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Showing 21 results for Stability

S.jadid, S.jalilzadeh,
Volume 1, Issue 2 (4-2005)

This paper presents a new composite index to analyze power system transient stability. Contingency ranking in power system transient stability is a complicated and time consuming task. To prevail over this difficulty, various indices are used. These indices are based on the concept of coherency, transient energy conversion between kinetic and potential energy and three dot products of the system variables. It is well known that some indices work better than others for a particular power system. This paper along with test results using two practical 230 kV Sistan and 400 kV Khorasan power system in Iran, and 9 bus IEEE test system demonstrates that combination of indices provides better ranking than a single one. In this paper two composite indices ( CI ) is presented and compared. One composite index is based on Least Mean Square algorithm (LMS) and other based on summing indices by equal weights. Numerical simulations of the developed index, demonstrate that composite index is more effective than other indices.
M. Gitizadeh, M. Kalantar,
Volume 4, Issue 4 (12-2008)

This paper presents a novel optimization based methodology to allocate Flexible AC Transmission Systems (FACTS) devices in an attempt to improve the previously mentioned researches in this field. Static voltage stability enhancement, voltage profile improvement, line congestion alleviation, and FACTS devices investment cost reduction, have been considered, simultaneously, as objective functions. Therefore, multi-objective optimization without simplification has been used in this paper to find a logical solution to the allocation problem. The optimizations are carried out on the basis of location, size and type of FACTS devices. Thyristor Controlled Series Compensator (TCSC) and Static Var Compensator (SVC) are utilized to achieve the determined objectives. The problem is formulated according to Sequential Quadratic Programming (SQP) problem in the first stage. This formulation is used to accurately evaluate static security margin with congestion alleviation constraint incorporating voltage dependence of loads in the presence of FACTS devices and estimated annual load profile. The best trade-off between conflicting objectives has been obtained through Genetic Algorithm (GA) based fuzzy multi-objective optimization approach, in the next stage. The IEEE 14-bus test system is selected to validate the allocated devices for all load-voltage characteristics determined by the proposed approach.
A. Ghaffari , N. Nasserifar ,
Volume 5, Issue 3 (9-2009)

In this paper a new mathematical model is developed for the dynamics between tumor cells, normal cells, immune cells, chemotherapy drug concentration and drug toxicity. Then, the theorem of Lyapunov stability is applied to design treatment strategies for drug protocols that ensure a desired rate of tumor cell kill and push the system to the area with smaller tumor cells. Using of this theorem a condition for drug administration to patients so that solution of the system of equations always tends to tumor free equilibrium point is proposed.
M. Sharma, K. P. Vittal,
Volume 6, Issue 4 (12-2010)

The recent trends in electrical power distribution system operation and management are aimed at improving system conditions in order to render good service to the customer. The reforms in distribution sector have given major scope for employment of distributed generation (DG) resources which will boost the system performance. This paper proposes a heuristic technique for allocation of distribution generation source in a distribution system. The allocation is determined based on overall improvement in network performance parameters like reduction in system losses, improvement in voltage stability, improvement in voltage profile. The proposed Network Performance Enhancement Index (NPEI) along with the heuristic rules facilitate determination of feasible location and corresponding capacity of DG source. The developed approach is tested with different test systems to ascertain its effectiveness.
M. Aghamohammadi, S. S. Hashemi, M. S. Ghazizadeh,
Volume 7, Issue 1 (3-2011)

This paper presents a new approach for estimating and improving voltage stability margin from phase and magnitude profile of bus voltages using sensitivity analysis of Voltage Stability Assessment Neural Network (VSANN). Bus voltage profile contains useful information about system stability margin including the effect of load-generation, line outage and reactive power compensation so, it is adopted as input pattern for VSANN. In fact, VSANN establishes a functionality for VSM with respect to voltage profile. Sensitivity analysis of VSM with respect to voltage profile and reactive power compensation extracted from information stored in the weighting factor of VSANN, is the most dominant feature of the proposed approach. Sensitivity of VSM helps one to select most effective buses for reactive power compensation aimed enhancing VSM. The proposed approach has been applied on IEEE 39-bus test system which demonstrated applicability of the proposed approach.
S. Haji Nasiri, M. K. Moravvej-Farshi, R. Faez,
Volume 8, Issue 1 (3-2012)

