针对不平衡负载三相逆变电源控制方法的研究

三相逆变电源一个最重要的性能是维持三相电压的对称输出,本文提出 了一种新的针对不平衡负载三相逆变电源控制方法,其通过对逆变电源交流中性点的控制,使得主电路中三相电压可以独立控制,所提出的方案在1KW,400Hz的三相逆变电源实现,在不平衡负载下,逆变电源能够维持很好的对称输出电压。     

Wyes and wye nots of three-phase distribution transformerconnections

The advantages and disadvantages of various three-phase transformer connections are summarized. Recommendations are provided regarding the standard types of connections a utility should offer for various service requirements. Warehousing costs, ferroresonance, neutral grounding, core construction, overvoltages, unbalanced loading, zero sequence source, and through faults are considered. Emphasis is placed on the exchange of information between customer and utility. An Application for Three-Phase Service form is provided. It can be used to determine service requirements including voltage, grounded, ungrounded, three-phase load, single-phase load, motor characteristics, and fault current interruption rating of secondary breakers and switchgear. Suggestions on utility-supplied information such as maximum available three-phase and line-to-ground fault currents and a discussion of the operation of ungrounded systems are also provided     

A novel three-phase four-leg inverter based load unbalance compensator for stand-alone microgrid

This paper proposes a three-phase four-leg voltage sourced inverter (VSI) based load unbalance compensator (LUC) including its control algorithm, which is a component of a microgrid. The purpose of proposed three-phase four-leg VSI based LUC is to improve power quality of the standalone microgrid. Power quality of the microgrid which was installed in Mara-island, Korea is analyzed using a real operational data. In this work, the microgrid in Mara-island which includes a photovoltaic power generation system, a diesel generator, a battery energy storage system, and a power management system is modeled in PSCAD/EMTDC, and proposed three-phase four-leg VSI based LUC is also modeled and applies to the modeled microgrid. Power flow and stability of the modeled microgrid with the LUC is analyzed under variable irradiance and unbalance loads. The results show that the proposed LUC helps to improve stability of the stand-alone microgrid. The proposed three-phase four-leg VSI based LUC and its control algorithm can be effectively utilized to the stand alone microgrid which has large unbalance loads.     

Dynamic load models in power systems using the measurement approach

Accurate dynamic load models allow more precise calculations of power system controls and stability limits, which are critical in the planning and operation of power systems. The development of dynamic load models using the measurement approach for the Taipower system are described. Two dynamic load model structures are developed. A procedure for applying a set of measured data from an online transient recording system to develop dynamic load models for the Taipower system is described. A technique based on the concept of confidence interval is used to validate the developed high-order load model structure. Case studies are also presented     

重庆电网多种负荷模型对系统稳定性影响机理

负荷特性是电压失稳过程中最活跃、最关键的因素,对于电压稳定研究具有重要意义。通过典型故障后电压恢复特性比较分析,研究了负荷特性和组成比例对系统故障后电压恢复特性的影响;通过计算典型故障极限切除时间,分析了负荷特性和组成比例对重庆电网系统稳定性的影响。经过计算可以得到,针对重庆电网的异步电动机负荷比静态负荷模型更有利于系统电压稳定。     

应用于三相潮流程序的变压器模型的实现

介绍一种三相潮流程序中的变压器模型的实现方法。其思路是将变压器分左右两块实现:左侧部分包含一次侧的联结组别和三相的变比值,右侧部分包含二次侧联结组别和变压器阻抗值,并分别以子程序实现。计算中根据实际变压器类型调用相应子程序,通过组合得到完整的变压器模型。该方法思路简洁,易于编程实现,能疗便地实现多种不同接法的变压器模型,可以很好地应用于三相潮流程序。通过一个简单算例对模型的有效性进行验证和分析。     

Inductive unbalance in power systems with untransposed transmission lines

In the early days of electric power systems, it was the usual practice to have frequent line transpositions to reduce capacitive and inductive unbalances on them. Transposition represents higher line costs owing to the presence of transposition towers and results in lower mechanical and electrical strength. These disadvantages make it desirable not to transpose lines if the resulting unbalances are not adverse.In this paper, the inductive unbalances of a single- and a double-circuit untransposed transmission line are analysed. Effects of loads, of line geometric mean radii and of line ground wire arrangements are included. Results for negative- and zero-sequence components which arise from such unbalances for power systems with typical lines are given and compared with those obtained using methods developed earlier.     

