At present, the firewalls of transformers and reactors in domestic UHV substations often adopt cast-in-place concrete structures or framed brick-and-powder structures, which have certain sound insulation effects. However, due to the low noise frequency of transformers and reactors and the long wavelength of sound waves, their noise diffraction and transmission capabilities are very strong, so the sound insulation and noise reduction effects are often poor. In addition, the current sound barriers and sound isolation covers of UHV substations are not specially designed for the low-frequency noise characteristics of the substation, and the noise reduction effect is not ideal. If we can take into account the characteristics of UHV substations dominated by mid- and low-frequency noise, and adopt a composite sound-absorbing structure that has both silencing and sound-absorbing functions, it is hoped that it can effectively reduce mid-frequency and low-frequency noise and also have flame retardant and insulation properties. This is of great significance for improving the acoustic environment quality of substations.

Based on this, this article analyzes the noise source intensity characteristics of the main equipment of the UHV substation. In view of the requirements for materials in the station to have flame retardant, insulating and other properties, we chose to carry out single and combined sound absorption experiments on melamine foam samples, micro-perforated plate samples, and foamed cement samples, and proposed a composite sound-absorbing structure suitable for noise reduction in UHV substations. This has important practical guiding significance for practical engineering applications.

Ⅰ. Noise measurement analysis of main equipment of UHV substation

During the operation of UHV substations, the main noise sources include high-voltage transformers, shunt reactors, substation structures, etc. The transformer noise includes body noise and cooling device noise, the shunt reactor noise is electromagnetic noise, and the substation structure noise is corona noise. Corona noise is generally medium to high frequency, attenuates quickly during propagation, and has a small impact range. The low-frequency noise generated by the internal structure of transformers and shunt reactors attenuates slowly and has a wide range of influence. Therefore, noise control of transformers and shunt reactors is the focus of noise control in substations.

1. Main transformer noise measurement analysis

The sound power level of the transformer is measured using the sound intensity method. Taking a single-phase transformer as the measurement object, the distance between the prescribed contour line and the reference emission surface is 2m, and two prescribed contour lines are set, located on the horizontal plane of 2m and 4m respectively. All measuring points are located on the specified contour line, and the horizontal distance between adjacent measuring points is 1m. Sound intensity probes are used to measure point by point.

When the power load of a single transformer group is 2000MW, the sound intensity measurement results of phase C of the transformer in Group I. The A-weighted sound power level is the largest in the 1/3 octave frequency band with the center frequencies of 100, 125, 160 and 200 Hz of the UHV transformer. According to the transformer noise generation mechanism, the noise in this frequency band mainly comes from the electromagnetic noise of the transformer body. Therefore, when reducing transformer noise, priority should be given to reducing the 100~200Hz frequency band. During the measurement, the power load of a single group of transformers was stable at 2000MW, and the corresponding total A-weighted sound power level of the transformers was 102.7dB(A).

2. Measurement and analysis of high-voltage reactor noise

The measurement method of reactor sound power level is the same as that of transformer. Taking the B phase of the parallel reactor in the UHV substation as the measurement object, the distance between the prescribed contour line and the reference emission surface is 1m, and two prescribed contour lines with heights of 1.5m and 3m are set respectively. The horizontal distance between adjacent measuring points is 1m.

The analysis shows that in the entire 1/3 octave band, low-frequency components account for a larger proportion, among which the sound power at the 100Hz frequency is the largest, reaching 96.4dB(A), which is 11.5dB(A) higher than the sound power level of 200Hz. The sound power in the 1/3 octave band where 100Hz is located accounts for 91.2% of the single-phase high anti-sound power. Therefore, it can be determined that the interference phenomenon in the reactor sound field distribution is caused by the 100Hz sound wave.

Ⅱ. Porous materials and micro-perforated plate experiments

In noise control engineering design, it is often difficult to achieve the expected noise reduction effect by using a single material for sound absorption and insulation treatment. Therefore, sound-absorbing material combination structures are often used to reduce noise. Especially in indoor spaces with larger volumes and longer reverberation times, the application is quite common. To study the sound-absorbing structure suitable for noise control in UHV substations, we must first consider the insulation and flame-retardant properties of the material, and secondly, we must combine specific measures to determine the sound-absorbing structure.

At present, perforated plates and sound-absorbing materials are used in the sound insulation enclosures of UHV substations to reduce noise, but the effect is not good in the low frequency band. This requires combining the sound source characteristics of the main equipment of the UHV substation. After determining the specific noise reduction measures, how to select porous sound-absorbing materials and how to determine the sound-absorbing structure becomes the key to noise control in the substation. In the following, three samples of micro-perforated plate, melamine foam, and foamed cement are selected to be combined with each other to carry out experiments.
1.Double-layer micro-perforated plate combination

Micro-perforated plate experimental samples, in which the perforation rate of micro-perforated plate sample 1 is 1.5%, the plate thickness is 1mm, and the pore diameter is 0.7mm; the perforation rate of micro-perforated plate sample 2 is 1.1%, the plate thickness is 1mm, and the pore diameter is 0.7mm , Micro-perforated plate sample 3, the perforation rate is 0.7%, the plate thickness is 1mm, and the hole diameter is 0.7mm.

