Sharif Digital Repository

This paper proposes a control strategy to improve the superimposed droop frequency control scheme in DC microgrids. Conventional droop-based schemes for DC microgrids control suffer from issues such as inaccurate power sharing among the sources, voltage control with insufficient quality, and adverse effect of the line resistance on the droop characteristic. To overcome these challenges, the superimposed droop frequency method has been introduced. However, the stable operation in terms of the load variations and improper voltage quality are still the challenges with this prior-art method due to the injected AC voltages. In this paper, a method is proposed to solve the stability problem and... 

This paper presents modeling of Distribution STATCOM (D-STATCOM) in load flow calculations for the steady-state voltage compensation. An accurate model for D-STATCOM is derived to use in load flow calculations. The rating of this device as well as the direction of required reactive power injection for voltage compensation in the desired value (1 p.u.) is derived and discussed analytically and mathematically by the phasor diagram method. Furthermore, an efficient method for node and line identification used in load flow calculations is presented. The validity of the proposed model is examined by using two standard distribution systems consisting of 33 and 69 nodes, respectively. The best... 

Distributed power generation and an ever growing load demand have caused the current level of fault to exceed the nominal rating of power system devices, and fault current limiters are needed even more. The Superconducting Fault Current Limiter, SFCL, forms an efficient category of current limiters. The superconductor part in SFCLs is the most costly part of the device, and minimizing its volume, while maintaining the required characteristics of the device, would be very beneficial. In this work, using a Simulated Annealing optimization algorithm, a method has been proposed to determine minimum required bulk superconductor material in inductive shield-type SFCL structures. The flux linkage... 

The application of flat superconductor rings has been investigated in the structure of inductive shield-type high temperature superconducting fault current limiters, HT c-SFCL. A laboratory scale inductive shield-type HT c-SFCL has been designed and fabricated using flat superconductor rings. The fabrication process has been fully presented. YBCO powder has been used for the fabrication of superconductor rings. This fabrication process, being quite innovative, is introduced completely. The method of the trapped field measurement has been used for the critical current density measurement of the fabricated superconductor rings. The device with nominal current of 2 A was tested in a 30 V... 

Distributed power generation and the ever-growing load demand have caused fault current levels to exceed the nominal rating of the power system devices, and fault current limiters are more needed. Superconducting fault current limiter (SFCL) forms an important category of current limiters. In this paper, a novel flux-based model for the inductive shield-type high-temperature SFCL is developed based on the Bean model. This model is employed to simulate the SFCL performance in a sample circuit. Utilizing the model, the signal characterization of the limited current is determined. A prototype laboratory scale SFCL has been fabricated with superconducting rings. Yttrium barium copper oxide... 

The fast and very fast transient overvoltages can endanger the insulation of transformer windings. Therefore, it is necessary to develop models, to study these transients. This paper compares the two well-known multiconductor transmission line (MTL) and lumped parameter detailed (RLC ladder network) models. The high-voltage (HV) winding of a 220/35-kV, 50-MVA transformer has been modeled using these models. The simulation results of the MTL and the RLC ladder network models have been compared with the measurements results in the frequency domain, by application of the Pearson's correlation coefficients. It is shown that the RLC Ladder Network model should be used for fast transient... 

This paper presents a novel approach to optimal placement of Phasor Measurement Units (PMUs) for state estimation. At first, an optimal measurement set is determined to achieve full network observability during normal conditions, i.e. no PMU failure or transmission line outage. Then, in order to consider contingency conditions, the derived scheme in normal conditions is modified to maintain network observability after any PMU loss or a single transmission line outage. Observability analysis is carried out using topological observability rules. A new rule is added that can decrease the number of required PMUs for complete system observability. A modified Binary Particle Swarm Optimization... 

Electric vehicles (EVs) usage is becoming ubiquitous nowadays. Widespread integration of electric vehicles into electric energy distribution systems (EEDSs) has a twofold impact: (1) It may impose a burden on EEDSs when EVs are charging, (2) EVs could discharge their battery capacity using V2G technology when required. To mitigate adverse effects of massive integration of EVs in EEDSs, EVs could be used as mobile energy storage devices (MESDs) to transfer electric energy throughout EEDSs using a proper charging/discharging scheme. In this paper, a mixed integer linear programming (MILP) model is proposed to control charging and discharging of EVs to improve EEDS performance. EVs are modeled... 

It is well known that photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. A closed form expression is found for the equivalent characteristic impedance and propagation constant of a uniform transmission line that can accurately model the transverse electromagnetic (TEM) propagation within nonuniform superconducting transmission lines with subwavelength inhomogeneities. It is shown that electromagnetic wave propagation, in... 

In this paper the impact of Static Synchronous Series Compensator (SSSC) on the impedance calculated by distance relay is investigated. Analytical results are presented and verified by detailed simulations. Six different phase to phase and phase to ground measuring units of the distance relay are simulated to resemble the behavior of the relay. It is shown in this paper that zero sequence of the injected voltage by 48 pulse SSSC converter has the most impact on the apparent impedance seen by the phase to ground fault measuring unit and cause under reaching of distance relay. It can be concluded from the results that SSSC located in the middle of the transmission line cause to divide trip... 

