Research on reliability evaluation model for frequency collapse of power grid

In order to comprehensively consider the possibility and the severity of frequency instability risk, this paper proposes the risk evaluation model and algorithm for frequency collapse of power grid. The uncertainties in generating dispatch, network topology and component outage are considered. The calculation method for system frequency variation is depicted while considering the active power deficiency arising from random failure of generating units, and then various probabilistic risk indices for frequency collapse are defined. The presented method can provide important reference information for evaluating the frequency collapse risk of power grid. The non-sequential Monte Carlo simulation-based results of IEEE-RTS79 show the correctness of the method.     

The power-supply of X-ray machine adopting PID control and high-frequency inverter technology

To solve the problem of low control accuracy and low stability of current X-ray machines, a kind of digital power supply for X-ray machine is introduced. In the digital power supply, some key technologies are used to improve the output stability of the high frequency inverter, including improved BUCK circuit for voltage regulating circuit, high frequency inverter and soft switching technology for the high-voltage generator. The PWM control technology is used to control the switch transistor of voltage regulating circuit and inverter circuit, which makes the drive waveform output stable and easy to control. In order to improve the control accuracy, reduce the volume and change high voltage consecutively, the software PID of the microcomputer is used to control the feedback circuit. The high-accuracy power chip is used for the filament power supply to make the filament electric current stable. Through the analysis of the circuit, the parameters of the circuit are calculated and the circuit is simulated by PSPICE software. The results show that the theoretical analysis is consistent with the practical simulation results. The design can meet the requirement of the technology indices of the power supply of the X-ray machines.     

Circulating current control method for inductive power transfer system

Inductive power transfer (IPT)technology allows electrical energy to be transferred between two loosely coupled inductors over relatively large air gaps based on electromagnetic induction. In current-fed IPT systems,resonant capacitor voltage distortions and current spikes on the semiconductor switches are easily caused when switches isn’t switched at zero crossing point of resonant voltage. As a result,the EMI level of the system and the switching loss are increased and thus the system transmission stability and efficiency decrease. The article analyzes the production mechanism of circulating current in load resonant high frequency inverter and points out that the resonant frequency change is the real cause. By analyzing the characters of circulating current before and after the zero-crossing points of resonant voltage and relationship between switching frequency and resonant frequency,this paper proposes a method of real-time detection for the direction of circulating current,and regulating the switching frequency in accordance with resonant frequency dynamically to ensure the switching of switch tubes at the zero-crossing points of the resonant capacitor voltage,and thus to inhibit circulating current. An experiental system with the platform of FPGA is set up to verify the effectiveness of this control method.     

Compensation trust evaluation model of grid service

It is difficult to distinguish the cheating and other malicious behaviours in the grid service trust evaluation. A trust evaluation model for grid service based on compensation is proposed.With market mechanism, service provider and demander present the expected profit and compensate price for their behaviours to maximize their own profit. The game theory is applied to balance the expected profit and compensate price. Based on the game theory, evaluation trade strategy achieves perfect Bayesian equilibrium. Service provider and demander can choose their own trade strategy to maximize profit. Theoretical analysis and simulation results show that the service demanders can abandon the cheating motivation voluntarily, while the service providers can make right decision to choose trust evaluation. Compared with the existing model, the precision and security of the trust evaluation is enhanced greatly, while the calculating and communication cost is reduced remarkably.     

Overall control strategy and operational performances of a doubly fed wind turbine generator system

In order to comprehensively and correctly analyze the real-time operational characteristics of a grid-connected wind turbine generator system, it is necessary to analyze the overall control strategies and operation performances of the wind turbine generator system. According to the type of a doubly fed wind turbine generator system, the mathematical models of wind turbine, drive train and doubly fed induction generator (DFIG) are presented. From the viewpoints of the maximum wind energy capture and safe operation of the wind turbine, a maximum power output control strategy is proposed by considering the DFIG loss minimization, and a variable pitch control strategy is also presented by considering generator rotational speed restriction and the output power restriction. Combined with a power decoupling strategy of DFIG, the overall operational performances of the doubly fed wind turbine generator system are simulated. By comparing the simulation, theoretical analysis and actual operational data, the results show that the presented model and overall control strategies of the doubly fed wind turbine generator system are valid.     

