Unsymmetrical short-circuit analysis for distribution system considering loads

Short circuit analysis is an essential tool for determining the short-circuit-current rating of the protective devices and different substation equipments to be installed in a distribution system as well as for co-ordination of the protective devices. Usually, the effect of load is neglected during short-circuit analysis, which introduces inaccuracy in the calculated values of short-circuit current. To address this issue, this paper proposes an efficient and accurate short-circuit fault analysis method for balanced and unbalanced distribution system considering the effect of loads. Comparison of the results obtained by the proposed method with those obtained by time-domain simulation using PSCAD/EMTDC establishes the accuracy of the proposed method.     

三相光伏并网发电系统的控制研究

介绍了三相光伏并网发电系统的工作原理,阐述了扰动观测法(P&O)实现最大功率跟踪控制(MPPT)的基本思想.对并网逆变器在d-q坐标系下的数学模型,采用逆系统方法进行线性化解耦,构造出伪线性系统.然后,运用变结构控制方法,设计出这个伪线性系统的变结构控制律.仿真结果表明该方法能够有效地控制并网电流波形趋于正弦波,并跟踪系统电压的相位,同时使系统具有较好的稳定性.     

Annual performance analysis of differentmaximum power point tracking techniquesused in photovoltaic systems

This paper presents an annual performance evaluation of three maximum power point tracking (MPPT) methods.The used MPPT techniques (Perturb and Observe, Incremental Inductance and Sliding mode) are evaluated underan annual data of atmospheric conditions of the target site. The main contribution of this work is to consider realfluctuation conditions of solar irradiations, ambient temperatures and wind velocities. It was found that the Slidingmode provides higher energy yields independently of the period. Compared to the basic P&O and the ICtechniques, sliding mode has the potential of generating up to 8.18% more electrical energy than other techniques.     

含UPFC的电力系统中的分歧研究

在建立统一潮流控制器UPFC(Unified Power Flow Controller)动态模型的基础上,利用分歧理论,揭示了含UPFC电力系统中的非线性奇异现象,并给出了分析这种系统位于非双曲平衡点邻域内稳定性的方法和步骤。计及分歧时的稳定与Lyapunov稳定有本质区别,后者是对一个给定系统改变初始条件研究系统的稳定性;而计及分歧时,系统的拓扑结构已发生变化,反映是系统状态的一种跳跃或突变,从而为含UPFC系统的安全稳定运行提供了一些新观点和新见解。     

Stability region and radius in electric power systems under sustained random perturbations

Two concepts are proposed to characterize the behavior of stochastic systems under sustained random perturbations in time: Using Lyapunov exponents we define the region where an electric power system can be operated under random perturbations without losing stability; and we characterize the maximum perturbation size that a system can sustain. The proposed methodology is applied to international test systems of nine and thirty-nine buses.     

Modeling and stability analysis of hybrid power systems for the more electric aircraft

This paper presents a detailed modeling and a comprehensive assessment of small-signal stability for a “more-electric” vehicular power system consisting of a synchronous variable-frequency generator which supplies power electronic controlled loads via an 18-pulse autotransformer rectifier unit for AC-DC conversion. Functional models for key power system components and loads are derived. Analytical derivations employed for small signal stability analysis based on linearized equations are described, and the influence of leading design parameters on system stability is evaluated.     

Power flow model/calculation for power systems with multiple FACTS controllers

This paper presents a new procedure for steady state power flow calculation of power systems with multiple flexible AC transmission system (FACTS) controllers. The focus of this paper is to show how the conventional power flow calculation method can systematically be modified to include multiple FACTS controllers. Newton–Raphson method of iterative solution is used for power flow equations in polar coordinate. The impacts of FACTS controllers on power flow is accommodated by adding new entries and modifying some existing entries in the linearized Jacobian equation of the same system with no FACTS controllers. Three major FACTS controllers (STATic synchronous COMpensator (STATCOM), static synchronous series compensator (SSSC), and unified power flow controller (UPFC)) are studied in this paper. STATCOM is modeled in voltage control mode. SSSC controls the active power of the link to which it is connected. The UPFC controls the active and the reactive power flow of the link while maintaining a constant voltage at one of the buses. The modeling approach presented in this paper is tested on the 9-bus western system coordinating council (WSCC) power system and implemented using MATLAB software package. The numerical results show the robust convergence of the presented procedure.     

光伏电站在线监测技术现状与进步趋势展望

随着并入电网的光伏电站规模逐年递增,其稳定性与安全性越来越受到重视.对光伏电站实施在线监测,能实时把握它的运行状态,为其运维提供必要支持,且有助于提升它的运行稳定性.文章从功率监测、环境监测和故障在线监测三个方面,对光伏电站在线监测技术研发应用现状进行了梳理,注意到其中的功率监测主要包括功率控制策略制定和短期功率预测技...     

