtle. Recent articles have demonstrated that minor shifting of the FRA traces can occur due to magnetic viscosity, temperature, and water content in insulation.[5,6] Therefore, it is questionable whether or not FRA testing is really capable of detecting minor winding movements or shifting within a transformer. A successful comparison of two FRA tests in the as shipped condition before and after transport may not necessarily prove that minor winding movement did not occur. The time to ship a transformer from the factory to the user introduces other factors that can influence the FRA test and result in a shifting of the FRA curves even though the windings may not have moved.

11

FRA TESTING OBSERVATIONS DEMYSTIFIED

Frequency response analysis (FRA) on power transformers is performed viewing the power transformer windings as an infinite collection of series/shunt reactive and resistive elements. Using this model, the theory postulates that these elements interact with each other collectively to cause resonance at specific frequencies. The theory logically assumes that the resonant conditions change as the transformer physical condition changes. There are many case studies demonstrating the effectiveness of FRA in diagnosing problems in power transformers that seem to support this theory.[2] The current theory does not seem to thoroughly explain the observations documented in the case studies. However, during the course of this research, a new theory was discovered that seems to more adequately explain these observations. This part of the paper will explain this new theory in an effort to share this knowledge within the industry and offer opportunities for further development. Test results used to validate if shipping damage can be detected using FRA are used to support this new theory.

The basis of this new theory is to view the transformer windings not as a complex network of series/shunt reactive and resistive elements, but instead as a transmission system and cavity resonator. The transmission system uses the conductor as a wave guide. The characteristic impedance is defined by the conductor geometry and the dielectric surrounding the conductor.

Angle (degrees) and Magnitude (dB) vs. Frequency (log scale) Test: Xcel-Energy_Maple-Grove_Kuhlman-Electric_257715_2010-01-28_08-30-33, H3-H2

FIGURE 9

The SFRA data shows what appears to be multiple passes through the zero phase angle. The phase angle is not actually going through zero multiple times, but instead is continuing to decrease. The test set can only measure the

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

Frequency

-120

-100

-80

-60

-40

-20

0

Angle Magnitude

Magnitude (dB) Other Influences

40.1kHz -360O

71kHz -720O

699Hz 34.3kHz

13.6kHz -180O

110kHz -1080O

Angle (Degrees)

12

output in relation to the input in 3600 increments. If the phase angle is properly represented by extending the angles in multiples of 3600 it would look like the green trace shown in Figure 9. This is an electromagnetic transmission phenomenon demonstrating multiple wave lengths traveling through the transformer winding.[8,9]

When the phase angle is viewed with a linear frequency scale (Figure 10), a periodicity of the 3600 transitions becomes apparent.

Angle vs. Frequency (Log Scale and Linear scale) Test: Xcel-Energy_Maple-Grove_Kuhlman-Electric_257715_2010-01-28_08-30-33

FIGURE 10

The most accurate periodic frequency measurements are taken from the valleys on the magnitude trace from 80kHz to 180kHz. These magnitude valleys actually correspond to null points. These are clearly defined points on the traces (Figure 11) that appear to be less influenced by the complex network interactions within the transformer.

Magnitude Traces (linear scale) of three tests Period frequencies shown FIGURE 11

90.9kHz, 121.1kHz, 147.8kHz Avg 28.5kHz

89.9kHz, 115.9kHz, 143.0kHz Avg 26.5kHz

109.6kHz, 141.4kHz, 172.6kHz Avg 31.5kHz

Blue In Tank, Oil, Tap E (65.2kV) Red In Tank, Oil, Tap A (72.4kV) Black Untanked, drained, Tap A

13

Each winding in the transformer can be viewed as a transmission line influenced by reactive and resistive components, nested coils, and the iron core. Figure 12 illustrates a simplistic view of the transformer as a transmission line neglecting influences of additional coils, core, and reflections. Consider a signal that is injected into the H3 terminal of a transformer and received at the H2 terminal. The signal in the transformer takes two different paths on its way to terminal H2, a direct path and a path that passes through terminal H1. The portion of the signal that passes through terminal H1 travels twice as far as the portion that travels the direct path. The signal observed at H2 is the combination of the portion of the signal traveling two wavelengths through H1 and the portion of the signal traveling one wavelength directly. A peak will occur on the test transformer at 31.5kHz, which is one wavelength per coil. Successive peaks will occur at multiples of the 31.5kHz fundamental frequency. For example, at two wavelengths per coil another peak will occur at 63.0kHz, at three wavelengths per coil another peak will occur at 94.5kHz, etc. Frequencies other than whole multiples of the fundamental wavelength traveling through the windings combine to result in lower received signals at H2. At some frequencies, the signals cancel, resulting in null points.

