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OMICRON electronics GmbH 2009 Workshop "Diagnostic Measurements on Power Transformers"

Noise in FRA Measurements: Sources, Effects and Suppression Methods

Juan L. Velsqueza, Michael Krgera, Sebastian knttera, Alexander Kraetgea, Samuel Galceranb, aOMICRON electronics GmbH, Oberes Ried 1 , A-6833 Klaus, Austria bCenter of Technological Innovation and Drives, Polytechnical University of Catalonia, Av. Diagonal 647, 08034 Barcelona, Spain

Abstract

The application of the FRA method in the last years as detection and diagnostic tool of mechanical deformations and electrical failures in the active part of power transformers has demonstrated its capabilities and its potential. The advances in this technique allow an excellent repeatability of results, however, similar to other on-site diagnostic methods performed, FRA measurements are also vulnerable to the harmful effects of noise. Due to noise important information for the assessment of FRA results can be lost. For this reason, a good understanding of the sources of noise, their effects as well as the suppression methods is imperative. In this contribution, these noise related-topics are theoretically explained and exemplified by real measurements performed in power transformers.

1. Introduction to the FRA method

The Frequency Response Analysis (FRA) has been proven to be a powerful tool for the detection and diagnosis of the active part of power transformers [1]. In contrast to traditional diagnostic methods, the FRA method is able to detect geometrical deformations in the windings before the occurrence of a major or catastrophic failure. When talking about FRA it is important to distinguish between Impulse Frequency Response Analysis (IFRA) and Sweep Frequency Response Analysis (SFRA). This work focuses the attention on the SFRA method. As illustrated in Fig.1, the SFRA consists in applying a frequency variable low-level sinusoidal signal "U" at one end of a winding and from this point a reference signal "U1"is measured. Simultaneously the output or response signal at the other end of the winding "U2" is measured. Subsequently, the transfer function H(f) is computed. It can be easily demonstrated that H(f) corresponds to the expression (1). This means that the H(f) is only dependant on the measurement resistance of the FRA instrument (Rm) and on the impedance of the transformer (Ztra). The most common way of representing the results is as bode diagrams as shown in Fig. 2. In the majority of the cases only the plot of the magnitude is used for interpretation purposes. Nevertheless, the plot of the phase also provides valuable information. The magnitude and the phase are computed according to the equations (2) and (3).

RMC

RLC Network

U1 Rref=50 U2 Rm=5050

U

CMC

Figure 1: Measurement setup

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

-100

-90

-80

-70

-60

-50

-40

-30

-20

H0 H1 H0 H2 H0 H3

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

100

150

Ma

gn

itu

de

(d

B)

Ph

as

e (

)

tram

m

ZR

R

fU

fUfH

+==

)(

)()(

1

2 (1)

)/(log20 1210 UUk = (2)

)/(tan 12

1UU = (3)

Figure 2: Graphical representation of FRA results

2. Introduction to noise in FRA results

Noise can be defined as unwanted disturbances that may be superimposed upon a useful (desired) signal. Noise tends to obscure the information content of the useful signal and for this reason its detection and mitigation is necessary. As in any other electrical diagnostic method, the FRA results can also be affected by noise. An understanding of the sources of noise, their effects and suppression methods is very important especially when FRA instruments of different manufacturers are compared. In this work, the relationship between the technical specifications of the FRA instrument and noise suppression capabilities is explained and exemplified by means of real FRA measurements in power transformers. Unfortunately, at present there are not international standards available in which the minimal acceptable specifications of the FRA instruments are stated. Only in China there is one standard [1] that was published in 2004. In Europe the only available document is the CIGR Report 342 that was published in April 2008 [2] and at the present the elaboration of the IEC standard 60076-18 in under elaboration. In America, the

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preparation of recommended practices is also under elaboration and at present there is only a draft that is no yet officially available [3]. Moreover, the literature on FRA has been more focused on case studies and interpretation of results and little importance has been given to the effects of noise in FRA results. Due to the lack of information on the subject noise in FRA results, this work consisting on the effects of noise in FRA results and suppression methods was formulated.

