Base-Frequency Negative Sequence Impedance Modeling and Sensitivity Analysis of DFIG

The phenomenon of three phase voltage imbalance frequently occurs in large-scale new energy grid connected areas in China; in severe cases, a large number of wind turbines will be disconnected from the grid. To solve the problem of the voltage imbalance at the point of common coupling (PCC), analyze the influence of generator parameters change on negative sequence voltage under the background of unbalanced power grid, a modeling method of base-frequency negative sequence impedance of doubly fed induction generator (DFIG) which including phase locked loop (PLL), rotor side converter (RSC) and grid side converter (GSC) is proposed. By establishing the negative sequence equivalent circuit of grid-connected system of DFIG, the relationship between the negative sequence voltage of PCC and the negative sequence impedance of DFIG is listed, and analyzing the sensitivity of control parameters link to base-frequency impedance, the parameter that has great influence on base-frequency negative sequence impedance of PCC is found out. Finally, the accuracy of impedance modeling and sensitivity analysis is verified by simulation studies.

Consistent Clustering Pattern of Prokaryotic Genes Based on Base Frequency at the Second Codon Position and its Association with Functional Category Preference

In 2002, our research group observed a gene clustering pattern based on the base frequency of A versus T at the second codon position in the genome of Vibrio cholera and found that the functional category distribution of genes in the two clusters was different. With the availability of a large number of sequenced genomes, we performed a systematic investigation of A2–T2 distribution and found that 2694 out of 2764 prokaryotic genomes have an optimal clustering number of two, indicating a consistent pattern. Analysis of the functional categories of the coding genes in each cluster in 1483 prokaryotic genomes indicated, that 99.33% of the genomes exhibited a significant difference (p < 0.01) in function distribution between the two clusters. Specifically, functional category P was overrepresented in the small cluster of 98.65% of genomes, whereas categories J, K, and L were overrepresented in the larger cluster of over 98.52% of genomes. Lineage analysis uncovered that these preferences appear consistently across all phyla. Overall, our work revealed an almost universal clustering pattern based on the relative frequency of A2 versus T2 and its role in functional category preference. These findings will promote the understanding of the rationality of theoretical prediction of functional classes of genes from their nucleotide sequences and how protein function is determined by DNA sequence. Graphical abstract

Bio-feedback of audio-entrainment by the pertinence of neuro-technology.

EEG is Electroencephalography, which is used to measure the activity of human brain. When two slightly difference pitches are played at the same time, the perceived pitch, or the base frequency, is halfway between the two tones. When a 300 Hz tone and a 310 Hz tone are played at the same time, the resulting pitch will be perceived as 305 Hz and the beats will occur at a frequency of 10 Hz. One may adjust an instrument to match the tone of a pitch pipe until the resulting beats disappear. These beats are referred to as monaural beats, because they can be heard with only one ear. However, one has a different experience when each tone is presented to each ear via stereo headphones. Beats perceived in this way are called binaural beats, the interaction of the two sounds and the perception of the beats occurs in the brain.

Bio-feedback of audio-entrainment by the pertinence of neuro-technology.

EEG is Electroencephalography, which is used to measure the activity of human brain. When two slightly difference pitches are played at the same time, the perceived pitch, or the base frequency, is halfway between the two tones. When a 300 Hz tone and a 310 Hz tone are played at the same time, the resulting pitch will be perceived as 305 Hz and the beats will occur at a frequency of 10 Hz. One may adjust an instrument to match the tone of a pitch pipe until the resulting beats disappear. These beats are referred to as monaural beats, because they can be heard with only one ear. However, one has a different experience when each tone is presented to each ear via stereo headphones. Beats perceived in this way are called binaural beats, the interaction of the two sounds and the perception of the beats occurs in the brain.

