Keyhan Sheshyekani

Evaluation of Lightning Electromagnetic Fields and Their Induced Voltages on Overhead Lines Considering the Frequency Dependence of Soil Electrical Parameters

This paper presents a comprehensive study on the effect of the frequency dependence of soil electrical parameters on the lightning radiated electromagnetic fields as well as their associated induced voltages on overhead lines. To this aim, a full-wave approach based upon the finite-element method (FEM) is utilized. In the analyses, frequency dependence of soil conductivity and relative permittivity is introduced, using available analytical formulae that is obtained from experimental data. It is shown that the radial electric field is the only component which is significantly affected by the frequency dependence of soil electrical parameters at observation points as close as some tens of meters from the lightning channel. The vertical component of the electric field and the azimuthal component of the magnetic field are not much influenced by this property of soil at moderate distances (up to several hundred meters) from the lightning channel. For distant observation points and for poorly conducting grounds, however, these components are also affected. It is also shown that for soils characterized by relatively moderate and low resistivity values (less than 1000 Ω.m), lightning-induced voltages are not significantly affected by the frequency dependence of soil electrical parameters. For poorly conducting soils, instead, the frequency dependence of soil electrical parameters results in a decrease of lightning-induced voltages.

The Effect of Frequency Dependence of Soil Electrical Parameters on the Lightning Performance of Grounding Systems

A rigorous full-wave approach based upon the finite-element method (FEM) is used for the analysis of grounding systems taking the frequency dependence of soil electrical parameters into account. The method uses minimum approximations to model the transient behavior of grounding systems. In this analysis, frequency dependence of the soil conductivity and relative permittivity is represented using available analytical formulae obtained from experimental data. It is shown that there are cases in which the frequency dependence of the soil conductivity and relative permittivity can affect the transient behavior of grounding systems. Our simulation results show that the effect of frequency dependence is more pronounced for grounding systems buried in low conductive soils in contrast with those buried in highly conductive soils and especially when these systems are subjected to lightning subsequent return stroke currents. In practice, it seems legitimate to disregard the frequency dependence of the soil conductivity and relative permittivity for grounding systems buried in highly conductive soils subjected to lightning first return stroke currents. Within this context, it is shown that the soil conductivity, length of grounding electrode, and the frequency content of the injected current are the main factors contributing to the beneficial effect of the frequency dependence of soil electrical parameters.

Evaluation of Lightning-Induced Voltages on Multiconductor Overhead Lines Located Above A Lossy Dispersive Ground

The paper presents a comprehensive study on the effect of soil dispersion (i.e., frequency dependence of soil conductivity and relative permittivity) on the lightning-induced voltages on single/multiconductor overhead distribution lines. For this aim, a full-wave approach based on the finite-element method is utilized. The soil dispersion is incorporated into the model using available analytical formulae obtained from experimental data. It is shown that for soils characterized by relatively moderate and low resistivity values (less than 1000 Ω.m), lightning-induced voltages are not significantly affected by the soil dispersion property. In the analyses, two different integration paths are used for obtaining the induced voltages by integrating the total vertical component of the electric field. It is shown that the soil dispersion effect on the induced voltages is negligible if the integration path continues vertically from the conductor surface to a point well below the ground surface at which the electric field vanishes. However, the soil dispersion for poorly conducting soils can markedly change the induced voltages if the integration path is considered as a vertical path between the conductor surface and the ground surface.

