Tao Zheng

A novel adaptive loss of excitation protection criterion based on steady-state stability limit

Among Loss-Of-Excitation (LOE) protections of synchronous generators, the impedance boundary criterion of steady-state stability limit (SSSL) is widely used to distinguish LOE from non-LOE conditions. But this criterion is sensitive to the variation of output active power of synchronous generator, and may mal-operate in external faults, power-swing and other non-LOE conditions. In this paper, a novel adaptive LOE protection criterion with an offset mho element based on SSSL is presented. The new criterion could automatically adapt to the system operating modes and correctly operate in LOE. Combine this criterion with some measures which prevent mal-operation in non-LOE conditions, a new LOE protection scheme is proposed. Simulation results demonstrated the effectiveness of the proposed adaptive LOE protection criterion and scheme.

Research on a mal-operation case of differential protection in UHV transformer

According to a field case on the mal-operation of transformer differential protection in the ultra-high voltage (UHV) experiment project in China, a new scheme of second harmonic blocking criterion is proposed in this paper. Based on the simulation model of UHV transformer established in MATLAB/SIMULINK, the causes of mal-operation is analyzed. The factors such as initial angle and remnant flux which could affect the attribute of inrush are studied. However, besides these possible factors, the special structure of UHV transformer may also result in mal-operation. The maloperation case indicates that the reliability of differential protection for voltage-regulating transformer is low. To avoid needless trip by inrush current, a comprehensive restraint scheme is proposed, which is based on a 2-out-of-3 method and takes the tendency of second harmonic ratio (2ndHR) as an auxiliary criteria to identify inrush. In addition, the reliability and selectivity of the proposed method is verified by the field recorded data and simulation tests respectively.

Analysis of the NDZ formulation theory of active frequency shift islanding detection method for grid-connected PV system

Active frequency drift islanding detection method is the most widely used in the distributed generator islanding detection. Now the domestic researches of islanding detection in China are base on algorithm Improvement but without very effective, because the fundamental reasons about the NDZ have not been researched and explained deeply. Therefore, the paper analyses the performances of the frequency drift islanding method from different aspects and deeply explains the reasons conducting the NDZ of the islanding detection from the aspects of load frequency characteristic and SFS method phase-frequency characteristic, which are the major factors affecting the phase-frequency characteristic of the islanding system.

An adaptive protection to single-line-to-ground fault in teed lines with wind power based on measuring impedance characteristic

With the development of wind power and the increase of grid density, it is a common style for wind power to be connected to the grid with T-type. The setting and configuration of protection is very difficult due to fault characteristics of wind power system and teed lines topology. Distance protection is an important research topic in teed lines with wind power. The adaptability of distance protection in teed lines with wind power is firstly analyzed. Then, a mathematical model of teed lines measuring impedance is built and its key influence factors which include wind power teed distance, positive sequence impedance of wind power, fault resistance and the relationship between the three side electromotive forces are extracted. Next, an adaptive pilot distance protection scheme based on measuring impedance characteristic is proposed. Finally, simulation is carried out and the research results verify the reliability and effectiveness of proposed protection scheme.

Configuration of the Wind Farm Reactive Power Compensation

This paper analyzes the influencing factors to the wind farm’s capacity of reactive power compensation, providing the reactive power calculation formulas of transformer, aerial cables and two main wind turbines, and estimates reactive power of different types of wind farms. Comparing four common reactive power compensation devices used in wind farms, the selection of device and method of the wind farm reactive power compensation are recommended, and its developing trend is also pointed out.

Diffusion Bonding of Mg/Cu Alloy with Zn Interlayer Metal

AZ31B Mg alloy and Cu were joined through diffusion bonding using a Zn interlayer film prepared. The element distribution, microstructure and hardness was studied by means of scanning electron microscope(SEM), Energy Dispersive Spectrdmeter (EDS) and Microhardness tester.The result show that SEM results indicate the diffusion zone of Mg/Cu diffusion bonded joint consists of intermetallic compounds. Via enhanced holding time,these intermetallic compounds go steady gradually. Thus the Mg/Cu diffusion bonded joint of strength can be improved. This work provides a theoretical reference and practical experience for Mg/Cu dissimilar metal connection.

The Analysis of the Performance of Active Frequency Drift Islanding Method

Active frequency drift islanding detection method is one of most widely used in the distributed generator islanding detection. Now domestic researches of islanding detection have not been done deep researched about the reason why islanding detection method will become invalid. Therefore, the paper analyses the performances of the frequency drift islanding method from load phase frequency characteristic and algorithm phase frequency characteristic, which are the major factors affecting the effectiveness of islanding detection method. At last, this paper proved the analysis results with MATLAB.