effect of load-generated transformer noise in a substation

EUROPEAN TRANSACTIONS ON ELECTRICAL POWEREuro. Trans. Electr. Power 2011; 21:596–607Published online 5 August 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.464

Effect of load-generated transformer noise in a substation

*CyE-

Co

Sanjay Patil1*,y, George G. Karady2 and Wesley Knuth3

1PJM Interconnection Operations Planning, Norristown, PA, U.S.A.2Department of Electrical Engineering, Arizona State University, Tempe, AZ, U.S.A.

3Salt River Project Apparatus Engineering, Tempe, AZ, U.S.A.

SUMMARY

The objective of the project was to assess the severity of load-generated transformer noise in neighborhoodsof the substation by performing the acoustic noise level dBA measurements around power transformer atdifferent loads and distances. The results showed that 125 Hz frequency noise becomes dominant at�80% ofthe full-load with fans out of operation and varies linearly with the load. The regression equations betweenMVA loading and the average sound pressure level at 2 and 70 m away from the transformer wereestablished. The increase in the average sound pressure level from no-load to full-load was found to be�3.60 dBA. When the fans were on, the increase in the average sound pressure level due to the loadwas negligible, as the fans’ noise was the dominant noise source. It was determined if the fans operate innight, it may cause some complaints from people living near the substation. Copyright # 2010 John Wiley &Sons, Ltd.

key words: transformer noise level; load-generated noise level; 120 Hz noise level; effect of noise level;sound pressure level; sound power level

1. INTRODUCTION

The choice of selecting the transformer depends on various factors, but these days

transformers noise level is becoming an important factor in its selection. The noise level

of the transformers located in the residential areas has become a point of concern for the

utilities [1]. Transformers produce a significant humming noise and the main causes of this

noise level are magnetostriction of the core, electromagnet forces in the winding, and cooling

equipments.

It was generally agreed that the no-load noise produced by the transformer core is the main source

of noise. Because of its severe requirement efforts had been made to reduce the no-load noise of

the transformer and a significant reduction in its level had been achieved [2]. However, the load-

generated noise in the transformer had not given importance in the past decades. It has been

observed that the noise level produced by the transformer increases as the load on the transformer

increases [3]. In the past, transformer manufactures provided only no-load noise emitted by the

transformer, whereas now many manufacturers also provide load-generated noise level of the

transformer.

The objective of the project was to measure the acoustic noise level dBA around several power

transformers at different loads and distances to achieve the following [4]:

� D

orr

m

py

evelop curves depicting the noise versus load in the near and far field.

� A
ssess the severity of load-generated transformer noise in the Salt River Project (SRP)
substations.

espondence to: Sanjay Patil, PJM Interconnection Operations Planning, Norristown, PA, U.S.A.

ail: [email protected]

right # 2010 John Wiley & Sons, Ltd.

EFFECT OF LOAD-GENERATED TRANSFORMER NOISE 597

2. TEST SETUP

Figure 1 shows the test site where sound pressure level measurements were taken. It can clearly be seen

that the test substation is located close to the residential area.

The nominal high/low voltage rating of the transformer is 230/70.8 kV, respectively. The MVA

rating of the transformer with respect to OA/FA/FA is 150/200/250 MVA at 458C and 168/224/

280 MVA at 558C, respectively [4].

The length of the prescribed contour of the transformer was measured to be 26 m. The prescribed

contour was selected at 2, 5, 8, and 16 m away from the principle radiating surface of the

transformer as shown in Figure 2. Some of the measurements were also taken 70 m away near the

residential area.

The sound pressure level measurements were taken on different dates and time with

the fans in or out of operation to get maximum exposure of the load, temperature, voltage,

and other factors. The drain valve (DV) was selected as the first position, and at each

successive clockwise position of 1 m (indicated as small boxes in Figure 2), the sound

pressure level measurements were taken facing the sound level meter microphone in front of

the transformer at a height of 2 m as suggested by Refs. [3,5]. The sound pressure level

predictions performed at 5, 8, and 16 m are not discussed in the paper due to document size

limitation.

Figure 1. Top view of the test site substation.

Figure 2. Field measurement locations in the substation site.

Copyright # 2010 John Wiley & Sons, Ltd. Euro. Trans. Electr. Power 2011; 21:596–607

DOI: 10.1002/etep

598 S. PATIL, G. G. KARADY AND W. KNUTH

3. TEST RESULTS

Figure 3 shows the calculated average sound pressure levels from 26 measurements taken around the

transformer at different distances and MVA loadings. Four noise level measurements taken with fans

on (FO) are also shown in figure and are indicated as FO in the x-axis. The x-axis shows the MVA

loading of the transformer and the y-axis shows the calculated average sound pressure levels of the

transformer at different distances.