Power Oscillations Damping in DC Microgrids

This paper proposes a new control strategy for damping of power oscillations in a multisource dc microgrid. A parallel combination of a fuel cell (FC), a photovoltaic system, and a supercapacitor (SC) is used as a hybrid power conversion system (HPCS). The SC compensates for the slow transient response of the FC stack. The HPCS controller comprises a multiloop voltage controller and a virtual impedance loop for power management. The virtual impedance loop uses a dynamic droop gain to actively damp the low-frequency oscillations of the power sharing control unit. The gain of the virtual impedance loop is determined using a small-signal analysis and the pole placement method. The Mesh analysis is employed to further study the stability of low-frequency modes of the overall dc microgrid. Moreover, based on the guardian map theorem, a robust stability analysis is carried out to determine a robustness margin for the closed-loop system. The main advantage of the proposed method is its robustness against uncertainties imposed by microgrid parameters. This feature provides DG units with plug-and-play capability without needing the exact values of microgrid parameters. The performance of the proposed control scheme is verified using hardware-in-the-loop simulations carried out in OPAL-RT technologies.

A Unified Framework for Participation of Responsive End-User Devices in Voltage and Frequency Control of the Smart Grid

The paper presents a unified control framework which allows the responsive end-user devices (REDs) such as inverter-based photovoltaic systems (PVs), plug-in hybrid electric vehicles (PHEVs), and domestic controllable loads at residential level to effectively participate in the voltage and frequency control of the smart grid. The presented control framework basically relies on extracting information from active and reactive power sensitivities at different buses. In this framework, for voltage control, two support groups namely the active support group and the reactive support group are dedicated to each transmission bus. However, for frequency control, only one active support group is defined for the entire system. The REDs used for voltage and frequency control are classified based on their controllability degree. The idea of selecting the most effective buses is also presented to minimize the burden of communication commands. Following the detection of voltage or frequency violation in the system, the targeted buses are identified and receive corrective control signals to accordingly change their active and/or reactive powers. To minimize the manipulated active and reactive powers, the whole process is formulated as a multiobjective problem solved by the particle swarm optimization. The control procedure involves a series of commands for which the incident command system is used as a secure communication structure.

A Combined MoM-AOM Approach for Frequency Domain Analysis of Nonlinearly Loaded Antennas in the Presence of a Lossy Ground

A modeling technique based on the frequency-domain spectral balance method is proposed for analyzing nonlinearly loaded antennas in the presence of a lossy ground. The modeling involves two stages. First, the problem is transformed into a nonlinear microwave equivalent circuit with the circuit parameters of the antenna extracted by the method of moments. Then, the nonlinear microwave equivalent circuit is treated using the arithmetic operator method. To efficiently perform basic arithmetic operations in the frequency-domain, a linear matrix transformation of the spectra is introduced. The main feature of the proposed technique is its efficiency in dealing with problems involving strongly nonlinear loads, the presence of a lossy ground, and multitone excitations. It is also suited for the analysis of array antennas, considering the mutual effects between array elements. The accuracy of the proposed model is confirmed by comparing the simulation results of several case studies with those obtained using the conventional methods available in the literature. Mutual coupling effects, multi-frequency excitation, strong nonlinear loads, and the presence of a lossy ground are included in the simulations to cover all of the asserted aspects of the paper.

Lightning Electromagnetic Fields and Their Induced Currents on Buried Cables. Part I: The Effect of an Ocean–Land Mixed Propagation Path

We use a full-wave finite-element-based solution of Maxwells equations for the evaluation of lightning electromagnetic fields inside a vertically stratified, two-layer ground (ocean-land mixed propagation path) and their induced currents on the shield of buried cables. For “normal” incidence (with respect to the ocean-land interface), it is shown that the vertical electric field is the component most affected by the ocean-land mixed path when the observation point is close to the ocean-land interface (i.e., 5 m or so). For “oblique” incidence, however, depending on the angle of incidence and the distance between the observation point and the ocean, all the field components are reduced by the ocean-land interface. For the calculation of induced currents, and for the case of a parallel layout (cable laying in parallel to the ocean-land interface); 1) for a strike to the land, when the cable is buried in the soil and the distance to the ocean is greater than about 100 m, the effect of the ocean is negligible. 2) For a strike to the ocean, the induced current magnitudes are appreciable only when the cable is entirely within the land. For the case of a perpendicular layout (cable perpendicular to the ocean-land interface); 1) for a strike to the ocean, when the cable is totally buried in the ocean, the effect of ocean-land mixed propagation is negligible. However, when the cable extends into the land through one end, the induced currents increase at both ends with increasing length of underland portion. 2) For a strike to the land, when the cable is located entirely inside the land, the effect of ocean-land mixed path on the induced currents at both ends is negligible. However, as the cable extends into the ocean, a remarkable enhancement in the induced currents is observed for the termination located inside the land. This enhancement can be as high as a factor of 2 with respect to the case of a cable in homogeneous soil characterized by the properties of the land.