Time domain analysis of multilayer graphene nanoribbon (MLGNR) interconnects, based on ‎transmission line modeling (TLM) using a six-order linear parametric expression, has been ‎presented for the first time. We have studied the effects of interconnect geometry along with ‎its contact resistance on its step response and Nyquist stability. It is shown that by increasing ‎interconnects dimensions their propagation delays are increased and accordingly the system ‎becomes relatively more stable. In addition, we have compared time responses and Nyquist ‎stabilities of MLGNR and SWCNT bundle interconnects, with the same external dimensions. ‎The results show that under the same conditions, the propagation delays for MLGNR ‎interconnects are smaller than those of SWCNT bundle interconnects are. Hence, SWCNT ‎bundle interconnects are relatively more stable than their MLGNR rivals.‎
M. Heydaripour, A. Akbari Foroud,
Volume 8, Issue 4 (12-2012)

Congestion in the transmission lines is one of the technical problems that appear particularly in the deregulated environment. The voltage stability issue gets more important because of heavy loading in this environment. The main factor causing instability is the inability of the power system to meet the demand for reactive power. This paper presents a new approach for alleviation congestion relieving cost by feeding required reactive power of system in addition to re-dispatching active power of generators and load shedding. Furthermore with considering different static load models in congestion management problem with both thermal and voltage instability criteria, tries to the evaluated congestion management cost become more real, accurate and acceptable. The voltage stability is a dynamic phenomenon but often static tools are used for investigating the stability conditions, so this work offers new method that considers two snapshots after contingency to consider voltage stability phenomena more accurate. This algorithm uses different preventive and corrective actions to improve unsuitable voltage stability margin after contingency. The proposed method is tested on IEEE 24-bus Reliability test system, the simulation results shows the effectiveness of the method.
M. Tolue Khayami, H. A. Shayanfar,
Volume 10, Issue 2 (6-2014)

This paper proposes a method for extending the ability of rotary power flow controller (RPFC) using tap-changer of the RPFC’s transformers. A detailed model of the device is presented to analyze the effects of the tap changer operation on the performance of the RPFC. To evaluate the results, the RPFC model is simulated using PSCAD/EMTDC software. Dynamic operation of the RPFC on a 400 kV transmission line is studied. Based on the results, using tap-changer of transformers can extend the RPFC ability to control the active power of the transmission line about 25%.
M. Hosseini Abardeh, R. Ghazi,
Volume 11, Issue 1 (3-2015)

The matrix converter instability can cause a substantial distortion in the input currents and voltages which leads to the malfunction of the converter. This paper deals with the effects of input filter type, grid inductance, voltage fed to the modulation algorithm and the synchronous rotating digital filter time constant on the stability and performance of the matrix converter. The studies are carried out using eigenvalues of the linearized system and simulations. Two most common schemes for the input filter (LC and RLC) are analyzed. It is shown that by a proper choice of voltage input to the modulation algorithm, structure of the input filter and its parameters, the need for the digital filter for ensuring the stability can be resolved. Moreover, a detailed model of the system considering the switching effects is simulated and the results are used to validate the analytical outcomes. The agreement between simulation and analytical results implies that the system performance is not deteriorated by neglecting the nonlinear switching behavior of the converter. Hence, the eigenvalue analysis of the linearized system can be a proper indicator of the system stability.
A. A. Khodadoost Arani, B. Zaker, G. B. Gharehpetian,
Volume 13, Issue 1 (3-2017)

The Micro-Grid (MG) stability is a significant issue that must be maintained in all operational modes. Usually, two control strategies can be applied to MG; V/f control and PQ control strategies. MGs with V/f control strategy should have some Distributed Generators (DGs) which have fast responses versus load changes. The Flywheel Energy Storage System (FESS) has this characteristic. The FESS, which converts the mechanical energy to electrical form, can generate electrical power or absorb the additional power in power systems or MGs. In this paper, the FESS structure modeled in detail and two control strategies (V/f and PQ control) are applied for this application. In addition, in order to improve the MG frequency and voltage stability, two complementary controllers are proposed for the V/f control strategy; conventional PI and Fuzzy Controllers. A typical low voltage network, including FESS is simulated for four distinct scenarios in the MATLAB/ Simulink environment. It is shown that fuzzy controller has better performance than conventional PI controller in islanded microgrid.

A. R. Moradi, Y. Alinejad-Beromi, K. Kiani,
Volume 13, Issue 1 (3-2017)

Congestion and overloading for lines are the main problems in the exploitation of power grids. The consequences of these problems in deregulated systems can be mentioned as sudden jumps in prices in some parts of the power system, lead to an increase in market power and reduction of competition in it. FACTS devices are efficient, powerful and economical tools in controlling power flows through transmission lines that play a fundamental role in congestion management. However, after removing congestion, power systems due to targeting security restrictions may be managed with a lower voltage or transient stability rather than before removing. Thus, power system stability should be considered within the construction of congestion management. In this paper, a multi-objective structure is presented for congestion management that simultaneously optimizes goals such as total operating cost, voltage and transient security. In order to achieve the desired goals, locating and sizing of series FACTS devices are done with using components of nodal prices and the newly developed grey wolf optimizer (GWO) algorithm, respectively. In order to evaluate reliability of mentioned approaches, a simulation is done on the 39-bus New England network.

