Liu Hongpeng

A novel bridgeless buck-boost PFC converter

Conventional cascade buck-boost PFC (CBB-PFC) converter suffers from the high conduction loss in the input rectifier bridge. To resolve the above problem, a novel bridgeless buck-boost PFC topology is proposed in this paper. The proposed PFC converter which removes the input rectifier bridge has three conduction semiconductors at every moment. Comparing with CBB-PFC topology, the proposed topology reduces the conduction semiconductors, reduces conduction losses effectively, improves the efficiency of converter and is suitable for use in the wide input voltage range. In this paper, the average current mode control was implemented with UC3854, the theoretical analysis and design of detection circuits was presented. The experimental prototype with 400 V/600 W output and line input voltage range from 220 VAC to 380 VAC was built. Experimental results show that the proposed converter can improve 0.8% efficiency comparing CBB-PFC converter.

The maximum power point tracking based on the double index model of PV cells

A maximum power point tracking (MPPT) circuit simulation model based on the double index model is proposed in this paper. By analyzing and comparing single index model and double index model, the double index model is more accurate and tallies closely with measured data. Because groups of photovoltaic (PV) cells can be packaged into array, the array model based on double index model can be established according to the number of series/parallel chains in the array. The output characteristic of PV array varies drastically with the solar insolation and the temperature, and presents a maximum power point under given atmospheric conditions. Thereby based on the perturbation and observation method (P&O), MPPT of PV array is analyzed and realized combining with the Boost dc/dc converter. The simulation results validate the accuracy of the model and the feasibility of the algorithm. The double index model is convenient to analyze and optimize the behavior of the power conversion system in the field of PV system.

Analysis of power losses in Z-source PV grid-connected inverter

Calculating power losses of the inverter plays an important role for improving system efficiency and power density, designing heat dissipation system and selecting the power devices. Generally, power losses include the main switch losses (conducting losses and switching losses), the drive losses, control circuit losses and losses of inductor. Among them, the losses of semiconductor power switching devices are the largest losses. This paper analyzed the power losses of blocking diode, switching devices of H-bridge, Z-source inductor and filter inductor in detail, combined operating principle with modulation strategy. And the practical formula is deduced. The experimental results verified the proposed method.

Light-Induced Agglomeration and Diffusion of Different Particles with Optical Tweezers

The dynamic process of light-induced agglomeration of carbon nanotubes (CNTs), C60 and Escherichia coli (E.coli) in aqueous solutions is demonstrated using an optical tweezers system. Based on the results, the diameter of the agglomerated region and the agglomeration rate increase with the increasing laser power. After the saturation-stable period, CNTs diffuse completely, C60 clusters only diffuse partially, and E. coli never diffuses in the agglomeration region. Theoretical analyses show that the molecular polarization and thermal diffusion of particles play crucial roles in the diffusion process. The results indicate the possibility of using light to aggregate and sort nanoparticles.

Harmonic suppression strategy for voltage-controlled grid-connected inverter based on parameter optimization

The injected grid current of single-phase grid-connected inverter will be distorted when the grid voltage contain any low-order harmonics. Thus, an effective harmonic suppression method should be designed to reduce total harmonic distortion for injected grid current, a novel method of harmonic suppression is proposed to achieve a low THD. In this paper. Harmonic components of grid voltage is extracted by the fourth order band-pass filter separately and injected into the reference of inverter output voltage for compensation. The particle swarm optimization is used to optimize the parameters of double loop control and the fourth order band-pass filter for a better effect of harmonic suppression. Simulation and experimental results validate the feasibility of the proposed method.

Stability analysis of improved Y-source inverter

The improved Y-source inverter (YSI) not only inherits the advantages of YSI, such as a simple structure, high voltage gain and low operating stress, but also solves the problems that the input current is discontinuous and mitigates the interference resulting from the leakage inductance. In recent years, Y source impedance network has become the main research direction of many scholars. Up to now, steady state and DC characteristics of improved YSI have been investigated in numerous articles. However, modeling analysis for improved YSI is rarely presented. It should be noted that due to the effective role of system modeling in closed-loop controller design, the modeling of improved YSI is critical. In this paper, a small-signal model of improved YSI with LC filter based on state-space averaging method is established. The transfer functions in continuous conduction mode (CCM) are further analyzed. Finally, in order to demonstrate the effect of circuit parameter variations on the system stability of improved YSI, the placement of poles and zeros of transfer functions is analyzed.

An improved droop control strategy of three-phase inverter for grid voltage unbalance compensation

In this paper, an improved control strategy for three-phase inverter is proposed, which ensures three-phase output voltage symmetry under unbalanced grid voltage. Firstly, the positive and negative sequence separation module is used to obtain unbalance output voltage and current during the grid faults. By calculating output power of the inverter, an improved P-ƒ, Q-U and Q-G droop control strategy is designed to adjust droop curve real-timely. Then three-phase symmetrical voltage reference could be generated. In order to improve the stability of parallel three-phase inverter system, the virtual impedance is further applied. The simulation results presented verify the feasibility of the improved control scheme. Experimental tests and analysis have been provided in this paper.

A suppression method of circulating current in parallel photovoltaic system based on virtual impedance

The equivalent impedance mismatch and dead time mismatch could result in circulating current among the parallel inverters. To suppress the circulating current, a current-sharing method based on virtual impedance is added to the traditional resistive droop control in this paper. The simulation and experimental results are provided to verify the correctness and feasibility of the suppression method.

Research on a novel islanding detection technique

Inadvertent islanding causes serious problems. For these reasons islanding detection is an indispensable feature for distributed generation systems. In this paper, the non-detection zones (NDZs) of over/under voltage (OUV) and over/under frequency (OUF) method are analyzed in ΔP versus ΔQ space. And the Slip-mode frequency shift (SMS) method is discussed. The non-detection zones of the SMS in the Qf versus f0 space is analyzed. In order to overcome the disadvantage of SMS, an adaptive sliding-mode frequency shift (ASMS) method is proposed. This method sets the starting angle. When the measured frequency deviation is beyond the error range, the maximum phase angle and starting angle will be increased in the positive feedback manner. Thus, the frequency deviation at the point of common coupling is accelerated, and the time of islanding detection is shortened. The Simulation and experimental results are given to verify the theoretical analysis.