Signal Travel Through the Delta Winding of the Test Transformer FIGURE 12

The actual transformer winding circuit is more complicated than the simplistic transmission model due to reflections within the transformer and at the terminals. Figure 13 is a more complex representation of the transmission model that takes into account the reflections within the transformer and at the terminals. The SFRA test set and cable impedance is 50 ohms which is not matched to the transformer winding impedance. The transformer tested had a calculated high side winding impedance of about 2 Ohms which also explains more than 50dB attenuation in the test traces. The impedance difference will cause the transformer to act as a cavity resonator, defined as A resonator formed by a volume of propagating medium bound by reflecting surfaces.[7] The propagating medium is the dielectric surrounding the winding conductor. The reflecting surfaces are due to different impedance transitions. Resonance does not occur due to the interaction of reactive components, but rather due to the cavity resonator.[8,9,10] The no load taps, when set other than at full tap, will act as stubs. The stubs introduce additional reflections and transmission differences which change the characteristics of the FRA trace.

More complex view of transformer as a transmission system and cavity resonator FIGURE 13

The traveling waves through this circular cavity resonator set up standing waves. Signals of different wavelengths traveling through the cavity resonator will result in different peaks, nulls (valleys), and nodes. A simulation of the

50 Load

50 Source and Cable Stubs, Un-Used

No-Load Taps

H1 H2

H3 Input Signal Sampled Here

H1 H2

H3

H1 H2

H3

14

received signal strength at different frequencies can be seen in figure 14. It can be seen that peaks will occur when the wavelength of the signal is equal to the length of the conductor in each winding. Large nulls (valleys) will appear with the same frequency periodicity.

Mathcad Signal Magnitude vs. Frequency of Simplistic View FIGURE 14

Key equations and components for the analysis of the three test curves shown Figure 11: Conductor Length of winding: l [m] = 5.5km (at full tap) Periodic Frequency: fp (obtained from traces) Relative Permittivity: r Phase Velocity: vp = l x fp = c = 3 x 108 m/sec

The transformer under test had 22 stacks of windings. Each stack consisted of 135 turns for a total of 2,970 turns on the 72.4kV tap. Each disk outer dimension was 27, the inner dimension was 19, with an average of 23. The total length of conductor in each HV winding was calculated to be approximately 5.5km.

Phase velocity can be calculated knowing the length of travel (5.5km, length of conductor in winding) and the frequency of the signal for one wave length. Thus

vp = 5.5km x 31.5kHz = 1.7325 x 108 m/sec.

Phase velocity, vp, is dependent on the relative permittivity of the medium of the traveling wave.[8,9,10] A primary theory is that the conductor acts as a wave guide for traveling electromagnetic waves. The energy of the signal travels through the insulating medium surrounding the conductor. The relative permittivity of the medium for a

0 1 2 3

0.5

0

0.5

1

Recieved SignalMagnitude of Received Signal

Simple Simulation of Received Signal Strength

Wavelength / Winding Length

Mag

nitu

de o

f Rec

eiv

ed Si

gnal

(N

orm

aliz

ed)

c

r

15

traveling wave with these parameters is calculated to be 3.00, which is a reasonable approximation for oil stained paper.

Test Case Description fp vp r 1 Transformer in tank, Full of Oil, Tap 65.2kV (assumed 90%

winding length) 28.5kHz

2 Transformer in tank, Full of Oil, Full Tap 72.4kV

26.5kHz 1.46x108 4.24

3 Transformer un-tanked well drained of oil, Full Tap 72.4k

31.5kHz 1.73x108 3.00

Observed Periodic Frequencies of Test Curves (Figure 11) Calculated Phase Velocity and Relative Permittivity TABLE 2

Table 2 shows the observed periodic frequencies of test cases (Figure 11) with calculated phase velocity and calculated relative permittivity. The calculated relative permittivity is 4.24 for Test Case 2, which is a reasonable approximation for oil impregnated paper. Phase velocity and relative permittivity were not calculated for Test Case 1. These values should be the same as Test Case 2. Reducing the conductor length to 90% through the no-load tap setting should result in an increase of the periodic frequency by 111%. The observed periodic frequency is actually about 107% greater than the periodic frequency at full tap. A possible explanation for the variance between the expected value and the observed value is the stubs of the unused tap sections.

