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A Comprehensive Review and Adequacy Evaluation of Voltage Stability Indices for Modern Distribution Systems
(PRAISE WORTHY PRIZE SRL, 2010) Eminoglu, Ulas; Hocaoglu, M. Hakan; Ayasun, Saffet
It is well known that the network variables and parameters contain sufficient information to obtain the most sensitive bus or line to the voltage collapse in power systems. Accordingly, there has been several static voltage stability indices developed for identifying the buses or lines on the verge of voltage collapse in the literature. In this paper, various voltage stability indices including those originally developed for transmission systems are first reviewed. Then, their adequacies are quantitatively evaluated on a modern distribution test system which has voltage controlled bus (PV-bus). From analyses, it is observed that most of the existing voltage stability indices would be inadequate for assessing the most sensitive bus or line on the verge of voltage collapse in such modern distribution systems. Copyright (c) 2010 Praise Worthy Prize S.r.l. - All rights reserved.
A comprehensive review and adequacy evaluation of voltage stability indices for modern distribution systems
(Praise Worthy Prize S.r.l, 2010) Eminoglu, Ulas; Hocaoglu, M. Hakan; Ayasun, Saffet
It is well known that the network variables and parameters contain sufficient information to obtain the most sensitive bus or line to the voltage collapse in power systems. Accordingly, there has been several static voltage stability indices developed for identifying the buses or lines on the verge of voltage collapse in the literature. In this paper, various voltage stability indices including those originally developed for transmission systems are first reviewed. Then, their adequacies are quantitatively evaluated on a modern distribution test system which has voltage controlled bus (PV-bus). From analyses, it is observed that most of the existing voltage stability indices would be inadequate for assessing the most sensitive bus or line on the verge of voltage collapse in such modern distribution systems. © 2010 Praise Worthy Prize S.r.l. - All rights reserved.
A sensitivity approach for computation of the probability density function of critical clearing time and probability of stability in power system transient stability analysis
(ELSEVIER SCIENCE INC, 2006) Ayasun, Saffet; Liang, Yiqiao; Nwankpa, Chika O.
This paper presents a linear approximation method to determine the probability density function (PDF) of the critical clearing time (CCT) and probability of stability for a given disturbance in power system transient stability analysis. The CCT is the maximum time interval by which the fault must be cleared in order to preserve the system stability. The CCT depends on the system load level and thus, is modeled as a random variable due to the probabilistic nature of system load demand. The proposed method first determines the sensitivity of the CCT with respect to the system load, and using these sensitivities it computes the PDF of the CCT based on the PDF of the system load. The probability of system being transiently stable for a particular disturbance and for a given fault clearing time is calculated using the PDF of CCT. This approach is verified to be accurate under the condition of small load deviation by Monte Carlo simulations method. Moreover, the proposed method reduces the computational effort significantly in Monte Carlo simulations indicating that it could be used in real-time on-line applications. (c) 2005 Elsevier Inc. All rights reserved.
Ağ Üzerinden Kontrol Edilen Yükselten DA-DA Dönüştürücünün Zaman Gecikmesine Bağlı Kararlılık Analizi
(2018) Sarı, Alperen; Sönmez, Şahin; Ayasun, Saffet
Bu çalışmada, yükselten doğru Akım (DA)-doğru Akım (DA) dönüştürücülerin ağ üzerinden kapalı çevrim kontrol edilmesi durumunda, kullanılan haberleşme ağının yapısına ve veri iletimine bağlı olarak sistemin dinamik performansını olumsuz etkileyecek haberleşme zaman gecikmeleri gözlemlenmektedir. Sistemin sınırda kararlı olacağı maksimum haberleşme zaman gecikmesinin hesaplanması, sistemin güvenilir ve kararlı bir biçimde kontrolünün yapılabilmesi için önemlidir. Bu çalışmada, ağ üzerinden kontrol edilen yükselten DA-DA dönüştürücünün zaman gecikmesine bağlı kararlılık analizi yapılmıştır. Bu amaçla, ilk olarak yükselten DA-DA dönüştürücünün denge noktası etrafında geçerli olan doğrusal zaman gecikmeli durum uzay denklem modeli elde edilmiştir. Daha sonra, oransal-integral (PI) denetleyicinin farklı değerleri için Kronecker çarpım ve temel dönüşüm metodu uygulanarak sistemin sınırda kararlı olacağı maksimum zaman gecikmesi değerleri analitik olarak hesaplanmıştır. Son olarak, bulunan teorik maksimum zaman gecikme değerlerinin doğruluğu, zaman gecikmeli karakteristik denklemlerin köklerini bulma algoritması ve zaman düzleminde yapılan benzetim çalışmaları yardımıyla gösterilmiştir.
