Konèar PowerTransformers Ltd. A Joint Venture of Siemens and Konèar
REDUCING TRANSFORMER HUM BASIC FACTS ABOUT TRANSFORMER NOISE There is an increased public awareness of all possible ways of pollution, and so of the transformer noise as one of the environmental environmental hazards, too. Although transformers are not the major cause of the increased noise in human environment, the problem is that a transformer produces noise during the whole night when other noise producers are silent. The noise a transformer produces in the night is especially disturbing also because it consists of several pronounced tones, which makes the noise even more annoying. of ONAN cooling the distance between the sound meter and the tank is 0.3 m, and in the case of ONAF and OFAF cooling the distance is 2 m. The result of measurement is the averaged A weighted sound pressure level (LpA ) expressed in decibels (dB). Recently, apart from the A-weighted sound pressure level, also the A weighted sound power level (LWA ) produced by a transformer is more frequently used. The unit of sound power level is also a decibel (dB). The sound pressure level LpA is meas ured, while the sound power level L WA is derived from the expressions defined in IEC 60551. The basic relation is: There are three main sources of noise in a transformer: · ·
·
0 0 0 2 . 2 0 d e h s i l b u P , E 9 0 0 T P I O T P K
core core vibr vibrat atio ions ns caus caused ed by magmagnetostriction and magnetic forces aero aerody dyna nami mic c nois noise e of the the fans fans in in the case of ONAF and OFAF cooling vibr vibrati ation ons s of of the the win windi ding ngs s caus caused ed by magnetic forces due to currents circulating in the windings.
The first two sources (the core and fans) are more interesting from the practical aspect because they produce noise during the night, while the vibration of windings cause significant noise only under full load of a transformer, which is generally during the day. For this reason the winding vibrations are, as a rule, disregarded when considering the total noise level of a transformer. Measuring the acoustic sound level
of a transformer is defined in detail in many national standards, whereby the international publication IEC 60551 underlies the majority of the national stand -ards. The acoustic sound level of a transformer is measured with an accurate sound level meter, using the so-called A-level weight ing. Namely, when meas uring the sound pressure level around a transformer, some tones are being weighted (filtered) to obtain by measuring the effective sound level approximately adjusted to the mans ear. It is known that the mans ear is most sensitive to tones of a frequency around 1000 Hz, and much less sensitive to lower frequencies (which represent a predominant share in the total noise produced by a transformer). Transformer sound level is measured at several points around a transformer at a distance from the tank which depends on the type of cooling. Thus, in the case
LWA = LpA + 10 lg S, where S represents the area of the hypothetical measurement surface surrounding a transformer, determined according to transformer type by means of the expression defined in IEC 60551. The value of the sound power level LWA is always higher than that of the sound pressure level LpA , and grows with the increase of transformer size. Typically, Typically, LWA is higher than LpA by 18-25 dB. By means of the known value of the sound power level of a transformer, transfo rmer, one can easily estimate the transformer noise at a larger distance (e.g. at a distance of 100 to 200 m from the transformer) because the following relation applies: LpAR = LWA - 10lg (2pR2), where LpAR is the A-weighted sound pressure level at a distance R from the transformer.
The effect of a constant market pressure is that modern transformers are substantially more silent than the standard solutions provided twenty-odd years ago. Today, the acousti c sound level of t ransformers with a rated power of 100 to 200 MVA is equal to the sound level of the earlier transformers with a rated power of 10 MVA and below. Although the noi se level of a transformer increases with its rating and size, today even a relatively large transformer (e.g. of 100 MVA) can have the sound pressure level lower than 60 dB, measured at 0.3 m.
B d ,
A p
NEMA TR-1,1980
80
L
60
40
20 Measured noise (ONAN)
· · · · ·
reduction of induction in the core adequate design and construction of the core insertion of vibration insulating components in the transformer tank selection of low-noise fans adequate design of the transformer tank.
With reduction of induction, the magnetostrictive and magnetic vibrations of the core are reduced, and thus selection of low induction represents a highly efficient means of reducing the acoustic sound level. To achieve a significant reduction of the noise, sometimes very low induction has to be applied, for instance of 1.3 T instead of the usual value of about 1.7 T. This, of course, increases the dimensions, the weight and the price of a transformer but parallelly reduces transformer no-load losses to a high extent. Since the core represents the major source of noise in the medium-size transformers, the selection of an adequate core type may influence the acoustic sound level as well. The application of the step-lap joint design in the core reduce the acoustic sound level by up to 6 dB. The number of revolutions is the most influential factor of the aerodynamic noise of the fans. Thus, to obtain low noise operation of the fans, one should choose fans with a low number of revolutions. To achieve a very low sound level fans driven by motors with 10 to 12 poles are required, which, naturally, reduces the cooling air supply of the coolers.
Address: Konèar
40
20
Low values of transformer noise are achieved by combining several design and construction measures:
Thus this measure also increases the size and the price of a transformer. An additional measure used only at extremely low noise requirements is mounting of sound insulating panels on the outer tank sides, which can additionally reduce the noise level even up to 10 dB. Sometimes, especially for large transformers additional external measures for reducing the noise level, such as insulating barriers and enclosures, are used. To ensure correct acoustic dimensioning of such elements, it is necessary to know the spectrum of the transformer noise. For this purpose, the octave-band sound spectrum is measured on the
B d
60
100 200 Rated power, MVA
transformer, in a frequency range from 63 to 8000 Hz. It is common to measure the noise spectrum by linear frequency response, i.e. without the A-filter. The noise of the core has a few pronounced tones in the frequency range from 100 to 300 Hz, whereas the noise spectrum of fans is broadband in character within the frequency range of 100 to 1000 Hz. Although modern transformers have a substantially lower noise level from the earlier ones as a standard, any requirement for extremely low noise level increases the transformer dimensions, weight and price. In extreme cases, the price may be up to 20% higher.
TYPICAL NOISE SPECTRUM
80
,
A p
L
70 60
ONAF
50 40
ONAN 30
63
PowerTransformers Ltd.,
Phone: (++ 385 1) 3794 222;
125
250
500
1000 2000 4000 Frequency, Hz
Hrvatska -10090 Zagreb, J. Mokroviæa 6
fax: 3794 045; E-mail:
[email protected]
Sales & Marketing: tel.: 3794 075, fax: 3794 153;
Purchasing: tel.: 3794 108, fax: 3794 152