3. Sources of noise in FRA measurements

In a substation environment the noise can be found basically in two forms, i.e., as a wideband or as narrowband noise. A typical narrowband noise is the power frequency noise (50 or 60 Hz noise). This kind of noise is called narrowband because its effect can be seen in the FRA plots normally in the frequency range from 30Hz till 100 Hz. At frequencies higher than 300 Hz is it very unusual to find narrowband noise. In substations with high harmonic pollution some narrowband noise at frequencies multiple of the power frequency could also be present. Other possible source of narrowband noise could be some communication signals in the substation, or noise generated by corona discharges, but these sources take place are higher frequencies and are very hardly found in the FRA plots. With respect to wideband noise, there will be always a noise floor that will affect the FRA plots. This presence of this noise is very close related to the dynamic range of the FRA instrument. For example, in FRA instruments with a dynamic range of 80 dB (+20 dB-60dB), below -60 dB the noise will be present. In this work the attention is focused on the two most important sources of noise, that is, power frequency noise and noise floor. In this sense, the noise can be characterized in the FRA plots in two regions as shown in Fig. 3.

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

-80

-70

-60

-50

-40

-30

-20

-10

Noise floor

Dynamic range of the FRA instrument

Power frequency noise band

30Hz100Hz

Figure 3: Characterisation of noise sources in a typical FRA plot

4. Effects of noise in FRA results

Both power frequency noise and noise floor can be present in FRA plots.

4.1 Effect of power frequency noise in FRA results

Power frequency noise is only present when measurements on-site in substations with high electromagnetic fields (typically in substations with live busbars with rated voltages above 380 kV) are carried out. The presence of this kind of noise is also dependant on the size of the windings. In Fig. 4 the frequency response of the windings of a 315 MVA transformer is illustrated. As can be seen, the plot of 400 kV windings presents some noise in the band between 30 Hz and 100 Hz, while in the plots of the 220 kV and 22 kV windings the effect of the narrowband noise is minimal. This is due to the high attenuation of the frequency response of the 400 kV winding. Around 50 Hz the attenuation is approximately -60 dB what makes the signal more sensitive to noise when compared to the 220 kV and 22 kV windings. It is also interesting to appreciate that the first resonance points of the 400 kV winding (in the range from 130 Hz till 180 Hz) are near to -90 dB and there is no noise at all.

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

-80

-70

-60

-50

-40

-30

-20

-10

400 kV Winding

220 kV Winding

22 kV Winding

Figure 4: Frequency response of a 315 MVA, 400/220/22 kV Transformer

The effects of the power frequency noise take place fortunately in the frequency range in which the linear behavior of the magnetizing inductance domains the response. From the interpretation of the results point of view, this noise is not so harmful since the useful information for the diagnosis remains intact. The first resonance points which take normally place above 130 Hz are not affected by the noise which allows a reliable assessment even when some 50 Hz noise is present. Nevertheless it is worth to mention that some winding's failure modes are visible at low frequencies, such as short-circuits between turns and opened-circuits, what makes the suppression of the power frequency noise very advisable. Noise at harmonic frequencies are not so usual but can also be present in the FRA plot. In substations with rated voltages higher than 400 kV the likelihood of finding this noise is higher. As example, in Fig. 5 the FRA plots measured in the 500 kV windings of a power

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transformer are illustrated. From 30 Hz till 100 Hz the effects of the noise are significant while at harmonic frequencies (250Hz and 350Hz) the effects smaller but can still be appreciated.

f/Hz30 50 70 100 200 300 500 700

dB

-90

-80

-70

-60

-50 250 Hz5th

350 Hz7th

H0 H3 H0 H1 H0 H2

Figure 5: Frequency response of a 500 kV windings

4.2 Effects of the noise floor in FRA measurements

In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system. This noise is normally present in the FRA response measured in transformers with high magnetizing inductance, in windings connected in delta and in capacitive inter-winding FRA measurements. Some examples of the effect of the noise floor in the frequency response in delta windings measured with an FRA instrument with a dynamic range of -80 dB are illustrated in Figures 6 and 7. In both cases it can be appreciated that below -80dB the frequency response is highly affected by the noise floor. This is due to the limited dynamic range of the FRA instrument with which the measurements were carried out. This indicates that the dynamic range of the FRA instrument used (80 dB) is not enough in many cases. In Figures 6 and 7 it can be also appreciated that in contrast to the power frequency noise, the effect of the noise floor is very harmful. The presence of this noise makes the assessment of the results difficult since several resonance points which contain very important information for the diagnosis are obscured by the noise.

