Transport of energy over long distances from remote natural sources to consumers in big cities requires technical, economic and environmental considerations. Uncertain wind flow, sunshine, faults and communication failures can compromise system safety, reliability, power quality during small signal and large scale power system oscillations. HVAC systems tackle power swing issue by electrical islanding protection schemes which would need to be revised for integrated AC/DC systems. Advent of DC transformers, CSC-HVDC, VSC-HVDC and MMC-HVDC technologies have enabled the enhancement of power delivery and integration of renewable energy sources under smart grid vision. HVDC transmission lines are recognized as an efficient alternative to HVAC lines. HVAC system power transfer capability is limited by reactance, whereas HVDC lines can be loaded up to the conductor thermal limit. This paper reviews power transfer capabilities of HVAC, HVDC and High Phase Order (HPO) HVAC lines to compare their relative performance under diverse conditions. Techno-economic analysis of converting existing AC lines into DC lines is presented. High voltage race has been analyzed in context of environmental concerns and HVDC transmission systems are concluded to be the right path to energy transition.

Comparative study of HVAC and HVDC transmission systems

Lightning surge characteristics of a transmission line comprising a tower, a shield wire and phase conductors are studied with the help of NEC-2. A tower struck by a lightning stroke behaves as an antenna until a traveling wave makes several roundtrips in the tower. During this interval, the tower footing impedance appears to be higher than the footing resistance, and the coupling coefficient between a shield wire and a phase conductor is much lower than that in the TEM mode. These affect the estimation of insulator voltages of transmission towers subject to lightning currents having short rise times. On the basis of these findings, the parameters in the multistory tower model, which has been used widely in EMTP multi-conductor analyzes, are newly proposed.

Numerical electromagnetic field analysis on lightning surge response of tower with shield wire

The emergence of power quality as a topical issue in power systems in the 1990s largely coincides with the huge advancements achieved in the computing technology and information theory. This unsurprisingly has spurred the development of more sophisticated instruments for measuring power quality disturbances and the use of new methods in processing and analyzing the measurements. Fourier theory was the core of many traditional techniques and it is still widely used today. However, it is increasingly being replaced by newer approaches notably wavelet transformand especially in the post-event processing of the time-varying phenomena. This paper reviews the use of wavelet transform approach in processing power quality data. The strengths, limitations, and challenges in employing the methods are discussed with consideration of the needs and expectations when analyzing power quality disturbances. Several examples are given and discussions are made on the various design issues and considerations, which would be useful to those contemplating adopting wavelet transform in power quality applications. A new approach of combining wavelet transform and rank correlation is introduced as an alternative method for identifying capacitor-switching transients.

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Wavelet transform for processing power quality disturbances

This paper reviews and compares several methods to analytically obtain the transient response of transmission lines in the time domain, in those cases where frequency independent parameters can be assumed. The distributed-parameter line is modeled by the cascaded connection of a number of lumped-parameter /spl pi/ circuits, each one representing a fraction of the line length, leading to a linear time-invariant (LTI) circuit. The associated state-space equations are formulated, allowing explicit expressions for the state variables to be written in the time domain. The solution is then obtained by means of three different approaches, all of them requiring that the natural frequencies be previously computed, namely: eigenvector-based procedure, Vandermonde matrix method, and Lagrange interpolation formula. Numerical integration by the trapezoidal rule is also considered for comparison. Two kinds of test results are presented. First, accuracy of the results provided by the LTI lumped-parameter model are compared with those obtained using the Electromagnetic Transients Program. Second, a comparison is performed in terms of the computational cost involved in each method. Two cases of practical interest are assessed, namely solving from scratch the state equations and updating the solution for a new set of initial conditions.

A comparison of techniques for state-space transient analysis of transmission lines

The transient caused by capacitor switching is one of the important power quality (PQ) problems. In particular, switching on the capacitor may result in an overvoltage to a sensitive load, serious disturbance to an adjustable-speed drive or a resonance in the system. This paper presents a new method to locate the positions of the transient sources with the help of the PQ monitoring system. The discrete wavelet transform is used to extract the features of transients caused by capacitor switching. The fuzzy-c-means is then used to determine the placement of PQ measurement facilities. The signal energies attained by wavelet coefficients serve as inputs to the hybrid principal component analysis neural network for locating the transient sources. The simulation results obtained from an 18-bus distribution system show the applicability of the proposed method