It can be noticed from Figure 3 that the sound levels decreases as the distance from the

transformer increases. It was expected that when the distance from the transformer doubles, the

sound pressure level would be reduced by 3 dBA in the near field and thereafter at a constant

interval of 6 dBA [6–8]. However, sound pressure level dBA measurements have shown that the

distance attenuation is more or less than expected attenuation when the distance is doubled. This

reveals that the sound pressure level is also dependent on factors other than MVA loading like

temperature, pressure, humidity, reflection, and diffraction due to the objects nearby the test field

etc. The effect of reflection and diffraction was not considered significant as the objects were far

away from the transformer and would have minimal effect on the measurements taken near the

transformer.

The calculated average sound pressure level without fans showed that as the MVA loading increases

the noise level around the transformer remained approximately the same at that particular distance. For

example, consider two measurements, one recorded in the summer with �199 MVA loading and the

other in the winter with �67 MVA loading on the transformer. The sound levels of the transformer are

recorded to be 63.57 and 63.72 dBAwith respect to the MVA loading of the transformer. Even the MVA

loading of the transformer was tripled or reached 70% of its full-load; the average sound pressure level

of the transformer did not change or changed very little. The reason was that the ambient was different

every time the measurement was carried out in the field and this ambient noise was also picked by the

sound level meter along with the sound pressure level of the transformer. The difference between

minimum and maximum average sound pressure level was found to be within 3 dBA at a particular

distance. Recorded sound pressure levels at individual frequencies were used to weaken the ambient

noise level from the overall sound pressure levels of the transformer using the method discussed in

Section 5.

Figure 3. Calculated average sound pressure level of the transformer at different distances and MVAloadings.


DOI: 10.1002/etep


It can also be noticed that the noise level recorded with fans is�7–10 dBA higher than the noise level

recorded without fans. Thus the fan noise is the dominant noise source in the transformer.

Another sound level meters are available in the market which records sound level using sound

intensity level technique and have better accuracy as it automatically eliminate the background noise

levels. In this project, sound pressure level method based sound level meter was used to carry out

measurements in the field. It also picked up background noise level in addition to the transformer noise

in the field.

4. MODEL FOR SOUND PRESSURE LEVEL MEASUREMENTS TAKEN AROUND THE

TRANSFORMER

To determine the dependency of noise level 14 factors (high voltage, low voltage, secondary current,

MVA loading, hot spot temperature, oil temperature, ambient temperature, pressure, relative humidity,

solar, wind direction, wind speed, FO/off, and distance) were initially considered. After applying the

linear regression analysis using Minitab, it was determined that only fans and distance are the

significant factors to determine the noise level of the transformer.

The linear regression equation of the model obtained is as follows:

Y ¼ 64:06 þ 8:52FS � 0:66X dBA (1)

The R2 of the model was found to be 0.9347, respectively. Equation (1) gives the relationship

between response (average sound pressure level) and the factors (FO/Off and distance). The

residual of the sound pressure level is close to 0, 54% of the time. Within �1.8 dBA change in the

residual of the sound pressure level, �91% of the data has been recovered. The model only

consists of 2, 5, 8, and 16 m distances. The linear regression equation obtained is limited to these

distances and does not take into account the MVA loading of the transformer. Therefore a

different approach is required to predict the average sound pressure level under different MVA

loadings and distances.

5. EVALUATION OF SOUND PRESSURE LEVEL MEASUREMENTS TAKEN AROUND

THE TRANSFORMER

Recorded sound pressure levels at different frequencies were used to evaluate the relation

between MVA loading and the average sound pressure level of the transformer and to assess the

severity of the problem. Using these frequency components, the ambient noise level in the

recorded sound pressure level was diminished to predict the overall sound pressure level of

the transformer.

The average sound pressure levels recorded at 31.5, 63, 125, 250, 500, 1000, and 2000 Hz without

and with fans at distance of 2 m are shown in Figures 4 and 5, respectively. The x-axis gives the

MVA loading and the y-axis gives the calculated average sound pressure levels at individual

frequencies.

By comparing Figures 4 and 5, it can be noticed that even when the fans were on, the lower

frequencies did not show any increase in the sound levels, that is, they did not contribute to it. In other

words, only high frequencies contribute to the fans noise of the transformer.

