General theory of instantaneous power for multi-phase systems with distortion, unbalance and direct current components

Active instantaneous currents are generally defined as those compensated supply-wire currents that deliver a given instantaneous power with minimum line losses, without a change in voltage. Since the concept was introduced 60 years ago, many theories have been proposed to enable the calculation of those optimum supply currents, for various conditions of the supply system. This paper shows how these optimal wire currents can be obtained with a single general formula applicable to all supply systems. The solution depends on the number of wires considered, their resistances, which need not be equal, and their respective voltages measured from a common reference. The formula is derived through the properties of linear algebra in vector space, and is a direct consequence of Kirchhoff's current law and the law of conservation of energy. All the existing theories can be identified as particular cases of the general formula and most can be grouped into three common categories.     

Three-phase unbalance of distribution systems: Complementary analysis and experimental case study

Three-phase unbalance is a familiar issue for power system researchers and engineers. This can introduce additional power losses in distribution network in steady states due to both negative and zero sequence components. It could also limit the loading capability of distribution transformers, well below their nominal ratings. There are many voltage and current unbalance definitions (e.g., IEEE and NEMA) for three-phase three-wire systems, assuming zero sequence currents to be of negligible practical value, for they cannot flow in three-wire systems. However, the zero sequence unbalance has significant current magnitude in three-phase with four-wire distribution networks, particularly in developing countries. Hence, this paper concentrates on the distribution unbalance, completing the available definitions in order to maintain tangible relationships between the level of unbalance and the cited consequences in distribution networks. Furthermore, practical works were performed on 11 selected 20 kV/0.4 kV substations within Tehran North-West Distribution System (TNWDS), where data loggers have been installed for 7 days to measure and record operating conditions of substations. Then, detailed analysis and assessment are suggested on empirical data to substantiate the presented complementary definitions and relationships.     

A general purpose definition of active current and non-active power based on German standard DIN 40110

Up to now reactive power and currents were only defined in common standards for symmetric systems of sinusoidal currents and voltages. Therefore a new German standard was developed, which includes the more frequent cases of asymmetric systems of non sinusoidal quantities concerning two-conductor- and multi-conductor-circuits. The new rules with the so called collective quantities are used to analyze and to estimate the power transfer between load and line. The results thereby obtained are the basis for the design of a compensator, as it is demonstrated by some examples.     

Load flow tracing in power systems with circulating power

Power flow tracing is very important to open access for loss allocation, congestion analysis and reactive power pricing, etc. In a previous paper, power flow tracing of power systems without circulating power has been implemented. In this paper, two lemmas have been proved first which show the reasons for circulating power. Then graph theory is applied to detect the existence of circulating power. After that optimal power flow (OPF) approach is suggested to eliminate circulating power. The sequential quadratic programming is used for OPF solution. The computer tests on a 6-bus system and the IEEE 30-bus power system yield satisfactory results. The suggested method is suitable for both active and reactive power flow tracings of power systems with circulating active and reactive power.     

Application of catastrophe theory to power systems

In recent publications dealing with nonlinear systems, nonlinearities existing in the systems under study have drawn much attention. Studies on the effects of nonlinearities in power systems are becoming an increasingly important part of the research on system stability. It is probable that heretofore undiscovered phenomena caused by the nonlinearities involved in load flow equations, generator swing equations and characteristics of control equipments and loads, etc., may be found. This paper presents a new Catastrophe Theory application to nonlinear power systems. Making use of the concept of Duffing's equation, it is shown that a Catastrophe Theory analogy can be used to interpret unstable phenomena caused by system nonlinearities from the viewpoint of oscillations. When considering system nonlinearities due to poor combinations of system parameters and periodic disturbances, there may exist the characteristic “jumps” in system ates that correspond to slow (quasi-dynamic) changes of the frequencies of periodic disturbances. With this Catastrophe Theory approach, a system bifurcation set can be identified to assess the unstable phenomena of power systems.     

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.     

