CHAPTER 5 SOUND LEVEL MEASUREMENT ON POWER TRANSFORMERS

Transcription

1 CHAPTER 5 SOUND LEVEL MEASUREMENT ON POWER TRANSFORMERS 5.1 Introduction Sound pressure level measurements have been developed to quantify pressure variations in air that a human ear can detect. The perceived loudness of a signal is dependent upon the sensitivity of the human ear to its frequency spectrum. Modern measuring instruments process sound signals through electronic networks, the sensitivity of which varies with frequency in a manner similar to the human ear. 5.2 Methods for Sound Level Measurement Sound power is the parameter, which is used for rating and comparing sound sources. Sound power can be calculated from sound pressure or sound intensity as discussed in chapter-1. Sound intensity measurements have the following advantages over sound pressure measurements [14]. i) An intensity meter responds only to the propagating part of a sound field and ignores any non-propagating part, for example, standing waves and reflections. ii) The intensity method reduces the influence of external sound sources, as long as their sound level is fairly constant. The sound pressure method takes the above factors into account through correction for background sound and reflections [14]. The sound intensity is the time-averaged product of sound pressure p and particle velocity v [14]. 5.1 Where, I=Intenisty, p=pressure, v=velocity 49

2 It is found that when converted in to sound power, the sound intensity measurement leads to values of 2 to 3 db less in comparison to the sound pressure measurement. As reported in [14], Measurements on 30 transformers in the range from 0.1 to 350 MVA, a mean difference of 3.8dB(A) was established between pressure and intensity measurement values. 5.3 Procedure for Sound Level Measurement as per Standards International standards IEC and IEEE standard C lay down the standard procedure for sound level measurements on transformers. Some of the salient points are extracted from the standards and presented here to introduce the subject. Ambient Sound Pressure Level The ambient sound pressure levels are measured immediately preceding and immediately following the sound measurements with the transformer energized. The ambient sound shall be measured at minimum four locations. Additional measurement may be made if the ambient measurements vary by more than 3 db around the transformer. At least one of the locations for measuring ambient sound pressure levels shall be on the center of each face of the measurement surface. Reference Sound-Producing Surface The reference sound-producing surface of a transformer is a vertical surface that follows the contour of a string stretched around the periphery of the transformer or integral enclosure. The contour shall include radiators, coolers, tubes, switch components, and terminal chambers, but exclude bushings and minor extensions, such as valves, oil gauges, thermometers, terminal boxes, and projections at or above cover height. First Measurement Position The first microphone location shall coincide with the main drain valve. Additional microphone locations shall be at 1m intervals in a horizontal direction, proceeding clockwise on the measuring surface. 50

3 Fig. 5.1 Microphone Location for Measuring Audible Sound on Transformer Locations of Microphones from Sound-Producing Surface The microphone shall be located on the measurement surface. As per IEEE Standard C , the microphone shall be spaced 0.3m away from the reference sound producing surface (principle sound radiating surface). When fans are in operation, the microphone shall be located 2m away from any portion of the radiator, coolers, or cooling tubes cooled by forced air. As per IEC , for measurements made with forced air cooling with auxiliaries out of service, the prescribed contour shall be spaced 0.3m away from the principle radiating surface and when these in service, the prescribed contour shall be spaced 2m away from the principle radiating surface. There shall be a minimum of six microphone positions. Generally, sound level is measured from sound-producing surface, apart at 0.3m for ONAN and 2m for remaining cooling scheme. When the distance of microphone becomes double, the drop in sound pressure level will be 6 db. Following formula is used for calculation of sound level at different distances from the sound emitting surface. 5.2 Where, d=reference measurement distance (m), d1=desired measurement distance (m) Height of Microphone Locations As per IEEE Standard C , and IEC , for transformers having an overall tank or enclosure height of less than 2.4m and 2.5m respectively, 51

4 measurements shall be made at half height. For transformers having an overall tank or enclosure height of 2.4m or more, measurements shall be made at one-third and at two-thirds height. Sound measurement of cooling system shall be made at locations only at one-half height. Correction Factors Measurement should be made in an environment having an ambient sound pressure level at least 5 db (as per IEEE standard) or 3 db (as per IEC-551 and CBIP manual) below the measured sound pressure level of the transformer. When the ambient sound pressure level is 3 db or more below the measured transformer sound level, the corrections shall be applied to the combined transformer and ambient pressure level to obtain the transformer sound pressure level. The various correction factors as defined in IEEE, IEC-551 and CBIP manual are reproduced below in Table 5.1. The measurements carried out on transformer with ambient sound level difference of less than 3 db are invalid. Table 5.1 Sound level Correction Factors as per Standards IEEE IEC 551 CBIP Difference between combined & Background Noise (db) Correction to be subtracted (db) Difference between combined & Background Noise (db) Correction to be subtracted (db) Difference between combined & Background Noise (db) Correction to be subtracted (db) Above

