Equivalent Circuit and Vector Diagram of an Autotransformer with Mapping of Magnetic Fluxes

A universal equivalent circuit of an autotransformer with explicit mapping of its main characteristic—transporting energy by two parallel paths (electrically and electromagnetically)—is presented. At the same time, a diagram displays the magnetic fluxes in the window sections and in separate sections of the magnetic circuit, as well as when the steel is saturated between it and the tank. A similar completeness of representation of the power and magnetic fluxes is provided by the vector diagram of autotransformer operating mode. With its help, a high degree of clarity was achieved when explaining overexcitation of individual parts of a magnetic circuit in a short-circuited autotransformer and when deriving formulas for determining super- and antisupercurrents in comparison with idle fluxes in this mode. The presented equivalent circuit is suitable for step-up and step-down autotransformers and is applicable to the study the steady-state and transient processes.     

The equivalent circuit of a two-winding autotransformer with mapping of magnetic fluxes

The fundamentals of a new theory of a two-winding autotransformer have been suggested, and a universal 4T-shaped equivalent circuit has been obtained. The circuit has a universal behavior, since it provides mapping of all the magnetic fluxes in the window and in the yoke of the autotransformer, as well as in the region between the yoke and autotransformer, tank in the case of saturation under any load. It is shown that super- and antifluxes with respect to the idle flux occur in the magnetic circuit of a short-circuited autotransformer. It has been shown that these fluxes can be much higher than the corresponding fluxes in shortcircuited transformers. It has been concluded that the autotransformer theory should be refined. It is shown that the solution of the problem requires a fundamental understanding of the operation of transformers and autotransformers.     

大容量电机启动方式对电网的影响

大容量同步电机直接启动时,启动电流很大,会引起电网电压急剧下降。为此,采用自耦变压器降压启动方式启动大型同步电机。建立了含大容量电机的系统潮流计算模型,分析了大容量电机启动方式对110 kV电网和用户变电站供电系统电压的影响。利用动态仿真软件,针对不同系统容量及电机采取不同启动方式进行了电压降落及启动过程的仿真。理论计算与仿真结果表明,大容量同步电机启动时需避开系统小运行方式,验证了自耦变压器降压启动对减小系统压降的可行性。同时,仿真模型可用来确定电机启动时系统的运行方式及降压启动时自耦变压器的抽头。为保证电网电压质量,与直接启动方式相比,大容量电机启动应该采用自耦变压器的降压启动方式,且需选择合适的降压抽头。     

Transient stability study of the large synchronous motors starting and operating for the isolated integrated steel-making facility

This paper presents the influence of the large motor starting and the large rolling mill operation on the isolated integrated steel cogeneration facility by executing the transient stability analysis. The system configuration of the steel facility is introduced first. Furthermore, the proper mathematical models and the accurate parameters of the cogeneration units, excitation systems, and governor systems are investigated in detail. Both the static and dynamic load models are applied in this paper. To reduce the starting current of large motors, the conventional step-down voltage method by adjusting the suitable tap position of the autotransformer and the constant magnetic flux control method by using the variable-voltage and variable-frequency inverter are considered in the computer simulations. Also, the system voltage and frequency fluctuations due to the operation of the rolling mill are observed and investigated carefully to verify that the isolated cogeneration can be operated well and safely.     

500 kV变电站自耦变压器公共绕组运行工况分析

针对电网内自耦变压器不同运行工况,详细分析了自耦变压器公共绕组的运行特性及其负载特性。以500kV自耦变压器为例,分析了在自耦变压器不同运行工况下,功率因数和低压侧无功补偿容量对公共绕组负载能力的影响,并得出结论:主变在中压侧向高压侧送电工况下投入低压电抗器,或在高压侧向中压侧送电工况下投入低压电容器,都会增加主变公共绕组的负担,使主变的负载能力下降。针对电网内现有的无功补偿装置可能产生的副作用,并根据电网的实际情况提出解决方案,即改善系统功率因数,减少主变低压侧无功补偿装置的投入。     

The windmill topology

A widely used 18-pulse clean power converter with windmill transformer topology is presented and analyzed. It provides a more economical solution to achieve harmonicmitigation in electric power systems when compared with some of the earlier methods. With the proliferation of nonlinear loads in industrial power systems, the advent of IEEE 519-1992 [1], and the increasing demand by utilities for power factor improvement, the specification of harmonic mitigation has become common [2], [3]. In the past 14 years, several patented 18-pulse converter methods have been accepted in the marketplace and are successfully applied in a variety of practical applications such as water management, oil field installations, etc., where adjustable speed drive (ASD) and harmonic mitigation are required. A threeto nine-phase unity-gain autotransformer topology with a ±20° phase shift between output voltages is first proposed in [4]. The step-up and step-down 18-pulse autotransformer topologies with a 40° phase shift between the output voltages are discussed in [5]?[9]. The winding current looping and sharing problems in early days have already been solved [10]. The challenging design problem becomes one of economics [11].     