The low voltage windings are much shorter than the high voltage windings resulting in a much higher periodic frequency. The low voltage winding tests (from a low side terminal to neutral) on a three phase grounded wye bank would seemingly be a more simple circuit to analyze (one signal path). However, low voltage windings are generally comprised of multiple conductors in parallel (for current capacity) tied together at termination points. Signals will also travel along the other two low voltage phase windings. This allows multiple paths of signal travel. Interactions with other elements of the transformer (HV windings and core) also add complexity to the FRA signal traces.

The view of the transformer as a transmission system and cavity resonator explains many of the observations associated with frequency response analysis. There are several applicable examples, a few of which are listed below:

Changing a high side no load tap from a lower to a higher voltage increases the total conductor length. This decreases the periodic frequency and the effect is that the test trace shifts to the left.

Removing oil from the transformer tank replaces the oil (r,oil = 2.2) with air (r,air = 1.0). The resulting decrease in relative permittivity increases the phase velocity. This increases the periodic frequency and the test trace shifts to the right.

Moisture in the insulation displaces oil (r,oil = 2.2) with water (r,water = 81). The relative permittivity increases, decreasing the phase velocity. This decreases the periodic frequency and the test trace shifts to the left.

Shorted or open turns change the signal path producing changes in the SFRA trace. Since low voltage windings are shorter, the periodic frequency would be much higher. An analysis similar

to that of Figure 9 for low the voltage winding would result in fewer transitions.

One piece of the frequency response puzzle is presented here. Frequency response analysis requires further study to better understand the test results. Items requiring better understanding include the wiggles in the magnitude trace as highlighted in figure 9. The wiggles correlate with deviations of the phase angle trace. They demonstrate a polarity associated with the direction of the phase angle shift. This is better observed by viewing the low side winding FRA trace as shown in figure 15. Cursor lines are shown to highlight the correlation between phase angle and magnitude. These small changes in magnitude and phase angle are most likely associated with waves traveling through the cavity resonator defined by the windings and termination points. It appears viewing both phase angle and magnitude together will provide better analysis of the FRA traces.

16

SFRA X3-X0 2010-01-28 08:47:14

-60

-50

-40

-30

-20

-10

0

10

20

100 1000 10000 100000 1000000

Frequency (Hz)

Mag

nitu

de (d

B)

-600

-500

-400

-300

-200

-100

0

100

200

An

gle

(deg

rees

)

Magnitude Angle

Angle (degrees) and Magnitude (dB) vs. Frequency (log scale) Test: Xcel-Energy_Maple-Grove_Kuhlman-Electric_257715_2010-01-28_08-47-14, X3-X0

FIGURE 15

755Hz 10.2kHz

34.3kHz

882Hz 6.85kHz

45.3kHz 2.33kHz

140kHz 108kHz

95.0kHz

17

REFERENCES

[1] Bocovich, Michael Swept Frequency Response Analysis, Realistic Expectations 2011 Doble Engineering Company 78the Annual International Doble Client Conference.

[2] Instruction Manual, "Doble SFRA User Guide" Copyright, 2006, Doble Engineering Company.

[3] IEEE PC57.149TM/D8, November 2009 Draft Trial-Use Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers.

[4] Instruction Manual, "M5100 SFRA Instrument Users Guide" Copyright, 2001, Doble Engineering Company.

[5] Lachman, Mark F, Fomichev, Vadim, Rashkovski, Vadim, and Shaikh, AbdulMajid Frequency Response Analysis of Transformers and Influence of Magnetic Viscosity 2010 Doble Engineering Company 77th Annual International Doble Client Conference.

[6] Reykherdt, Andrey A., Davydov, Valery Case Studies of Factors Influencing Frequency Response analysis Measurements and Power Transformer Diagnostics IEEE Electrical Insulation Magazine, January/February 2011, Volume 27 Number 1.

[7] IEEE Std 100-1992 The New IEEE Standard Dictionary of Electrical and Electronics Terms Fifth Edition.

[8] Balanis, Constantine A. Advanced Engineering Electromagnetics John Wiley & Sons, Copyright 1989.

[9] Johnk, Carl T.A. Engineering Electromagnetic Fields and Waves John Wiley & Sons, Copyright 1975.

[10] Yariv, Amnon, Yeh, Pochi Photonics Optical Electronics in Modern Communications Sixth Edition, Oxford University Press, Copyright 2007.

BIOGRAPHY

Mike Bocovich is currently employed at Xcel Energy. Mike is a Principal Engineer in the Substation Maintenance Engineering Department. He has also held positions at other utilities and served as an officer in the US Navy. Mike earned his Bachelor of Science, Electrical Engineering from the University of Colorado in 1982. He also earned his Masters of Engineering, Electrical Power Engineering from Rensselaer Polytechnic Institute in 1987. He is currently a PhD student at the University of Minnesota.