Liu Jianzheng

A Single-Stage Three-Phase Grid-Connected Photovoltaic System With Modified MPPT Method and Reactive Power Compensation

Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency, etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Simulation and experimental results show the high stability and high efficiency of this single-stage three-phase grid-connected PV system.

Comparison of three PWM strategies-SPWM, SVPWM & one-cycle control

Sinusoidal pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) are the most popular modulation strategies applied to variable frequency and adjusting speed systems, and there are diverges in comparing the merits and demerits of their own. Also there is another modulation strategy called one-cycle control. This paper gathers the three strategies and does some comparisons through simulation based on a simulation tool-PSIM, and has drawn some profitable conclusions.

Modified MPPT strategy applied in single-stage grid-connected photovoltaic system

Single-stage grid-connected photovoltaic (PV) system has advantages such as simple topology, low cost, etc. However, since this kind of system has only one stage of power conversion, all the control objectives need to be considered simultaneously, such as maximum power point tracking (MPPT), synchronization with the utility voltage and harmonics reduction for output current, so that the complexity of control scheme is much increased. This paper presents a modified MPPT strategy, which can remarkably improve the stability of the single-stage grid-connected PV system during rapidly changing process of light intensity, besides realizing the control objectives above. Comparing it with traditional constant-step MPPT strategy, this paper illustrates the improvement of the novel strategy based on PSIM simulation platform. A 300 Wp grid-connected PV system is also established in laboratory to verify the stability of this modified MPPT strategy, in which DSP (TMS320F2407) is employed to implement the proposed MPPT controller. Simulation and experimental results show the high stability and high efficiency of this modified strategy applied in single-stage grid-connected PV system

Implementation of a stand-alone photovoltaic lighting system with maximum power point tracking and high pressure sodium lamp

This paper presents a high-efficiency stand-alone photovoltaic (PV) lighting system which can provide functional illumination based on high-pressure sodium (HPS) lamp. HPS lamp in the system works at very high efficiency with specially designed ballast and ignitor. In order to provide a steady-state operating environment for HPS lamp, a DC-DC converter and a DC-AC inverter are also implemented. A high frequency also remarkably reduces the size and the operating noise of the ballast. Maximum power point tracking (MPPT) technique is also implemented in this system. Experimental results will show the performances of the photovoltaic lighting system.

Experiment and analysis for single IGCT equipped in MV three-level inverter

In order to ensure the reliability and dynamic characters of IGCTs equipped in MV (medium voltage) three-level inverter, the switching experiments for single IGCT in the inverter are proposed in this paper. The experiment scheme is described in detail, including the topology, key parameters and the control method. Based on the experiment results, the switching behavior for every IGCT in the inverter is compared, and the influence factors to the behavior are analyzed, such as the inverters structure, topology position of IGCTs, the stray inductance in commutating loop, etc Finally, the advantages of the experiment are summarized in the conclusions.

Insulator leakage current analysis based on time-frequency and HHT methods

Leakage current (LC) passing through the contaminated insulator surface under wet conditions was frequently analyzed by FFT, STFT, spectrogram and wavelet transform, in order to reflect the surface condition of insulators, while more or less, these methods have some limitations on nonstationary and nonlinear signals. A recently reported technique, termed HHT, was applied to provide time-frequency information about typical LC waveforms. Comparative study found HHT is more adaptive and has higher frequency resolution than other time-frequency methods, hence more suitable to study the discharge property of LC waveforms. It was shown that the decrease in the ratio between RMS and absolute maximum value of LC indicates the presence of partial arcs. According to this, this paper first separated the nonlinear LC into transient impulses and noisy signal, then employed HHT and wavelet transform to analyze each parts separately. Present study showed that wavelet transform is sensitive to high frequency impulses, and the discontinuity of fundamental harmonic together with the dramatically enlarged high frequency harmonics indicate LC gets into danger region.

Accurate flux observer based on distributed multi-processor in MV adjustable speed drive system

This paper presents an accurate flux observer used in distributed multi-processor (DMP) adjustable speed drive systems. The stator flux linkage is estimated with a full-order observer, while the rotor speed is identified by using Lyapunov theorem. In order to improve the system performance in a wide speed range, the stator resistance is estimated simultaneously. In this paper, the structure of the DMP system, the principle of the full-order observer and the method for identifying the rotor speed and state resistance are all described in details. Relevant simulation results based on MATLAB/SIMULINK are given to prove the performance of the method. Experiments of a real system also verify the feasibility of this scheme

Multifunctional directional transversal difference protection device by microcomputer

This paper describes the first successful power system device with multifunctional directional transversal difference protection in China. It realized some protection action performances which are difficult for conventional relay protection. Especially, eliminating both the voltage dead-zone and increasing action sensitivity and selectability are important improvements over conventional relay protection. By using the advantages of microcomputers, this device has created some new functions which are impossible for conventional protection, and has progressed toward the direction of multifunctionality and automation. It has been successfully in field operation in a 66 kV substation in Liaoning Province since May of 1992.<<ETX>>