Multi-objective congestion management incorporating voltage and transient stabilities

Congestion in a power system is turned up due to operating limits. To relieve congestion in a deregulated power market, the system operator pays to market participants considering their bids to alter their active powers. After relieving congestion, the network may be operated with a reduced voltage or transient stability margin because of hitting security limits or increasing the contribution of risky participants. The proposed multi-objective framework for congestion management in this paper simultaneously optimizes competing objective functions of congestion management cost, voltage security, and dynamic security. The voltage stability margin and corrected transient energy margin are employed as indices to be incorporated into the multi-objective congestion management. A fuzzy decision maker is proposed to derive the most efficient solution among Pareto-optimal solutions of multi-objective mathematical programming problem. Results of testing the proposed method on the New-England test system elaborate the efficiency of the proposed method.     

The impact of large-scale distributed generation on power grid and microgrids

With the widespread application of distributed generation (DG), their utilization rate is increasingly higher and higher in the power system. This paper analyzes the static and transient impact of large-scale DGs integrated with the distribution network load models on the power grid. Studies of static voltage stability based on continuous power flow method have shown that a reasonable choice of DG's power grid position will help to improve the stability of the system. The transient simulation results show that these induction motors in the distribution network would make effect on the start-up and fault conditions, which may cause the instability of DGs and grid. The simulation results show that modeling of distributed generations and loads can help in-depth study of the microgrid stability and protection design.     

发电机无功功率与机端电压对系统暂态功角稳定性的影响

阐述了系统暂态功角稳定的物理意义,运用扩展等面积法则(EEAC)理论详细分析了发电机的无功功率和机端电压水平对电力系统暂态功角稳定性的影响。指出提高领先群机组的无功出力和机端电压水平有利于电力系统的暂态功角稳定;而提高余下群机组的无功出力和电压水平则不利于电力系统的暂态功角稳定。通过安徽电网实例仿真计算对结论的正确性进行了论证。     

Representing wind turbine electrical generating systems in fundamental frequency simulations

Increasing numbers of wind turbines are being erected. In the near future, they may start to influence the dynamics of electrical power systems by interacting with conventional generation equipment and with loads. The impact of wind turbines on the dynamics of electrical power systems therefore becomes an important subject, studied by means of power system dynamics simulations. Various types of power system dynamics simulations exist and the approach depends on the aspect of power system dynamic behavior being investigated. In this paper, the focus is on fundamental frequency simulations, also known as electromechanical transient simulations. In this type of simulation, the network is represented as an impedance matrix and only the fundamental frequency component of voltages and currents is taken into account in order to reduce the computation time. This simulation approach is mainly used for voltage and angle stability investigations. Models of wind turbine generating systems that match the fundamental frequency simulation approach are presented and their responses are compared to measurements.     

Evaluating the benefits of an electrical energy storage system in a future smart grid

Interest in electrical energy storage systems is increasing as the opportunities for their application become more compelling in an industry with a back-drop of ageing assets, increasing distributed generation and a desire to transform networks into Smart Grids. A field trial of an energy storage system designed and built by ABB is taking place on a section of 11 kV distribution network operated by EDF Energy Networks in Great Britain. This paper reports on the findings from simulation software developed at Durham University that evaluates the benefits brought by operating an energy storage system in response to multiple events on multiple networks. The tool manages the allocation of a finite energy resource to achieve the most beneficial shared operation across two adjacent areas of distribution network. Simulations account for the key energy storage system parameters of capacity and power rating. Results for events requiring voltage control and power flow management show how the choice of operating strategy influences the benefits achieved. The wider implications of these results are discussed to provide an assessment of the role of electrical energy storage systems in future Smart Grids.     

Voltage control performance of AWS connected for grid operation

A scheme for connecting a marine power plant (MPP) to a distribution grid in local shore is presented. In this scheme, power generated from two wave energy generators based on Archimedes wave swing (AWS) is fed through a combination of dc/dc converter and inverter to the grid. Suitable controls of the dc/dc converter and the inverter are suggested to ascertain satisfactory performance of the proposed scheme under different operating environments. The MPP is controlled to regulate the bus voltage of the grid. The performance of the scheme is demonstrated in detail through PSCAD/EMTDC simulation. The simulation resulting in two distribution networks show good transient performance of the MPP under different types of network faults. The level of harmonics generated by the MPP systems is also acceptable.     