Lightning Electromagnetic Fields and Their Induced Currents on Buried Cables. Part II: The Effect of a Horizontally Stratified Ground

We use a finite element method (FEM) to evaluate the effect of a horizontally stratified two-layer ground on underground lightning electromagnetic fields and their induced currents on the shield of buried cables. It is shown that the azimuthal component of the magnetic field in the upper soil layer is affected by the soil stratification only when this layer is more conductive than the lower soil layer. On the other hand, inside the lower soil layer, this component is always affected by the soil stratification. The vertical electric field in the upper soil layer is mainly determined by the conductivity of the same layer in particular at close observation points. However, this component inside a more conductive lower soil layer is identical to that corresponding to a homogeneous soil with the same property of the lower soil layer. The horizontal electric field inside a stratified ground always takes values in between the electric fields corresponding to one-layer homogenous grounds. We also present a comparison with available experimental data on induced currents of a shielded buried cable and show that, in agreement with recent studies, taking the soil stratification into account allows to improve the late-time response of the induced currents. We also show that the horizontal stratification of soil may result, in some cases, in an enhancement of the induced currents with respect to the case of a homogeneous ground characterized by the electrical properties of either of the two layers.

Application of the Matrix Pencil Method to Rational Fitting of Frequency-Domain Responses

This paper presents a general methodology based on the matrix pencil method (MPM) for the fitting of frequency-domain responses in order to be properly represented in time-domain analysis. By virtue of the proposed method, a rational fitting of the frequency-domain responses can be inferred, which, in turn, helps with their inclusion into time-domain calculations. The proposed technique is well suited for the estimation of any type of function including the case of those with superimposed noise. The main feature of the method is its direct solution, hence avoiding any iteration in the estimation process. Moreover, the method does not require starting poles as opposed to the vector-fitting (VF) method. This paper presents the validation of the proposed approach by fitting the frequency responses of general a priori known functions, artificially created noisy functions and those of power components, namely: the elements of a power transformer admittance matrix and the characteristic admittance and propagation functions of a single-core sheathed cable. Finally, a time-domain analysis referring to lightning transients on a transmission line is presented.

Coordination of Distributed Energy Resources and Demand Response for Voltage and Frequency Support of MV Microgrids

This paper presents a framework for the coordination of distributed energy resources (DERs) and demand response (DR) for voltage and frequency support of islanded microgrids. The proposed method basically relies on extracting information from real and reactive power sensitivities at different buses for minimizing the voltage and frequency deviations of the islanded microgrid. To this aim, a new power flow procedure is adopted in which the frequency deviation appears as an additional state variable. This helps to calculate the required setpoints for the DERs as well as the amount of the demanded power curtailed through the controllable loads to meet the overall goal. The loads are classified based on their a-priori known controllability degree. To minimize the manipulated load, the most effective buses are selected based on their associated sensitivity values. In the grid-connected mode, the total operation cost is minimized, while the microgrid bus voltages are maintained within the pre-specified acceptable range. In both modes, the whole process is formulated as a multiobjective problem solved by the particle swarm optimization (PSO). The control procedure involves a series of commands for which the incident command system (ICS) is used as a secure communication structure. The performance of the proposed control framework is evaluated for the case of a typical MV microgrid in both grid-connected and islanded modes.