V. Ghaffari,
Volume 15, Issue 4 (12-2019)

In this paper, a chattering-free sliding-mode control is mainly proposed in a second-order discrete-time system. For achieving this purpose, firstly, a suitable control law would be derived by using the discrete-time Lyapunov stability theory and the sliding-mode concept. Then the input constraint is taken into account as a saturation function in the proposed control law. In order to guarantee the closed-loop system stability, a sufficient stability condition would be addressed in the presence of unstructured uncertainties. Hence the states of the discrete-time system are moved to a predefined sliding surface in a finite sampling time. Then the system states are asymptotically converged to the origin through the sliding line. The suggested SMC is successfully applied in two discrete-time systems (i.e. regulation and tracking problems). The effectiveness of the proposed method will be verified via numerical examples.

N. Bensaker, H. Kherfane, B. Bensaker,
Volume 16, Issue 2 (6-2020)

In this study, delay-dependent robust stability problem is investigated for uncertain neutral systems with discrete and distributed delays. By constructing an augmented Lyapunov-Krasovskii functional involving triple integral terms and taking into account the relationships between the different delays, new less conservative stability and robust stability criteria are established first using the delay bi-decomposition approach then generalized with the delay N-decomposition technique. Some integral inequalities are employed to deal with the cross terms and few free weighing matrices are introduced to reduce the conservatism. The proposed criteria are expressed in terms of linear matrix inequalities. The effectiveness of the proposed stability conditions is illustrated by a numerical example.

Z. Rafiee, M. Rafiee, M. R. Aghamohammadi,
Volume 16, Issue 3 (9-2020)

Improving transient voltage stability is one of the most important issues that must be provided by doubly fed induction generator (DFIG)-based wind farms (WFs) according to the grid code requirement. This paper proposes adjusted DC-link chopper based passive voltage compensator and modified transient voltage controller (MTVC) based active voltage compensator for improving transient voltage stability. MTVC is a controller-based approach, in which by following a voltage dip (VD) condition, the voltage stability for the WF can be improved. In this approach, a voltage dip index (VDI) is proposed to activate/deactivate the control strategy, in which, two threshold values are used. In the active mode, the active and reactive power are changed to decrease the rotor current and boost the PCC voltage, respectively. Based on the control strategy, in a faulty grid, DFIG not only will be able to smooth DC-link voltage fluctuations and reduces rotor overcurrents but also it will increase the voltage of point of common coupling (PCC). Therefore, it improves transient voltage stability. The simulation results show the effectiveness of the proposed strategy for improving voltage stability in the DFIG.

A. H. Poursaeed, F. Namdari,
Volume 16, Issue 3 (9-2020)

In this paper, a novel method is proposed to monitor the power system voltage stability using Support Vector Machine (SVM) by implementing real-time data received from the Wide Area Measurement System (WAMS). In this study, the effects of the protection schemes on the voltage magnitude of the buses are considered while they have not been investigated in previous researches. Considering overcurrent protection for transmission lines not only resolves some drawbacks of the previous studies but also brings the case study system closer to the realities of actual systems. Online monitoring of system stability is performed by prediction of the Voltage Stability Index (VSI) and carried out by using Support Vector Regression (SVR). Due to the direct effect of appropriate SVR parameters on the prediction quality, the optimum value is chosen for learning machine hyperparameters using Differential Evolution (DE) algorithm. The obtained simulation results demonstrate high accuracy, effectiveness, and optimal performance of the proposed technique in comparison with Back-Propagation Neural Network (BPNN) and Adaptive Neuro-Fuzzy Inference System (ANFIS) approaches. The presented method is carried out on the 39 bus New England system.

H. Shayeghi, A. Younesi,
Volume 16, Issue 4 (12-2020)

The main objective of this paper is to model and optimize the parallel and relatively complex FuzzyP+FuzzyI+FuzzyD (FP+FI+FD) controller for simultaneous control of the voltage and frequency of a micro-grid in the islanded mode. The FP+FI+FD controller has three parallel branches, each of which has a specific task. Finally, as its name suggests, the final output of the controller is derived from the algebraic summation of the outputs of these three branches. Combining the basic features of a simple PID controller with fuzzy logic that leads to an adaptive control mechanism, is an inherent characteristic of the FP+FI+FD controller. This paper attempts to determine the optimal control gains and Fuzzy membership functions of the FP+FI+FD controller using an improved Salp swarm algorithm (ISSA) to achieve its optimal dynamic response. The time-domain simulations are carried out in order to prove the superb dynamic response of the proposed FP+FI+FD controller compared to the PID control methods. In addition, a multi-input-multi-output (MIMO) stability analysis is performed to ensure the robust control characteristic of the proposed parallel fuzzy controller.

