Harmonic Analysis and Neutral-Point Potential Control of Interleaved Parallel Three-Level Inverters for Flywheel Energy Storage System

Flywheel energy storage system is a popular energy storage technology, in which inverters are the center of electrical energy conversion, directly affecting the power capacity. Parallel operation of three-level inverters is an effective approach to achieve larger motor drive power and the interleaved operation can improve the harmonic characteristics. However, harmonic analysis models of the interleaved parallel three-level inverters are rare in the literature and how the neutral-point potential imbalance affects the harmonics characteristics has not been discussed. This article establishes the harmonic calculation for balanced and unbalanced neutral-point potential through the five-level voltage capability of the interleaved parallel three-level inverters. Moreover, a neutral-point potential control method based on zero-sequence voltage injection is proposed. The implement process of the method is proposed, and how the operating frequency affect the ability of the neutral-point potential balance is studied. Finally, the simulation and experiment results verify the feasibility and practicability of the established harmonic analysis models and the neutral-point potential control method.

HARMONIC ANALYSIS AND MODELLING OF ANNUAL SOIL TEMPERATURE VARIATIONS

Harmoni c analysi s using se ries is applied to th e annual variation s of so il tem pe rature From th esurface down to I III depth. TIle fir st harmo nic alon e expla ins about 99% of the variation .II is also shO\\TI th at heat conduction is nor valid in a soil med iuIIIunder exothermic conditions. especially in th euppermost layer of abom 20 em. as neither the thermal properties. exp ressed by the damping depth (D), nor th e meansoil temperature (nremai n constant with depth . and th e am plitude ofoscilla tio n (.4:) does nOI vary strictly expcnen rially.Thi s is as sumed to be con sider ably reduced soil moisture of thi s in compari son with deeper layers,Finally. cons truction of a mod el to predict mon thly or ten-d ay period mean soil temper atures is based' onwr-at h e r vari ab les (difference between precipitation and pan evaporation), providing an ind ex for the soil moisturede ficit and an est imate of D. and overcomes the difficulties of ch angi ng T and A: with depth.

Improving the transmission error source tracing method for gear hobbing machines

By considering the uncertainness of initial measuring position of encoders and signal sidebands caused by the fault gear pair, this paper presented a new comprehensive harmonic analysis method for the transmission error of gear hobbing machine. Based on that, a test platform was established, in which two circle grating encoders were connected to the hob spindle and workpiece spindle respectively. With the help of this new harmonic analysis method as well as the self-developed test platform, a new improved transmission error fault diagnosis method was developed for the gear hobbing machines. To verify its accountability, a case study was conducted on a YS-type gear hobbing machine. According to the spectrum amplitude comparison and the analysis of harmonic frequency distribution, the fault transmission gear pair was successfully located. This improved transmission error source tracing method was very helpful for quantifying both the manufacturing qualities and assembly qualities of parts and locating potential error source for new gear hobbing machines.

Analysis of soil temperature at various depths by Fourier technique

Harmonic analysis of weekly means of soil temperatures at 5,.15 and 30 cm; depths have been done for seven stations of .India. The corresponding amplitudes and phase angles In respect of different harmonics are presented- The warmest soil near the soil surface (5 cm depth) occurs during the period 16th to 19th week. While the highest maximum occurs during the period 20th to 26th week (30 cm depth).

Decadal variations of Hadley and Walker circulations in the tropics

Preliminary estimate of divergent Hadley and Walker circulation associated with inter-decadal variations in the tropics is made with 50-year reanalysis data and compared with their inter-annual counterparts. Interdecadal and inter-annual components are separated using harmonic analysis and meridional and zonal mass flux stream functions are used to calculate the strength of Hadley and Walker circulations. The magnitude of inter-decadal Hadley and Walker circulation anomalies are shown to be comparable to those associated with dominant inter-annual variations. How superposition of inter-decadal and inter-annual divergent circulations may influence regional climate is discussed.

Estimation of soil temperature by Harmonic analysis

Soil temperatures recorded at 5.10 and 20 cm depth at Anand station of Gujarat state for the period of 1982-93 were analyzed by harmonic analysis and tested it for the year 1998. The amplitude of the soil temperature wave for the various depths decreases from a maximum of 5.76° C in the first harmonic to 0.96°C in the third harmonic. First, second and third harmonic explained 55 to 76 percent, 18 to 37 percent and 2 to 4 percent of the total variance, respectively. Total percentage of total variance covered by the three harmonics at different depths are in the range of 95 to 97 percentage for the period of 1982-93 normalized data series which shows the applicability of harmonic analysis in describing soil temperatures. The weekly observed and computed soil temperatures by harmonic analysis were in agreement and their differences were non significant by Chi-Square test.

