An approach for the integration of renewable distributed generation in hybrid DC/AC microgrids

This paper presents an investigation of a hybrid DC/AC integration paradigm to establish microgrids (MGs) by using a conventional three-phase local power delivery system. This approach adds an additional DC power line to the local power distribution system in order to collect energy generated by distributed domestic renewable sources. The local renewable distributed generation (DG) works in conjunction with the conventional grid utility to reduce the power draw from the grid. Researchers designed an energy conversion station to mix energy from the local DGs with energy from the grid utility. This approach, therefore, uses a continuous energy mixing strategy for DC integration of local generation and grid energy to supply energy to MG consumers via the conventional three-phase power distribution system. Thus, local distributed renewable generators do not have to contend with AC integration problems, such as AC stability and line synchronization. This approach can facilitate the transformation of conventional local power distribution systems into reliable MGs in an affordable way for stakeholders and it is a step towards construction of future smart grids.     

一种关于电弧炉调节器的新思路

在电弧炉电极升降调节器中,通过对三相电流和电压瞬时值的检测,就可计算出系统的有功功率、无功功率和功率因数.瞬时功率理论为今后电极升降调节器理论的发展提供了新的理论基础.     

交直流线路并行架设时的电磁环境横向分布监测

随着电网的不断建设,交直流输电线路同走廊并行架设已在多处出现。目前,交直流混合电磁环境监测多以单点离散测量为主,无法实现横向分布的连续实时测量。通过建立工频电场、无线电干扰、合成电场等测量设备沿同一路径的分布式实时数据采集系统,实现交直流并行线路电磁环境移动式横向分布测量及数据的动态管理,并以典型合成电场数据为例,研究了垂直于线路走廊155m的横向断面合成电场分布趋势。结果表明,实测数据与仿真计算结果趋势较吻合,为交直流并行线路合成电场研究提供了参考。     

A novel exact and universal approach for calculating the differential leakage related to harmonic waves in AC electric motors

In this paper, the distribution pattern of harmonic waves in AC electric motors is investigated. On the basis of this, a novel exact and universal approach for calculating differential leakage related to the harmonic waves was developed. This approach applies to any AC electric motor, regardless of winding type, single-phase or three-phase, regular or nonregular, integral-slot or fractional-slot, symmetrical or asymmetrical. Since this novel approach substitutes the approximation method adopted by conventional approaches with an exact one, it distinguishes itself by offering precise results, general applicability, and fast calculation.     

Sensorless operation of surface mount permanent magnet AC (PMAC)motors

A sensorless field oriented control scheme for a surface mount permanent magnet AC (PMAC) motor with split phase stator windings is presented. This motor is obtained by splitting the phase windings of a conventional three phase motor. The six-phase motor, however is run as a three-phase motor by connecting the split phase stator windings in series, while the taps are made available for voltage measurements. By measuring the terminal voltages and the line currents, absolute position of the permanent magnet AC motor driven by a current regulated PWM inverter with a hysteresis controller is estimated. The estimated position information is independent of the stator resistance, thus this scheme is even applicable at low speeds. Results are presented to show the effectiveness of the new controller, and it is also shown that the position error is negligible     

Triumph of AC. 2. The battle of the currents

This is the second part of a two-part article on the early development of electric power systems and the competition between advocates of direct current (DC) and of alternating current (AC) for the generation, transmission, distribution, and utilization of electrical energy. The first part discussed the invention of the practical incandescent light; the development, chiefly by Thomas Edison, of low-voltage DC power systems; and the work of Nicola Tesla, William Stanley, George Westinghouse, and others in Europe and in North America that led to the advent of AC power systems. This second part of the article discusses what became known as "the battle of the currents.".     

Analysis on the fault characteristics of three-phase short-circuit for half-wavelength AC transmission lines

Half-wavelength AC transmission (HWACT) is an ultra-long distance AC transmission technology, whose electrical distance is close to half-wavelength at the system power frequency. It is very important for the construction and operation of HWACT to analyze its fault features and corresponding protection technology. In this paper, the steady-state voltage and current characteristics of the bus bar and fault point and the steady-state overvoltage distribution along the line will be analyzed when a three-phase symmetrical short-circuit fault occurs on an HWACT line. On this basis, the three-phase fault characteristics for longer transmission lines are also studied.     