Frequency shift of the spectrum of an incident optical pulse by an intense THz pulse inducing cross-phase modulation (XPM) in a nonlinear dielectric slab is analyzed. The effect is predicted with a high degree of accuracy using the well-known transmission line matrix (TLM) technique. In this research, to model the THz-induced temporal and spatial variation of the dielectric permittivity of the nonlinear dielectric slab, the transmission lines of the TLM method are loaded with open shunt stubs. The parameters of the stubs are modified in accordance with the refractive index variation of the dielectric slab, here ZnTe, induced by the strong THz pulse. The obtained numerical results are... 

This article proposes a tunable composite right-/left-handed transmission line consisting of a transversely magnetized ferrite substrate periodically loaded by microstrip inductors. The gapless transition between the left- and right-handed bands (balanced response) is the main characteristic of this structure. This composite right-/left-handed transmission line is proposed in both reciprocal and nonreciprocal versions. Based on periodic structure analysis, the dispersion relation is estimated theoretically and validated numerically by the finite-element method in the reciprocal case. Theoretical and numerical results demonstrate the tunability of the proposed structures by changing the DC... 

The modal analysis of a lossy coupled plasmonic waveguide consisting of two graphene sheets sandwiched among three SiO 2 layers is presented. This analysis extracts even and odd complex propagation constants in addition to even and odd characteristic impedances at various Fermi levels. It is shown that the first even mode is lossless in theory, but other modes are very lossy. Taking into account the losses, first, the transmission line (RLCG) model, including self and mutual elements, is constructed for various Fermi levels and frequencies. Then, the length of a lossy plasmonic directional coupler based on the coupled plasmonic waveguide is designed for a variety of Fermi levels. The... 

Large amplitude oscillation of the power transmission lines, which is also known as galloping phenomenon, has hazardous consequences such as short circuiting and failure of transmission line. In this article, to suppress the undesirable vibrations of the transmission lines, first the governing equations of transmission line are derived via mode summation technique. Then, due to the occurrence of large amplitude vibrations, nonlinear quadratic and cubic terms are included in the derived linear equations. To suppress the vibrations, arbitrary number of the piezoelectric actuators is assumed to exert the actuation forces. Afterwards, a Lyapunov based approach is proposed for the robust adaptive... 

In this paper, extended composite right/left-handed (E-CRLH) transmission line (TL) metamaterial structures, with two left-handed (backward) and two right-handed (forward) pass bands, are investigated. Also, design procedures in order to design dual- and quad-band E-CRLH-TLs are presented in detail and the parameters of these structures are extracted by clean formulas, while satisfying arbitrary phase shifts at the operating frequencies. Finally, the dispersion and characteristic impedances of these transmission lines are derived and plotted. The results of this paper can be applied to any type of TL-based dual- and quad-band microwave component  

Transmission line formulation is used to analyze two-dimensional low-loss metallic gratings at optical frequencies when plasmonic waves propagate in the structure. This method, like the Fourier modal method, suffers from numerical instabilities when applied to such structures. A systematic approach to avoid these instabilities is presented. These numerical artifacts are attributed to the violation of Li's inverse rule and the appearance of higher-order spurious modes. In this paper, a new approach is proposed to identify and to greatly reduce the effect of these spurious modes based on the accuracy by which these modes are satisfying the conservation of momentum. Furthermore, the proposed... 

An approximate and accurate enough multi-conductor transmission line model is developed for analysis of side-coupled metal-insulator-metal (MIM) waveguides. MIM waveguides have been already modeled by single conductor transmission lines. Here, the side coupling effects that exist between neighboring plasmonic structures are taken into account by finding appropriate values for distributed mutual inductance and mutual capacitance between every two neighboring conductors in the conventional single-conductor transmission line models. In this manner, multi-conductor transmission line models are introduced. Closed-form expressions are given for the transmission and reflection of miscellaneous MIM... 

In this paper, distribution of voltage along a general nonuniform transmission line is expanded in an appropriate form, and by employing an approach similar to conventional and modified differential transfer matrix methods already proposed for optical structures, analytical expressions are obtained for voltage/current distributions and reflection/transmission coefficients. This method shows great accuracy in different test cases and has been found to be superior to the well-known analytical method of small reflections. Notwithstanding, the overall accuracy of proposed approach is further improved by introducing the technique of multiple divisions. In particular, lossy/lossless tapered... 

A new analytical method for finding the general solution of the nth-order linear differential equation with variable coefficients is given based on generalizing the idea of differential transfer matrix method already proposed for solving the second order Helmholtz equation. Our generalization has two aspects. First, the given formulation copes with the nth-order linear differential equations, rather than the special case of second order wave equations. Second, the proposed approach is generalized in several different ways each yielding different types of differential transfer matrices with correspondingly different numerical accuracies. The presented methods can be applied to problems such... 

The deployment of flexible ac transmission system (FACTS) devices, despite miscellaneous benefits and applications, has been restricted mainly due to their large investment costs. Accordingly, the notion of distributed FACTS (D-FACTS) was recently proposed. Distributed static series compensators (DSSCs), as a new member of D-FACTS, are directly clamped to conductors of transmission lines. The possibility of controlling the compensation rate of DSSCs from the control center affords the active control of network power flow. This paper presents a comprehensive reliability model for the DSSC. The model is then reduced and a state-space representation is extracted for a transmission line with a...