With the growing consciousness on the unkind effects of noise pollution, many users are specifying lower noise levels for transformers. While the trend of ever increasing transformer ratings implies a corresponding rise in noise level, noise reducing measures have to be adopted to make the transformer less noisy. The noise pressure generated by vibration of core and windings is transmitted to tank surfaces though the oil medium. Since the oil is relatively incompressible, the noise is transmitted without appreciable damping. The tank responds to these noise waves depending on its natural frequencies and complete shapes of vibrations. The principal sources of transformer noise, the magnetic core influence to control the noise of the equipment are discussed.
The primary source of acoustic noise generation in a transformer is the periodic mechanical deformation of the transformer core and the winding coils. This is due to the influence of fluctuating electromagnetic flux associated with these parts. The flow line of the acoustic energy generated in the transformer is represented in the figure.1. The propagation of noise from the core and windings through the oil medium is spread to external environment .The spectrum of noise can be captured by the advanced noise level measuring instruments. The physical phenomena associated with this noise generation can be briefed as follows:
• The material of a transformer core exhibits magnetostrictive properties. The vibration of the core is due to its magnetostrictive strain varying at twice the frequency of the alternating magnetic flux. The frequencies of the magnetic flux are equal to the power system supply frequency and its harmonics.
• When there are residual gaps between laminations of the core, the periodic magneto-motive force may cause the core laminations to strike against each other and produce noise. Also, the periodic mutual forces between the current-carrying coil windings can induce vibrations if there are any loose turns of the coil.
Figure 1: Acoustic Energy Flow in an Oil Cooled Transformer
This noise is emitted by a loaded transformer in addition to its no-load noise. It is caused by electromagnetic forces between the windings resulting from the leakage fields and is proportional to the square of the load current. These forces cause the winding vibrations and acoustic radiations having frequency of 100 or 120 Hz that is twice the power frequency. The contribution of load controlled noise to the overall noise level of the transformer becomes significant even if the operating flux density in the core is lower. Generally, at lower value of flux density, noise from the core is considerably reduced.
The other sources of noise are the vibrations of tank walls and the magnetic shunts placed on them. If the magnetic shunts are rigidly anchored to the tank wall, the noise due to their vibrations is usually low. The vibration amplitudes produced by a given axial compressive force corresponding to a load current flowing in a winding, depend on the winding properties, like mass, modulus of elasticity and damping. Pressboard and other insulating materials play an important role in deciding the winding response. The winding noise can be kept as low as possible by using a pressboard material with a high damping coefficient and applying a proper value of pre-stress to the winding.
The winding natural frequencies should be fairly away from the frequencies of the exciting compressive forces that is twice the power frequency and its multiples, since a resonance will amplify the vibrations and noise. The loaded transformer represents a typical magneto-mechanical system immersed in a fluid (oil). For developing the numerical method for accurate calculation of the load-controlled noise, the electromagnetic field, mechanical displacement field, acoustic pressure field and their couplings have to be considered as one system. Due to the complexity of this multi-field problem, a combination of the finite element and boundary element methods are used in for the prediction of the load-controlled noise of transformers.
Figure 2: Frequency Spectrum of Noise
Noise due to cooling equipment
Cooling is one of the most essential process for the effective operation of the transformers in the field. For dry type, large distribution and power transformers cooling method selection is vital as it contributes significant levels of noise. The typical frequency spectrum of noise produced by power transformer is shown in the figure 2. Fan noise is a result of vortex flows in the vicinity of its blades. The noise is a function of air delivery, blade size and speed. While the noise due to core produces frequencies in the range of 100 to 600 Hz. The load noise due to cooling equipment like fans and pumps are usually below or above core frequency range in the sound spectrum. In general, high flow speed of cooling medium of fans and pumps should be avoided. Since the fan noise is a function of its speed and circumferential velocity, a low speed fan has a smaller noise level.
As the speed is lowered, air delivery also reduces necessitating an increase in number of fans. Many times, the noise level specified is so low that it may not be possible to get such a low noise fan. Therefore, ONAN cooling should be specified and used in place of mixed ONAN/ONAF cooling for small and medium rating power transformers, even if it results in increase number of radiators. A radiator noise is caused by the tank vibration transmitted through cooler pipes connecting the tank and radiator. Pipe-work and supporting structures should be designed such that there is no resonance.