基于瞬时功率理论的输电线路分相电流差动保护

为了消除分布电容电流对电流差动保护的影响,提出一种基于瞬时功率理论的输电线路分相电流差动保护原理。对线路两端的三相瞬时电流进行傅里叶变换,并对变换后的电流进行低通滤波后得到线路两端的三相基波有功电流分量,利用其构成差动电流保护判据。利用PSCAD软件建立500 k V输电线路仿真模型对所提差动保护原理进行仿真验证,仿真结果表明,其不受过渡电阻和分布电容电流的影响,可快速、准确地区分各种故障类型。     

Optimal power flow with UPFC using security constrained self-adaptive differential evolutionary algorithm for restructured power system

The restructuring of the power industry begins with the 21st century and this restructuring of the power system requires a careful analysis because of the nature of the real-time operations. For the restructuring power system (RPS), the self-adaptive differential evolutionary (SADE) algorithm is proposed for enhancing and controlling the power flow using Unified Power Flow Controller (UPFC) under practical security constraints (SCs). The new formulas for the tuning parameters of Differential Evolutionary technique are designed in such a manner that they become automatically adaptive throughout the whole iteration. The UPFC is modeled considering losses of the both converters, transmission loss in UPFC and losses of the coupling transformers. The unique mathematical modeling of the cost function is developed considering practical SCs. The proposed algorithm and other evolutionary algorithms are applied on the IEEE standard and ill-conditioned test systems. With and without UPFC, the power flow and line losses are observed for the three sets of user-defined active and reactive power. The use of UPFC not only enhances the power flow but also reduces the total line losses. Comparing characteristics, convergence rate, success rate for all cases, the best performances are observed for the proposed Security Constraint SADE (SCSADE) algorithm.     

ANN for multi-objective optimal reactive compensation of a power system with wind generators

In this paper, we develop a method aimed to impose an acceptable voltages profile and to reduce active losses of an electrical supply network including wind generators in real time. These tasks are ensured by acting on capacitor and reactor banks implemented in the load nodes. To solve this problem, we minimize multi-objective functions associated to the active losses and the compensation devices cost under constraints imposed on the voltages and the reactive productions of the various banks. The minimization procedure was realised by the use of evolutionary algorithms. After a training phase, a neural model has the capacity to provide a good estimation of the voltages, the reactive productions and the losses for actual curves of the load and the wind speed, in real time.     

Data acquisition,power forecasting and coordinated dispatch of power systems with distributed PV power generation

The integration of photovoltaic (PV) power generation with highly random and intermittent characteristics has posed significant challenges to the safe and economic operations of power systems. This paper establishes an entire operation structure covering PV data acquisition, PV power forecasting, and coordinated dispatch of power systems with large-scale behind-the-meter distributed PV units. The PV data acquisition method provides training data for the power forecasting model, and then the forecasting results serve as the references for developing coordinated dispatch strategies. Real-world systems and numerical studies validate the effectiveness and superiority of the proposed operation structure.     

Recognition of coherent swing in power systems and detection of step-out using power and current measured on tie lines

When a fault occurs in a large interconnected power system, several generators may swing coherently and form a group. Loss of synchronism may occur among groups of these generators. In order to develop a stabilizing control system, two new methods are proposed in this paper. The first method is to select generators which swing coherently. The method uses a new index that is based on the physical relation: when two generators swing coherently, the phase angle of a generator terminal voltage increases when the angle of the other generator increases, and decreases when the other decreases. To detect a loss of synchronism, the power P and current I measured on a line with the interval of a certain time period are used. By using this method, the position of the operating point of power system after disturbance on the power angle curve can be estimated. This means that the degree of stability can be observed with a small number of observations measured on a line. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 119(4): 22–31, 1997     

协整理论在华东电网中长期电力需求预测中的应用研究

通过研究1978～2005年华东电网四省一市的经济发展和电力需求状况,运用协整理论分析了电力发展与经济发展之间的关系,找出关键变量,运用相关计量经济理论建立用电量与经济变量多方程联立模型,在此基础上提出华东地区电力需求一经济发展模型.     