It can also be noticed from Figure 4 that only 125 Hz sound pressure level has shown linear

increase as the MVA load increases and other frequencies are dependent on the other factors. The

increase in the 125 Hz level is from 50.70 to 59.89 dBA, which is a significant increase. The 125 Hz

is almost the dominant noise source of the transformer compared to the other frequencies. It can be

predicted that with a little increase in the MVA loading, the 125 Hz sound pressure level would

increase and so will the overall sound pressure level of the transformer. This reveals that the load

frequency component is adding to the no-load noise and would increase the overall noise level

emitted by the transformer.


DOI: 10.1002/etep

Figure 4. Average sound pressure levels at individual frequencies without fans at 2 m.

Figure 5. Average sound pressure level at individual frequencies with fans at 2 m.


The 125 Hz sound pressure level theoretically should increase linearly as the load increases. The

prediction of the 125 Hz component at a distance of 2 m with load was done in MS Excel by both linear

and second order polynomial regression analysis and is shown in Figure 6. Both linear and polynomial

regression analyses showed a possible increase in the load component frequency and were initially

considered to test the validity.


DOI: 10.1002/etep

Figure 6. Regression analysis of average sound pressure level-125 Hz at 2 m.


5.1. Determination of average no-load and load-generated sound pressure level of the

transformer without fans at 2m

To predict the average no-load and load-generated sound pressure level without fans at 2 m, the

125 Hz frequency was varied and the other frequencies were chosen to be the minimum, obtained

from the measurements taken around the transformer without fans. At first the sound pressure levels

for all the frequencies, except the 125 Hz frequency were converted to sound power levels using

Ref. [9].

The average no-load sound pressure level for 125 Hz frequency was interpolated to be 48.20

and 43.00 dBA for linear regression and polynomial regression, respectively, from Figure 6.

The average full-load sound pressure level for 125 Hz frequency was interpolated to be 62.67

and 69.00 dBA for linear regression and polynomial regression, respectively, from Figure 6. The

predicted 125 Hz sound pressure level at low-load for polynomial regression was not used as

its contribution is less compared to the other frequencies and will not affect the calculation

of overall sound pressure level of the transformer. Thus the 125 Hz frequency value was varied

from 48.00 to 62.67 dBA for the linear regression prediction and from 48.00 to 69.00 dBA for

the polynomial regression prediction at step rate of 1 dBA. At each step increase, the corresponding

MVA loading of the transformer was interpolated and the 125 Hz sound power levels were

obtained using Ref. [9].

The variable 125 Hz sound power levels were then added to the other frequencies sound power levels

obtained earlier and the total sound power level of the transformer for individual interpolated MVA

loadings were acquired. The total sound power levels were then converted back to sound pressure

levels at different MVA loadings using Ref. [9] and are shown in Figure 7 for only linear regression

analysis.

The predicted average no-load Lp noload and full-load Lp load sound pressure level using linear

regression was found to be 61.51 and 65.12 dBA, respectively. The predicted average no-load and full-

load sound pressure level for polynomial regressions was found to be 61.51 and 69.68 dBA,

respectively. Note the no-load sound pressure levels achieved for linear and polynomial regression

analysis are same because the no-load sound pressure level at 125 Hz was varied starting at 48.00 dBA

for polynomial regression analysis.


DOI: 10.1002/etep

Figure 7. Regression analysis of the predicted average sound pressure level without fans at a distance of 2 mfor different MVA loadings.


Using method described in Ref. [5], the average load-generated sound pressure level at \approx 78%

rated value of the transformer can be used to calculate the average full-load generated sound pressure

level and was found to be:

Lp load ¼ 65:15 dBA (2)

The average full-load sound pressure level at a distance of 2 m obtained from the linear regression

prediction is 65.12 dBA, which is close to as obtained by Equation (2). It revealed that the 125 Hz

sound pressure level of the transformer is proportional to the MVA loading and exhibits a linear

relationship.

The increase in the average sound pressure level from no-load to full-load at a distance of 2 m was

3.61 dBA, which can be significant in highly urbanized area.

The third order polynomial regression line was also drawn and an equation was obtained using MS

Excel for the predicted average sound pressure level at 2 m without fans and is shown in Figure 7. The

R2 for the third order polynomial regression was found to be 1. Within 0.00 to + 0.03 dBA change in the

residual of the predicted average sound pressure level all the data was recovered.