Noise Level Reduction
As the noise has become one of the key issues, it should be addressed starting from the design stage by the effective techniques. A lower value of the operating flux density also results in higher material cost and size of the transformer. Hence, other cost-effective noise reduction methods are commonly used which are now described. There are different ways, by which the noise can be reduced. Methods like stiffening the bracing or supporting parts and adding cushions between parts of a transformer have long been known and used for reducing vibrations and noise. Barrier walls and total sound-proof enclosures have also been commonly used. An easy but expensive way would be to put the transformer in a closed room whose walls and floor are massive. The noise reduces as it tries to pass through a massive wall. The noise can also be reduced by building a free-standing enclosure of concrete and steel plates around the transformer. However, this method has some disadvantages like a large area is needed for the transformer installation.
Use of sound insulation panels is another way of getting reduced noise levels without any additional space requirement. The assembly consists of a resilient steel sheet, a steel plate and weights. The steel sheet connects the steel plate to the stiffeners. The weights are placed at the boundaries of the plate and sheet to avoid the transmission of structure borne vibrations from the stiffeners to the steel plate. The noise level reduction of 14 dB is reported by the use of these insulation panels. Development of a vibration controlled sound insulation panel, capable of reducing the noise generated from a transformer by 12 to 13 dB. A substantial reduction of noise of the order of 15 dB can also be obtained by using a double tank design. Both the tanks are suitably insulated from each other to reduce the structure borne sound. Glass wool is placed in the space between the two tanks for the effective noise reduction. Active noise control is one more technique for the noise level reduction, in which an anti-phase noise is generated and superimposed on the noise emitted by the transformer. It requires very sophisticated instrumentation and computational facilities. The active control scheme is implemented with Digital Signal Processing (DSP). It is reported that a noise level reduction of 5 to 15 dB can be achieved depending upon the effective implementation of the technique.
Dry type distribution transformers are made up of resin impregnated or cast resin. Due to the absence of oil and presence of openings or perforations on the tank for effective air circulation and cooling, the noise level can be higher. Hence, the core limbs can be of bolted construction in addition to the bolted yokes to give more rigidity to the core structure and reduce the noise emanating from it. The noise reduction techniques can be summarized as below:
• Reduction in core flux density: This gives noise reduction of 3 to 5 dB for a reduction in flux density by 10% or approximately 2 dB per flux density reduction of 0.1 T. The method has adverse effects on the cost and size of transformers.
• Hi-B grade and scribed core materials give 2 to 3 dB reduction as compared to non Hi-B grades.
• Increased core damping: By application of suitable viscoelastic or adhesive coating to the core laminations, the noise level can be reduced. It should be ensured that any links or attachments to the core are flexible so that they do not transmit the vibrations.
• Use of step-lap joint: It gives reduction by about 4 to 5 dB as compared to the mitred construction for the commonly used flux densities (1.6 to 1.7 T). The corner protrusions of the built core should be cut since they may contribute to noise due to vibrations.
• To reduce the structure borne vibrations, the core-winding assembly should be isolated from the tank base by use of oil compatible anti-vibration pads between them. Use of anti-vibration pads is also made between frames and tank. Such isolations can give a noise level reduction of 2 to 4 dB.
• Use of sound insulation panels between tank stiffeners can give 5 to 15 dB reduction. An increased tank wall mass, by use of sand in hollow braces on the wall, can give appreciable noise level reduction.
• Use of double tank design: Inner and outer tanks are suitably insulated from each other to eliminate structure borne vibrations. Also, suitable sound absorbent wool is placed between the two tanks. The noise reduction is about 15 dB.
• Use of active phase cancellation technique: The sound emitted by a transformer is overlaid by externally applied anti-phase sound. A noise level reduction of 5 to 15 dB may be possible.
Some precautions which need to be taken at the site for noise level control are:
• The reflecting surfaces should not coincide with half the wavelength of frequencies of noise emitted by the transformer to avoid standing waves and reverberations or echoes.
• Fire walls are sometimes placed adjacent to the transformer. It may not be possible to place them at a location so that no undesirable reflections occur. In such cases a sound-absorbent material, suitable for outdoor use, may have to be applied on the walls.