An Exact Method for Computing Delay Margin for Stability of Load Frequency Control Systems With Constant Communication Delays
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2016) Sonmez, Sahin; Ayasun, Saffet; Nwankpa, Chika O.
The extensive usage of open communication networks in power system control causes inevitable time delays. This paper studies impacts of such delays on the stability of one-area and two-area load frequency control (LFC) systems and proposes an analytical method to determine delay margins, the upper bound on the delay for stability. The proposed method first eliminates transcendental terms in characteristic equation of LFC systems without making any approximation and transforms the transcendental characteristic equation into a regular polynomial. The key result of the elimination process is that real roots of the new polynomial correspond to imaginary roots of the transcendental characteristic equation. With the help of new polynomial, it is also possible to determine the delay-dependency of system stability and root tendency with respect to the time delay. An analytical formula is then developed to compute delay margins in terms of system parameters. For a large set of controller gains, delay margins of LFC systems are computed to investigate the qualitative effect of controller gains on the delay margin. Finally, simulations studies are carried out to verify the effectiveness of the proposed method.
Comprehensive gain and phase margins based stability analysis of micro-grid frequency control system with constant communication time delays
(INST ENGINEERING TECHNOLOGY-IET, 2017) Guenduez, Hakan; Soenmez, Sahin; Ayasun, Saffet
This study presents a comprehensive delay-dependent stability analysis of a micro-grid system with constant communication delays. First, an exact method that takes into account both gain and phase margins (GPMs) is proposed to determine stability delay margins in terms of system and controller parameters. The method implements an elimination procedure to transform transcendental characteristic equation into a standard polynomial of the crossing frequency. The real roots of this new standard polynomial exactly match with the purely imaginary roots (crossing frequencies) of the original characteristic equation with transcendental terms. Second, an effective and simple graphical method is implemented to compute all stabilising proportional integral (PI) controller gains for a given time delay. The approach is based on extracting stability region and the stability boundary locus in the PI controller parameter space having user defined GPMs, and relative stability. The time-domain simulation studies indicate that the proposed schemes give better desired dynamic performance as compared with the recently developed schemes for micro-grid with communication delays.
Computation of All Stabilizing PI Controller Parameters of Hybrid Load Frequency Control System with Communication Time Delay
(IEEE, 2017) Erol, Halil; Sezer, Halil; Ayasun, Saffet
This paper presents an effective and simple graphical method to compute all stabilizing Proportional Integral (PI) controller gains of a hybrid load frequency control (LFC) system that contains time delays due to use of open and distributed communication network. The method is based on extracting stability region and the stability boundary locus in the PI controller parameters. The accuracy of the proposed method is verified by time-domain simulations.
Computation of Robust PI-Based Pitch Controller Parameters for Large Wind Turbines
(IEEE Canada, 2020) Turksoy, Omer; Ayasun, Saffet; Hames, Yakup; Sonmez, Sahin
This paper deals with the computation of all proportional-integral (PI)-based pitch controllers which achieve the desired frequency-domain specifications, namely, gain and phase margins (GPMs) of a large wind turbine (LWT) with communication delays. An efficient graphical method based on extracting the boundaries of stability regions in PI controller parameter space having user-defined GPMs has been employed to determine GPM-based stability regions for a wide range of time delays. The theoretical region boundaries are validated by using a powerful numerical method known as the quasi-polynomial mapping-based root finder (QPmR) and time-domain simulations. Results indicate that the proposed scheme gives an improved dynamic response compared to the recently developed scheme based on stability only for the pitch control of LWTs with communication delays.