ANN based pattern classification of synchronous generator stabilityand loss of excitation

The paper presents a novel artificial intelligence-based neural network (ANN) pattern classification and online detection scheme for a single machine infinite bus system. The proposed online relay and dynamic pattern classifier utilizes specific frequency spectra of the hyperplane discriminant vector of machine rotor angle, speed, accelerating power, instantaneous power, voltage, and current using either a perceptron single layer detection scheme or a two layer feedforward ANN for online classification and detection of fault condition causing first swing transient stability or loss of excitation. Other relay binary outputs include fault type and allowable clearing time identification. The detection accuracy is improved by utilizing the cross spectra of discriminant vector input variables correlations. The proposed pattern classification technique can be extended to interconnected multimachine power systems by using relative rotor angles, frequency deviations, tie-line powers, and their cross spectra variables     

Combined operation of DC isolated distribution and PV systems for supplying unbalanced AC loads

This paper describes a DC isolated network which is fed by distributed generation (DG) from photovoltaic (PV) renewable sources to supply unbalanced AC loads. The battery energy storage bank has been connected to the DC network via DC/DC converter called storage converter to control the network voltage and optimize the operation of the PV generation units. The PV units are connected to the DC network via its own DC/DC converter called PV converter to ensure the required power flow. The unbalanced AC loads are connected to the DC network via its own DC/AC converter called load converter without transformer. This paper proposes a novel control strategy for storage converter which has a DC voltage droop regulator. Also a novel control system based on Clarke and Park rotating frame has been proposed for load converters. In this paper, the proposed operation method is demonstrated by simulation of power transfer between PV units, unbalanced AC loads and battery units. The simulation results based on PSCAD/EMTDC software show that DC isolated distribution system including PV units can provide the balanced voltages to supply unbalanced AC loads.     

Energy storage-based power buffering control for photovoltaic power generation system

光伏发电系统输出的最大功率随外界环境变化而波动,无法满足负荷的供电需求,针对该问题,建立了基于储能系统的光伏发电系统结构,介绍了光伏发电系统运行原理,分析了系统功率与直流母线电压的关系,设计了无源式储能系统和有源式储能系统对功率进行缓冲以满足控制目标.仿真结果表明,有源式储能系统较无源式储能系统有更好的功率调节作用,通过双向DC-DC变换器的储放能量自动切换控制,使得光伏发电系统输出的功率与负荷需求功率良好匹配,直流电压稳定.     

Enhancement of transient stability of an industrial cogeneration system with superconducting magnetic energy storage unit

This paper has developed the coordination of load shedding scheme and superconducting magnetic energy storage (SMES) unit to enhance the transient stability of a large industry cogeneration facility. The load-shedding scheme and the tie line tripping strategy by using the frequency relays have been designed to prevent the power system from collapse when an external fault of utility power system occurs. An actual external fault case and a simulated internal fault case have been selected to verify the accuracy of the load shedding scheme by executing the transient stability analysis. To improve the frequency and voltage responses, an SMES unit with various control modes has been installed in the cogeneration system. The sensitivity analysis of the SMES unit with different parameters is applied to achieve better system responses. Besides, an SMES unit with active power deviation as feedback signal is also considered to improve the electric power fluctuation of the study plant with rolling mills. It is found that the SMES system will enhance the electric power quality and minimize the economic losses of the cogeneration facility due to unnecessary load shedding.     