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-90

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Power frequency noise

Noise floor effect

CA phase

BC phaseAB phase

Figure 6: Example of the simultaneous effect of noise floor in FRA measurements in delta connected windings

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BC phaseAB phase

CA phaseNoise floor effect

Figure 7: Example of the effect of noise floor in FRA measurements in delta connected windings

5. Noise suppression methods in FRA results

There are different methods that can be used for suppressing the effects of noise in FRA results. These methods can be structured in three groups: hardware based methods, software based methods and connection technique.

5.1 Hardware based methods

Hardware based methods consists in the capacity of the FRA instrument of minimizing the presence of noise during the performance of the FRA measurements. There are two key factors that are defined in the technical specifications of the FRA instruments that are close related to its capabilities of suppressing noise, these are the output voltage of the instrument and the input filters. Next an explanation of the effect of these two important specifications is presented. Other typical specification that is also here discussed is the integration time in instruments that use the sine correlation as noise suppression tool. 5.1.1 Output voltage It is well known that transfer function measurements (as the case of FRA) are not voltage dependant since a power transformer winding is considered as a linear system. Nevertheless, the output voltage is also related to the signal to noise ratio, especially around 50/60 Hz since as lower the output voltage, weaker the signal, which could make the FRA measurements more sensitive to noise. For illustrating the effect of the output voltage in the frequency response, let's consider the equivalent circuit of typical SFRA measurements depicted in Fig. 8. In this figure, Zs represents the internal impedance of the source, Zr is the reference impedance, Zt is the impedance of the transformer and Zm is the measurement impedance.

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Zr UmUr Zm

U

Zs

Itot ImZt

Figure 8: Equivalent circuit of FRA measurements

The current Itot coming from the FRA instrument may be computed according to (1). As can be seen, the current is proportional to the output voltage U. In this manner the effect of the output voltage in the flowing current during the measurements is evident. But from the point of view of FRA measurements, what is important to know is the effect of U in the voltage drops in the impedances Zr and Zm, since the transfer functions depends on the quotient (Um/Ur). These voltages Um and Ur can be computed by (3) and (5) respectively. As can be appreciated in (6), the transfer function (Um/Ur) depends only on the impedance of the transformer (Zt) and the measurement resistance (Zm) which is typically 50 . From this it can be concluded that the output voltage has no effect in the transfer function.

UZZZZZZZ

ZZZI

mrtsrmt

tmrtot ++++

++=

)()((1)

tot

rmt

rm I

ZZZ

ZI

++= (2)

mtot

rmt

rm ZI

ZZZ

ZU

++= (3)

tot

rmt

mtr I

ZZZ

ZZI

++

+= (4)

rtot

rmt

mtr ZI

ZZZ

ZZU

++

+= (5)

ttm

m

r

m

ZZZ

Z

U

U

+=

+=

50

50(6)

In summary, the frequency response or transfer function is not dependant on the output voltage but the signal (Itot) can be increased by increasing the output voltage according to (1) what can be used for improving the noise to signal ratio and therefore for avoiding noise at power frequency. However, by increasing the output voltage of the FRA instrument it is not possible to reduce the wideband noise that as before shown is very harmful for the assessment of FRA results. Moreover even measurements carried out with FRA instruments with output voltages higher than 10 V can

also be sensitive to power frequency noise. In the majority of the cases it is impossible to completely remove the noise by just increasing the output voltage. For investigating the effect of the output voltage, FRA measurements were performed in a 400 kV winding with FRA instruments with different output voltages. As can be appreciated in Fig. 9, the measurement with an instrument with 20 Vpp presents lesser noise around 50 Hz than the measurement carried out with the instrument with 2.83 Vpp. Nevertheless, the plot corresponding to the instrument with 20 Vpp present much more noise bellow -65 dB. It was found that the reason of this was a limited dynamic range of the instrument.

