Harmonics and interharmonics adversely affect power grids. The fast Fourier transform (FFT) algorithm is one of the most commonly used methods for harmonic analysis. However, in practical applications, the accuracy of harmonic analysis can be seriously affected by fence effect and spectral leakage, which are undesired characteristics inherent to discrete Fourier transforms. Moreover, when non-synchronous sampling is carried out, the phase measurement is not accurate enough, and there is a large error in the identification of interharmonics. In order to improve the measurement precision, the method of allphase spectrum analysis is used, since it has the characteristics of phase invariance and good spectral leakage suppression. A novel method based on improved Nuttall double-window all-phase FFT is proposed by improving the window function and the spectrum correction method for achieving higher precision. Through simulation and experimental verification, the proposed algorithm has proven to perform better than the traditional algorithms both for the detection of harmonics and interharmonics. In addition, the computation burden is not considerably increased when compared to such algorithms, which allows the on-line use of the proposed algorithm.

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In summary, there are two main issues facing high voltage parameter online measurement: obtaining of high-voltage side working power, and signal transmission between high and low voltage. To resolve these issues, we propose and design the high voltage parameter online measurement sensor system whose light source and light signal transmission is based on a sealed space. The sensor system contains high-voltage side power supply system and signal measurement and transmission system that are sealed inside a layer insulator. The sealed light source and signal transmission makes sure the high efficiency and reliability of power source at high-voltage side signal sampling and processing circus. It also guarantees the safety, reliability and accuracy of signal transmission. This article introduces the working principle, the structure and the design of the sensor system. Keywords-sealed; photoelectric sensor; high-voltage; online measurementI.