Carlos Portela

Computing small-signal stability boundaries for large-scale power systems

This paper describes two algorithms for determining the value of a given system parameter that causes the crossing of a complex-conjugate eigenvalue pair through the small-signal stability boundary (Hopf bifurcation). A large-scale test system was utilized to validate the two proposed Hopf bifurcation algorithms. The results presented demonstrate the computational efficiency and numerical robustness of the algorithms.

Going the Distance

The transmission of bulk power over long distances is a very current topic in the worldwide energy sector. An excellent example is the transmission system of Madeira River Hydropower Generation Project in Brazil, which includes two 2,400-km transmission lines, designed to transmit 3,150 MW through each HVDC link. In fact, long transmission trunks enable the use of natural resources that are far from major load centers as well as provide the interconnection of large power systems.

Six-phase transmission line-propagation characteristics and new three-phase representation

A new component transformation is proposed for six-phase systems, applicable for six-phase transmission lines studies, allowing its representation by two uncoupled three-phase lines. Cyclic transposition allows additional properties of the two three-phase groups that are presented. Transposed and nontransposed six-phase lines are analyzed, and the main characteristics of their natural modes are discussed in the frequency domain. An eigenvector normalization method for nontransposed lines that assures null modal conductance and, as a consequence, positive modal resistance, is proposed. Numeric results are presented in relation to an example line of 500 kV/sub rms/, phase to ground, with characteristic power of 5176 MW. >

A new procedure to derive transmission-line parameters: applications and restrictions

The objective of this paper is to show an alternative methodology to calculate transmission-line parameters per unit length. With this methodology, the transmission-line parameters can be obtained starting from impedances measured in one terminal of the line. First, the article shows the classical methodology to calculate frequency-dependent transmission-line parameters by using Carsons and Pollaczecks equations for representing the ground effect and Bessels functions to represent the skin effect. After that, a new procedure is shown to calculate frequency-dependent transmission-line parameters directly from currents and voltages of an existing line. Then, this procedure is applied in a two-phase and a three-phase transmission line whose parameters have been previously calculated by using the classical methodology. Finally, the results obtained by using the new procedure and by using the classical methodology are compared. The article shows simulations results for a typical frequency spectrum of switching transients (10 Hz to 10 kHz).

Quasi-modes multiphase transmission line model

Abstract This article presents a new model to represent transmission lines including the frequency dependence of longitudinal parameters. The model uses the natural modes, for ideally transposed lines, and ‘quasi-modes’ for non-transposed lines, and is applied to lines that have a vertical symmetry plane. The line is represented through π -circuits, with one π -circuit for each mode. The transformation matrix is modeled using ideal transformers. The model is described for three-phase lines, dc lines, double three-phase lines and six phase lines. A 440 kV three-phase transmission line illustrates it and is compared with a frequency dependent EMTP line model, the Semlyen one.

Behavior of overhead transmission line parameters on the presence of ground wires

Initially this paper shows the ground wire reduction process for generic multiphase transmission lines and after, the ground wire reduction process for a specific 440-kV three-phase overhead transmission line. Following this, the influence of the ground wire reduction process considering two situations is shown: first, considering frequency independence and second, when these parameters are considered as frequency dependent. This paper presents analytical results for generic multiphase transmission lines. For a specific 440-kV three-phase overhead transmission line, analytical and graphic results are shown considering real data for every frequency between 10 Hz and 1 MHz.

Half-Wave Length Line Energization Case Test - Proposition of a Real Test

In Brazil big blocks of energy will be transported through distances between 2500 and 3000 km to the strong network nodes. Among the AC transmission systems alternatives being analyzed the half-wave length transmission seems to be the natural solution as the lengths involved are around a half-wave length of a 60 Hz frequency system, as the Brazilian one. As there is no half-wave transmission system in operation in the world, there is a major sense of caution in order to be the first to construct and use this new AC-link. In order to give some support a field test with a set of existing similar 500 kV lines that could be connected in series was proposed to simulate the AC-link behavior under some controlled switching. The proposed AC-link test was simulated in PSCAD/EMTDC.

Grounding Systems Modeling Including Soil Ionization

This paper presents a hybrid frequency-time domain methodology for the modeling of grounding systems considering the nonlinear effect of soil ionization. The method is validated by analyzing two typical grounding systems under high soil ionization effects. It is clear from the gotten results the reduction effect of the so-called grounding system ldquoequivalent impedancerdquo when soil ionization takes place.

Extra Long-Distance Bulk Power Transmission

This paper presents a solution for bulk ac power transmission over extra long distance based on an unconventional transmission line that is little longer than a half wavelength (λ/ 2+). This solution is very convenient to be applied in countries where the generation sites are very distant from the consumption centers. However, it has some restrictions regarding feeding loads at intermediate points. To overcome this drawback, an HVAC tap based on power-electronics converters is proposed. This tap allows feeding local loads as well as integrating power plants to the main circuit without changing the transmission systems electrical characteristic. This paper also presents an application example of a λ/ 2+ line with an HVAC tap draining and injecting energy from the line.

A 25-MW soft-switching HVDC tap for /spl plusmn/500-kV transmission lines

This paper presents an optimization of a previously proposed high-voltage dc current transmission system (HVDC) tap system where the power rating of the soft-switching dc-dc converter has been increased to supply medium loads (up to 25 MW) from /spl plusmn/500-kV HVDC transmission lines. This dc-dc converter is connected in series with a pole of the HVDC transmission line. It drains energy to a dc capacitor through an air-core transformer and a single-phase diode rectifier connected at the secondary. A pulsewidth-modulation (PWM) voltage-source converter is directly connected to this dc capacitor to supply ac loads. The HVDC tap comprises all those components and can supply high-quality power to isolated or interconnected load areas. The controller of the HVDC tap is independent from the control systems of the main HVDC converter stations. It does not need any communication channel to the control of the main HVDC converter stations. A complete bipolar /spl plusmn/500-kV HVDC system, together with a refined model of dc transmission line were implemented to simulate the performance of the proposed HVDC tap. It has revealed to be very robust and to be an attractive solution for feeding small loads from long dc transmission lines.