直驱型风力发电变流器低压穿越控制策略研究

研究了直驱型风力发电变流器系统低压穿越控制策略。首先提出了一种对三相电量进行快速准确的正负序分离软件锁相环。在此基础上,为消除直流电压的二次谐波,采用正、负序双电流内环控制不对称运行控制策略。正负序分离软件锁相环采用了正负序级联延时信号消除法,能够实现对三相电压电流基波正负序分量在同步旋转坐标下的快速提取,并且通过选择不同的参数,可以滤除任何次数谐波的干扰。该方法无需采用滤波器,从而同时具备了稳态精确性和动态快速性。现场实验结果表明,该软件锁相环为三相并网型风力发电变流器在电网发生跌落及谐波畸变时提供了良好运行控制提供保障,正负序双电流内环不对称运行的控制策略保证了在电网电压不对称跌落时的正负序分离控制,消除了直流电压的二次谐波。     

基于混合储能的并网光伏发电系统功率控制研究

针对并网光伏发电系统输出功率的波动,提出利用混合储能系统对功率进行平抑.介绍了光伏最大功率跟踪和并网逆变的控制,为实现发电和并网功率的匹配,考虑蓄电池和超级电容器各自特性的优势,对混合储能系统提出了三级式功率分配策略;通过设计相应的控制方法和以功率分配单元的输出功率作为参考值,混合储能系统控制变换器进行合理充放电.混合储能系统不仅保证了并网功率按计划运行,而且稳定了直流母线电压、满足了随机负荷供电.通过仿真验证,三级式功率分配策略有效,控制方法可行.     

A review on the design and control strategy of energy storage system in wind power application

随着风力发电大规模入网,其随机性,波动性和间歇性特征对电力系统调频,调峰等有功平衡手段及电压稳定的影响越来越严重.储能系统能够在一定程度上控制风场的输出功率,平抑风电功率波动,改善风机低电压穿越能力,甚至为系统提供辅助服务,是从风场侧提高系统对风电的接纳能力的可行解决方案之一.作者在简要的介绍了风场储能技术应用现状的基础上,重点针对储能型风场内蓄电池储能系统的设计方案,容量优化及控制策略的研究现状及关键问题进行综述及探讨.     

The concept of distribution flexible network PV system

In stand-alone photovoltaic (PV) systems, when the battery is fully charged, the excess generated power is wasted. To solve the problems of wasted excess power, a distributed flexible network photovoltaic (DFNPV) system is studied. It incorporates many PV subsystems each consisting of PV panel, DC/DC converter, and load, and are connected to each other with shared batteries. The excess generated power of the subsystem is transferred between PV subsystems to compensate the lack of power in other subsystems.The control method of transferring power is based on simple voltage control of the subsystems. The output voltage in a given subsystem decreases if a transient excessive load is larger than the generated power; as a result excess power is transferred from another subsystem that has sufficient power and higher voltage output. In this study, this proposed operation method is demonstrated by simulation of power transfer between two subsystems and among four subsystems. Furthermore, to estimate the size of the DFNPV system within an acceptable voltage drop, the relationships between cable length, power loss, and cable types are discussed.     

考虑暂态电压稳定性的电力系统多目标无功备用优化

鉴于目前电力系统无功备用优化方法较少涉及暂态电压稳定性,提出一种同时提高电力系统静态/暂态电压稳定性的无功备用优化模型,该模型首先基于数据密度提出一种无功电压控制分区方法,分别在静态/暂态电压的薄弱区域内定义静态/暂态无功备用,然后结合所建立的两个目标建立多目标无功备用优化模型,最后提出一种基于多阶段的多目标优化求解方法。通过求解所建模型,获得能够同时提升系统静态/暂态电压稳定性的无功备用配置方案。对IEEE 39节点系统的仿真分析证明了所提方法的有效性。     

PSCAD/EMTDC-Based Modeling and Analysis of a Gearless Variable Speed Wind Turbine

This paper presents dynamic modeling and simulation of a grid connected variable speed wind turbine (VSWT) using PSCAD/EMTDC, a widely used power system transient analysis tool. The variable speed wind system with a direct-drive generator and power electronics interface is modeled for dynamic analysis. Component models and equations are addressed and their implementations into PSCAD/EMTDC are described. Controllable power inverter strategy is intended for capturing the maximum energy from varying wind speed and maintaining reactive power generation at a pre-determined level for constant power factor or voltage regulation. The component models and entire control scheme are constructed by user-defined function provided in the program. Simulation studies provide control performance and dynamic behavior of a gearless VSWT under varying wind speeds. In addition, the system responses to network fault conditions have been simulated. This modeling study can be employed to evaluate the control scheme, output performance and impacts of a VSWT on power grid at planning or designing stage.     