The predicted average sound pressure level without fans Y2 load at a distance of 2 m from the

transformer can be obtained by inputting the load values in Equation (3):

Y2 load ¼ ð61:51 þ 1 � 10�7L3 þ 8 � 10�6L2 þ 29 � 10�4LÞ dBA (3)

5.2. Determination of average no-load and load-generated sound pressure level of the transformer

with fans at 2m

The variation in the sound pressure levels from no-load to full-load with fans can also be predicted by

using the recorded frequencies of the transformer. The average sound pressure levels at 31.5 and 63 Hz

were kept the same as described in Section 5.1 as fan noise does not contribute to the lower frequencies

sound levels. The sound pressure levels with FO at frequencies 250 Hz and onwards have been chosen

minimum from the measurements performed around the transformer with FO. The value of the 125 Hz

sound pressure level had been predicted for only linear regression using the Figure 6. The same method


DOI: 10.1002/etep

Figure 8. Regression analysis of the predicted average sound pressure level with fans at a distance of 2 m fordifferent MVA loadings.


was used as described in Section 5.1 to obtain predicted average sound pressure level with fans at a

distance of 2 m for different MVA loadings and is shown in Figure 8.

The R2 for the third order polynomial regression was found to be 0.9996. The residual of the

predicted average sound pressure level was found to be within the range of �0.11 to 0.00 dBA using the

equation obtained from the third order polynomial regression:

Y2 fan load ¼ ð70:60 þ 3 � 10�8L3 � 4 � 10�6L2 þ 6 � 10�4LÞ dBA (4)

The regression Equation (4) can be used to determine the predicted average sound pressure level

with fans Y2 fan load at a distance of 2 m from the transformer.

The predicted average no-load and full-load sound pressure level with fans was found to be

70.60 and 71.22 at a distance of 2 m, respectively. The increase in the sound pressure level

from no-load to full-load is small because higher frequencies have more contribution to

the overall sound pressure level of the transformer than 125 Hz or lower frequencies. The increase

in the predicted no-load and full-load average sound pressure level due to fans is 9.09

(Y2 fan load � Y2 load at L¼ 0) and 6.10 (Y2 fan load � Y2 load at L¼ 280) dBA at a distance of 2 m,

respectively. Therefore, it can be concluded that the fans noise is the dominating noise source

in the transformer.

5.3. Determination of average no-load and load-generated sound pressure level of the

transformer with and without fans at 70m

The predicted average sound pressure level in the near field can be used to predict the transformer noise

level in the far field at different MVA loadings as described in Refs. [3,5]. The transformer has been

considered as a point source. The measured height and perimeter of the transformer is 4.78 and 26 m.

The distance from the transformer to the far field (near residential area) has been measured to be around

70 m.

Using predicted average sound pressure levels at 2 m from Figures 7 and 8, the predicted average

sound pressure level in the far field has been calculated at different MVA loads without and with fans as

shown in Figures 9 and 10, respectively [3,5]. The third order polynomial regression line and equation


DOI: 10.1002/etep

Figure 9. Regression analysis of the predicted average sound pressure level without fans at a distance of70 m for different MVA loadings.

Figure 10. Regression analysis of the predicted average sound pressure level with fans at a distance of 70 mfor different MVA loadings.


obtained has also been shown in the figures. The R2 for the third order polynomial regression was found

to be 1 and 0.9996, respectively. The residual of the sound pressure level was found to be within the

range of �0.01 to + 0.01 and �0.11 to + 0.00 dBA using the third order polynomial regression

equation, respectively.


DOI: 10.1002/etep


The predicted average sound pressure level of the transformer without fans Y70 load and with fans

Y70 fan load at a distance of 70 m from the transformer can be calculated by inputting the load values in

obtained Equation (5) and Equation (6), respectively:

Y70 load ¼ ð39:09 þ 1 � 10�7L3 þ 8 � 10�6L2 þ 28 � 10�4LÞ dBA (5)

Y70 fan load ¼ ð48:17 þ 3 � 10�8L3 � 4 � 10�6L2 þ 6 � 10�4LÞ dBA (6)

The predicted average no-load LpAR noload and full-load LpAR load sound pressure level in the far field

without fans in operation was found to be 39.09 and 42.70, respectively.

The predicted average no-load LpAR fan noload and full-load LpAR fan load sound pressure level in the

far field (70 m) with fans in operation was found to be 48.18 and 48.80 dBA, respectively.

6. ASSESSMENT OF THE DISTURBING EFFECT OF THE NOISE

In general, the response to the noise level at a point may vary from person to person. However

limits are usually set by cities for the maximum allowable noise levels in residential, industrial,

and commercial areas. The maximum limit may also vary from city to city, depending on the

condition of the surrounding areas. According to the City of Tempe Code, the noise level near

residential area should not exceed 45 and 55 dBA from 2200 to 0700 hours and 0700 to

2200 hours, respectively [10].

Table I. The expected sound pressure level in the far field and complaint levels.

