Impact of FACTS controllers on the stability of power systems connected with doubly fed induction generators

The increasing power demand has led to the growth of new technologies that play an integral role in shaping the future energy market. Keeping in view of the environmental constraints, grid connected wind turbines are promising in increasing system reliability. This paper presents the impact of FACTS controllers on the stability of power systems connected with wind energy conversion systems. The wind generator model considered is a variable speed doubly fed induction generator model. The stability assessment is made first for a three phase short circuit without FACTS controllers in the power network and then with the FACTS controllers. The dynamic simulation results yield information on (i) the impact of faults on the performance of induction generators/wind turbines, (ii) transient rating of the FACTS controllers for enhancement of rotor speed stability of induction generators and angle stability of synchronous generators. EUROSTAG is used for executing the dynamic simulations.     

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.     

Phase-domain power flows in the rectangular co-ordinates frame of reference including VSC-based FACTS controllers

This paper presents a method for systematic modelling of VSC-based FACTS controllers within three-phase power flows in rectangular co-ordinates. Among the FACTS controllers modelled are the STATCOM, SSSC, UPFC and HVDC-VSC. The approach taken is to represent the fundamental frequency operation of each power converter as a three-phase voltage source behind a leakage reactance, where one or two of them may be connected in either series or parallel depending on the FACTS controller being modelled. Active and reactive power flow equations are developed for each voltage source circuit together with constraint equations to account for co-ordinated operation of two converters, such as in modelling of UPFC and HVDC-VSC. The power flow equations representing the VSC-based FACTS controllers are combined with the nodal power equations of the power network for a combined iterative solution using a Newton–Raphson three-phase power flow algorithm in rectangular co-ordinates enabling robust and efficient solutions of three-phase power networks with any number and kind of VSC-based controllers.     

扩展等面积法暂态稳定分析的FACTS控制器模型

提出了应用扩展等面积法（EEAC）暂态稳定分析的FACTS控制器统一模型,将FACTS对暂态稳定的作用通过对电力系统的串联和并联控制来描述。研究了这一模型对电力系统网络方程、转子运动方程和EEAC单机无穷大系统等值的影响,推导了适合于EEAC各种方法和各类FACTS控制器的单机无穷大（OMIB）系统等值统一表达式。该模型被应用于伊-冯500 kV可控串补的EEAC暂态稳定分析,并应用中国电力科学研究院电力系统分析综合程序进行了校核,结果证明了这种FACTS模型是有效和正确的。     

在dq0坐标系建立的VSC-HVDC控制策略

对电压源换流器的暂态数学模型和控制策略进行了研究,建立了dq0坐标系下VSC-HVDC系统的暂态数学模型,并设计了相应的dq解耦控制器,实现了系统有功功率和无功功率的独立调节。利用PSCAD／EMTDC仿真软件对所设计的控制器进行了稳态和动态仿真,结果证明所设计的控制器是正确的,该控制器在系统稳态和动态情况下均具有良好的控制性能。     

A three-phase SSVS model for power flow studies in unbalanced transmission networks

The goal of this paper is to develop a steady-state mathematical model of the new generation of power electronic-based plant components presently emerging as a result of the concept of flexible AC transmission systems (FACTS), namely the series synchronous voltage source (SSVS) controller. The modelling is carried out in the phase co-ordinate frame of reference to study the SSVS power flow control performance in three-phase transmission networks. In this context, the controller power flow equations are integrated into an existing three-phase Newton–Raphson power flow program. Guidelines and methods for implementing the proposed model are described for highly robust and unified iterative solutions. The validity of this model is verified by a numerical example. The paper concludes with an example of power flow control in a network operating under unbalanced conditions.     

