Time‐frequency analysis of a pressure wave caused by impulse tree using wavelet transform

This paper describes results of time‐frequency analysis of a pressure wave caused by impulse tree extension using wavelet transform. Pressure waves were measured by a piezo sensor which was located on the back of the plane electrode when high‐voltage nanosecond pulses were applied to a point electrode in a low‐density polyethylene block. Frequency of the pressure wave from induced charges on the plane electrode was higher than that of the pressure wave from tree. In addition, the frequency of the pressure wave from tree tended to decrease with increasing gap length because of the dispersion of the pressure wave during its propagation. When a large tree extension occurred between a 5‐mm gap, pressure waves of about 10 MHz propagated from the tree tip and a pressure wave of 1 MHz propagated from the tree stem. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(1): 1–7, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10097     

Auto‐detection of partial discharges in power cables by discrete wavelet transform

One of the serious problems that can occur in power XLPE cables is destruction of insulator. The best and conventional way to prevent this is ascertaining partial corona discharges occurring at small voids in organic insulators. However, there are some difficulties in detecting those partial discharges due to the existence of external noise in detected data, whose patterns are hardly identified at a glance. For this reason, there have been a number of researches into detecting partial discharges by employing a neural network (NN) system, which is widely known as a system for pattern recognition. We have been developing an NN system for auto‐detection of partial discharges, and have input numerical data of the waveform itself and obtained appropriate performance. In this paper, we employed the discrete wavelet transform (DWT) to acquire more detailed transformed data in order to use them in the NN system. Employing the DWT, we were able to express the waveform data in time–frequency space, and achieved effective detection of partial discharges by the NN system. We present herein the results using DWT analysis for partial discharges and noise signals which we obtained. Moreover, we present results out of the NN system which dealt with those transformed data. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(1): 24–30, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10315     

A delta‐method technique in the wavelet domain to determine statistical quantities of the response of electromagnetic devices with uncertain parameters

In this paper a method to determine the mean value and the variance of the response of a system with uncertain parameters is proposed. With the term ‘response of a system’ we indicate either the value of a single parameter that represents a figure of merit of a device (e.g. the efficiency of a transmission system or the band width of a communication channel) or a characteristic function of the system (e.g. the impulse or the frequency response). In the latter case we estimate the mean value and the variance at every sample of the response. The estimate is performed by using the delta method, a technique for approximating expected values of functions of random variables when the direct evaluation is not feasible. Two examples of the application of the proposed procedure are reported and the results are compared with simulations performed by a Monte Carlo analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

A novel method to improve low‐voltage ride‐through capability of wind turbine generators

With increasing penetration of wind farms, power grids have responded by developing specific grid codes to maintain their stability. One of the main grid codes is the low‐voltage ride‐through (LVRT) capability, which requires the wind generator to remain connected when the grid voltage sags for a certain time period. A wind farm with squirrel cage induction generators suffers this LVRT problem the most because of their direct connection to the grid and reactive power consumption. In this paper, a new method is proposed to solve this problem by shunt‐connecting a motor‐driven mechanical load to the cage wind generator. For driving mechanical loads, the induction motor is most widely used in industries. This paper studies the terminal voltage holding effect of an induction machine during grid voltage sag due to the magnetic flux holding effect and the saturation characteristic. Taking advantage of this effect, the induction motor that is used for driving mechanical load is then proposed to improve the LVRT capability of wind turbine generators. Furthermore, the change of the rotating speed or slip of the induction machine is found to have a great impact on improving the LVRT. By adding some inertia to the motor‐driven mechanical load, an enhanced voltage holding effect, and therefore LVRT improvement, is expected for the wind farm. Both simulation and experimental results prove the effectiveness of the proposed method. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.