Operation of Grid-Connected Inverter under Unbalanced Grid Conditions Using Indirect Voltage Sensoring

Abstract The grid connected voltage source inverter is now the most widely used interface for connecting renewable power generation to the grid. Control of this device is a key aspect to ensure the performance, reliability and life span of the renewable power generation system. Conventionally, the current control of the grid connected inverter is based on the measured grid side voltage. The power and the power factor at the receiving end, which is usually defined as the point of common coupling, can be controlled accurately. This controller topology has been widely used and many control methods have been developed aiming at objectives such as increasing system stability, decreasing harmonic injection, and improving transient response of the system. However, in case of the voltage measurement is not available, i.e. a faulty voltage sensor, the conventional current control topology will be disabled for lack of information of the grid voltage. This would decrease the reliability and efficiency of the system thus should be improved. voltage-sensor-less In this research, a current control system for the grid connected inverter system not relying on the information provided by the a.c. side voltage sensors will be developed with compliance to the recommendations issued to the performances of the distribution generations such as the harmonic limitations and the fault-ride-through capabilities. Three problem will be addressed and solved. Firstly, the a.c. side voltage should be acquired without the use of a.c. side voltage sensors. This is achieved by adopting an a.c. side voltage estimation algorithm. Secondly, the grid connected inverter should be able to start-up without synchronising to the grid while keep the current injected in a safe range. This is achieved by the newly designed start-up process. Thirdly, the grid connected inverter should be able to ride-through grid faults and providing support to the grid. The transient response of the grid connected inverter is the key measure to define the performance. In this study, a faster symmetrical component decomposition method is proposed to improve the transient response of the current control, without relying on grid voltage sensors. The proposed system is verified by both simulation and experimental tests, with analyses and insight aiming at general applications of the proposed method and algorithms

Low Correlation Interference OFDM-NLFM Waveform Design for MIMO Radar Based on Alternating Optimization.

The OFDM chirp signal is suitable for MIMO radar applications due to its large time-bandwidth product, constant time-domain, and almost constant frequency-domain modulus. Particularly, by introducing the time-frequency structure of the non-linear frequency modulation (NLFM) signal into the design of an OFDM chirp waveform, a new OFDM-NLFM waveform with low peak auto-correlation sidelobe ratio (PASR) and peak cross-correlation ratio (PCCR) is obtained. IN-OFDM is the OFDM-NLFM waveform set currently with the lowest PASR and PCCR. Here we construct the optimization model of the OFDM-NLFM waveform set with the objective function being the maximum of the PASR and PCCR. Further, this paper proposes an OFDM-NLFM waveform set design algorithm inspired by alternating optimization. We implement the proposed algorithm by the alternate execution of two sub-algorithms. First, we keep both the sub-chirp sequence code matrix and sub-chirp rate plus and minus (PM) code matrix unchanged and use the particle swarm optimization (PSO) algorithm to obtain the optimal parameters of the NLFM signal's time-frequency structure (NLFM parameters). Next, we keep current optimal NLFM parameters unchanged, and optimize the sub-chirp sequence code matrix and sub-chirp rate PM code matrix using the block coordinate descent (BCD) algorithm. The above two sub-algorithms are alternately executed until the objective function converges to the optimal solution. The results show that the PASR and PCCR of the obtained OFDM-NLFM waveform set are about 5 dB lower than that of the IN-OFDM

Parallel filtering scheme for fast symmetrical component extraction based on asynchronous coordinate transformation

The common shortage of the existing methods for extracting symmetrical components from unbalanced and distorted three-phase voltages is that they introduce significant time delay before reaching an accurate result. Therefore, this paper presents a new filtering method to fast extract the positive sequence component based on asynchronous coordinate transformation. Unlike the conventional methods that filter out the harmonics in sequence, resulting in cascading delay, the proposed method has two signal streams operating in parallel, which in particular refer to the negative sequence component extraction and harmonic filtering, respectively, and then the positive sequence component can be easily derived by skillfully combining the results of two signal streams. In theory, the proposed method only suffers the largest delay introduced by these two streams. By carefully designing the asynchronous coordinate transformation, the delay can be restricted, thus improving the overall performance. Simulation and experimental results verified the performance of the proposed method

Nano-spheroid formation on YAG surfaces induced by single ultrafast Bessel laser pulses

International audienceWe report on single pulse ultrafast Bessel laser beam processing of YAG ceramic surfaces as a method for producing contamination-free submicron particles and size adjustable surface hemisphere structures of different curvature signs. The micro-machining process was performed in both air and liquid environments, leading to a variety of surface structures depending on the processing parameters. Particularly, a transformation of the surface structure morphology from micro-hole profiles to hemisphere extrusions was observed. The size of the hemisphere structure is found to be highly sensitive to laser parameters, such as pulse energy, pulse duration and beam focusing position. Through careful analyses of the influence of the laser pulse parameters, a precise regulation of the lateral diameter and height of the hemisphere structure was achieved. Large area hemisphere arrays with low standard deviation in size were fabricated. The detachment of the emerging structures and subsequent particle deposition can be observed in liquid environments when the height-diameter aspect ratio of the hemisphere exceeds a factor of 0.65. The Mechanisms for the formation and detaching of the hemisphere structure are discussed with cross-sectional and morphology imaging via Scanning Electron Microscopy and Atomic Force Microscopy. As a preliminary step towards submicron particle generation in liquid environments, the observation of surface hemispheres has interest in exploring the initial mechanisms of particles formation under laser ablation in liquids. The presented method allows for the fabrication of contamination-free and size adjustable YAG submicron convex structures which have potential applications in integrated optics, biotechnology and other advanced processing techniques