Sharif Digital Repository

Of particular interest to restrict the short-circuit level of interconnected power systems is to exploit fault current limiter (FCL) technologies. FCLs let the system planners devise new reliable and rather economical substation configurations and provide the possibility of proposing a promising cost-effective and prompt solution to the fault current over duty problem in the existing substations. This paper attempts to assess reliability of substation architectures accommodating the FCL operation and, besides, numerically investigates the FCL's impacts on the substation reliability indices. In order to clarify the proposed approach, two case studies with and without FCL are analyzed and... 

This paper presents an optimization model for the calculation of the minimum number of phasor measurement units (PMUs) in electrical power networks. The problem constraint is a predefined probability of observability associated with each bus. The mixed-integer programming is used for the proposed optimization and an efficient linearization technique is proposed to convert the nonlinear function representing the probability of observability into a set of linear expressions. The PMU placement is staged in a multi-year planning horizon due to financial and physical constraints. The average probability of observability is maximized at the intermediate planning stages, subject to a limited number... 

This paper enhances the observability of power networks by taking into consideration random component outages. The architecture of wide-area measurement system (WAMS) is analyzed in order to identify components that would affect the network observability. An iterative framework is devised to calculate a bus index in power networks equipped with phasor measurement units (PMUs) and conventional measurements. The average of bus indices represents a system index which provides an overall insight on the power network observability. The system index is utilized as a criterion to distinguish among multiple optimal PMU placements. Conventional bus injection and line flow measurements and the effect... 

Probabilistic analyses of a wide-area measurement system (WAMS) would require equivalent reliability models for its components which include phasor measurement units (PMUs). In this paper, the reliability modeling of PMU is proposed and the proposed model is extended to consider options for the PMU hardware. The Markov process is employed to analyze the proposed model and to present an equivalent two-state model of PMUs. Reliability parameters of PMU are estimated with major difficulties associated with limited and uncertain data. In this paper, uncertainties are taken into account to achieve more realistic estimates of PMU characteristics. Fuzzy sets along with reliability analyses and... 

In this study, a hierarchical optimal robust controller is presented for the power system load-frequency control problem. In this approach, the multi-area power system is first decomposed into several sub-systems (areas). Then by using a two-level control strategy, the overall optimal solution is obtained. At the first level, the optimal control of each area is obtained with respect to local information and also considering interactions coming from other areas. At the second level, by using a coordination strategy, the local controllers will converge to global solution and optimal robust controller is achieved. This approach is applicable to both single-area and multi-area interconnected... 

This paper enhances the observability of power networks by taking into consideration random component outages. The architecture of wide-area measurement system (WAMS) is analyzed in order to identify components that would affect the network observability. An iterative framework is devised to calculate a bus index in power networks equipped with phasor measurement units (PMUs) and conventional measurements. The average of bus indices represents a system index which provides an overall insight on the power network observability. The system index is utilized as a criterion to distinguish among multiple optimal PMU placements. Conventional bus injection and line flow measurements and the effect... 

There is a global tendency towards using Distributed Generation (DG) and renewable energy resources. Considering low utilization cost and low undesirable environmental effects, wind farms have become a considerable resource for producing electrical energy in many countries. Since Wind farms are not programmable and their power output which depends on weather and wind speed is uncertain, they create problems for utilizing electricity system. There are different methods for preserving system stability against the uncertainty of wind power plants. Optimal utilization of pumped storage and gas resources is one of the approaches for reducing production risk of wind farms. In this paper, it is... 

Different analysis methods have been used for voltage stability assessment. In comparison with static analysis methods, little work has been done on dynamic analysis of large interconnected power systems. Voltage instability can be studied effectively with a combination of static approaches and time simulations. This paper discusses voltage stability assessment using mixed static and dynamic techniques. Using static methods, a voltage stability based ranking is carried out to specify faint buses, generators and links in power system. The system is analyzed for most severe conditions. Then, time domain simulation is performed for the conditions determined by voltage instability ranking. The... 

This paper is concerned about the optimal placement of phasor measurement units (PMUs) so as to make a system completely observable. Observability assessment is done by the aid of the topological observability rules. Moreover a new rule is added which can decrease the number of required PMUs for complete system observability. A modified binary particle swarm is used as an optimization tool for obtaining the minimal number of PMUs and corresponding configuration. In order to improve the speed of convergence, an initial PMU placement is provided by graph-theoretic procedure. The simulation results of proposed approach are presented for several IEEE test systems  

The necessity of dynamic equivalents for power system analysis has been well known since the expansion of large interconnected power networks, and has been discussed during the last decades. The present paper proposes a new method for constructing dynamic equivalents of power systems. In this method, at the first step the "study system" is modeled completely via a single machineinfinite bus modeling procedure. Then the "external system" is identified as a MIMO feedback block of this model in such a manner that can include dynamic effects of the latter on the behavior of the former. The method is successfully experimented on a part of the Iranian southern network. Copyright © 2007 The... 

