# Research on a Partial Discharge Expert System for the Diagnosis of Damaged Transformation Equipment
## 2 Materials and Methods
### Functions and Processes of System Software and Data Acquisition Systems
#### 2.2.3 Noise-Suppression Technology  
The noise-suppression technique is applied in the design planning of the raw discharge waveform-detection system for PD. It is a method and technology used to reduce or eliminate unnecessary signals, interference, or background noise. Filtering noise in the input of the analog-to-digital converter (ADC) is a common but important problem. Such converters are used to convert analog signals into digital signals, but they may introduce various types of noise in the process. Two main specified requirements for bandpass and bandstop filters are the cutoff frequency at \(-3\) dB and the stopband frequency \(\mathsf{f_{N}}\). The variability of the bandwidth and stopband frequency in the filter will change the behavior of the filter. Filters with adjustable bandwidth and stopband frequency are known as variable filters or adjustable filters, and they play a very important role in digital signal-processing applications. There are various designs for variable bandpass and bandstop filters, with one of the previously proposed designs in the literature suggesting replacing almost all delay elements in the digital low-pass prototype filter with second-order digital all-pass filters and then modifying the coefficients, which can be computationally complex [21; 22]. To efficiently reduce computational complexity, one method is to use a Taylor series expansion of the transfer function coefficients, but truncating the approximated linear terms can degrade the filter's amplitude-frequency response. Another computationally simpler  
Figure 7: Simulated discharge waveform of power transmission transformation equipment. method adjusts only the center frequency f0 while keeping the bandwidth constant and is designed primarily for narrowband digital filters. This method adjusts the bandwidth of variable filters based on the cutoff frequency fc of the digital low-pass prototype filter, resulting in a limited range of frequencies and increased complexity for higher-order filters. Another significant issue to consider is that the design of most digital bandpass and bandstop filters using the center frequency f0, which is the geometric mean of the lower frequency f1 and the upper frequency fU, applies primarily to analog filter design. It is well known that using the center frequency f0 for designing digital bandpass and bandstop filters has limitations because f0 is not the stopband frequency fN and is only an approximation for the design of narrowband digital filters. The results of capturing high-frequency signals from the raw discharge waveform of PD are shown in Figure 8. It is evident from the figure that the signal levels contain high-frequency noise. Filters can be used to create low-pass filters at different output points, primarily designed to allow low-frequency components to pass through while reducing or blocking high-frequency components. A low-pass filter can filter out all frequency components above a certain specific cutoff frequency, allowing only components below that frequency to pass. This is included in various applications of analog-to-digital converters. Additionally, it can be used to construct various broadband filters with independently controllable lower and upper cutoff frequencies, as well as to ensure frequency doubling and one-third octave filter sets in the form of tunable bandpass filters. The main drawback of tunable filters based on typical filter sections is that they do not allow for high stability at high frequencies. Figure 9 shows the results of an RC filter. Figure 8: Original discharge simulation waveform. Figure 9: Simulation waveform noise suppression results using RC filtering. Finite impulse response (FIR) filters are characterized by having a finite-length impulse response, where the input signal is convolved with a set of fixed coefficients to obtain the output signal.