Monocyclic power: a novel but short-lived power distribution system

This paper provides a brief history of the monocyclic power system, which was rendered obsolete because of its technical complexity and by the development of the three-phase power distribution system. Primarily developed for the General Electric Company as a means of avoiding possible infringement of the Tesla patents, the monocyclic power system was decidedly inferior to the power transmission efficiency of the three-phase system, which eventually became universal for power delivery purposes. Nevertheless, the Steinmetz monocyclic circuit, which converts constant voltage into constant current and vice versa, remains a basic electrical circuit concept that may still have applications in various types of today's electronic circuitry.     

PWM整流器在变速恒频风力发电系统中的应用

变速恒频风力发电系统要求励磁电源有良好的输入、输出特性和能量双向流动的能力.交-直-交双PWM变频器是理想的励磁电源,而PWM整流器是其中一项关键技术.文章从三相电压型PWM整流器主电路拓扑结构出发,建立了基于三相静止坐标系和两相同步旋转坐标系的数学模型.阐述了电压、电流双闭环控制的基本原理和空间矢量脉宽调制技术在三相PWM整流器中的应用.在Matlab/Simulink环境下,对其进行了单位功率因数整流和逆变的仿真研究.结果证明,该整流器是满足交流励磁需要的理想整流电源.     

A multi-function grid connected PV system with three level NPC inverter and voltage oriented control

In this paper a grid connected photovoltaic (PV) system is presented. The grid integration of the PV system is carried out via a three phase three level neutral point clamped (NPC) inverter. To control the inverter a modified version of voltage oriented control (VOC) method and the space vector pulse width modulation (SVPWM) technique have been applied. With the proposed modification the PV system operates as a shunt active power filter (SAPF), a reactive power compensator, and a load’s current balancer simultaneously. In this way the PV system operates more efficiently compared to the conventional PV systems and offers ancillary services to electric power system. The effectiveness of the proposed control scheme is established through simulation results with Matlab/Simulink in steady state and transient response of the electric power distribution system.     

新型铁路电力光伏三相并网逆变器

针对传统的逆变器技术谐波大、电能质量不稳定等问题,基于单周期控制原理对铁路电力光伏三相并网逆变器进行了研究。介绍了单周期控制的基本原理,分析了单周期控制下的三相逆变器工作原理,推导了三相逆变器的开关数学模型和等效控制方程,建立了用于铁路电力的三相光伏并网逆变器及其控制系统,并通过Matlab/Simulink对三相光伏逆变器建模仿真。结果表明,该三相光伏并网逆变技术可快速有效地消除谐波和校正功率因数,具有良好的性能。     

Performance evaluation of series compensated self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents the steady-state behavior of a series compensated (short-shunt) self-excited six-phase induction generator (SPSEIG) configured to operate as stand-alone electric energy source in conjunction with a hydro power plant. A purely experimental treatment is provided with the emphasis placed on operating regimes that illustrate the advantages of using SPSEIG. In particular, it is shown that the SPSEIG can operate with a single three-phase capacitor bank, so that the loss of excitation or fault at one winding does not lead to the system shutdown. The generator can also supply two separate three-phase loads, which represent an additional advantage. Experimental results include loading transients with independent three-phase resistive and resistive–inductive load at each of the two three-phase winding sets, and measured steady-state characteristics for various load and/or capacitor bank configurations. Practical results for long-shunt configuration are also given for comparative performance evaluation of series compensated SPSEIG.     

Early three-phase power [History]

The early decades of what became known as the "Electric Age" were marked by the efforts of outstanding engineers, inventors, and entrepreneurs on both sides of the Atlantic Ocean to develop a power supply system to best meet electrical generation, transmission, distribution, and customer utilization requirements. This article is a fascinating account of the development and evolution of alternating current (ac) and polyphase systems with special emphasis on the development of three-phase ac power, the prevailing system that today enjoys worldwide use.     