Grasshopper optimization algorithm optimized multistage controller for automatic generation control of a power system with FACTS devices

This paper uses a Grasshopper Optimization Algorithm (GOA) optimized PDF plus (1+ PI) controller for Automatic generation control (AGC) of a power system with Flexible AC Transmission system (FACTS) devices. Three differently rated reheat turbine operated thermal units with appropriate generation rate constraint (GRC) are considered along with different FACTS devices. A new multistage controller design structure of a PDF plus (1 + PI) is introduced in the FACTS empowered power system for AGC while the controller gains are tuned by the GOA. The superiority of the proposed algorithm over the Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) algorithms is demonstrated. The dynamic responses of GOA optimized PDF plus (1+ PI) are compared with PIDF, PID and PI controllers on the same system. It is demonstrated that GOA optimized PDF plus (1+ PI) controller provides optimum responses in terms of settling time and peak deviations compared to other controllers. In addition, a GOA-tuned PDF plus (1 + PI) controller with Interline Power Flow Controller (IPFC) exhibits optimal results compared to other FACTS devices. The sturdiness of the projected controller is validated using sensitivity analysis with numerous load patterns and a wide variation of parameterization. To further validate the real-time feasibility of the proposed method, experiments using OPAL-RT OP5700 RCP/HIL and FPGA based real-time simulations are carried out.     

Coordinated application of FACTS controllers to damp out inter-area oscillations

This paper proposes a systematic analysis and design procedure for simultaneously determining the best locations and input signals of flexible AC transmission system (FACTS) devices in order to damp out inter-area oscillations. First, a modified modal power flow oscillation flow method is developed to determine the nature of the energy exchange over the transmission network in the presence of FACTS devices. With this method, the modal distribution along critical system paths is identified, and the contribution of each machine and network device is computed. Controllability and observability studies are then used to assess the effect of existing FACTS controllers on system damping as well as to identify optimal locations for new devices. The proposed procedure is demonstrated on a 46-machine, 190-bus reduced-order equivalent model of the Mexican interconnected system that includes several static VAR compensators.     

FACTS装置对含风电互联系统低频振荡特性分析

随着电力系统互联加强,风电并网容量的增加对电网的稳定运行影响越来越大。构建了以UPFC(Unified Power Flow Controller, UPFC)和SVC(Static Var Compensator, SVC)为代表的FACTS装置与含风电系统的数学模型。采用留数指标定位FACTS装置,设计了附加阻尼控制器(Additional Damping Controller, ADC)。基于IEEE 2区域4机互联系统,从特征根分析和时域仿真两个方面分析了FACTS装置对含风电互联系统低频振荡特性的影响。研究结果表明,加装附加阻尼控制器的FACTS装置能够维持母线电压以及发电机转速的稳定,增加互联系统联络线功率传输范围,抑制低频振荡引起的电网参数波动,改善了含风电电力系统的低频振荡特性。     

Determination of Maximum Loadability by a Mixed Complementarity Formulation of the Adjusted Power Flow Problem

The ever-increasing demand for power leads to increased loading on the power system. It is necessary to determine the maximum loadability of power systems so that a sufficient margin can be maintained. Maximum loadability can be determined by solving the power flow equations successively while increasing the loading gradually; conventionally, the unadjusted power flow problem is solved. This also helps in determining the voltage profile with respect to loading. However, determination of loadability margin by the solution of adjusted power flow problem is challenging in the presence of certain limits such as the limits of tap-changing and phase-shifting transformers, and the operating limits of Flexible AC Transmission Systems (FACTS) controllers. In the course of determination of maximum loading, it is likely that these limits are encountered. A novel method for systematic handling of these limits is presented in this paper. A Mixed Complementarity Problem formulation is proposed for inclusion of the limits of tap-changing and phase-shifting transformers, and FACTS controllers. The proposed method is tested on the IEEE 300 bus system and the WSCC 9 bus system in the presence of FACTS controllers.     

Dynamic ATC enhancement through optimal placement of FACTS controllers

Flexible AC Transmission System (FACTS) controllers offer an effective means to enhance the power transfer capability of the network. However, the extent to which a FACTS controller can enhance Available Transfer Capability (ATC) depends on its optimal location in the system. This paper has proposed sensitivity analysis of structure preserving energy margin with respect to the control parameters of FACTS controllers for their optimal placement in the network. Two types of FACTS controllers, viz. Static Synchronous Compensator (STATCOM) and Unified Power Flow Controllers (UPFC) have been considered. The hybrid approach, combining a structure preserving and a time domain simulation method, has been utilized to compute the dynamic ATC in presence of these controllers and their impact on dynamic ATC has been analyzed. The potential energy, contributed by the STATCOM and the UPFC, has also been included in the structure preserving energy function to include their influence on transient stability. The proposed method has been tested on 39-bus New England system and a practical 246-bus Indian system.     

