Zhu Yongqiang

Fault diagnosis method and simulation analysis for photovoltaic array

Based on the distinguishing characteristics of the output current and voltage for photovoltaic array working in different states, a novel voltage and current detection method was proposed in this paper compared to the original current and voltage method. Of this method, the fault position can be located precisely and quickly by analyzing the current and voltage variation of each branch, according to the characteristic that the output current as well as voltage of the fault branch is less than the normal ones of the same number modules. The feasibility of this novel method is proved by the simulation results given under the Matlab/Simulink environment.

Comprehensive evaluation of power quality based on meaningful classification

In a liberalized electricity market, it is not surprising that different customers need different power quality levels with different price. So how to achieve the synthetic grading evaluation of power quality becomes a concerned problem. In this paper, the idea of classified evaluation of power quality is emphasized, and the major differences between the continuous and event-based power quality problems both in the characteristics and time scales are analyzed. In addition, least squares support vector machine (LSSVM) theory is applied to the comprehensive evaluation of continuous power quality. Meanwhile, power supply reliability index is introduced to evaluate the overall quality level of the event-based power quality. On this basis, subjective weighting method is used to achieve the comprehensive evaluation of two types of power quality problems. Comprehensive evaluation of power quality based on meaningful classification not only highlights the two types of power quality problems, but also achieves the overall level of power quality.

Study of Nonlinear Model of Shunt Reactor in 1000kV AC Transmission System

Modeling of accurate nonlinear shunt reactor is basis of harmonic analysis of 1000kV AC transmission system. Based on the electromagnetic relations of single-phase shunt reactor and the effect of iron core nonlinearity, a new model of single-phase shunt reactor used for harmonic analysis and computer simulation is presented. It presented shunt reactor by a simple equivalent circuit consisting of an impedance representing the leakage flux losses in series with a parallel combination of a nonlinear inductance (accounting for the saturation of the iron core) and a nonlinear resistance (accounting for the iron loss).The nonlinear impedance is calculated by the and curves which had been used for modeling of transformer excitation impedance. The result of simulation in PSCAD/EMTDC using the parameters of shunt reactor in Chinas UHV AC Pilot Project have proved that this model accurately reflect the nonlinear characteristics of shunt reactor, which is the prerequisite to complete the simulation model of UHV AC transmission system and analyze the harmonic characteristic of the system.

A Stages Compensation and Control Strategy for Series Power-Quality Regulator

A novel stages compensation and control strategy suitable for a series power-quality regulator (SPQR) based on a voltage-source converter (VSC) inverter is proposed in which steady operation stage, overrun operation stage, and recovery operation stage are included. The presag voltage compensation, minimum energy compensation, and maximum energy compensation strategies are adopted to the three stages, respectively, based on the characteristics of the three stages. The double closed-loop control with feedbacks from load voltage and filter capacitor current is used in each stage. Moreover, the dc capacitor voltage regulation control for SPQR is applied in the steady operation stage. To avoid switching disorder during the stage change, a criterion based on voltage magnitude of the dc capacitor and source side is proposed. Simulation results in EMTDC/PSCAD show that voltage disturbances of the steady and transient state can be compensated by using the proposed compensation and control strategy and, at the same time, the dc capacitor voltage can be stabilized. The proposed strategy makes it possible to use the dc capacitor as the energy source of SPQR with the merit of a simple main circuit topology.

Evaluation system for reliability of grid-connected wind farms based on fuzzy analytic hierarchy process

With wind power develops rapidly, there are lots of problems appear when wind power connects grid, of them, the most important thing is reliability. The paper proposes a tree-like four-story evaluation system, which uses fuzzy-AHP as evaluation method. All these form a complete reliability evaluation system in theory. The evaluation system has advantage, which considers the economy as an important factor affecting the reliability. All these meet the market requirements. Also, the paper makes evaluation example to demonstrate the application process, so that research has a certain theoretical level and practical value. These evaluation data and analysis results provide an important reference to study economic and true value of wind power. They can be used as an important reference for wind farm investors, as well as some help for the design of wind farms. Besides, they not only run certain significance for the wind farm planning, but also are the basis for the Government to formulate policies and the power companies to determine electricity price. Lastly, they make greatly contribution for wind power to healthy development.