京沪高速铁路对江苏电网影响及治理措施

为研究高速电气化铁路接入江苏电网对电能质量的影响,以京沪高速铁路为例,分析了交-直-交型电力机车的负荷特点,基于ETAP软件平台建立了京沪高铁江苏段各牵引变电站及其周边网络的仿真模型,对谐波和负序进行定量评估,并针对电能质量问题,根据牵引侧三相SVC装置的补偿原理,提出了采用基于系统变换方法的动态无功补偿治理措施。结果表明,该方法谐波和负序补偿效果明显,各项指标均满足现行国标规定。     

交流电弧炉电弧功率软测量模型

电弧功率对于电弧炉冶炼时间、吨钢电耗有着直接的影响,所以对电弧功率进行在线监测是十分重要的。但是由于现阶段检测技术的限制,对电弧功率进行直接测量比较困难,因此建立电弧功率软测量模型有着十分重要的现实意义。电弧功率软测量模型是以三相不平衡电路计算为基础,选择电炉变压器二次侧出口线电压和相电流作为辅助变量,从而实现了对电弧功率的在线测量。通过与实测数据进行对比,说明了模型具有较好的精度,可以用于对电弧功率进行在线监测。     

Improved particle swarm optimization for photovoltaic system connected to the grid with low voltage ride through capability

Grid connected photovoltaic (PV) system encounters different types of abnormalities during grid faults; the grid side inverter is subjected to three serious problems which are excessive DC link voltage, high AC currents and loss of grid-voltage synchronization. This high DC link voltage may damage the inverter. Also, the voltage sags will force the PV system to be disconnected from the grid according to grid code. This paper presents a novel control strategy of the two-stage three-phase PV system to improve the Low-Voltage Ride-Through (LVRT) capability according to the grid connection requirement. The non-linear control technique using Improved Particle Swarm Optimization (IPSO) of a PV system connected to the grid through an isolated high frequency DC–DC full bridge converter and a three-phase three level neutral point clamped DC-AC converter (3LNPC²) with output power control under severe faults of grid voltage. The paper, also discusses the transient behavior and the performance limit for LVRT by using a DC-Chopper circuit. The model has been implemented in MATLAB/SIMULINK. The proposed control succeeded to track MPP, achieved LVRT requirements and improving the quality of DC link voltage. The paper shows superiority of IPSO than Incremental Conductance (IC) method during MPPT mode of PV system.     

A three-phase current-fed dc/dc converter with a three-leg high frequency transformer for fuel cells

In this paper, a three-phase current-fed dc/dc converter with an active clamp is introduced, and a new three-phase three-leg high frequency transformer is proposed for the converter. The three-phase dc/dc converter transfers power through transformer leakage inductances in the discontinuous current mode; a single common active clamp branch is employed for zero-voltage switching (ZVS) in all active switches. Further, the converter's three-phase power configuration increases power transfer, and it reduces the rms current per phase, thus reduces conduction losses. Moreover, a delta–delta connection on the three-phase transformer provides parallel current paths and reduces conduction losses in the transformer windings. A three-phase transformer can be constructed by connecting three discrete single-phase transformers, but this process results in a higher volume and higher material costs. Therefore, a new three-phase three-leg high frequency transformer is designed with three discrete cores integrated into a single transformer core. The proposed transformer is analyzed according to the several operating modes of the converter, and its design rules are determined. Experimental results are obtained on a 500-W prototype unit; the design is fully verified and analyzed.     

A six-phase synchronous generator for stand-alone renewable energy generation: Experimental analysis

This paper presents the steady-state behavior of a SPSG (six-phase synchronous generator) configured to operate as a stand-alone electric energy source in conjunction with a hydro power plant. A purely experimental treatment is provided with the emphasis placed on operating regimes that illustrate the advantages of using SPSG. In particular, it is shown that the generator can supply two separate three-phase loads which represent an additional advantage. Last but not least, outputs of the two three-phase windings can be used to supply a single three-phase load through an interconnecting six-phase to three-phase transformer, in which case failure of one three-phase winding does not lead to the system shutdown and the load can be still supplied from the remaining healthy winding. Experimental results include loading transients with independent three-phase resistive load at each of the two three-phase winding sets, and measured steady-state characteristics for various configurations and operating conditions.     

基于Z源逆变器的离网光伏系统仿真设计

对于偏远或电网未覆盖的区域,设计优良的离网光伏系统可解决本地负载的供电问题。将高效Z源逆变器应用于离网光伏系统,提出了系统整体控制策略,在控制离网系统三相交流电压稳定的基础上,同时进行光伏系统最大功率点跟踪（MPPT）,额外能量为蓄电池组充电,实现了能源的有效利用。系统仿真结果表明,当交流负载阶跃变化时,系统输出交流电压及MPPT控制仍能快速恢复稳定,蓄电池充电电流也能自行调整,验证了该离网光伏系统的正确性与可行性。