Comparative study on transient stability analysis of wind turbine generator system using different drive train models

A huge number of wind generators are going to be connected with the existing network in the near future. Therefore it is necessary to analyse the transient stability of power systems, including wind turbine generator systems (WTGS). It has already been reported that one-mass or lumped model of wind turbine system is insufficient to analyse the transient behaviour of WTGS. It has also been reported that for the precise transient analysis of WTGS, a six-mass drive train model is needed. The reduced order models (three-mass and two-mass) have also been adopted so far for transient behaviour analysis. But the transient stability analysis of using six-mass, three-mass and two-mass drive train models has not been reported sufficiently so far in the literature. The authors have conducted an analysis using these methods. First, a detailed transformation procedure is presented from six-mass drive train model to two-mass model, which can be used in the analysis of transient stability simulation with sufficient accuracy. It is then determined which drive train model is appropriate for transient stability analysis of grid-connected WTGS. The effects of drive train parameters (such as inertia constant, spring constant and damping constant) on stability are examined using the above mentioned types of drive train models. Moreover, different types of symmetrical and asymmetrical faults at different wind generator power levels are considered in the simulation analyses with and without considering damping constants in six-mass, three-mass and two-mass shaft models. Considering the simulation results, it can be concluded that two-mass shaft model is sufficient for the transient stability analysis of WTGS.     

Study on the operation control strategies of microgrid islanding of photovoltaic and energy storage

主要讨论了光储微网在孤岛运行模式下的控制策略,分别对光伏逆变器进行PQ控制,对储能变流器进行恒压恒频控制实现微网在孤岛模式下对负载的稳定供电.在Simulink中搭建仿真平台对所提出理论进行仿真,对仿真结果进行分析,验证了所提控制策略的正确性和有效性.     

A novel line drop secondary voltage control algorithm for variable speed wind turbines

This paper addresses the design and implementation of the line drop secondary voltage control (LDSVC) for the doubly fed induction generator‐wind turbine (DFIG‐WT) complemented with reactive power allocation algorithm to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the electric power system. The LDSVC is used to generate the local voltage reference, providing an improvement for overall voltage profile. The paper presents the influence of the integration of variable speed wind turbines‐based doubly fed induction generator (DFIG) while employing LDSVC for increasing the transient stability margin. This paper proposes an improved voltage control scheme, based on a secondary voltage controller complemented with an automatic gain controller (AGC). The scheme is applied to a wind energy system incorporating DFIG‐based wind turbines. The controller structure is developed and the performance of the self‐tuning AGC scheme is developed and analysed. The proposed controller is tested in response to system contingencies for different short circuit ratios. The performance of the secondary voltage control without and with AGC is verified. The influence of the AGC in improving the transient response and damping of voltage oscillations is verified. Copyright © 2010 John Wiley & Sons, Ltd.     

A versatile method for computation of power pulsations in DFIG under grid imperfections

In the context of a Doubly Fed Induction Generator (DFIG) connected to the utility grid under unbalanced voltage conditions, the controller design needs to ensure additional challenges such as restricting the Total Harmonic Distortion (THD) in grid current, minimizing the pulsations in generated power, torque, dc link voltage etc. apart from facilitating the generator power control. Thus the schemes for generating reference currents for rotor converters need to incorporate a measure of power pulsations to what is required for steady state power flow control. This paper proposes a versatile scheme for computing the power pulsations in a DFIG connected to grid under unbalance voltage conditions. The active and reactive power oscillations are computed in a simple and straight forward manner using the measured stator voltage and currents in the positive d-q frame without using flux estimation. The scheme is free from flux integration or differentiation, rotor position computation and independent of machine parameters. Further, the worst case error in computation is bound within 3% considering 30% voltage dip, 7% of harmonics, ±10°phase jump or ±10% dc offset in the grid voltage. The effectiveness of the scheme is validated through PSCAD/EMTDC simulations and experimental results for a 2.3 kW DFIG test setup.