This paper focuses on the problem of frequency estimation of noise-contaminated sinusoidal. A basic tool to solve this problem is the interpolated discrete Fourier transform (DFT) algorithms, in which the influences of the spectral leakage from negative frequency are often neglected, resulting in significant errors in estimation when the signals contained small cycles. In this paper, analytic expressions of the interference due to the image component are derived and its influences on the traditional two-point interpolated DFT algorithms are analyzed. Based on the achieved expressions, the interpolated DFT algorithms are generalized and a novel frequency estimator with high image component interference rejection is proposed. Simulation results show that the frequency errors returned by the new algorithm are very small even though only one or two cycles are obtained. Comparative studies indicate that the new algorithm also has a good performance in the noise condition. With the advantages of high precision and strong robustness against additive noise, the proposed algorithm is a good choice for frequency estimation when the negative frequency interference is the dominant error source.
This paper investigates the design and implementation of an improved series-parallel inductor-capacitor-inductor (LCL) resonant circuit power supply for excitation of electromagnetic acoustic transducers (EMATs). The main advantage of the proposed resonant circuit is the absence of a high-permeability dynamic transformer. A high-frequency pulsating voltage gain can be achieved through a double resonance phenomenon. Both resonant tailing behavior and higher harmonics are suppressed by the improved resonant circuit, which also contributes to the generation of ultrasonic waves. Additionally, the proposed circuit can realize impedance matching and can also optimize the transduction efficiency. The complete design and implementation procedure for the power supply is described and has been validated by implementation of the proposed power supply to drive a portable EMAT. The circuit simulation results show close agreement with the experimental results and thus confirm the validity of the proposed topology. The proposed circuit is suitable for use as a portable EMAT excitation power supply that is fed by a low-voltage source.
This study proposes a simple mixed droop-v/f control strategy for the master inverter of a microgrid to achieve seamless mode transfer between grid-connected and autonomous islanding modes. The proposed control utilises (i) a modified droop control in grid-connected mode to achieve seamless transfer from grid-connected mode to islanding mode in case of intentional and unintentional islanding events, and (ii) v/f control in islanding mode, which is realised by simply reducing the droop coefficient to zero gradually after islanding, to provide voltage and frequency references to the slave inverters. Presynchronisation on the voltage and frequency between microgrid and utility grid is used for the reconnection of them in a seamless manner. The rationale and detailed design of the system with the proposed control strategy are elaborated, after which its effectiveness is proved by simulations and experiments.
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