Applied Mechanics and Materials Vols. 446-447

Abstract: It is necessary that the grid-connected inverter was set up in wind generation system (WGS), in order to convey energy generated in WGS to the electrical grid. There are many sorts of inverters, but only two sorts of inverters suit wind generation system, which are current-source and voltage-source inverter, and only the latter is being widely used. The configuration, principle, control and characteristic of the two inverters were analyzed. Results have shown that, voltage-source grid-connected inverter has been the core of the commutation control configuration of direct-driven WGS, which is decided by the peculiarity of itself.

Abstract: As a solid oxide fuel cell (SOFC) system is under dynamic development for a profitable use in the future, it is very important to understand its transient characteristics. In this paper, the modeling and simulation of an energy integrated SOFC system has been performed to analyze the dynamic characteristics of the heat integration. The lumped models of the energy integrated SOFC system are developed and simulated to present the results of open loop response of the temperatures of SOFC, steam reformer and air inlet stream. The simulation results showed that the air flowrate has a dominant effect on the temperature of SOFC while the valve position and the heater power supply noticeably affect the reformer temperature and air inlet temperature, respectively. Therefore, the match of major influence of input step change on the temperatures of SOFC, steam reformer and air inlet stream in the energy integrated system is very practical for an effective control strategy in the development of control system design.

Abstract: In order to reveal the commutation failure process and the reason for DC differential protection (87DCM) unwanted action of Yunnan-Guangdong UHVDC in 9.12^th 2012, and to lessen the unnecessary outage and guarantee a safe and stable operation, the 9.12 outage is deeply studied, including commutation failure phenomena and equivalent circuit analysis, high voltage DC current, low voltage DC current and their differential current analysis and reason for 87DCM unwanted action, and the final conclusions come out that: (1) 9.12 commutation failure includes V1 failing to communicate to V3 of Y bridge and V2 failing to communicate to V4 of D bridge; (2) during commutation failure process, high voltage DC current and low voltage DC current all raise rapidly and both contain DC component, AC component and no-circle rising component, and theoretically high voltage DC current equals to low voltage DC current; (3) failure in low pass filter circuit of pole 1 low voltage CT leads to measuring differential current between high voltage and low voltage DC current, and finally leads to unwanted action of 87DCM.

Abstract: A solid oxide fuel cell (SOFC) is widely recognized to be an attractive energy conversion device owing to its fuel flexibility and high efficiency. Hydrogen-rich stream produced by fuel processor, especially steam reforming process, is used as fuel carrier converting to generate electrical energy. This paper presents a steady state modeling of SOFC system with an integrated steam reformer fueled by ethanol. The main objective of this study is to analyze the steady state characteristics of this system. The basic operating principle of SOFC is briefly introduced and the steam reformer converting ethanol as the fuel source into hydrogen-rich gas is also discussed. Based on thermodynamic analysis, the equilibrium compositions of produced gas fed into SOFC can be predicted by using the method of Gibbs free energy minimization. The effects of main operating parameters of steam reformer including the temperature and water-to-ethanol molar ratio on the SOFC power generation are investigated. In the SOFC operation, an increase of SOFC temperature causes a decrease of cell voltage and an increase of excess air entering a fuel cell to maintain the adiabatic temperature of SOFC stack. The steady state conditions for the SOFC and ethanol steam reformer systems are summarized in this paper as well.

Abstract: This paper designed a large-capacity Li-ion battery formation system, and illustrated its composition and working principle. The formation system uses a programmable voltage-limiting and constant-current control circuit with voltage-limiting protection function to prevent Li-ion battery over-charge and over-discharge. The system consists of a upper computer (server) and a number of lower computers (formation units) linked with 485 bus. The server can monitor formation process and record formation data in real-time. By testing, the system has good characteristics, such as large charge and discharge current, high control accuracy, multiple-step programming, etc., and it can meet the needs of a large-scale power Li-ion battery formation.

Abstract: It is quite convenient and flexible to adopt the equivalent enthalpy drop method, in order to analyze a thermal system. In this report, a 330MW cogeneration unit will be chosen, as a case in point, to provide quantitative calculation and qualitative analysis in both global and local aspects. According to our research, this cogeneration unit is designed to operate in a double pumping status. It has such properties as follows: Under 85%BMCR double pumping condition, its highest efficiency of power generation is obtained. In pure condensing conditions with around 70% load, the steam consumption rate reaches minimal level. The reason for this phenomenon lies in the maximal net equivalent enthalpy drop of new steam, although with 70% load, the power generation efficiency is much lower than that in rated pure condensing conditions.

Abstract: One dimensional steady state mathematical model has been developed for study the thermal behavior of two types of flat-plate solar air heating collectors. The model allows demonstration the influence of the collector's dimension leading to optimize the length of the collector. The model implemented numerically using finite-difference technique with mass flow rate of 0.01 kg/s per unit area of solar collector and the incident solar radiation equal to 900W/m². The analysis has been done for a large range of areas. Illustrative results such as the temperatures of the components of the collectors (glass cover, absorber plate, air flowing through the collector and the rear plate), efficiency and useful heat extracted from the collector are graphically presented. The obtained results from both the energetic and economic analysis showed that, for the first type of the flat-plate solar air heating collector, the optimum length is 4.05 meters, moreover, it dawdles its energy collected past for lengths greater than 6 meters. There is no optimum length for the second type, but there is no much energy gain for lengths greater than 6 meters anyhow, the economic analysis showed that the length must be greater than 2.5 meters for both types of collectors.

Abstract: Analytic Hierarchy Process (AHP) is presented in this paper to measure the relative criticality for transmission lines, considering the changes of power system’s status about transmission lines overload, central buses voltage over-limit and transient stability. At first hierarchy structure is modeled, and then indexes of criteria Layer including transmission line overload index, central buses voltage over-limit index and transient stability index were presented. The simulation results show that, the AHP assessment method can give a full consideration to transmission lines’ function, which of huge significance for identifying critical lines and increasing the security of power system.

Abstract: This project is to investigate the counter electrode material from two different carbon sources to fabricate dye sensitized solar cell (DSSC). The carbon sources are extracted from battery and pencil lead. The method to prepare the DSSC is through the conventional Dr.Blading method. The same method is also used for the counter electrode which uses carbon from recycle batteries, the carbon from pencil lead are scribbled onto the ITO glass to get a uniform coating. Both thickness of the counter electrode vary accordingly. The solar cells are then placed under outdoor solar irradiation and the output is taken every 10 minutes. Based on observation, the solar cells which have the carbon from batteries shows higher cell efficiency which is 8.2 % with lower FF of 0.78, compared to by using the pencil lead, the cell efficiency is only 7.23% but with a higher FF of 0.93.

Abstract: Distribution transformer constitutes a significant asset for any distribution power system. Short circuit currents or forces can cause mechanical faults in transformer winding. In order to discover the hidden trouble of the transformer, in this paper, on-line monitoring system based on FRA method is developed, and signal source, signal injection, system software are introduced. Furthermore, a nanosecond impulse signal generator is used to inject impulse signals through a noninvasive capacitive sensor, to realize on-line monitoring for winding deformation of distribution transformer. Finally, the monitoring system is validated by a corresponding experimental study.