Electromagnetic unbalance in power systems with untransposed transmission lines

Transposition of power transmission lines represents high costs due to the required transposition towers and also results in low mechanical and electrical strengths. These disadvantages make it desirable not to transpose lines if the resulting unbalances are not adverse. This paper proposes a method to predict the electromagnetic (combination of inductive and capacitive) unbalance of untransposed lines. The method is validated by its application to typical lines under various load conditions and various geometric mean radii.     

Overview of wind power intermittency impacts on power systems

Although integrating a large amount of wind power is technically possible, higher integration costs might be incurred when the penetration level of this intermittent power increases. This paper reviews wind power variability and its different impacts on power systems. In addition, an up-to-date overview of wind power balancing costs is presented.     

一种改善三相不平衡下逆变器并网系统电能质量的新方法

针对典型的逆变器并网系统,研究三相不平衡度对公共并网点(PCC)电能质量的影响。三相不平衡电压经序分量分解后,由瞬时功率可推导其对并网逆变器直流侧二倍频波动量的影响,结合开关函数的傅里叶表达式,分析逆变器交直流侧交互影响机理,同时经过数学推导得出公共并网点的谐波特性。基于此谐波特性,建立由有源滤波器和无源滤波器组成的混合电力滤波器(HPF),达到改善并网电能质量的目的。利用Matlab/Simulink软件搭建三相不平衡并网系统,检测特定次谐波与理论推导值作对比,考虑并网系统具有的谐波特性,建立HPF滤波模块,验证了HPF在提高电网电压三相不平衡时并网系统的电能质量方面具有优越性。     

Unified power flow controller models for three-phase power flow analysis

In this paper, three models of the unified power flow controller (UPFC) suitable for three-phase power flow analysis in polar coordinates are presented. The symmetrical components control model can be used to control the positive-sequence voltage of the shunt bus and the total three-phase active and reactive power flows of the transmission line while the injected shunt voltages and the series voltages are balanced, respectively; the general three-phase control model can be used to control the three shunt phase voltages and the six independent active and reactive power flows of the transmission line; the hybrid control model can be used to control the positive-sequence voltage of the shunt bus and the six independent active and reactive power flows of the transmission line. The proposed UPFC models were successfully implemented in a three-phase Newton power flow algorithm in polar coordinates. In the implementation of these UPFC models, transformers of some common connection types, which connect the UPFC with the network, are explicitly represented. Numerical results based on a five-bus system and the modified IEEE 118-bus system are given to illustrate the UPFC control models and demonstrate the computational performance of the three-phase Newton power flow algorithm.     

基于PSS/E的电力系统机电暂态负荷模拟

介绍了PSS/E仿真软件所提供的LDFR模型、IEEL模型等静态负荷模型,CIM5模型、CIMW模型、CIM6模型等感应电动机模型和CLOD综合负荷模型,叙述了各个负荷模型在PSS/E 14机59节点系统实例的机电暂态仿真中的具体实现方法,仿真结果说明了不同的负荷模型对电力系统动态仿真的影响.     

A survey of neutral currents in three-phase computer power systems

It is shown that the magnitude of the neutral current in three-phase computer power systems depends on the harmonic content and phase balance of the load currents. While very high neutral currents are possible due to the additive nature of triplen harmonic currents, a low percentage of data processing sites in the United States are actually experiencing neutral currents in excess of the rated phase current. However, trends in computer systems make high harmonic neutral currents more likely. Power system problems associated with high harmonic neutral currents include overloaded transformers, voltage distortion, and common mode noise. Whenever three-phase, four-wire power systems are used to supply power to computer systems or other similar electronic loads, the power system design should allow for the possibility of high harmonic neutral current to avoid potential problems     

New digital filter for unbalance distorted current and voltage estimation in power systems