Generalization of the Y-differential autotransformer for 12- and 18-pulse converters

This work proposes a methodology to generalize the Y-connections for 12- and 18-pulse autotransformers. A single mathematical expression, obtained through simple trigonometric operations, represents all the connections. The proposed methodology allows choosing any ratio between the input and the output voltages. The converters can operate either as step-up or as step-down voltage. To simplify the design of the windings, graphics are generated to calculate the turn-ratio and the polarity of each secondary winding, with respect to the primary winding. A design example, followed by digital simulations, illustrates the presented steps. Experimental results of two prototypes (12 and 18 pulses) are presented. The results also show that high power factor is an inherent characteristic of multi-pulse converters, without any active or passive power factor pre-regulators needs.     

特高压变压器调压方式的探讨

针对特高压变压器的型式等问题进行了探讨分析。特高压变压器在考虑体积、造价及可靠性的情况下,采用单相自耦变压器成为必然。对于特高压中采用的自耦变压器来说,其调压方式有自身特殊的地方。在一般的双绕组变压器中,有载调压装置往往连接在接地的中性点上,这样调压装置的电压等级可比在线端调压时低。而自耦变压器中性点调压则会带来相关调压问题。故自耦变压器调压时,常采用线端调压方式。1000kV自耦变压器因其电压等级的原因,中压线端调压方式很难实现。在对中压线端调压和中性点调压方式,有载和无励磁两种调压方式进行分析比较的基础上,对特高压自耦变压器采用中性点无励磁调压方式的合理性进行了分析;考虑到特高压变压器在系统中的重要性和可靠性,对单独设置调压变压器的必要性进行探讨;对补偿原理进行了说明。     

Calculations of the Electromechanical Processes during Parametric Optimization of the Distribution Systems of Active and Reactive Loads of Ship Power Plants

A mathematical model of a ship power plant (SPP) to study dynamic modes of the active and reactive load distribution system using the parametric optimization of adjustable parameters has been presented. The algorithm that minimizes the time of transient processes in different modes of SPP operation has been developed for parametric optimization of the active and reactive load distribution system. The calculations of electromechanical processes during of parametric optimization of the load distribution system of an SPP have been carried out in the MATLAB package using mathematical models and numerical methods.     

Power Quality Assessment of Large Motor Starting and Loading for the Integrated Steel-Making Cogeneration Facility

This paper presents the power quality assessment of large synchronous motor starting and loading in the integrated steel-making cogeneration facility. To execute the transient stability analysis, the proper mathematical models, and the accurate parameters of the cogeneration units, excitation systems, governor systems, load, and static var compensators (SVCs) are investigated in detail. Four case studies with or without considering the connection of the power grid, the installation of autotransformer (AT) starter, and SVC are performed to demonstrate the dynamic responses of system frequency, voltage, and cogeneration units due to motor starting and loading. Also, the voltage sag ride-through curve of sensitivity load has been included, and a power quality index (PQI) due to voltage variation in the assessment period has been proposed to find the impact of motor starting and loading on the power quality of the cogeneration system. It is concluded that the system dynamic responses and PQI values have better performance if the AT starter is applied with either the regulation of the SVC system or connection to the bulk power grid     

电力系统无功补偿优化规划

建立无功补偿优化规划的数学模型,采用年电能损失费用与折合为等年值的新增无功补偿设备的投资费用之和最小为目标。在约束条件中,考虑了几种不同负荷的运行方式。根据分解协调原理,把这一复杂问题分解为最大负荷方式,一般负荷方式和最小负荷方式三个子模型,其投资费用在最大负荷方式下考虑,对于三个子模型用最大负荷损耗时间来协调统一,使得模型更符合实际情况,避免了只考虑单一负荷方式的片面性,具体求解方法采用非线性规     

500 kV及220 kV自耦变压器对电网单相短路电流的影响

江苏部分500kV枢纽变电站220kV母线单相短路电流大于三相短路电流较多,且已经逼近了断路器的额定开断能力。主要是由于大量采用自耦变压器引起的,考虑更换自耦变为非自耦变以降低单相短路电流,实质是减少零序网接地支路。通过序网图原理分析发现更换500kV自耦变为非自耦变比更换220kV站内的自耦变为非自耦变对单相短路电流的抑制作用大,算例也证明了理论分析的正确性。但是更换500kV自耦变在技术和经济上可行性都不强,所以抑制单相短路电流的最有效措施是500kV自耦变中性点加装小电抗。     

输入侧断相对自耦型12脉波整流器的影响

分析了输入侧断相对使用升压型三角形联结自耦变压器的12脉波整流器的影响,给出正常工作和断相运行时整流器各处的电压和电流特征。理论分析和实验结果表明,两整流桥输出电压的瞬时差是形成12脉波整流的关键;断相时,12脉波整流器等效为两个具有相同输入电压的单相全桥整流电路的并联,两整流桥输出电压瞬时差等于零,导致负载电压为2脉波,直流侧电能质量显著降低;所断相的输入电流等于零,整流器工作于严重的不对称状态。     