Quantitative evaluation of generator power control effect of FACTS controllers for power system stabilization

The advancement of power electronics technologies has significantly developed the power system stabilizing controllers. Quantitative as well as qualitative evaluation of their effectiveness in power systems is a matter of great importance for the feasibility investigation of these apparatus. In this paper, the possible control region of FACTS controllers with series and/or shunt configuration in a single machine to infinite bus system is formulated in the powerangle curve with a set of algebraic equations. The effectiveness of TCPST (Thyristor‐Controlled Phase Shifting Transformer), SSSC (Static Synchronous Series Compensator), and TCSC (Thyristor‐Controlled Series Compensator) for the improvement of the transient stability is evaluated quantitatively as a numerical example. The correctness of the proposed method has been confirmed by analysis based on the electromagnetic transients simulation with a detailed system model. © 2001 Scripta Technica, Electr Eng Jpn, 138(3): 43–51, 2002     

计及FACTS装置的概率特征根分析

概率特征根分析计及了较宽范围的系统运行方式变化,利用柔性交流输电系统FACTS(Flexible AC Transmission System)装置上的附加控制器可以改善系统的动态特性,将现有的概率特征根分析扩展到包含FACTS功能模块。以并联型静态无功补偿器SVC(Static Var Compensator)和串联型可控串联补偿器TCSC(Thyristor-Controlled Series Capacitor)为例,在原有概率特征根模型的基础上,依据具体的元件模型和控制器表达,确定了形成系统状态空间方程时的相应线性化表达式;详细讨论了功能增加后系统状态方程矩阵的形成;通过补充相关的灵敏度计算,完成了计及FACTS装置的电力系统多运行方式下的小干扰稳定性分析。最后,在一个八机系统上进行了试算。在选定的附加控制器参数下,比较了增加SVC前后的系统主导特征根的变化,考察了附加控制器增益变化对临界特征根的影响。     

基于RGA的FACTS控制器之间的交互影响研究

多台FACTS控制器之间的交互影响与协调是FACTS技术应用中亟待解决的问题,介绍了采用相对增益矩阵(RGA)方法,分析了多机电力系统中多台柔性交流输电系统(FACTS)控制器之间的交互影响。通过对4机两区域系统以及新英格兰系统两个实例的分析研究,分别采用频域分析、特征值分析以及时域仿真三种方法来验证RGA所得的结论,证明了RGA方法可以用来分析FACTS间的交互现象。     

Decoupling Controller Design and Controllable Regions Analysis for the Space Vector Modulated Matrix Converter-Unified Power Flow Controller in Transmission Systems

The flexible alternating current transmission systems (FACTS) devices are essential components of the transmission system to enhance the controllability and elevate the transfer capacity of the network. The unified power flow controller is known as the most versatile device in the FACTS family. This work studies a distinctive unified power flow controller (UPFC) structure based on the direct matrix converter to regulate the active and reactive power in a transmission system. In contrast to the conventional UPFC, there is no requirement for a bulky energy storage element in this structure. This results in various benefits including: decreased system volume, improved efficiency, prolonged lifetime, reduced maintenance and removal of the DC-link control. The full power controllable regions are analyzed and graphically obtained for the MC-UPFC, which facilitates the selection of proper UPFC ratings. The working principles and a model of the MC-UPFC are put forward and discussed, followed by explanations of direct space vector modulation (SVM) for this application. Based on the SVM modulation scheme, PID controllers are developed to control power flows in a double-line transmission system. In addition, decoupling controllers are derived by feeding back the coupling components into controllers. The numerical simulation results for a double-line transmission system corroborate the feasibility and effectiveness of the proposition.     