• Dry type distribution transformers are mostly located in a room inside a building. With the walls of the room having a low sound absorption coefficient, the sound emitted by the transformer reflects back and forth between the walls. This may lead to a considerable increase of noise level.
These aspects should be duly considered by the users while designing the room and manufacturers while designing the transformer.
Figure 3: Noise level measurement in outdoor space
Noise Level Measurement Facility at CPRI
The noise level is commonly measured in decibels (dB) by comparing the pressure generated by a noise source with some standard level. The noise level is measured basically two methods, sound pressure measurement and sound intensity measurement. The Noise level measurements in CPRI are carried out by using sound pressure method. The test methods and acceptable test environment conditions are specified in standard IEC: 60076–10. The methods are applicable to transformers, reactors and their cooling devices also. Sound pressure level is a scalar quantity and requires simple instrumentation. Sound intensity is a vector quantity and the method measures directional sound. It is, therefore, less affected by a background noise. Hence, the sound intensity method can give more accurate measurements in the presence of background noise.
However, sound intensity measurements require higher skill and more sophisticated instrumentation. Information about the location and characteristics of noise sources can be obtained by studying the frequency spectrum. Apart from design challenges, the measurement of low noise poses a difficult problem. The minimum level of noise which can be measured is limited by the ambient noise conditions in the test area. Special enclosures may have to be used to shield the instruments that are the test set-up and transformer from the high ambient noise. The noise level measurements can also be performed in large room or open areas where the interference of background noises are much minimal as shown in Figure 3. Where conditions are close to free-field, essentially undisturbed by reflections from nearby objects and the environment boundaries, as sometimes achieved for outdoor measurements, then the value for ‘K’ would tend to zero and no environmental correction is necessary.
Sound power at no-load excitation
Sound power due to no-load excitation has to be regarded for all types of transformer. The excitation voltage shall be of sinusoidal or practically of sinusoidal waveform and rated frequency. The usual condition for sound power level determination of transformers at no-load excitation refers to rated voltage at an untapped winding. Other excitation conditions may occur in service leading to lower or higher sound power levels and might also be the condition for a guarantee and if so shall be specified by the purchaser. For transformers designed to operate with variable flux, the sound power at no-load excitation is strongly impacted by the tapping position. The tapping position for the sound measurement has, therefore, to be agreed between manufacturer and purchaser during tender stage. The measurements shall be made on the principal tapping of the transformer as possible. The purchaser may also specify a value for the sum of the sound power levels at no-load excitation, Noise due to cooling device and also due to load current. The transformer shall be located so that no acoustically reﬂecting surface is within 3 m of the measuring microphone, other than the ﬂoor or ground.
Figure 4: Prescribing contour representation from principal radiating surface
Measurements should be made in an environment having an ambient sound pressure level at least ﬁve decibels below the combined sound pressure level of the transformer and the ambient sound pressure level. For one-third octave or narrow band measurements, the ﬁve-decibel difference shall apply to each frequency band in which measurements are being made. When ambient sound conditions do not comply with the above, suitable corrections may be feasible when the ambient sound conditions are steady and discrete frequency sound levels are measured. The environmental correction factor ‘K’ shall preferably be determined by measurement techniques. However, for the purpose of this standard estimation for ‘K’ is allowed by use of absorption coefficients. In order to account for the effect of these sound reflections, the IEC Standards provides the following calculation formula for what is termed as the “Environmental factor K” .The calculated value as per equations (1) & (2) is to be subtracted from the measured value of the noise level.
α = Average Acoustic Absorption coefficient
SV = Total Area of the Surface of the test room
S = Transformer Measurement Surface Area
The maximum value allowed for this Environmental factor correction is 7 dB, otherwise measurements would be considered invalid. For a test room to be satisfactory, A/S shall be ≥1. This will give a value for the environmental correction factor K of ≤7 dB.
For distribution type transformers, where test facilities are available anechoic chambers are used for sound measurements.
