# Thermal and Acoustic Design of a Shelter for High-Voltage Electrical Equipment
## 5 Acoustic Aspects  
### Noise Characterisation  
To characterise the noise sources from the electrical devices, it was necessary to measure the noise profile generated by the switches. Inside the shelter, there are three identical switchbears, representing the only sources emitting noise. A field experiment was performed on a switchbear of the same type to be installed in the shelter at a substation in South Tyrol on 19 June 2023. The result of the characterisation of the noise emitted by this source was applied to each one of the three sources placed in the shelter. The investigated frequency range was from 20 Hz to 10 kHz. To measure the noise emitted by the electrical devices, an OROS Type OR36 multi-channel analyser was used. A total of three microphones were located around the switchgear placing them on a reference surface. The sound pressure level on the top face could not be measured for safety reasons, being the device on and under voltage. One of the long sides was not measured because of the symmetry of the noise source. The microphones used are BSWA Type MPA 416 and were calibrated before and after the measurements with a Larson Davis calibrator Type CAL200. The OROS multi-channel analyser was connected to a laptop equipped with the OROS NVGate Version 11.10 software. A reference measurement surface was set to evaluate the sound energy levels (Figure 10). Given the device dimensions, it was decided that the maximum height of the surface (L3 + D\({}_{\mathrm{T}}\)) would be 3 m, while the first height for positioning the microphones, H2, was 2.3 m due to safety reasons, which is also the height of the midsection of the horizontal isolators. The dimensions of the considered imaginary measurement surface are (L1, L2, L3) = (5 m, 4.2 m, 1.8 m). The distance between the envelope surface of the switchgear and the measurement surface (D) was 1.5 m in both directions. A second set of measurements was performed at a different height to get a better average. The height chosen for measurement is H1 = 1.5 m, as it corresponds to the height of the control boxes placed directly below the noisy part of the switchgear. For the measurements performed on the left and right sides of the switchgear as in Table 2, the microphones are placed at a distance D1 of 0.7 while for the measurements performed on the front and rear sides, the microphones were placed at a distance D2 of 0.6 m. A picture of the measurement surface is given in Figure 10.  
A total of six measurements were performed for each face, as shown in Table 3. Each of the three microphones has two measurements of the same \"event\" for two different heights from the ground. Moreover, two events corresponding to two different types of operation were characterised: an Open operation and a Close operation. The second column of Table 3 determines the side of the switchgear along which the microphones are placed when facing the device as it is represented in Figure 10.  
Figure 10: Schematic design of the measurement surface encompassing the electrical device. After setting up all the equipment, the microphones were auto ranged. The \(L_{j}\) of each face was found for each one-third octave band in the frequency range of interest. With the help of Equations (4)-(8), the \(L_{j}\) of every single operation, i.e., O, C and the overall energy level for the composed operation formed by the sequence open-close-open-close-open (OCO2 CO), can be easily calculated. The computed sound energy level (\(L_{j}\)) with its spectrum was later used as an input in a noise modelling software to predict the sound pressure levels around the installation. Table 4 gives overall \(\overline{L_{E}}\) and \(L_{j}\) levels of O, C, and OCO2 CO operations. The spectra of the three types of operations are given in Figure 11. Looking at the spectra it can be stated that the contribution of the bands above 10 kHz can be neglected since they are at least 10 dB lower than the ones giving the highest contribution. The final step is to define the sound energy levels of the noise sources.