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dB

-85

-80

-75

-70

-65

-60

-55

Output voltage 20 Vpp

Output voltage 2.83 Vpp

f/Hz30 50 70 100 200 300 500

-150-100

-50

Figure 9: Experimental investigation of the effect of the output voltage in the noise

Regarding standards requirements, neither the CIGR Report 342 [2] nor the Chinese standard [1] imposes minimal requirements for the output voltage of the FRA instrument. The output voltage of the most popular FRA instruments is presented in Table 1. Table 1. Output voltage of most popular FRA instruments FRA Instrument Output voltage Impedance

OMICRON FRAnalyzer

2:83 V peak to peak

50 Ohm

M5200/M5300 of Doble Engineering

20 V peak to peak 50 Ohm

FRAX-101 of Megger-PAX Diagnostics

10 V peak to peak 50 Ohm

12 V peak to peak 50 Ohm FRA 5310 of Tettex

instruments 24 V peak to peak 1 MOhm

5.1.2 Input filters Normally at the inputs of the FRA instruments, there are filters that have as objective to suppress noising signals. The noise suppression capabilities of such filters can be controlled by the bandwidth. The selection of the bandwidth is a compromise between measurement time and noise. An optimal setting of the bandwidth can also be obtained by adapting automatically the receiver bandwidth as function of the

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attenuation of the signal. In Fig. 10, this concept is illustrated.

if

BW

if: intermediate frequency

BW: bandwidthSampling points

Figure 10: Illustration of the noise filtering methods bay

The Chinese standard [1] states that the testing instrument shall have frequency selective filtering function and the bandwidth shall be less than 5% of the centre frequency of frequency selector filter. It can be demonstrated that by a proper setting of the bandwidth it is possible to suppress both the power frequency noise and the noise floor. The suppression of the noise floor leads to an increase the dynamic range of the instrument, which is one of the most important specifications of an FRA instruments. The dynamic range is normally defined as the absolute value of the negative measurement range plus 20 dB. This definition is graphically illustrated in Fig. 11. According to the CIGR report 342 [2], a measurement range of -100 dB to +20 dB should be enough to cover all the cases. The only FRA standard available at present [1] also states that the FRA instrument shall have a dynamic range of -100 dB ~ 20 dB. The dynamic range of the most popular FRA instruments is presented in Table 2.

Noise floor

20 dB

Dynamic Range of the

FRA instruments:

+20dBNoise floor

Figure 11: Illustration of the dynamic range concept

Table 2. Dynamic range of the most popular FRA instruments

FRA Instrument Dynamic range

OMICRON FRAnalyzer >120 dB M5200/M5300 of Doble Engineering >90 dB FRAX-101 of Megger-PAX Diagnostics >130 dB

FRA 5310 of Tettex instruments 90 dB

The dynamic range of one FRA instrument can be easily measured by connecting the instrument as shown in Fig. 12. The dynamic range measured in the

OMICRON's FRAnalyzer is presented in Fig. 13. As can be observed, thanks to the narrow bandwidth of the instrument, it is possible to achieve a dynamic range of at least 130 dB (-110 dB+20 dB).

Output Reference Measurement

FRA instrument

Figure 12: Connection scheme for measuring the dynamic range of FRA instruments

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006

dB

-160

-150

-140

-130

-120

-110

Figure 13: Dynamic range of the OMICRON's FRAnalyzer

5.1.3 Integration and sine correlation This method is based on the fact that the noise is sometimes positive, sometimes negative, at random. For this reason, when random positive and negative numbers are added together, they "eat" each other up. It has been established that when the sum of n periods of signal with uncorrelated noise is performed, the

signal/noise ratio is increased by a factor n [4]. Sine correlation is a noise suppression method based on the integration principle before mentioned. A detailed explanation of this method is presented in [5]. The sine correlation analyzer uses one sine channel to calculate the real part of the response R(T), and one cosine channel to calculate the imaginary part I(T) as shown in Fig. 14. The response signal coming from the device under test (DUT) is multiplied by the sin and cosine of the output signal of the instrument. Then real and imaginary parts of the signal are integrated in order to average the signal along T seconds. As the averaging or integration time is increase, the unwanted frequency components, i.e., noise, decreases.

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SignalGenerators

H(j)DUT X R(T)

X R(T)

Asintsint

cost

Multipliers Integrators Figure 14: Sine correlation circuit

5.2 Software based methods

Software based methods consists in removing the noise of an already measured FRA plot by means of signal processing techniques. Some methods have been found in the literature for the detection and suppression of noising signals. The method described in [6] is based in variance analysis which was developed for detecting and quantifying the nonlinear distortions and the disturbing noise in frequency response measurements. Other examples are the application of the Wavelet transformation that was used in [7] for suppressing noise in time domain FRA measurements and the application of Kalman filters [8] for eliminating the narrow-band noise and wideband noises from FRA plots. Other methods of easier implementation consists in typical averaging filters such as the well know moving average filter, exponential weighted moving average filter, etc. Applications of fittings algorithms such as vector fitting [9] can also be used for reconstructing noising FRA plots. Some commercially available FRA instruments, such as the OMICRON FRAnalyzer, have some signal processing tools implemented in the software that can be used for removing noise in FRA plots. As shown in Fig. 15.