Computation of Stability Delay Margin of Time-Delayed Generator Excitation Control System with a Stabilizing Transformer
(HINDAWI PUBLISHING CORPORATION, 2014) Ayasun, Saffet; Eminoglu, Ulas; Sonmez, Sahin
This paper investigates the effect of time delays on the stability of a generator excitation control system compensated with a stabilizing transformer known as rate feedback stabilizer to damp out oscillations. The time delays are due to the use of measurement devices and communication links for data transfer. An analytical method is presented to compute the delay margin for stability. The delay margin is the maximum amount of time delay that the system can tolerate before it becomes unstable. First, without using any approximation, the transcendental characteristic equation is converted into a polynomial without the transcendentality such that its real roots coincide with the imaginary roots of the characteristic equation exactly. The resulting polynomial also enables us to easily determine the delay dependency of the system stability and the sensitivities of crossing roots with respect to the time delay. Then, an expression in terms of system parameters and imaginary root of the characteristic equation is derived for computing the delay margin. Theoretical delay margins are computed for a wide range of controller gains and their accuracy is verified by performing simulation studies. Results indicate that the addition of a stabilizing transformer to the excitation system increases the delay margin and improves the system damping significantly.
Computation of time delay margin for power system small-signal stability
(JOHN WILEY & SONS LTD, 2009) Ayasun, Saffet
With the extensive use of phasor measurement units (PMU) in the wide-area measurement/monitoring systems (WAMS), time delays have become unavoidable in power systems. This paper presents a direct and exact method to Compute the delay margin of power systems with single and commensurate time delays. The delay margin is the maximum amount Of time delay that the system can tolerate before it becomes unstable fora given operating point. First, without using any approximation or substitution, the transcendental characteristic equation is converted into a polynomial without the transcendentality such that its real roots coincide with the imaginary roots of the characteristic equation exactly. The resulting polynomial also enables us to easily determine the delay dependency of the system stability and the sensitivities of crossing roots with respect to time delay. Then, an expression in terms of system parameters and imaginary root of the characteristic equation is derived for computing the delay margin. The proposed method is applied to a single-machine-infinite bus (SMIB) power system with an exciter. Delay margins are computed for a wide range of system parameters including generator mechanical power, damping and transient reactance, exciter gain, and transmission line reactance. The results indicate that the delay margin decreases as the mechanical power, exciter gain and line reactance increase while it increases with increasing generator transient reactance Additionally, the relationship between the delay margin and generator damping is found be relatively complex. Finally, the theoretical delay margin results are validated using the time-domain simulations of Matlab. Copyright (C) 2008 John Wiley & Sons, Ltd.
Computation of time delay margins for stability of a single-area load frequency control system with communication delays
(World Scientific and Engineering Academy and Society, 2014) Sönmez, Şahin; Ayasun, Saffet; Eminoğlu, U.
Time delays have become unavoidable in power systems since communication links are extensively used for sending and receiving control signals. This paper investigates the effect of time delays on the stability of a single-area load Frequency Control (LFC) system. A direct and exact method to compute delay margins is presented. The delay margin is the maximum amount of the time delay that the system can tolerate before it becomes unstable for a given operating point. The proposed method starts with the determination of all possible purely imaginary characteristic roots for any positive time delay. To achieve this, Rekasius substitution is first used to convert the transcendental characteristic equation of the LFC system into a polynomial. Then, Routh stability criterion is applied to determine the critical root, the corresponding oscillation frequency and the delay margin for stability. For a wide range of controller gains, delay margins of LFC system are determined to find out the qualitative effect of controller gains on the delay margin. Finally, theoretical delay margin results are verified by using the time-domain simulation capabilities of Matlab/Simulink.