A New Harmonic Balance Approach Using Multidimensional Time

Over the past years, nonlinear frequency-domain methods have become a state-of-the-art technique for the numerical simulation of unsteady flow fields within multistage turbomachinery. Despite this success, it still remains a significant challenge to capture nonlinear interaction effects within the context of configurations with multiple fundamental frequencies. If all frequencies are integer multiples of a common fundamental frequency, the interval spanned by the sampling points typically resolves the period of the common base frequency. For configurations in which the common frequency is very low in relation to the frequencies of primary interest, many sampling points are required to resolve the highest harmonic of the common fundamental frequency and the method becomes inefficient.To overcome the issues regarding multi-frequency problems described above, a new harmonic balance approach based on multidimensional Fourier transforms in time is presented. The basic idea of the approach is that, instead of defining common sampling points in a common time period, separate time domains, one for each base frequency, are spanned and the sampling points are computed equidistantly within each base frequency's period. Since the sampling domain is now extended to a multidimensional time-domain, all time instant combinations covering the whole multidimensional domain are computed as the Cartesian product of the sampling points on the axes. In a similar fashion the frequency-domain is extended to a multidimensional frequency-domain. In this way the proposed method is capable of integrating the nonlinear coupling effects between higher harmonics of different fundamental frequencies.

A New Harmonic Balance Approach Using Multidimensional Time

Over the past years, nonlinear frequency-domain methods have become a state-of-the-art technique for the numerical simulation of unsteady flow fields within multistage turbomachinery as they are capable of fully exploiting the given spatial and temporal periodicities, as well as modelling flow nonlinearities in a computationally efficient manner. Despite this success, it still remains a significant challenge to capture nonlinear interaction effects within the context of configurations with multiple fundamental frequencies. If all frequencies are integer multiples of a common fundamental frequency, the interval spanned by the sampling points typically resolves the period of the common base frequency. For configurations in which the common frequency is very low in relation to the frequencies of primary interest, many sampling points are required to resolve the highest harmonic of the common fundamental frequency and the method becomes inefficient. In addition when a problem can no longer be described by harmonic perturbations that are integer multiples of one fundamental frequency, as it may occur in two-shaft configurations or when simulating the nonlinear interaction in the context of forced response or flutter, then the standard discrete Fourier transform is no longer suitable and the basic harmonic balance method requires extension. One possible approach is to use almost periodic Fourier transforms with equidistant or non-equidistant time sampling. However, the definition of suitable sampling points that lead to well-conditioned Fourier transform matrices and small aliasing errors is an intricate issue and far from straightforward. To overcome the issues regarding multi-frequency problems described above, a new harmonic balance approach based on multidimensional Fourier transforms in time is presented. The basic idea of the approach is that, instead of defining common sampling points in a common time period, separate time domains, one for each base frequency, are spanned and the sampling points are computed equidistantly within each base frequency’s period. Since the sampling domain is now extended to a multidimensional time-domain, all time instant combinations covering the whole multidimensional domain are computed as the Cartesian product of the sampling points on the axes. In a similar fashion the frequency-domain is extended to a multidimensional frequency-domain by the Cartesian product of the harmonics of each base frequency, so that every point defined by the Cartesian product is an integer linear combination of the occurring frequencies. In this way the proposed method is capable of fully integrating the nonlinear coupling effects between higher harmonics of different fundamental frequencies by using multidimensional discrete Fourier transforms within the harmonic balance solution procedure. The aim of this paper is to introduce the multidimensional harmonic balance method in detail and demonstrate the capability of the approach to simultaneously capture unsteady disturbances with arbitrary excitation frequencies. Therefore the well established aeroelasticity testcase standard configuration 10 in the presence of an artificial inflow disturbance, that mimics an upstream blade wake, is investigated. The crucial aspect of the proposed testcase is that a small ratio of the frequency of the inflow disturbance and the blades vibration frequency is chosen. To demonstrate the advantages of the newly proposed multidimensional harmonic balance approach, the results are compared to unsteady simulations in the time-domain and to state-of-the-art frequency-domain methods based on one-dimensional discrete Fourier transforms.