Application Of Digital Filtering Techniques Research Articles

Application of digital filtering techniques to the determination of partial discharge location in transformers

The capacitative network representation of transformer windings has long been used for the study of pulse distribution along the windings. However, the author's test results and those published in the literature show that theoretical analysis based on this equivalent network may not be applicable. It was found that the capacitive network approximations were valid only within certain ranges of frequency. The frequency ranges were determined through the analysis of terminal measurements. The capacitatively transferred components of the input pulse along the windings in the appropriate frequency range were then extracted by the aid of digital filtering techniques, and good agreement between measured and theoretical distributions was obtained. These techniques, in conjunction with a new straight-line interpolation method, were employed to locate simulated partial discharges in a 66 and 132 kV transformer winding. Measurement results showed that an accuracy of better than 5% of winding length was obtained. >

Active control of 2-D instability waves on an axisymmetric body

Abstract : Experimental studies have demonstrated the feasibility of Tollmien-Schlichting (T-S) disturbance cancellation in flat plate boundary layers using a variety of active sources. What is detailed herein is the first experimental study of the generation and cancellation of laminar-instability (T-S) waves by adaptive heat addition to a laminar boundary layer of an axisymmetric body. Tests were performed in a water tunnel environment on a 9:1 fineness ratio ellipsoid utilizing strip heaters to create and actively attenuate laminar instability waves. The attenuation signal was synthesized by a modified adaptive filter. The filter was able to actively adapt the attenuation signal to changes in amplitude and frequency of the instability wave with no loss in attenuation downstream. Two important distinctives between Liepmann's work and the current study are the method of signal processing used and the introduction of three dimensionality in the flow. In our work the application of digital filtering techniques to the synthesis of the cancellation signal is unique. Further the flow geometry, although not a fully three dimensional, has elements of three dimensionality that further complicate the utilization of selective heat addition. The current experimental program has established that active heat addition controlled by a digital adaptive filter can successfully attenuate T-S waves in an axisymmetric geometry.

Catheter smearing correction using inverse filtering techniques.

The application of digital filtering techniques to correct for the instrumental error, known as ``smearing,'' which occurs when a long catheter of narrow bore is used to sample the concentration of an indicator substance dissolved in blood, is described.

A nonrecursive digital filter for data transmission

High-speed digital data transmission systems often require special filter functions for spectral shaping with prescribed phase characteristics. One such filter is the cosine roll-off filter, which has applications in both serial and parallel data transmission systems. This filter is of special interest, because in such applications it is common to utilize two identical cosine roll-off filters in a cascade connection, i.e., one in the transmitter and one in the receiver. The composite response of the cascade connection has the raised-cosine roll-off characteristic, which yields low intersymbol interference. Because a linear phase characteristic is further required, the digital filter type chosen for this application is nonrecursive. The difference equation coefficients are determined from the coefficients of a Fourier series expansion of the magnitude-frequency characteristic in the form of a cosine series. The linear phase characteristic is the direct result of setting the sine terms of the Fourier series to zero. Two criteria have been selected as a measure of the digital filter performance: minimum stop-band attenuation and an RMS measure of intersymbol interference. Performance was calculated as a function of the number of terms in the cosine series used to approximate the desired function. Performance graphs, which summarize the results of this investigation, are included. The application of digital filtering techniques in the field of data transmission systems is shown to have practical value.