Damping Based Relative Stability Regions in Load Frequency Control System with Plug-in Electric Vehicles and Communication Delays
(IEEE, 2020) Naveed, Ausnain; Sonmez, Sahin; Ayasun, Saffet
This paper presents a damping based stability analysis of a time delayed single-area load frequency control (LFC) system with plug-in Electric Vehicles (EVs) Aggregator by employing a graphical method. The proposed technique computes all the stabilizing gain values of Proportional Integral (PI) controller of the LFC with plug-in EVs (LFC-EVs) system. The proposed method relies on identifying stability region and the stability boundary locus in the PI controller parameter plane having user defined relative stability. These damping based stability regions are obtained and the accuracy of their Complex Root Boundary (CRB) and Real Root Boundary (RRB) is validated by an independent algorithm and time-domain simulations. Moreover, a simple and effective analytical approach known as Weighted Geometrical Center (WGC) is used for tuning the stabilizing controller parameters to achieve better system performance.
DC motor speed control methods using MATLAB/Simulink and their integration into undergraduate electric machinery courses
(JOHN WILEY & SONS INC, 2008) Ayasun, Saffet; Karbeyaz, Gueltekin
This paper describes the MATLAB/Simulink realization of the DC motor speed control methods, namely field resistance, armature voltage and armature resistance control methods, and feedback control system for DC motor drives. These simulation models are developed as a part of a software laboratory to support and enhance undergraduate electric machinery courses at Nigde University, Nigde, Turkey. (c) 2007 Wiley Periodicals, Inc.
Delay - Dependent Stability Analysis of Load Frequency Control Systems Considering Wind Power Participation
(IEEE, 2024) Gul, Kubra Nur; Sonmez, Sahin; Ayasun, Saffet
This study presents the delay-dependent stability analysis of the load frequency control (LFC) system enhanced by dynamic participation of wind turbine (WT). The development of the energy conversion technologies in the variable speed WTs provides the inertia support based on rotational kinetic energy and primary frequency reserve for the grid. However, the extensive utilization of communication networks to exchange data for managing and control of generation units causes the network-induced delays which negatively affect the frequency stability of the system. In addition, the LFC-WT systems face with the parametric uncertainty issue due to increasing the complexity of the LFC system and uncertainties in the WT. In this regards, the study aims to obtain the stability delay margins of the LFC system with WT deloading operation by time-domain simulation and Kharitonov theorem. The obtained stability delay margins are verified by Quasi-Polynomial mapping Root (QPmR) finder algorithm. The obtained findings show that the integration of WT including inertia control and deloading control loops into LFC system improves the delay margins and robustness of the system.
Delay-Dependent Stability Analysis of Multi-Area LFC-EVs System
(IEEE-Inst Electrical Electronics Engineers Inc, 2023) Sari, Alperen; Sonmez, Sahin; Ayasun, Saffet; Kabalci, Yasin
In this study, the delay-dependent stability of a multi-area Load Frequency Control (LFC) system with Electric Vehicle (EV) aggregators is investigated with the help of the Advanced Clustering with Frequency Sweeping (ACFS) method for incommensurate time delays. Both Integer-Order (IO) and Fractional-Order Proportional Integral (FOPI) controllers are utilized as a controller. The communication infrastructure used in LFC systems induces time delays resulting in deteriorations in the system stability. Even if the maximum allowable delay margin limits are not exceeded, these inevitable time delays could cause undesired frequency deviations and tie-line power fluctuations. The ACFS method is employed in this study to investigate impacts of time delays and to ensure better controller performance objectives taking into account the effects of time delays. Firstly, 2-dimensional (2D) stability delay maps are obtained for various LFC-EVs system parameters. The stability regions are then verified by the Quasi-Polynomial Mapping Root (QPmR) finder algorithm and MATLAB/Simulink-based time-domain simulations. The results clearly show that the participation of the EV aggregators in traditional LFC systems improves the frequency regulation and tie-line power-sharing in the system. Finally, it is concluded that the stability regions are enhanced as the fractional order of the FOPI decreases.