5 5.4 Scales for Sound Level Measurement There are three types of scales for sound level measurement: (1) A-weighting scale (2) B-weighting scale (3) C-weighting scale. The typical frequency response waveforms for the above weighting scales are represented in Fig.5.2. Fig Frequency Response for the A, B and C Weighting Networks A-weighting scale A-weighted scale is generally accepted for measurement of transformer sound because the purpose is to measure the sound level frequency within the band, which can be sensed by the human ear. Sound measurements made with the A-weighting scale are designated as db (A). B-weighting scale It is used to predict the performance of loudspeakers and stereos, but not industrial sound. 53

6 C-weighting scale The C-weighting scale includes the sound at low frequency range than the A and B scales. Table 5.2 Sound level Conversion Chart Frequency (Hz) Decible (db) A Weighting B -Weighting C -Weighting

7 For conversion of db to A, B & C weighted scale, add factor from conversion chart Table 5.2 with respect to frequency of that db level. 5.5 Sound Level Measurement on Different Scales In order to understand the difference and verify the explanation given earlier as to why sound level measurements are specified according to A-weighted scale, a measurements of sound level with A and C-weighted scales were carried out on a 10 MVA, 110/33/11kV, 3-Phase, 50 Hz Power transformer. 55

8 Table 5.3 Sound level Measurement on Different Scales Test Background No-load Condition No-load Condition Load Condition Measured Sound level (Corrected) 60.8 db(a) 63.4 db(a) 72.4 db(c) 61.8 db(a) From the above experimentation, it is inferred that, (a) sound level is high on C- weighted scale as compared to that on A-weighted scale, because C-weighted scale includes sound at lower frequency, which can not be sensed by the human ear. (b) A- weighted scale is generally accepted for measurement of transformer sound because the purpose is to measure the sound level frequency within the band, which can be sensed by the human ear. (c) The sound level under load when excited at impedance voltage during works testing is significantly lower than no-load sound level. 5.6 Case Study - Load Sound Level Measurement Load sound level measurement was carried out on four power transformers during load loss measurement at factory in order to establish the influence of load sound level on overall transformer sound. Fig Load Test of Transformer at Industry It is well known that during load loss measurement at works while the windings carry full load current, the transformer is excited only partially at impedance voltage, which is merely percent of the voltage. As such negligibly small amount of flux is produced in the core. Accordingly, the sound produced by the core would also be 56

9 negligible. However, it is noticed that transformer produces sound during load loss measurement. This is mainly due to magneto-motive force due to current flowing through the windings and causing vibrations therein. The measured load sound levels along with transformer details are shown in Table 5.4 below. The core sound level of the above transformers could not be measured at rated excitation. Therefore this value was calculated using the modified empirical formulae. Comparison of the measured load sound level and the calculated core sound level suggest that the former is significantly lower and hence could be ignored, for all practical purpose Table 5.4 Load Sound level Rating Measured Sound level Calculated Sound level 10 MVA, 110/30/11 kv, 52.49A db(a) 68 db(a) 50 MVA, 132/33 kv, A db(a) 71 db(a) 12.5MVA, 132/34.5 kv, A db(a) 70 db(a) 31.5 MVA, 132/34.5kV, A db(a) 71 db(a). 5.7 Case Study No-Load Sound Level Measurement During no-load test, primary winding is supplied with full voltage and other winding is kept open. In this condition the transformer core carries rated flux, which results in significant core vibrations thereby producing sound. Various experiments to establish behavior of the sound were carried out by changing excitation voltage and frequency. Characteristic of Sound level as a Function of Flux density In order to establish the behavior of sound level as a function of flux density, following experiments were carried out, A Change in excitation voltage keeping the supply frequency constant B Change in frequency keeping the excitation voltage constant 57

10 A) Change in excitation voltage keeping the supply frequency constant The sound level measurements were performed on the following power and distribution transformers raging from 100MVA to 2.5MVA (see Table 5.5) by changing excitation voltage keeping the frequency constant. Table 5.5 Transformer Details-1 Transformer No. MVA Rating kv Class CRGO Lamination Thickness Flux Density T /132 MOH 0.27 mm 1.6 T T /33 MOH 0.23 mm 1.59 T T /0.725 M mm 1.7 T The measured sound levels in db(a) at varying flux density are reported in Table 5.6 below and presented graphically in Fig.5.3. Table 5.6 No-load Sound level Vs. Flux Density Sr. No. Flux Density (T) Measure sound level of T-1 Measure sound level of T-2 Measure sound level of T db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) 58

11 Fig. 5.4 Sound level Vs, Flux Density It is observed from the fig 5.4 that (a) Sound level decreases with decrease in flux density and doesnt have linear relationship, (b) Sound level changes by 2-3dB(A) on changing the flux density value by 0.1T in the flux density zone closer to rated flux density. B) Change in frequency with constant excitation voltage The sound level measurement is carried out on Power transformers by changing frequency (in-turn flux density) ranging from 48 Hz to 53 Hz with constant excitation voltage. The transformer details are given in Table 5.7 below. Table 5.7 No-load Sound level Vs. Frequency Sr. No. Frequency (Hz) Measure sound level of T-4 Measure sound level of T-5 Measure sound level of T db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) db(a) 59