Figure 15: Noise removal in FRA plots with average filtering

The noising plot shown in Fig. 9 was cleaned up by using the averaging filter of the FRAnalyzer software. A comparison of the plots before and after noise removal is presented in Fig. 16.

f/Hz30 50 70 100 200 300 500

dB

-70

-60

-50

-40

-30

-20

-10

N W Before noise removal

N W After noise removal

f/Hz30 50 70 100 200 300 500

-150-100

-50

Figure 16: Comparison of the FRA plot before and after noise removal

5.3 Connection technique

The connection technique can also help to reduce the noise. In Fig. 17 the FRA plots measured in a substation with very high noise interferences using aluminium braids and wire as connection technique are compared. It is clear that the use of aluminium braids also helps to reduce the noise.

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

-70

-60

-50

-40

-30

-20

N C with Alu braid N C with Wire

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

100150

Figure 17: Connection technique and noise suppression

6 Conclusions The effects of the power frequency noise takes place around 50/60 Hz. Because in this frequency range the linear behavior of the magnetizing inductance domains the response, this kind of noise is not as harmful as the noise floor for the assessment and interpretation of the FRA results and its suppression can be easily achieved. It was demonstrated that the output voltage of the FRA instrument can help in reducing power frequency noise,

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however depending on the electromagnetic environment of the substation even measurements carried out with FRA instruments with output voltages higher than 10 V are sensitive to noise. Neither the Chinese standard nor the CIGR Report establishes minimal requirements for the output voltage of the FRA instrument. The effects of the wideband noise or noise floor of the FRA instrument are very critical. The frequency response of transformers with big magnetizing inductances or of delta connected windings as well as capacitive inter-winding requires a dynamic range of al least -100 dB20 dB. If the FRA instrument has not enough dynamic range, the high content of noise obscures the frequency response signal which makes difficult the assessment and interpretation of the results. It was also found that the only available standard establish as requirement that the dynamic range of the FRA instruments shall be of at least -100 dB20 dB. The CIGR Report 342 also agrees with this dynamic range requirement.. There are different methods of mitigating the effects of noise in FRA results. With a narrow bandwidth of the FRA instrument the noise can be highly mitigated during the measurement. In cases in which the noise cannot be completely suppressed during the measurement, there are software based methods such as averaging filters that can be used for removing the noise from FRA results.

Literature

[1] Frequency Response Analysis on Winding Deformation of Power Transformers, The Electric Power Industry Standard of Peoples Republic of China, Std. DL/T911-2004, ICS27.100, F24, Document No. 15182-2005, June 1st, 2005.

[2] CIGRE Report 342 WG A2.26, "Mechanical condition-assessment of transformer windings using Frequency Response Analysis, April 2008.

[3] IEEE PC57.149/D6, Draft Trial Use Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers April. 2009.

[4] E. Brasseur, "How does a receiver work?", 1997, http://www.4p8.com/eric.brasseur/receiv.html

[5] N.D. Cogger, R.V.Webb, Frequency Response Analysis, Solartron Analytical, Technical Report 10, 1997.

[6] Tom Dhaene, Rik Pintelon, Johan Schoukens, Els Van Gheem, 'Variance Analysis of Frequency Response Function Measurements Using Periodic Excitations', IEEE Transactions on Instrumentation and Measurement, Vol. 54, No. 4, August 2005.

[7] R. Wimmer, K. Feser, S. Tenbohlen, M. Krger, 'Erhhung der Reproduzierbarkeit von FRA-Messungen durch Standardisierung', HV Simposium 2006.

[8] A.R. Moniri, S. Farshad, 'Modeling the Frequency Response Movements in PowerTransformers for Predicting Purposes'Iranian Journal of Electrical&Electronical Engineering, Vol. 2, No. 1, Jan. 2006', pp. 26-33.

[9] B. Gustavsen, A. Semlyen: Rational Approximation of Frequency Domain Responses by Vector Fitting. IEEE Transactions on Power Delivery, 14:1052{1061, 1999.

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