PLC在一台自耦变压器起动多台电动机中的应用

结合目前大功率电动机用自耦变压器降压起动的方法 ,从经济、可靠的角度考虑 ,提出用PLC来控制一台自耦变压器分时起动 5台电动机 ,并给出系统主回路和控制电路接线图 ,以及PLC部分主要程序 ,并具体阐述了其实现方法。     

Dynamic processes in abnormal operating conditions of power supplies of welding electron-beam guns

The dynamic processes in power supplies of welding electron guns caused by abrupt change in the parameters of load, currents, and voltages have been considered. A mathematical model for the study of abnormal modes of power supplies that are part of the power unit of an electron-beam welding installation with regard to the mutual influence of their parameters has been proposed. Results of experimental studies of the operation of an inverter prototype have been compared with the simulation results.     

之字形自耦变压器移相角影响分析及Matlab仿真

自耦变压器较传统隔离式变压器在一定程度上减小了整流系统的体积,同时自耦变压器由于其对称的设计结构使其在消除谐波、降低负载电压纹波系数方面也优于传统隔离式变压器。为适应升压场合,优化了传统移相角。分析了移相角对12脉波整流系统输入侧电流和负载电压的影响,及对12脉波整流系统之字形自耦变压器等效容量和平衡电抗器等效容量的影响。给出了新的移相角,使之字形自耦变压器能适用于二次侧电压稍大于一次侧电压的升压场合。仿真结果表明,在同匝比条件下,之字形自耦变压器较传统隔离式移相变压器容量小33%且结构对称。     

Analysis,modeling, and implementation of a new transformerless semi-quadratic Buck–boost DC/DC converter

This paper presents a novel transformerless semi-quadratic buck-boost converter (SQBuBoC). In the proposed SQBuBoC, two power switches with simultaneous operation are used and a higher step-up/step-down voltage conversion ratio is achieved compared with the traditional buck-boost, Cuk, single-ended primary-inductor converter, and Zeta converters. The positive polarity of the output voltage, along with low ripple continuous input current and common ground between the source and the output voltages, are some features that make the suggested topology more suitable for many applications with wide range of output voltage such as photovoltaic systems. Moreover, the total voltage stress across the power switches in this converter is lower than the cascade boost, and the traditional buck-boost converters led to power MOSFETs selection with lower drain-source ON resistance (R_ds) and efficiency improvement. All the steady-state analysis and comparisons in continuous conduction mode (CCM) are discussed in details. In addition, to study the low frequency behavior of the SQBuBoC by means of the state-space averaging technique, the small and large signal models of this converter in CCM are presented. Finally, the SQBuBoC analysis is justified using experimental results of a 50 W step-up 25 V to 120 V and a 28 W step-down 25 V to 14 V laboratory prototypes.     

微型燃气轮机发电建模与仿真研究

针对发电用微型燃气轮机,介绍了不同模式下不同控制方式的数学模型,基于PSCAD/EMTDC仿真平台构建其在不同运行模式下的仿真模型。仿真结果验证了在联网时功率参考值变化和孤岛时负荷需求变化的条件下,微型燃气轮机的动态响应性能良好,输出功率均能迅速准确地跟踪需求,暂态过程持续时间短暂。     

考虑风电大规模接入的电力系统调峰平衡分析

电力系统调峰平衡是其维持静态稳定的关键,在风电大规模接入的形势下,亟需研究风电接入对电力系统调峰平衡的影响。以适用于含风电电力系统调峰平衡计算的数学模型及算法为基础,研究了风电接入对电力系统调峰平衡的影响,并提出相应计算方法。在此基础上,以区域电网为例,根据其实际情况设计电网的调峰运行方式以及风电接入方式,基于电网实际数据,仿真计算了其在3种风电接入方式下的调峰盈余,分析了风电大规模接入对电网调峰平衡的影响,结果表明,风电大规模接入将对电力系统的调峰平衡带来显著负面影响,而影响程度与风电接入电网的方式密切相关。     

Modeling the saturation processes of a power-transformer core under simultaneous direct and alternating current passing through the winding

A mathematical model of a three-phase two-winding step-up power transformer with the winding circuit Δ/Yo with simultaneous ac and dc currents passed through the grounded winding has been developed. The proposed model is represented by a system of differential equations, which are nonlinear because of the nonlinearity of the inductive resistances of the magnetization curve. The calculation results for a TDTs-400000/242/20 shell–core transformer are given. The model takes into account the effect of dc on the saturation of the power-transformer core, which is especially important under asymmetrical modes that can occur during thunderstorms, seismic activities, or space-weather disturbances.