Effect of load increase and power system stabilizer on stability delay margin of a generator excitation control system
(Scientific and Technological Research Council of Turkey, 2016) Sönmez, Şahin; Ayasun, Saffet
This paper studies the impact of load increase and a power system stabilizer (PSS) on the stability delaymargin of a single-machine-infinite-bus system including an automatic voltage regulator. An analytical method isproposed to determine the stability delay margin of the excitation control system. The proposed method first eliminatestranscendental terms in the characteristic equation of the excitation system without making any approximation and transforms the transcendental characteristic equation into a regular polynomial. The key result of the elimination process is that the real roots of the new polynomial correspond to the imaginary roots of the transcendental characteristic equation. With the help of the new polynomial, it is also possible to determine the delay dependency of system stability and the root tendency with respect to the time delay. Delay margins are computed for various loading conditions and PSS gains. It is observed that the delay margin generally decreases as the PSS gain and load demand increase, resulting in a less stable system.
Effects of algebraic singularities on the voltage dynamics of differential-algebraic power system model
(WILEY-BLACKWELL, 2008) Ayasun, Saffet
Voltage stability analysis of differential-algebraic equation (DAE) power system model is more complicated than for systems described by ordinary differential equations (ODEs). In addition to unstable equilibria, algebraic singularity plays a crucial role in assessing the voltage stability. This paper explores the algebraic structure and singularities of the DAE model to investigate its influence on the voltage stability. Singular points are identified at various load levels and are illustrated together with the equilibria using nose Curves. Then, at a given load level, the constraint manifold is decomposed into the voltage casual regions and singular points connecting them. Time-domain simulations initiated in the vicinity Of Singular points are performed to determine how singular points affect system dynamics. It is shown that depending on the relative location of initial points with respect to singular points, trajectories of bus voltages may settle to an infeasible low voltage stable equilibrium point, which may cause a further disturbance in the system leading a voltage stability problem. Copyright (C) 2007 John Wiley & Sons, Ltd.
Effects of PWM Chopper Drive on the Torque-Speed Characteristic of DC Motor
(IEEE, 2008) Gelen, Ayetuel; Ayasun, Saffet
This paper describes a MATLAB/Simulink realization of the DC motor speed control method by controlling the voltage applied to the armature circuit using a pulse width modulated (PWM) chopper drive. Torque-speed characteristics are obtained for different values of switching frequency to demonstrate its effect on the linearity of the characteristic. The proposed simulation model is developed as a part of a software laboratory to support and enhance undergraduate electric machinery courses at Nigde University, Nigde, Turkey.
Gain and phase margin based stability analysis of time delayed single area load frequency control system with fractional order PI controller
(Gazi Univ, Fac Engineering Architecture, 2019) Sonmez, Sahin; Ayasun, Saffet
The study investigates gain-phase margin (GPM) based stability analysis for a time-delayed single-area load frequency control (LFC) system with fractional order proportional-integral (FOPI) controller. The extensive usage of open communication networks in power system control causes inevitable time delays. To compute such delays, an analytical direct method determined the imaginary axis crossing of roots of the characteristic equation is proposed for desired gain and phase margins (GPMs) and a large set of FOPI controller. Finally, for time-delayed single area LFC system with FOPI controller, results obtained are verified by using the time-domain simulation studies in Matlab/Simulink and the quasi-polynomial mapping-based root finder (QPMR) algorithm which locates quasi-polynomial roots in time delay systems.
Gain and Phase Margins Based Delay- Dependent Stability Analysis of Two-Area LFC System with Communication Delays
(IEEE, 2017) Sonmez, Sahin; Ayasun, Saffet
This paper investigates the effect gain and phase margins (GPM) on delay-dependent stability analysis of a two-area load frequency control (LFC) system with communication delays. An frequency-domain based exact method that takes into account both gain and phase margins is utilized to determine stability delay margins in terms of system and controller parameters. A gain-phase margin tester (GPMT) is introduced to the LFC system as to take into gain and phase margins in delay margin computation. For a wide range of proportional - integral (PI) controller gains, time delay values at which LFC system is both stable and have desired stability margin measured by gain and phase margins are computed. The time-domain simulation studies indicate that delay margins must be determined considering gain and phase margins to have a better dynamic performance in term of fast damping of oscillations, less overshoot and settling time.

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