Two sets of experiments were carried out on the double-layer micro-perforated plate combination. Experiment 1 was a combination of micro-perforated plate sample 2 and micro-perforated plate sample 1, and experiment 2 was a combination of micro-perforated plate sample 1 and micro-perforated plate sample 3. Two sets of experiments on the double-layer micro-perforated plate combination structure show that the sound absorption coefficient at 100Hz is around 0.7, and the maximum value of the sound absorption coefficient appears at 500Hz to 600Hz in both sets of experiments. In the frequency band above 600Hz, the sound absorption coefficient gradually decreases.

2.Combination of foamed cement and micro-perforated board

First, a sound-absorbing structure combining foamed cement and a single-layer micro-perforated plate was used. Two experiments with different parameters were carried out by combining foamed cement and a single-layer micro-perforated plate. In both experiments, the foamed cement was placed in front of the micro-perforated plate.

There is almost no big change between the data of Experiment 3 and Experiment 4. It can be seen that the sound absorption performance of the combined structure basically does not change with slight changes in the thickness of the foamed cement and the distance between the foamed cement and the micro-perforated plate. At 100Hz, the sound absorption coefficient is around 0.4, at 250Hz, the sound absorption coefficient reaches its maximum, and at around 900Hz, it reaches the maximum value of the sound absorption coefficient again.

3. Melamine foam and micro-perforated panel combination

Two sets of experiments were conducted on the combination of melamine foam and single-layer micro-perforated plates. In both sets of experiments, melamine foam was placed before the micro-perforated plates. The sound absorption coefficients at 100Hz in both groups of experiments are all around 0.7, and between 100Hz and 500Hz, the sound absorption coefficients are all above 0.7. Around 500Hz, the sound absorption coefficient decreases, but as the frequency increases, the sound absorption coefficient gradually increases. This combination has a relatively ideal sound absorption effect in both low-frequency and high-frequency bands.

Ⅲ. Analysis of experimental results

According to the characteristics of the noise source of the main equipment of the UHV substation that is prominent in the low frequency band, experiments were carried out by selecting a micro-perforated plate composite structure, a foamed cement plus micro-perforated plate composite structure, and a melamine foam plus micro-perforated plate composite structure. The test results show that all three combination schemes can achieve a certain sound absorption effect in the low frequency band, but how to apply it in actual engineering requires specific noise reduction measures.

1. Micro-perforated plate combination structure

Double-layer micro-perforated boards can broaden the sound absorption frequency band of single-layer micro-perforated boards. Based on the principle of impedance matching, the principle of combination design is that the perforation rate of the front layer of micro-perforated boards is greater than the perforation rate of the back layer of boards. The combined design of micro-perforated panels can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 150-800Hz when the thickness of the entire combined structure is 20cm. When the thickness of the entire combined structure is 30cm, it can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 100-800Hz.

2.Foamed cement plus micro-perforated plate composite structure

Adding foaming cement in front of the micro-perforated board can broaden the sound absorption frequency band of the micro-perforated board, and the foamed cement can also play a fireproof role.

Foamed cement plus a single-layer micro-perforated board can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 150-1000Hz when the thickness of the entire combined structure is 20cm. Foamed cement plus double-layer micro-perforated panels can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 150-1400Hz when the thickness of the entire combined structure is 20cm. When the thickness of the entire combined structure is 30cm, it can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 100-1400Hz. At the same time, the smaller the bulk density of foamed cement, the greater the porosity, and the better the low-frequency sound absorption effect.

3. For the combined structure of sound-absorbing material and micro-perforated plate, the sound-absorbing material can expand the sound-absorbing frequency band of the micro-perforated plate and increase the distance between the sound-absorbing and micro-perforated plate. The sound absorption coefficient of medium and high frequencies increases, and the sound absorption peak moves to low frequencies. By increasing the thickness of the sound-absorbing material, the low-frequency sound absorption coefficient decreases slightly, and the mid- and high-frequency sound absorption coefficient increases. Increasing the thickness of the cavity layer behind the micro-perforated plate can increase low-frequency sound absorption. Sound-absorbing materials plus double-layer micro-perforated panels have better sound absorption and wider sound-absorbing frequency bands than sound-absorbing materials plus single-layer micro-perforated panels.

It can be seen from the analysis of experimental data that melamine foam plus a single-layer micro-perforated plate can achieve a sound absorption coefficient of more than 0.6 in the frequency range of 200-1600Hz when the thickness of the entire combined structure is 20cm. When the thickness of the entire combined structure is 30cm, a sound absorption coefficient of more than 0.6 can be achieved in the frequency range of 100-1600Hz. Melamine foam plus double-layer micro-perforated panels can achieve a sound absorption coefficient of more than 0.7 in the frequency range of 200-1600Hz when the thickness of the entire combined structure is 20cm. When the thickness of the entire combined structure is 30cm, a sound absorption coefficient of more than 0.7 can be achieved in the frequency range of 100-1600Hz.

Based on the aforementioned analysis of the noise frequency characteristics of the UHV main transformer and high-voltage shunt reactor, the noise energy of the UHV main transformer is mainly concentrated at 100-200Hz, and the noise energy of the high-voltage reactor is mainly concentrated at 100Hz. To select a noise-reducing material, it is necessary to combine the frequencies of the two main noise sources. At the same time, noise reduction methods also need to be combined with the on-site layout of the substation to control noise with the optimal combination of materials and methods.

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