计及电网停电风险的输电线路动态融冰决策方法

电网发生大面积冰灾情况时,多条输电线路存在覆冰故障隐患,电网稳定运行受到严重威胁,需要提前制定输电线路融冰方案,合理安排融冰顺序。针对此问题,分析输电线路覆冰继续运行给电网带来的停电风险,将与待融冰输电线路相关联的变电站的电气主接线展开,将传统削负荷模型改进后应用于输电网与变电站主接线的组合网络,建立断路器可靠性模型,计算靠后融冰线路因继续运行的电网停电风险;利用计及风速及降雨影响的输电线路覆冰增长预测模型,计算输电线路的覆冰率,根据覆冰率门槛值确定待融冰线路集,基于覆冰严重度模型构建系统全局的覆冰指数指标;综合考虑电网停电风险及系统全局覆冰指数建立输电线路融冰紧迫度指标,动态更新待融冰线路并决策融冰顺序。最后,以IEEE RBTS系统进行算例分析,验证了所提方法的可行性与合理性。     

Modeling and simulation of a grid connected wind turbine withdoubly fed induction generator

In order to fully research the effects of external excitations, such as wind speed, wind direction and voltage disturbance, on a grid connected wind turbine with doubly fed induction generator, a co-simulation model of MW-class wind turbine system is established by using Simpack and Matlab/Simulink software. Also, by considering the actual control strategies in the process of simulation, the relationship between external excitations and mechanical system is studied. According to time-domain simulation under the turbulent wind condition, the dynamic meshing forces of gears are analyzed. Besides, the vibration responses of tower are calculated under the different external excitations. Research results lay a foundation for dynamic performance optimization and reliability design of a wind turbine.     

A new control design for grid integration ofDFIG wind farm with VSC-HVDC

This paper proposes a control scheme for a doubly-fed induction generator (DFIG) wind farm with VSC-HVDC grid integration to ensure secure and reliable operation of power system with fluctuated wind power penetration. The whole system dynamic models are respectively described. Based on vector control system with a virtual voltage orientation, a steady-state voltage control block and a dynamic voltage control block with a cross-product term of d-q currents are separately designed for WFVSC to control the corresponding parts of the wind farm voltage. Meanwhile, with the consideration of AC and DC side parameter variation and external disturbance, an improved backstepping control scheme with Lyapunov stability provement is also proposed for the GSVSC. The combination of the two schemes makes up the new control design of this study. Besides, to verify the performance of the proposed new control design, three different cases are suggested which contain the AC and DC side parameter variation and external disturbance such as local load cut off-in, grid voltage harmonics. Finally, numerical simulations using MATLAB/Simulink are presented for a 200MW DFIG wind farm with VSC-HVDC grid integration to validate the proposed control scheme and demonstrate the improved system operation performance. Moreover, the corresponding discussions are also presented for further illustrations.     

刚性连体位置对非对称双塔连体结构动力可靠度的影响

考虑地震动的非平稳性,变化连体位置,对非对称双塔连体结构运用虚拟激励法进行非平稳随机激励下的动力可靠度研究。采用刚度退化的Bouc-Wen模型模拟塔楼各楼层的滞变特性,建立非线性化动力方程。运用混合精细积分法对每一时刻的响应进行求解,得到连体位置变化时非对称连体结构在非平稳随机激励下的时变方差。基于首次超越破坏准则与Markov假定,研究非平稳随机地震激励下连体结构的动力可靠度。运用上述理论,在8度罕遇地震作用下对某非对称双塔连体结构进行随机地震响应与动力可靠度分析。研究结果表明,地震作用下结构的层间位移响应呈现强烈的非平稳性,变化连体位置对连体结构的随机地震响应与动力可靠度将产生显著影响。     