H. Shayeghi, Y. Hashemi,
Volume 17, Issue 3 (9-2021)

The main idea of this paper is proposing a model to develop generation units considering power system stability enhancement. The proposed model consists of two parts. In the first part, the indexes of generation expansion planning are ensured. Also, small-signal stability indexes are processed in the second part of the model. Stability necessities of power network are supplied by applying a set of robustness and performance criteria of damping. Two parts of the model are formulated as two-objective function optimization that is solved by adaptive non-dominated sorting genetic method-III (ANSGM-III). For better decision-making of the final solution of generation units, a set of Pareto-points have been extracted by ANSGM-III. To select an optimal solution among Pareto-set, an analytical hierarchy style is employed. Two objective functions are compared and suitable weights are allocated. Numerical studies are carried out on two test systems, 68-bus and 118-bus power network. The values of generation expansion planning cost and system stability index have been studied in different cases and three different scenarios. Studies show that, for example, in the 68-bus system for the case of system load growth of 5%, the cost of generation expansion planning for the proposed model increased by 7.7% compared to the previous method due to stability modes consideration and the small-signal stability index has been improved by 6.7%. The proposed model is survived with the presence of a wide-area stabilizer (WAS) for damping of oscillations. The effect of WAS latency on expansion programs is evaluated with different amounts of delay times.

Y. McHaouar, A. Abouloifa, I. Lachkar, H. Katir, F. Giri, A. El Aroudi, A. Elallali, C. Taghzaoui,
Volume 18, Issue 1 (3-2022)

In this paper, the problem of controlling PWM single-phase AC/DC converters is addressed. The control objectives are twofold: (i) regulating the output voltage to a selected reference value, and (ii) ensuring a unitary power factor by forcing the grid current to be in phase with the grid voltage.  To achieve these objectives, the singular perturbation technique is used to prove that the power factor correction can be done in the open-loop system with respect to certain conditions that are not likely to take place in reality. It is also applied to fulfill the control objectives in the closed-loop through a cascade nonlinear controller based on the three-time scale singular perturbation theory. Additionally, this study develops a rigorous and complete formal stability analysis, based on multi-time-scale singular perturbation and averaging theory, to examine the performance of the proposed controller. The theoretical results have been validated by numerical simulation in MATLAB/Simulink/SimPowerSystems environment.
Keyhan Hosseini,
Volume 18, Issue 2 (6-2022)

Anisotropic media appear regularly in electromagnetic wave engineering. The finite-difference time-domain (FDTD) method is a robust technique to model such media. However, the value of the time step in the FDTD algorithm is bounded by the Courant-Friedrichs-Lewy (CFL) condition. In this paper, a simple analytical approach is developed using the Gershgorin circle theorem to derive a point-wise closed-form relation for the CFL condition in bounded inhomogeneous anisotropic media. The proposed technique includes objects of arbitrary shapes with straight, tilted, or curved interfaces located in a computational space with uniform or adaptive gridding schemes. Both axial and non-axial anisotropies are considered in the analysis. The proposed method is able to investigate the effect of boundaries and interfaces on the stability of the algorithm. It is shown that in the presence of an interface between two anisotropic media, the von-Neumann criterion is not able to predict the stability bound for specific ranges of the permittivity tensor components and unit cell aspect ratios. Exploiting the proposed closed-form formulations, it is possible to tune the CFL time step and avoid the temporal instability by the wise selection of the gridding scheme especially in curved boundaries where subcell modelings such as Yu-Mittra formalism are applicable. Some illustrative examples are provided to verify the method by comparing the results with those of the eigenvalue analysis and time-domain simulations.

A. Zakipour, K. Aminzare, M. Salimi,
Volume 18, Issue 3 (9-2022)

Considering the presence of different model parameters and controlling variables, as well as the nonlinear nature of DC to AC inverters; stabilizing the closed-loop system for grid current balancing is a challenging task. To cope with these issues, a novel sliding mode controller is proposed for the current balancing of local loads using grid-connected inverters in this paper. The closed-loop system includes two different controlling loops: a current controller which regulates the output current of grid-connected inverter and a voltage controller which is responsible for DC link voltage regulation. The main features of the proposed nonlinear controller are reactive power compensation, harmonic filtering and three-phase balancing of local nonlinear loads.  The developed controller is designed based on the state-space averaged modelling its stability and robustness are proved analytically using the Lyapunov stability theorem. The accuracy and effectiveness of proposed controlled approach are investigated through the PC-based simulations in MATLAB/Simulink.

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