The prescribed contour shall be spaced 0.3 m away from the principal radiating surface for oil cooled transformers as shown in the figure 4. For measurements made with forced air cooling devices in service, the prescribed contour shall be spaced 2 m away from the principal radiating surface. In case of dry-type transformers without enclosure the principal radiating surface is the surface obtained by the vertical projection of a string contour encircling the dry-type transformer excluding protrusions such as bushings, turrets and other accessories. For dry-type units with and without enclosure, the prescribed contour shall be spaced 1 m away from the principal radiating surface. The principal radiating surface shall include cooling devices attached to the transformer, if any. For transformers with a height less than 2.5 m, the prescribed contour shall be on a horizontal plane at half the height. For transformers with a height greater than or equal to 2.5 m, two prescribed contours shall be used which are on horizontal planes at one third and two-thirds of the height of the transformer under test.
Figure 5: Audible Sound Levels for Liquid Immersed Distribution Transformers
Sound pressure method
The intention of this test is to report the total spatially averaged A-weighted sound pressure level for each energisation option accompanied with a single spatially averaged frequency spectrum. The same test procedure either walk-around procedure or point-by-point procedure applies for both background noise measurements and test measurements. A total spatially averaged background noise level and the corresponding frequency spectrum shall be recorded immediately before and after each test measurement sequence. If the background noise level is at least 10 dB below that of the test object then the background noise can be measured at only one location on the prescribed contour and a background noise correction is not necessary.
Figure 6: Audible Sound Levels for Dry-Type Transformers 15000-Volt Nominal System Voltage and below for ventilated & sealed
Calculation of average sound pressure level
The uncorrected average A-weighted sound pressure level , shall be calculated from the A-weighted sound pressure levels LpAi, measured with the test object energized by using equation as per equation (3).
Where N is the total number of measuring positions.
The average A-weighted background noise pressure level , shall be calculated separately before and after the test sequence using equation (4).
Where M is the total number of measuring positions
LbgAi is the measured A-weighted background noise pressure level at the ith measuring position.
If the initial and final average background noise pressure levels differ by more than 3 dB and the higher value is less than 8 dB lower than the uncorrected average A-weighted sound pressure level, the measurements shall be declared invalid and the test repeated except in cases where the uncorrected average A-weighted sound pressure level is less than the guaranteed value. In this case, the test object shall be considered to have met the guaranteed level. While the standard permits a small difference between the background noise level and the combined sound level of the background and the test object, every effort should be made to obtain a difference of at least 6 db. The corrected average A-weighted sound pressure level , shall be calculated by using equation (5).
where is the lower of the two calculated average A-weighted background noise pressure levels. As per the standard, the maximum allowable value of the environmental correction ‘K’ is 7 db. The A-weighted sound power level of the test object, LWA, shall be calculated from the corrected average A-weighted sound pressure level , according to equation (6).
where S is derived from the following equation (7) and S0 is equal to the reference area (1 m2).The area S of the measurement surface, expressed in square meters, is given by
S = 1.25hml (7)
h is the height in meters of the transformer tank .
lm is the length in meters of the prescribed contour.
1.25 is an empirical factor intended to take account of the sound energy radiated by the upper part of the test object.
Limits of sound power level
The distribution transformers noise level should not exceed the values specified in accordance with the conditions outlined in ANSI/IEEE C57.12.90-1993 as shown in the figures 5 & 6. National Electrical Manufactures Association (NEMA) Standards publication No.TR1-1993 (R2000) is also specified the limits for noise level of transformers.
In addition to the design challenges, the measurement of low noise poses a difficult problem. The minimum level of noise which can be measured is limited by the ambient noise conditions in the test area. Special enclosures may have to be used to shield the instruments and transformer from the high ambient noise. Several sound reduction methods have been explored in various literatures. However, in order to make these methods to be effective, the transformer manufacturer needs to understand completely the nature and origin of transformer noise and should be able to handle through proper selection of the core and flux density. Dry transformers are increasingly used in urban areas, because they are environmentally favorable, and have significant noise levels . Studies have proved that in the dry transformers up to 1000 kVA, noise levels can be reduced up to 10 dB by using Neoprene noise dampening pads to isolate the core and coil from the enclosure leads to reduce the noise than the oil transformers of same power. In addition, accurate measuring and calculating techniques as well as detailed modelling of the transformer are critical considerations when designing transformers with low noise emission.
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