# Application Study of Adaptive Tracking Algorithm in Active Noise Control System of Transformer
## 2 Substation Sound Field Characteristics  
For the ANC system, the key factors of noise reduction effect are to design a targeted control algorithm and a secondary source distribution scheme, according to the types and characteristics of noise sources [13]. The control strategy and target of ANC algorithm are designed on basis of acoustic characteristics of noise. The secondary sources placement is based on the substation sound field distribution. Therefore, the first job of this paper is to research the characteristics of transformer noise and the substation sound field distribution. Operating transformer makes a kind of electromagnetic noise. The reason is that alternating magnetic flux produces magnetostrictive effect which causes vibration of the core laminations. Since the magnetostrictive period is half of the power period, the fundamental frequency of core noise is twice of the power frequency [14]. The difference of magnetic flux path along the core frame and the inner frame, the core hysteresis loss and the non-linearity of the eddy current loss cause the high-order harmonic flux in the core, which produces higher order harmonic noise. The frequency of such noise is usually an integer multiple of the frequency of fundamental frequency noise [15]. In addition, the transformer cooling device produces fluid noise whose energy is evenly distributed over a wide frequency band with no significant peaks. The noise level of such noise is much lower than that of the transformer ontology noise. A 220 kV substation with a high level of transformer noise located in residential area is taken as the research object. Such industrial noise is a typical acoustic pollution source and needs to be governed. Transformer noise is collected and measured, and then the acoustic characteristics, frequency characteristics and noise distributions are analyzed. The analysis result is used to guide the ANC system control algorithm and the secondary acoustic source placement. The schematic diagram of the substation structure and probe location is shown in Figure 1.  
The width of the substation is 60 m, and the length is 70 m. A residential building is 15 m from the west boundary of the substation. Transformer No. 3 is 25 m from the residential building. This transformer noise is a major source of nuisance noise. H series measuring points are placed surrounding by transformer No. 3 and No. 2. Each measuring point is 0.2 m from the transformer, and the measuring point to each other is 1.5 m apart on each side. Transformer noise characteristics are analyzed by the collection of noise signal and sound pressure level of H series measuring points. A column measuring points are arranged in the west boundary, and each point is 7 m apart. A to  
Figure 1: Sketch of substation structure and location of measuring point. E column, the interval between each column is 10 m. The sound pressure levels of these points are collected as the references of substation sound field distribution. Each measurement point sampled the sound pressure level 10 times, and the interval of each sampling is 10 min. The transformer noise level is measured at 10 min per interval for 600 min at H series and A8 to A10 measurement points, and the noise signal was collected for 15 s. The type of measuring instrument is AWA6921, which is a general-purpose 1 sound measurements to the latest international standards. Figure 2 shows the noise level variation at H2 and A9 in 600 min. Figure 3 shows the time domain and the spectrum of noise signal of H2 within one second. Table 1 shows the noise pressure level of 100 Hz to 500 Hz frequency component at H2. As shown in Figure 2, the transformer noise is maintained at \(64\pm 1\) dB at H2. The noise level at A9 is maintained at \(55\pm 1\) dB(A), and the variation range is less than 1 dB(A). According to the environmental quality standard for noise in China [16], residential buildings belong to the Category 1 acoustic environment functional area. Such areas require the environmental noise limit to be 55 dB (A) during the day.