Harmonic

About: Harmonic is a research topic. Over the lifetime, 44833 publications have been published within this topic receiving 495922 citations. The topic is also known as: overtone & partial.

Optical wavelength converting apparatus

Abstract: In an optical wavelength converting apparatus, a fundamental wave impinges upon a crystal of a nonlinear optical material, the type II of phase matching between the fundamental wave and its second harmonic is effected, and the second harmonic of the fundamental wave is thereby radiated out of the optical wavelength converting apparatus. Two crystals constituted of the same material are employed as the crystal. The two crystals have equal lengths and are located in orientations such that corresponding optic axes may be shifted 90° from each other. The optical wavelength converting apparatus yields the second harmonic having the maximum possible output power and yet can be kept small in size and low in cost.

Practical approaches for AC system harmonic impedance measurements

Abstract: AC supply system harmonic impedance measurements are discussed. On the basis of a given and known switchable shunt impedance and/or nonlinear load and site measurements, three methods are presented. The first approach assumes a no-load distorted busbar and uses basic circuit theory to calculate the required harmonic impedance. The second method disregards any previous busbar distortion and the harmonic injection but it takes into account any busbar distortion prior to the nonlinear load connection. Using a laboratory model and by comparing harmonic impedance results it is shown that both no-load busbar distortions and the correct measurement of harmonic phase angles may strongly affect the accuracy of calculation. >

PMSM Sensorless Control by Injecting HF Pulsating Carrier Signal Into ABC Frame

Abstract: Spatial-saliency-tracking-based high-frequency (HF) pulsating carrier injection method is widely employed in permanent-magnet synchronous machines sensorless control. In order to avoid the problems of long convergence time, potential start-up failure and limited system stability existing in tracking observers, a strategy to inject three HF pulsating voltages with different frequencies and amplitudes into ABC frame is proposed in this paper. With the proposed method, three frequency components in the response current signals are demodulated and then combined together to calculate the rotor position directly. Different from the conventional method, the proposed solution utilizes both d- and q-axis carrier current instead of only the q-axis carrier current to enhance the acquisition of the rotor position information. Meanwhile, a new signal processing strategy is developed to completely eliminate the effects of system delay. Furthermore, the compensation of cross-saturation effect is deduced in detail to improve the accuracy of rotor position estimation. In addition, magnetic polarity detection is implemented with the same proposed solution by demodulating the second harmonic components from the response current signals, which simplifies the magnetic polarity process. Finally, the experimental results demonstrate that the proposed strategy can obtain an accurate rotor position with good steady-state and dynamic performance.

An Adaptive Virtual Impedance Control Scheme Based on Small-AC-Signal Injection for Unbalanced and Harmonic Power Sharing in Islanded Microgrids

Abstract: To address the unbalanced and harmonic power sharing issue among parallel inverters caused by feeder impedance mismatch in islanded microgrids, an adaptive virtual impedance control method is proposed based on the injection of an extra small ac signal (SACS) in the output voltage of each inverter. Similar to the principle of active power–frequency droop, the frequency of the injected signal droops with the output unbalanced and harmonic power, while the active power produced by the injected SACS is detected to adjust the virtual impedance at the fundamental negative sequence and selected harmonic frequencies, which will tune the distribution of unbalanced and harmonic power in the system. When the injected SACSs of each inverter synchronize with each other and reach a common frequency in steady state, the virtual impedance of each inverter will be matched to each other for evenly sharing the unbalanced and harmonic power. This proposed method requires neither communication links among parallel inverters nor feeder impedance information. Furthermore, the control parameter design method based on modeling and stability analysis of the proposed control structure is discussed in detail. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed scheme.

Stator Interturn Fault Detection of Synchronous Machines Using Field Current and Rotor Search-Coil Voltage Signature Analysis

Abstract: Our recent observations suggested that harmonics in the field current are very promising to detect stator interturn faults in synchronous machines. So far, an increase in some of the even harmonics in the field current has been reported to detect such faults. However, no explanation has been provided for the cause of these harmonics. Moreover, the even harmonics can significantly increase with supply unbalance as well as time harmonics, which can lead to a serious confusion. Hence, in this study, an in-depth investigation was conducted to determine the origin of various harmonic components in the field current and their feasibility to detect stator faults. It was found that, owing to structural asymmetries of the field winding, some of these components clearly increased with stator interturn fault. The findings are helpful to detect faults involving few turns without ambiguity, in spite of the presence of supply unbalance and time harmonics. Both simulation and experimental results are presented in this paper. The diagnosis results have also been verified using a rotor-mounted search coil, which can also be used to detect even a one-turn stator fault very effectively.