Study of a major oscillations event in northeastern area of the Iranian power network

Iranian power system encountered major oscillations in January 2008 in the northeastern area with an amplitude of about 120 MW. Since not all the events and variables had been recorded, a scenario to simulate the recorded oscillations and results of studies conducted to reproduce the oscillations by simulation are discussed in the first part. Tuning of supplementary controllers, such as PSSs, on the generating units and the use of reactive power compensators in the long transmission line to enhance stability and eliminate severe oscillations between the north and eastern areas are investigated in the second part. Eigen-value analysis and participation factors are used to appreciate the nature of oscillations and the required PSS settings. The results show that, by using the properly tuned PSSs and accurate compensation of the reactive power, transient stability and damping of oscillations are considerably improved.     

Impact of Distributed Rooftop Photovoltaic Systems on Short-Circuit Faults in the Supplying Low Voltage Networks

This article evaluates the effect of randomly distributed, residential single-phase rooftop photovoltaic systems in the low voltage residential networks, during short-circuit faults on the overhead lines. The important parameters such as the fault current, the current sensed at the distribution transformer secondary, and the voltage profile along the feeder during the fault are examined. A sensitivity analysis is carried out in which the rating and location of the photovoltaic systems in the feeder, as well as the fault location and type, are the considered variables. Moreover, to demonstrate the effect of multiple photovoltaic systems with different ratings and penetration levels when distributed unequally among three phases of the network, a stochastic analysis is carried out. The article summarizes the outcomes of these two analyses to provide a better understanding of the impact of single-phase rooftop photovoltaic systems on the residential feeders during short-circuit faults.     

Modeling and fault diagnosis of a photovoltaic system

In this paper, a circuit-based simulation model of a photovoltaic (PV) panel by PSIM software package is developed, firstly. Then, a 3 kW PV arrays established by using the proposed PSIM model with series and parallel connection is not only employed to carry out the fault analysis, but also to represent its I–V and P–V characteristics at variable surface temperatures and isolations under normal operation. Finally, a novel extension diagnosis method based on the extended correlation function and the matter-element model was proposed to identify the faulting types of a 3 kW PV system. The simulated results indicate that the proposed fault diagnosis method can detect the malfunction types correctly and promptly.     

Multiconductor analysis of underground power transmission systems: EHV AC cables

It is widely ascertained that the multiconductor analysis is a powerful tool which can solve any structurally complex circuit (e.g., high-speed railway supply system, gas insulated lines, etc.). Moreover, this method can be also used as an in-depth analysis of the electric networks after the power-flow studies when they introduce simplifying hypotheses especially in the presence of asymmetry. In this paper, the multiconductor cell analysis has been applied to AC underground cable lines (UGC). This multiconductor procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of sheath bonding, possible multiple circuits, allows predicting the steady-state (and faulty) regime of any cable system. The method calculates the proportion and behavior of the phase currents carried by each parallel conductor, the circulating current in the sheath of each cable and the stray current in the earth. Moreover, some comparisons have been made with traditional programs showing the great accuracy of multiconductor cell model.     

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.     

Scenario selection in composite reliability assessment of deregulated power systems

Power market analysis should be incorporated in reliability assessments of deregulated power systems. For the Nordic power system, this is done by using The Multi-area Power-market Simulator (EMPS) for long-term power market analysis, where EMPS finds the optimal socio-economic dispatch on a weekly basis, with respect to, e.g., hydro reservoir levels. The EMPS analysis results in a set of load and generation scenarios, and these scenarios are interpreted as a sample of future power market behaviour, and is used as basis for a reliability assessment. These load and generation scenarios are referred to as power market scenarios.The power market analysis produces a large number of power market scenarios, and to include all these scenarios in a reliability assessment results in excessive computation time. The scenario selection method is presented and discussed. Scenario selection is used to pick out a subset of the generated power market scenarios, to only use this subset of scenarios as a basis for the reliability assessment. The paper provides some general guidelines for application of the scenario selection method. It is shown that the scenario selection method can reduce the scenario set by about 90%, with little loss of accuracy in the reliability assessment.     

A perturbation analysis of harmonic generation from saturated elements in power systems