12 db(a) db(a) db(a) db(a) db(a) db(a) Fig. 5.5 Sound level Vs. Frequency Sound level is increases at frequencies below and above rated frequency. Higher sound level at lower frequency than rated frequency could be attributed to higher flux density. Higher sound level at high frequency could be attributed to higher magnetostriction frequency. 5.8 Case Study - Comparison between Load and No-load Sound level Measurement of Load and No-load sound level was carried out on different transformer to find out the influence of load & No-load sound level on the overall transformer sound level. Fig. 5.6 No-load and Load Test of Transformer at Industry 60

13 Table 5.8 Measured Load and No-load Sound level MVA Rating Measured Load Sound level Measured No-Load Sound level 50 MVA, 132/33 kv, db(a) db(a) 100 MVA, 230/110/11 kv db(a) db(a) 10 MVA, 110/33/11 kv 61.8 db(a) 63.4 db(a) It is observed from the Table 5.8 that, Load sound level is lower than No-load sound level and No-load sound level (Core sound) gives the major effect on overall transformer sound. 5.9 Measurement of Magnetostriction Several techniques have been successfully used for the measurement of magnetostriction in silicon Iron laminations, but probably the simplest method is a technique used by Neurath to measure it in localized areas using displacement transducers. The sensitivity and accuracy of the technique was improved by Browney and Maples who measured the fundamentals component of magnetostriction using a ceramic displacement transducer capable of measuring ac displacements in the range 10-4 to 10-8 cm. Simmons and Thompson made a systematic study of the use ceramic transducers and showed ways of avoiding errors produced by transverse or vertical vibrations. This was done by either using three separately calibrated transducers or one stereo cartridge whose output were connected in opposition to produce a signal independent of vertical vibrations. The magnetostriction of individual samples is best measured using the trnaducer technique by clamping one end of the sample and measuring the displacement of several points along its length. A plot of displacement against distance alog the sample is almost linear, slight undulations are present due to localized variations in magnetostriction. The method is accurate to within +/-0.02 x 10-6 strain. It is more convenient to measure the displacement of two pints, say 20 cm apart, by subtracting 61

14 the outputs of two fixed transducers of the same sensitivity. In this way the average magnetostriction of large samples can be quickly recorded in one reading. In order to investigate the localized variations in magnetostriction, Simmons went on to construct a double pickup built from two mono cartridges in which the pickup points were only 0.3 cm apart, small enough to measure the magnetostriction in individual grains. A commercial double pickup with an accuracy better than 10% is now available constructed from a piezoelectric ceramic 1.1 cm long cemented to pickup needled 2.3 cm apart. The measurement of the harmonics of magnetostriction is more difficult because their magnitudes are very small, but the same technique can be used, the output of the device being filtered to obtain the appropriate harmonic. The harmonics like the fundamental component of magnetostriction are extremely stress-sensitive and it is this factor which undoubtedly controls the noise output due to magnetostriction Sound Level Determination Report as per IEC Standards Typical report of sound level determination Contract: Manufacturer: Site: Place of manufacture:.. Details of Transformer Serial number: MVA: Tapping range: Voltage ratio: kv Connections: Rated frequency: Hz Rated current: ka Rated voltage: kv.. 62

15 Details of guaranteed level Sound pressure level: db(a) Measurement distance: m TRANSFORMER WITH/WITHOUT COOLERS Details of measurement method Measurement standard: Sound pressure A-weighted Test object TRANSFORMER WITHOUT COOLERS TRANSFORMER WITH COOLERS COOLERS WITHOUT TRANSFORMER Plan of test object Height of microphone above ground: Test conditions Excitation voltage: Frequency: kv Hz 63

16 Tap position: Measurement distance: Length of prescribed contour(s), lm: Height of test object, h: m m m Area of measurement surface, S : m 2 Sound pressure method is used A-weighted sound pressure levels of the background noise Plan position At start of tests At end of tests Average, LbgA A-weighted sound pressure levels Plan position Height 1 Height

17 Average, LpAO LpAO --- LbgA (must be 3 db(a)): Environmental correction (must be 7 db): Corrected average A-weighted sound pressure level, LpA db(a) db db(a) 5.11 Conclusion Sound energy is cause and sound pressure is the effect. The sound power is rather the unique descriptor of the noisiness of a sound source. Measurement of transformer sound level in A-weighted scale with Decibel (db) unit. Sound level measurement procedure is as per International standards IEC and IEEE standard C Sound level decreases with decrease in flux density and doesn t have linear relationship. Sound level behavior with respect to frequency, no significant change in sound level for supply frequency within a range of ± 3%. 65