Prediction of terrestrial and extraterrestrial parameters by modelling and extrapolating their natural regularities

Extrapolation of dominant modes of fluctuations after fitting suitable mathematical function to the observed long period time series is one of the approaches to long-term weather or short-term climate prediction. Experiences suggest that reliable predictions can be made from such approaches provided the time series being modeled possesses adequate regularity. Choice of the suitable function is also an important task of the time series modelling-extrapolation-prediction, or TS-MEP, process. Perhaps equally important component of this method is the development of effective filtering module. The filtering mechanism should be such that it effectively suppresses the high frequency, or unpredictable, variations and carves out the low frequency mode, or predictable, variation of the given series. By incorporating a possible solution to these propositions a new TS-MEP method has been developed in this paper. A Variable Harmonic Analysis (VHA) has been developed to decompose the time series into sine and cosine waveforms for any desired wavelength resolution within the data length (or fundamental period). In the Classical Harmonic Analysis (CHA) the wavelength is strictly an integer multiple of the fundamental period. For smoothing the singular spectrum analysis (SSA) has been applied. The SSA provides the mechanism to decompose the series into certain number of principal components (PCs) and then recombine the first few PCs, representing the dominant modes of variation, to get the smoothed version of the actual series. Twenty-four time series of terrestrial and extraterrestrial parameters, which visibly show strong regularity, are considered in the study. They can be broadly grouped into five categories: (i) inter-annual series of number of storms/depressions over the Indian region, seasonal and annual mean northern hemisphere land-area surface air temperature and the annual mean sunspot number (chosen cases of long term/short term trends or oscillation); (ii) monthly sequence of zonal wind at 50- hPa, 30-hPa levels over Balboa (representative of quasi-biennial oscillation); (iii) monthly sequence of surface air temperature (SAT) over the India region (strongly dominated by seasonality); (iv) monthly sequence of sea surface temperature (SST) of tropical Indian and Pacific Oceans (aperiodic oscillations related to El Nino/La Nina); and (v) sequence of monthly sea level pressure (SLP) of selected places over ENSO region (seasonality and oscillation). Best predictions are obtained for the SLP followed by SAT and SST due to strong domination of seasonality and/or aperiodic oscillations. The predictions are found satisfactory for the lower stratospheric zonal wind over Balboa, which displays quasi-periodic oscillations. Because of a steep declining trend a reliable prediction of number of storms/depressions over India is possible by the method. Prediction of northern hemisphere surface air temperature anomaly is not found satisfactory.

Assessing Waterlogging Stress Level of Winter Wheat from Hyperspectral Imagery Based on Harmonic Analysis

Frequent waterlogging disasters can have serious effects on regional ecology, food safety, and socioeconomic sustainable development. Early monitoring of waterlogging stress levels is vital for accurate production input management and reduction of crop production-related risks. In this study, a pot experiment on winter wheat was designed using three varieties and seven gradients of waterlogging stress. Hyperspectral imagery of the winter wheat canopy in the jointing stage, heading stage, flowering stage, filling stage, and maturation stage were measured and then classified. Wavebands of imaging data were screened. Waterlogging stress level was assessed by a combined harmonic analysis method, and application of this method at field scale was discussed preliminarily. Results show that compared to the k-nearest neighbor and support vector machine algorithms, the random forest algorithm is the best batch classification method for hyperspectral imagery of potted winter wheat. It can recognize waterlogging stress well in the wavebands of red absorption valley (RW: 640–680 nm), red-edge (RE: 670–737 nm), and near-infrared (NIR: 700–900 nm). In the RW region, amplitudes of the first three harmonic sub-signals (c1, c2, and c3) can be used as indexes to recognize the waterlogging stress level that each winter wheat variety undertakes. The third harmonic sub-signal amplitude c3 of the RE region is also suitable for judging stress levels of JM31 (one of the three varieties which is highly sensitive to water content). This study has important theoretical significance and practical application values related to the accurate control of waterlogging stress, and functions as a new method to monitor other types of environmental stress levels such as drought stress, freezing stress, and high-temperature stress levels.