Locating Switched Capacitor Using Wavelet Transform and Hybrid Principal Component Analysis Network

A new method has been developed to study the transient behavior of nonuniform transmission lines whose parameters can have any arbitrary space variation. Using the exponential line as a building block, s-domain expressions are obtained for the voltage and current at any point along the nonuniform line. An efficient numerical Laplace-transform inversion technique is then used to compute the time-domain solution. Computer simulations and comparisons with other solution methods are included. >

Transient analysis of lossless single-phase nonuniform transmission lines

This paper addresses the effect of the electrostatic field due to extra high voltage (EHV) overhead transmission lines on pipelines in the vicinity of those power lines. Two measures for this effect are used: the maximum electric field on the pipe surface and the total electric charge on the pipeline per unit length. A mathematical model is presented for assessing these two measures. The results of applying the model to a situation involving a single-circuit, six-bundle, three-phase, 750-kV EHV line with flat conductor configuration are presented and discussed. The first set of results refers to a parallel pipeline of radius 0.5 m. The dependence of both the maximum electric field (at the top of the pipe) and the charge per meter on the distance between the pipeline and the tower center is similar to the distribution of the electric field beneath the tower at the ground surface. It shows maxima of both quantities if the pipe is exactly under one of the outer phases of the power line. At a distance of about 35 m from the tower center, both the electric field and the charge per unit length drop to 50% of their maximum values. The electric field is found to increase almost linearly with the clearance between the pipeline and the ground surface. The charge changes in a more complicated way with the clearance. It decreases if the pipe clearance increases from 0 to 0.2 m, then increases steadily beyond this value. For a given distance from the 750-kV line and for a fixed clearance from the ground, both the electric field and the electric charge per unit length on the pipeline will increase with the pipe radius. This paper will deal also with the impact of the pipeline on the nearby EHV power line and its associated network. The results will show that for a solidly earthed power network, the presence of the pipeline will be accompanied by a slight increase in the neutral current. On the other hand, for a power network with an inductively earthed neutral, there will be a tendency toward a parallel resonance that can occur for particular values of the neutral inductance and pipe radius. This resonance will result in an increase in the system's neutral potential, which will be primarily limited by the network losses.

The capacitive coupling between EHV lines and nearby pipelines

The analysis of electromagnetic transients on nonuniform lines is addressed with special attention paid to those transients relevant to power networks. Traveling-wave and time-domain solutions are discussed, and the applicability of a proposed s-domain, distributed parameter direct-simulation technique is demonstrated. It is shown that this approach yields exact solutions in cases where the characteristic impedance at any line point can be represented by some analytic function of the coordinate of that point. Two examples of this are discussed. Comparisons between the other techniques and the advantages and limitations of the suggested technique are given. >

s-domain analysis of electromagnetic transients on nonuniform lines

This paper deals with the analysis of transients initiated by the switching of shunt capacitors in power networks. These transients will propagate through the network along the transmission elements and will, accordingly, be felt at other locations far from the capacitors, such as load terminals. This can lead to adverse effects on the supply quality at those load buses. Based on a typical radial system, the distributed-parameter analysis will give analytical expressions for the expected transient voltages at the load terminals in terms of both the size and location of the switched capacitor. In its second part, this paper presents simple closed-form expressions for the accurate identification of the shunt capacitor's size and location, in terms of easily measurable parameters of the voltage signal at the load terminals.

Capacitor switching transients: analysis and proposed technique for identifying capacitor size and location

A method for the exact computation of long-line fault transients is presented. The use of a numerical inverse Laplace transform algorithm makes it possible to analyze faults on lines of any length, while the derivation of the equations in the s-domain makes it possible to represent such lines by a single two-port network for each one. For convenience, the approach is first illustrated for a single-phase representation, which is valid for symmetrical faults. Then a generalized approach for three-phase modeling is derived to simulate any type of fault. The advantages of this approach are its accuracy and simplicity as well as its inherent capability for giving the values of the different currents and voltage at different network locations at preselected time points. It is believed that one of the major applications of this method will be the testing of different digital protection algorithms and strategies. >

M.M. Saied

Papers

Transient analysis of lossless single-phase nonuniform transmission lines

The capacitive coupling between EHV lines and nearby pipelines

s-domain analysis of electromagnetic transients on nonuniform lines

Capacitor switching transients: analysis and proposed technique for identifying capacitor size and location

A method for the computation of fault transients in transmission lines