Nonlinear phenomena such as saturation of magnetic flux have considerable effects in power systems analysis. It is reported that a failure in a real 500‐kV system triggered islanding operation, where resultant even harmonics caused malfunctions in protective relays. It is also reported that the major origin of this wave distortion is nothing but unidirectional magnetization of the transformer iron core. Time simulation is widely used today to analyze phenomena of this type, but it has basically two shortcomings. One is that the time simulation takes too much computing time in the vicinity of inflection points in the saturation characteristic curve because certain iterative procedures such as N‐R (Newton–Raphson) must be used and such methods tend to be caught in an ill‐conditioned numerical hunting. The other is that such simulation methods sometimes do not aid an intuitive understanding of the studied phenomenon because all of the nonlinear equations are treated in matrix form and are not properly divided into understandable parts, as is done in linear systems. This paper proposes a new computation scheme that is based on the so‐called perturbation method. Magnetic saturation of iron cores in a generator and a transformer are taken into account. The proposed method has a special feature to deal with the first shortcoming of the N‐R‐based time simulation method stated above. The proposed method does not use an iterative process to reduce the equation residue, but uses perturbation series, so that it is free of the ill‐conditioning problem. The user need only calculate the perturbation terms one by one until the necessary accuracy is attained. In a numerical example treated in the present paper, first‐order perturbation can achieve reasonably high accuracy, which means very fast computing time. In a numerical study, three nonlinear elements are considered. The calculation results are almost identical to the conventional N‐R‐based time simulation, which shows the validity of the method. The proposed method can be effectively used in screening where many case studies are needed. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 35–42, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20895     

Stochastic maximum power point tracking of photovoltaic energy system under partial shading conditions

A large portion of the available power generation of a photovoltaic (PV) array could be wasted due to partial shading, temperature and irradiance efects, which create current/voltage imbalance between the PV modules. Partial shading is a phenomenon which occurs when some modules in a PV array receive non-uniform irradiation due to dust, cloudy weather or shadows of nearby objects such as buildings, trees, mountains, birds etc. Maximum power point tracking (MPPT) techniques are designed in order to deal with this problem. In this research, a Markov Decision Process (MDP) based MPPT technique is proposed. MDP consists of a set of states, a set of actions in each state, state transition probabilities, reward function, and the discount factor. The PV system in terms of the MDP framework is modelled frst and once the states, actions, transition probabilities, and reward function, and the discount factor are defned, the resulting MDP is solved for the optimal policy using stochastic dynamic programming. The behavior of the resulting optimal policy is analyzed and characterized, and the results are compared to existing MPPT control methods.     

基于多重故障和混合仿真算法的风电系统概率短路分析

多重故障、风电并网增加了概率短路计算难度。引入双馈风电机组潮流算法,计及风速不确定性,得到概率短路前初始运行方式。引入多重短路算法,计算节点电压和支路电流的期望、方差以及概率分布。对枚举故障线路进行概率加权,抽样确定其他故障参数,建立混合仿真算法。综合考虑解析枚举和概率抽样特点,提出混合概率仿真方差计算公式。算例分析对比了风电出力波动、双重短路对概率短路计算结果的影响,比较了三序电压和电流的收敛性。研究结果为电气设备选择和电网安全分析提供了参考数据。     

Interaction between phase shifting transformers installed in the tie-lines of interconnected power systems and automatic frequency controllers

FACTS devices like TCPAR can be used to regulate the power flow in tie-lines of interconnected power system. The transient state power flow occurring after power disturbances can be influenced by using TCPAR equipped with power regulator and frequency-based stabilizer. The analysis of a simple interconnected power system consisting of two power systems has shown that the control of TCPAR can force a good damping of both power swings and oscillations of local frequency. In the case of a larger interconnected power system consisting of more than two power systems, the influence of the control of TCPAR on damping can be more complicated. Strong damping of local frequency oscillations and power swings in one tie-line may cause larger oscillations in remote tie-lines and other systems. Hence the use of devices like TCPAR as tools for damping power swings and frequency oscillations in a large interconnected power system must be justified by detailed analysis of power system dynamics. In this paper, some results of time-domain simulations of a three-system area are presented. These results have proven that it is possible to obtain a good damping of tie-line power and frequency swings by optimizing the main parameters of TCPAR installed in tie-lines. The results have been confirmed by an eigenvalue analysis of linearized model of interconnected system consisting of three subsystems.     

光伏发电系统宽电压开关变换器参数设计

光伏电池输出电压特性较软使得系统中DC-DC开关变换器需要有宽范围的工作电压,而开关变换器参数合理设计是其安全可靠工作的前提条件。本文以独立光伏发电系统中广泛应用的Buck-Boost变换器为例,分析了其工作原理,讨论了电感和电容对变换器工作模式和输出纹波电压的影响。在此基础上给出了宽输入和宽输出电压范围的电感、电容参数设计方法。通过仿真分析,验证了宽电压范围Buck-Boost变换器参数设计的合理性。     

Rational Reliability Centered Maintenance Optimization for power distribution systems

Reliability Centered Maintenance Method (RCM) currently used for maintenance optimization based on the analysis of modes of dysfunctional system failures (FMEA), and after determining the criticality of the system, an adequate maintenance plan is adopted. Following the succession of several applications of this method, in several areas, along twenty years of experience, it was proved that the use of this approach was incomplete for other analysis and evaluation criteria were completely ignored especially in electrical systems. Our view is to apply this method rationally by introducing a whole dependability study (RAMS).