New Idea of Comprehensive Evaluation of Power Quality

The idea of meaningful classification of power quality is proposed, the definitions and physical meanings of both continuous and event-based power quality problems are given, and the major differences between them in both the characteristics and time scale are analyzed. In addition, two-level evaluation of power quality based on meaningful classification is presented in the paper. It not only retains the physical meanings of two types of power quality problems, but also achieves the overall evaluation of power quality, and it should be an important innovation.

Economic loss evaluation and selective treatment of power quality

The importance and necessity of economic loss evaluation of power quality is discussed in the paper. In order to scientifically evaluate the economic loss of power quality indexes, the idea of meaningful classification of power quality problems is emphasized and the quality loss function is introduced into the evaluation models. According to the characteristics of continuous power quality problems, all indexes are divided into two intervals, namely qualified and unqualified, and evaluation methods based on quality loss function are proposed to calculate the economic loss caused by the exceeding indexes. In addition, effects of event-based power quality problems are fully considered, and their direct and potential economic loss evaluation models are determined according to their influence. On this basis, the concept of selective treatment of power quality is proposed, which solves the balance problem between technical and economic efficiency. It has highly practical operability and a light application prospect.

The Research of Shunt Active Power Filter Compensation Strategy

The high capacity active filter have the problem of high cost, high power loss, strong electromagnetic interference and so on, the compensation capacity of the device using in practical is limited, how to choose the right strategy to achieve optimal compensation plays an important role in the process of the device using in practical. This paper discussed strategy of shunt active filter compensation, and combined the choice of compensation strategy of active filter with the compensation ability of devices, and discussed superiority and inferiority of the compensation strategy in two situations, one is that the compensation capacity can meet the needs of compensation, and the other is that the compensation capacity is less than the compensation. And this paper suggested that using genetic algorithm in the active filter compensation strategy to optimize the distribution of the compensation capacity of each harmonic, and through the simulation, verified that the method can insure that the filtering effect achieve optimal results in a limited compensation capacity.

Simulation and Harmonic Analysis on Ultra High Voltage AC Transmission System

Against the background of 1000kV AC Pilot Project from Jindongnan to Jingmen via Nanyang which has been completed in January 2009, based on accurate modeling of the nonlinear equipments such as autotransformer and shunt reactor in 1000kV system, the harmonic characteristics of 1000kV system are calculated and analyzed in PSCAD/EMTDC. Simulation results show that for the 1000kV system, the total harmonic distortion of voltage (THDu) is higher, in which 3rd and 5th harmonics are the main components and change with the variation of places. The reason why the THDu of 1000kV system is higher is mainly that the permeation of harmonic voltage in 500kV system into 1000kV system is stronger, and the particularity of harmonic impedance and transmission characteristic of 1000kV system will amplify certain order harmonic voltages or currents, which further raises the harmonic level of 1000kV system.

Analysis of impact of DG on LVDC distribution network protection

Compared with traditional AC distribution network, DC distribution network, which stands for future distribution network is suitable for distribution generation (DG) to multiple access. DG access changes the structure of low-voltage DC (LVDC) distribution network, affecting the reliability of relay protection. To eliminate the influence, a protection strategy which is based on Hall DC transformer is proposed. First, study about the effect of DG access on relay protection when DG is connected is discussed in detail for different capacities and locations. Then analysis shows that the integration of DG will cause rejection or malfunction, which may threaten the reliability of LVDC distribution system. Finally, the analysis shows that the optimization strategy based on Hall DC transformer can eliminate the effect by changing operation conditions.