This paper probes the impact of utilizing an IPFC on the reliability indices of interconnected power systems. First, a concise presentation of IPFC and its structure are provided and the reliability model of two unequally-rated parallel transmission lines equipped with IPFC is then extracted. The assumed IPFC is composed from two parallel converting bridges associated with each line. Afterwards, based-on equivalent assisting unit approach, different commonly-used adequacy indices including the loss of load expectation (LOLE), loss of energy expectation (LOEE) and system minutes (SM) are calculated. A set of numerical analyses are conducted to illustrate the sensitivity of these indices with... 

This paper investigates the application of immunity genetic algorithm (IGA) for the problem of optimal placement of phasor measurement units (PMUs) in an electric power network. The problem is to determine the placement sites of the minimal set of PMUs which makes the system observable. Incorporating immune operator in the canonical genetic algorithm (GA), on the condition of preserving GA's advantages, utilizes some characteristics and knowledge of the problems for restraining the degenerative phenomena during evolution, so as to improve the algorithm efficiency. This type of prior knowledge about some parts of optimal solution exists in the PMU placement problem. So, the IGA is adopted in... 

This paper presents a control scheme for microgrids with passive loads. The existing control techniques for distributed generation systems are designed to operate either in the grid-connected or in the islanded mode. In the grid-connected mode of operation, a currentcontrolled scheme based on d-q variables is used for the regulation of real and reactive powers of the converter. In the islanded mode of operation, the converter is controlled in the voltage-controlled mode. This paper deals with the modeling of both control schemes. Furthermore, reconnection from the islanded mode to the grid-connected mode based on the use of adequate Phase-Locked Loops (PLLs) has been demonstrated  

Probabilistic analyses of a wide-area measurement system (WAMS) would require equivalent reliability models for its components which include phasor measurement units (PMUs). In this paper, the reliability modeling of PMU is proposed and the proposed model is extended to consider options for the PMU hardware. The Markov process is employed to analyze the proposed model and to present an equivalent two-state model of PMUs. Reliability parameters of PMU are estimated with major difficulties associated with limited and uncertain data. In this paper, uncertainties are taken into account to achieve more realistic estimates of PMU characteristics. Fuzzy sets along with reliability analyses and... 

This paper describes an application of intelligence- based predictive scheme to load-frequency control (LFC) in a two-area interconnected power system. In this investigation, at first, a dynamic model of the present system has to be considered and subsequently an efficient control scheme which is organized based on Takagi-Sugeno-Kang (TSK) fuzzy-based scheme and linear generalized predictive control (LGPC) scheme needs to be developed. In the control scheme proposed, frequency deviation versus load electrical power variation could efficiently be dealt with, at each instant of time. In conclusion, in order to validate the effectiveness of the proposed control scheme, the whole of outcomes are... 

This paper presents an optimization model for the calculation of the minimum number of phasor measurement units (PMUs) in electrical power networks. The problem constraint is a predefined probability of observability associated with each bus. The mixed-integer programming is used for the proposed optimization and an efficient linearization technique is proposed to convert the nonlinear function representing the probability of observability into a set of linear expressions. The PMU placement is staged in a multi-year planning horizon due to financial and physical constraints. The average probability of observability is maximized at the intermediate planning stages, subject to a limited number... 

Of particular interest to restrict the short-circuit level of interconnected power systems is to exploit fault current limiter (FCL) technologies. FCLs let the system planners devise new reliable and rather economical substation configurations and provide the possibility of proposing a promising cost-effective and prompt solution to the fault current over duty problem in the existing substations. This paper attempts to assess reliability of substation architectures accommodating the FCL operation and, besides, numerically investigates the FCL's impacts on the substation reliability indices. In order to clarify the proposed approach, two case studies with and without FCL are analyzed and... 

In this paper a novel approach based on the emotional learning is proposed for improving the load-frequency control (LFC) system of a two-area interconnected power system with the consideration of generation rate constraint (GRC). The controller includes a neuro-fuzzy system with power error and its derivative as inputs. A fuzzy critic evaluates the present situation, and provides the emotional signal (stress). The controller modifies its characteristics so that the critic's stress is reduced. The convergence and performance of the proposed controller, both in presence and absence of GRC, are compared with those of proportional integral (PI), fuzzy logic (FL), and hybrid neuro-fuzzy (HNF)... 

In large-scale power systems, classical centralized control approaches may fail due to geographically distribution of information and decentralized controllers result in sub-optimal solution for load-frequency control (LFC) problems. In this paper, a two-level structure is presented to obtain optimal solution for LFC problems and also reduce the computational complexity of centralized controllers. In this approach, an interconnected multi-area power system is decomposed into several sub-systems (areas) at the first-level. Then an optimization problem in each area is solved separately, with respect to its local information and interaction signals coming from other areas. At the second-level,... 

This paper presents an integer linear programming (ILP) framework for the optimal placement of phasor measurement units (PMUs), in the presence of conventional measurements. Furthermore, by the proposed method, the power system remains completely observable during all possible single contingencies for lines and measurement devices. In doing so, the potential of circuit equations associated with both PMUs and conventional measurements as well as the network topology are fully utilized by a system of equations to reach the minimum possible numbers of required PMUs. The limitation of communication channels is also taken into account in the proposed ILP-based framework. The method is implemented...