Analysis of Theory and Method about the Corrosion as well as the Broken Point of the Grounding Grid

The paper studies the theory about the corrosion as well as the broken point of the grounding grid in power plants and substations. By using sensitivity analysis, failure diagnosis equation is put up through rigorous theoretical analysis. Meanwhile the problem of morbid equation is solved by introducing lowest energy theory and technique of optimiation to diagnosis equation solution of grounding grid combined with definite constraint condition and linear or non-linear objective function, the fault diagnosis equation is discussed. The results of emulation calculation and analogous experiments verify the correctness,feasibility and practicality of the theory and method.     

The method of dynamic aperture control technology based on ultrasound imaging system

Dynamic aperture control technology is one of the most important parts in ultrasonic imaging system. Excellent control technology of dynamic aperture can effectively improve the quality of ultrasonic imaging. Based on deep analysis of ultrasonic beam distribution, a control method for dynamic aperture which can eliminate the scanning dead zone of focused beam is presented. The detection region is even divided according to the number of probe array elements. The desired focused beam width is calculated between the next two scan lines at different depths, and then according to the beam width, the required aperture size for different depths is determined. The method is not affected by the probe parameters or detection objects, and it can be widely employed in ultrasonic imaging system. The simulation experiments indicate that the proposed method can effectively improve the whole quality of ultrasonic imaging.     

Simulation analysis of control strategy for electric power steering system

The mathematical model of the electric power steering (EPS) system is developed,and control strategy is designed by the way of frequency analysis using Bode diagram,on a basis of steering feel expression．A complete model used for analysis of automotive steering characteristic,with four tire and three freedom vehicle models is built．In order to meet optimal goals,including steering agility,return ability and steering effort requirements of drivers, the rule of controller parameter values under the condition of different speeds and steering torques is concluded through numerous simulation tests．     

Modeling and control of An LCL bi-directional inductive power transfer system

Since there exist high-frequency switching networks and complicated operating modes in bi-directional inductive power transfer (IPT) system, it is easy to result in high harmonics and EMI interference. An LCL filter network is introduced to effectively filter out the unwanted harmonics in track current and reduce EMI interference. Based on a mathematical model of the power transmission established by complex analysis of the AC variables, under the condition of maximum transmission efficiency, the phase angle differences between the primary input voltage and secondary output voltage U·so are ±90° and the energy is respectively transferred in forward or backward direction. Moreover, the power magnitude can be controlled by adjusting the modulus of U·pi and U·so. In addition, to keep the track current constant, different phase-shifting control strategies are designed in the primary and secondary parts. Simulation and experimental results show that these strategies can not only adjust track current to keep constant but also dynamically change the direction and magnitude of power transmission according to the working conditions and actual needs of the system load to improve the system operating efficiency.     

Analysis of reasonable wind farm energy storage capacity range

The wind farm output power fluctuations caused by changing wind speed can be improved by configuring the appropriate storage energy capacity .For the problem that how to select the storage capacity more economical, two indicators for the judgment of the optimization wind power output are mentioned, which is based on a method calculating storage energy capacity. By detailed analysis of the all kinds of factors that can influence the reasonable value of storage capacity, and by using the cost of storage capacity and the smoothing effect of wind farm output as assistant criterion, a reasonable storage capacity range is discussed.     

Comparative study of low voltage ride-through techniques for doubly-fed induction generator wind turbines under asymmetrical grid faults

Asymmetrical grid faults occur more frequently and have more adverse effects on the doubly fed induction generator (DFIG) than symmetrical grid faults in the transmission system. The transient response of DFIG under asymmetrical grid faults is analyzed firstly. Meanwhile, operation behaviors of several low voltage ride-through (LVRT) techniques under severe asymmetrical grid faults are given by simulation with the Matlab/Simulink software. Then the characteristics of these LVRT techniques are further researched and analyzed based on the simulation results. Finally, the economies of these LVRT techniques are discussed. The conclusion lays a certain foundation for engineering development of these LVRT techniques.     