This paper presents a novel algorithm for identifying and measuring the symmetrical components of distorted three phase voltage or current waveforms in electrical power systems. The proposed algorithm is capable of estimating the symmetrical components as well as the harmonic contents of a given unbalanced distorted signal. The proposed technique is based on stochastic estimation theorem. The problem is formulated as an estimation problem and presented in state space form. The proposed algorithm used to estimate the positive, negative and zero components of unsymmetrical waveforms as well as the harmonic content of a given distorted signal. Application of the proposed algorithm has been conducted on various test cases. Among which a practical simulated power system has been implemented using EMTP. Various scenarios are carried out to simulate realistic situations of unsymmetrical waveforms. Effects of bad data on the solution accuracy are also studied. The speed of convergence is examined by changing the estimator initial conditions. Results obtained show that the proposed technique can estimate and track the symmetrical components of non-stationary three phase unbalance voltage or current waveforms in noisy environments. Fast accurate solutions are guaranteed regardless of the initial conditions. It is also shown that bad measurements have no effects on the final accuracy of the estimation.     

Structural stability in power systems-effect of load models

In this paper, a new notion of power system stability is introduced, namely structural stability, and its significance in the context of load modeling is examined. It is shown that beyond certain values of load indices (in the case of static models) and beyond certain value of the dynamic component (in loads such as induction motors), the power system may become structurally unstable. The immediate application of these results is in voltage stability problems     

Effect of load characteristics on frequency control in power systems

The load-frequency control (LFC) problem is investigated on assumptions considered realistic for most power systems. Voltage and frequency dependence of system loads, and load inertia effects, are taken into account. Results include system dynamic response to changes in load, and effects of LFC parameter settings. Using a performance index based on squared tie-line flow error plus squared area frequency error, the effect of load representation on optimum integrator gain is examined.     

配电网络拓扑分析与三相潮流计算的实现

根据配电网三相潮流计算的基本原理,按面向对象技术设计并实现了配电网拓扑分析和三相潮流计算算法.潮流计算算法采用实用高效的前推回推法,拓扑分析则根据前推回推法的特点采用支路分层的方法,两者结合有效地保证了算法的效率.在实现中,按面向对象技术设计了支路、节点、层和馈线类,并在这些类的基础上实现了拓扑分析和潮流计算.利用33节点、123节点和292节点系统对实现的算法软件进行了性能测试,结果表明,算法软件具有很高的效率.     

Parallel operation of three-phase power transformers with different short-circuit voltages

Parallel operation of three-phase power transformers with different short-circuit voltages has been studied using a lab bench. The circuit diagram of the bench and experimentally measured vector diagrams of the currents in primary and secondary windings of the transformers have been presented. It has been found that, when the difference of short-circuit voltages of parallel operating transformers is more than 10%, both an arithmetic difference of currents and a great phase shift of secondary-phase voltages occur. As a result, one of the transformers becomes overloaded, while the other is underloaded. It has experimentally been found that the load current is determined by the geometrical sum of currents of secondary windings in transformers. It has been shown that one of the studied transformers works in the conversion mode and its currents are shifted by 180° with respect to the phase, which is the main reason for failure of the normal operational conditions of parallel transformers.     

Automatic load shedding in multiarea elastic power systems

A procedure for the design of automatic load-shedding in an elastic multiarea power system is proposed. Assuming that the power—frequency transfer functions of the various areas are similar, and that the interconnection network can be approximated by a purely reactive network, separate specifications can be given with regard to the control of the mean frequency transient and that of the interarea electromechanical oscillations. While the first determines the total load to be shed, the second determines its optimal subdivision in order to maximize the stability margin in the large.     

电力系统低频减载优化研究

低频减载作为电力系统的第三道防线,是一种有效防止系统频率崩溃的手段。结合江苏电网的实际情况,研究电力系统低频减载的优化配置。采用多机模型的时域动态仿真方法,用故障仿真法确定适合低频减载分析的系统模型和参数,通过选择江苏电网的一个典型分区作为研究对象,提出轮级协调动作配合后加速动作来优化调整现有低频减载的方案,以获得更高的安全性和经济性。