Embedding sensitivity theory in ordinal optimization for decentralized optimal power flow control

In this paper, the decentralized optimal power flow with continuous and discrete control variables problem is firstly formulated as a NP-hard optimization problem - Block Additive constrained with Continuous and Discrete variables (BACD) problem. Secondly, an algorithm of embedding sensitivity theory (ST) in ordinal optimization (OO), abbreviated as STOO, is proposed for solving this NP-hard optimization problem. The STOO algorithm consists of three stages and three models of performance evaluation. The proposed method not only copes with the computational complexity due to huge solution space but also obtains a good enough solution with high probability guaranteed by the OO theory. Finally, this work demonstrates the computational efficiency of the STOO algorithm via various tests on the IEEE 118-bus and 244-bus systems partitioned into four subsystems using a 4-PC network and compares the results with those obtained using other heuristic methods, Genetic Algorithm, Tabu Search, Ant Colony Optimization and Simulated Annealing. Test results show the validity, robustness and excellent computational efficiency of the STOO algorithm for obtaining a good enough solution.     

Three-dimensional visualization and animation for power systems analysis

This paper describes a novel approach for three-dimensional visualization and animation of power systems analyses. The paper demonstrates that three-dimensional visualization of power systems can be used for teaching and can help in easily understanding complex concepts. The solutions of power flow analysis, continuation power flow, optimal power flow and time domain simulations are used for illustrating the proposed technique. The paper presents a variety of examples, particularly oriented to education and practitioner training. Conclusions are duly drawn.     

基于瞬时无功功率理论的谐波电流检测方法及实现

对比分析各种谐波电流检测方法的基础上,提出一种基于瞬时无功功率理论的谐波电流检测方法及实现方法,详细分析了谐波电流的检测算法和数字锁相环的设计实现。最后,利用MATLAB／Simulink对所提出检测方法进行了仿真验证,并设计了一台基于DSP（Digital Signal Processor数字信号处理器）的实验样机。仿真和样机实验结果证明了所提方法的可行性和正确性。     

非线性电力系统中对称分量法的应用

运用对称分量法原理对经过Furrier变换后的三相不对称,非正弦电压和三相线性不平衡负载电流进行分解,并与其他功率理论方法如电流物理份量法和瞬时无功功率理论对系统的功率现象和定义进行分析和比较.通过比较可见在频域上,对于线性负载系统,应用对称分量法对所有三相四线电压不对称,负载不平衡的线性系统的分解与定义较其他的功率方...     

基于Matlab的电力系统谐波电流实时检测方法仿真

讨论了2种基于瞬时无功功率理论的谐波电流检测方法,给出了检测系统的框图,并应用matlab仿真平台进行了仿真研究,给出了仿真波形,验证了该方法的正确性.     

基于三相-单相电力电子变压器的牵引供电系统负载不平衡、无功和谐波潮流分析

将三相-单相电力电子变压器应用于贯通式同相供电系统,对比计算了传统变压器、电力电子变压器的单相侧和三相侧之间负载不平衡、无功和谐波的潮流关系。基于三电平电路研究了隔离DC-DC变换器和多模块级联整流器的控制方法:整流器采用有功无功解耦控制,实现了三相网侧单位功率因数运行;隔离DCDC变换器采用双边三电平控制,减小了流过中频变压器电感电流有效值。仿真和实验结果验证了基于三相-单相电力电子变压器的贯通式同相供电系统可有效消除负载不平衡、无功和谐波对三相电网电能质量的不良影响,系统动静态性能良好。     

基于瞬时无功功率理论的谐波电流检测改进方法及其数字低通滤波器的优化设计

针对有源滤波器实际工程的需要,提出一种基于瞬时无功功率理论的改进ip-iq谐波和基波无功电流检测方法,可简化复杂的数学变换,能直接应用于三相三线制、三相四线制和单相系统谐波和基波无功电流的检测.并综合考虑FIR滤波器与IIR滤波器的优缺点,进一步提出均值滤波器与Elliptic滤波器串联滤波的二级滤波新方法.仿真结果表明该方法检测实时性好、精度高、易实现,具有实用价值.