二维高炉区域BFC网格生成技术及其Matlab实现

利用数学模型对高炉冶炼过程进行模拟是高炉炼铁新工艺研发的有效方法,网格生成技术是数值模拟过程中重要的前处理过程,是高炉模拟计算的先决条件。生成网格的质量对高炉模型模拟的精度、效率以及收敛性具有重要影响,因此,建立优质的网格对高炉数学模型的求解具有重要意义。文中提出了一种适用于高炉数学模型的适体坐标系(BFC)网格的生成方法,从求解区域的划分、椭圆型方程的转换、椭圆型方程的离散及BFC网格生成步骤等方面进行了研究,并把死料区的边界作为BFC网格计算的边界条件,使数学模型的求解过程得以简化。采用带有源项的泊松方程作为变换方程,网格的正交性和疏密程度便于控制。该网格生成算法原理简单、易于编程、网格生成效率高,生成的网格能够满足数学模型求解的要求。     

高充电效率的恒流太阳能路灯控制器

针对传统太阳能路灯控制器主要存在充电效率低、恒流精度低的问题,提出基于多路脉冲宽度调制(PWM)恒流技术的新型太阳能路灯控制器的设计方案。该控制器采用复合式DC-DC变换器,通过控制多路PWM信号,实现高效地充放电功能。充电过程以改进的扰动观察法实现最大功率点跟踪(MPPT),提高充电效率;放电过程通过实时检测LED路灯的实际电流,动态调整相应PWM信号的占空比实现高精度恒流驱动LED路灯。实物测试结果表明该控制器能有效地改善充电效率和恒流源精度,充电速度提高了4%~7%,恒流源精度高达2.5%。     

Performance analysis of a concentrating solar hybrid photovoltaic thermal system

The photovoltaic components of a hybrid photovoltaic thermal (PV/T) system are combined with a solar thermal collector, which generates both thermal and electrical energy simultaneously with a higher integrated efficiency. A three dimensional steady model of the concentrating solar PV/T system which has a plate type metallic thermal collector with a serpentine rectangular channel is developed. The processes of photovoltaic and thermal transformation and heat transfer in the PV/T collector are calculated and simulated. The effects of the optical concentrating ratio, coolant mass flow rate, ambient air velocity and glass cover on both thermal and electrical performance of the collector are presented and discussed.     

Research on the improvement of the convergencespeed for CDMA system power control

Optimal power control with game-theory has been a hot research topic in code division multiple access (CDMA) system. However, the convergence speed of the power control algorithm based on traditional static game-theory is not fast enough and the power fluctuation is large during the iterative process. In order to solve this problem, dynamic game-theory is introduced into CDMA system power control algorithm. The simulation results show that the proposed algorithm, compared with the distributed power control algorithm based on the traditional static game theory, can improve the convergence speed obviously with the same power spending.     

An improved rotor side control strategy of double fedwind turbine generation system

Based on a on site wind turbine’s output active power vs. generator speed curve, the shortage of conventional power control strategies of doubly fed wind turbine generation system are analyzed. Considering the generator speed restriction and active power fast track control requirement, the power decoupling control strategy in the rotor side converter and variable pitch control strategy are improved, that is, an output mechanical torque of wind turbine and the generator speed of DFIG are used in the rotor side active power control loop in order to achieve an accurate optimal speed control, as well as a direct active power control method is also adopted in the variable pitch control strategy. Finally, the overall operational performances of DFIG wind turbine generation system using the proposed control strategies are simulated, and also compared with the conventional control strategies. The results show that the improved control strategy is more valid and correct.