MVA loading Fans Predicted average sound pressure levels Day complaint levels Night complaint levels

0 Off 39.09 No No0 On 48.18 No Low280 Off 42.70 No No280 On 48.80 No Low

Figure 11. Predicted average sound pressure levels around the transformer at different distances and MVAloadings.


DOI: 10.1002/etep


The assessment of the predicted average sound pressure levels at a distance of 70 m should be

checked with the city ordinance limits. Table I shows the predicted average sound pressure levels under

different conditions and the expected complaint levels from the residents.

When the transformer is under no-load and full-load condition without fans, the predicted average

sound pressure level in the far field is below the city ordinance limits during the day and the night.

Under the no-load and full-load condition with FO, the predicted average sound pressure level is not a

problem during the day time; however during the night, the sound pressure level under the no-load and

full-load conditions is higher by 3.18 and 3.80 dBA, respectively, from the city ordinance limits.

Therefore, the sound pressure level generated by the transformer may cause some complaints from the

residents near the substations.

Using method described in Section 5, the predicted average sound pressure levels of the

transformer at different distances and MVA loadings was calculated and is shown in Figure 11.

In addition to the MVA loading values during which the data was recorded in the field

around the transformer, MVA loading values were extended to full-load of the transformer.

By comparing Figures 2 and 11 it can be clearly seen that the distortion in the sound

pressure has been weakened by using the method described in the paper. The predicted

average sound pressure level shows distinguish variation in figure with the distance and MVA

loading change.

The above method only takes the effect of distance attenuation of the sound pressure level.

The sound pressure level at a distance is not only dependent on the MVA loading, distance, and

fans, but also on the propagation path of the sound. Factors like temperature, wind direction,

wind speed, relative humidity, pressure, and refraction and diffraction from the objects nearby

the transformer were not included in the calculation method. The sound pressure level in the far

field may vary from the predicted average sound pressure level if the above factors were taken

into account.

7. CONCLUSIONS

The results achieved from the research should be fairly applicable to the other substations with similar

transformer and surroundings. The conclusions drawn from the research performed on the test

transformer are as follows:

Co

� T

py

he evaluation of frequency components proved that the 125 Hz frequency component varies

linearly with the load.

� T
he regression equations had been established to predict the average sound pressure level at a
distance of 2 m (near field) and 70 m (far or near residential field) with and without fans for

different MVA loadings on the test transformer.

� T
he results revealed that the increase in overall sound pressure level from no-load to full-load is
\approx 3.60 dBA without FO the test transformer. However when the fans are on, the increase in

overall sound pressure level from no-load to full-load is \approx 0.60 dBA as the fan noise is the

dominant noise source in the transformer.

� T
he no-load and load-generated sound pressure levels without fans are below the city ordinance
limit and there should be no complaints from the neighborhood. Also if the fans are on during the

day time, the sound pressure level is under the city ordinance limit. However if the fans turns on in

the night, the sound pressure level of the transformer will exceed the city ordinance limit and low

complaints can be expected.

� T
he results did not include the effect of atmospheric conditions. The model built for 30 m away
from the transformer revealed that the sound pressure level can be varied by 8 dBA, if the

atmospheric condition were considered [4]. Therefore the sound pressure level at 70 m away from

the transformer can also vary and may cause more complaints. The precise variation of sound

pressure level with the distance considering the atmospheric conditions near residential area has

not been investigated.

right # 2010 John Wiley & Sons, Ltd. Euro. Trans. Electr. Power 2011; 21:596–607

DOI: 10.1002/etep


8. LIST OF SYMBOLS AND ABBREVIATIONS

ASU A

Copyrigh

rizona State University

dBA a
-weighted sound pressure level
FS f
ans status (on or off)
L lo
ad (MVA)
SRP S
alt River Project
SPL s
ound pressure level
X d
istance (m)
ACKNOWLEDGEMENTS

I wish to thank Salt River Project, for funding this research work. Special thanks to Yan Ma for her wisdom andvaluable time, my family, and fellow students for their encouragement.

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Arizona State University, Tempe, AZ, December, 2006.

5. IEEE standard test code for liquid-immersed distribution, power, and regulating transformers, IEEE Standard

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7218, December, 1970; 1–11.

10. Ordinance No. 2000. 01, City of Tempe, 2000. Available Online: www.tempe.gov/codecompliance/pdf/

ord.2000.01.pdf.

t # 2010 John Wiley & Sons, Ltd. Euro. Trans. Electr. Power 2011; 21:596–607

DOI: 10.1002/etep

effect of load-generated transformer noise in a substation

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