Mathematical model of five-level ac/dc converter in abc reference frame

This paper proposes a mathematical model for three-phase multilevel diode clamped ac/dc converter (DCC) in abc-reference frame, in order to study dynamic responses and stability of the system, where multilevel converters are employed. An attempt is made to model the converter adopting 5-level DCC and can be easily extended to DCC with any number of levels. The converter output and dc capacitor voltages are expressed in terms of input source currents and switching functions; which are basic inputs provided to a converter. Connection and capacitor currents of the converter have been analyzed. The effectiveness of the proposed mathematical model is verified by the computer simulations and experimental results. The obtained mathematical model has been validated with the physical converter model in MATLAB/Simulink environment and the prototype model using dSPACE DS under closed loop control system. The developed mathematical model equations are able to represent the physical converter with similar characteristics.     

Enhancement of interconnected power system stability using a control strategy involving static phase shifters

The static phase shifting transformer is one of the potential options of the recently proposed FACTS (flexible AC transmission systems). Promising results have been obtained for enhancing the small-disturbance and the transient stability of interconnected power systems.

In this paper, the important concept of involving in the same control strategy both generating units and static phase shifters has been considered. A systematic procedure for designing co-ordinated and decentralized controllers of these components is provided to assure a satisfactory dynamic performance of an interconnected power system under both small and large perturbations. The approach uses optimal control theory as a basis for the co-ordination of static phase shifter and governor controllers. A suboptimal decentralized control scheme is derived from the designed optimal controller by using a ‘minimum norm’ nearness criterion. The resulting feedback control signals for each generating unit and for each phase shifter is expressed in terms of measurable and local variables only.

Test results show the effectiveness of the proposed control strategy and the usefulness of control actions on static phase shifters.     

Optimal controllers based on FACTS devices intended for decentralized control of integrated large electrical power systems

The decentralized synthesis of stabilizing controllers based on FACTS devices in an integrated electrical power system (IEPS), which secures minimum energy consumption for control, has been examined. This task can be solved with the help of decomposition and aggregation of the initial model of the IEPS to regional electric power systems (REPS) (subsystems). For aggregated models of the subsystems, a control is synthesized based on Riccati’s quadratic equations or, in the case of asymptotical stability of aggregated models of the sub-system, based on Lyapunov’s linear equations.     

抑制区域间低频振荡的FACTS阻尼控制

随着电力系统规模的不断扩大,区域间低频振荡正成为限制电网传输能力的瓶颈。对少数发电机组安装PSS来抑制区域间低频振荡很难有好的效果。但FACTS因其安装地点的灵活性及良好的动态性能而给抑制区域间振荡提供了新的手段。为此,利用相角补偿原理,设计控制器持续减小区域间的振荡能量,以此来实现区域间阻尼控制。以SVC为例详细说明附加阻尼控制器的设计,通过PSASP软件下的仿真结果表明,具有附加阻尼控制作用的SVC能有效地抑制区域间低频振荡。另外,对其它几种常用的FACTS器件也设计了阻尼控制器,并同样通过了PSASP下的仿真验证,阻尼效果很好。以上结果证明利用FACTS可实现区域间低频振荡的阻尼控制。     

双馈入直流输电系统中VSC-HVDC的控制策略

针对多馈入直流输电(multi-infeed direct current,MIDC)系统的稳定性问题,提出将基于电压源换流器的高压直流输电(voltage source converter based HVDC,VSC-HVDC)引入到MIDC系统中,用以改善MIDC系统公共连接母线的电压特性。建立HVDC和VSC-HVDC双馈入系统的物理模型,导出相应的数学模型。并通过坐标变换得出VSC功率传输方程的直角坐标形式。采用多变量非线性控制的逆系统方法,设计VSC-HVDC系统的非线性控制器。PSCAD/EMTDC 环境下的仿真实验表明,所设计的VSC-HVDC非线性控制器不仅能有效改善VSC-HVDC的动态特性,而且在交流系统发生扰动时能有效稳定系统电压,减少HVDC逆变站发生换相失败的几率,提高HVDC系统的运行可靠性。