José L. Silvino

A flux observer for induction machines based on a time-variant discrete model

An alternative approach to obtaining a discrete-time counterpart to the two-axis continuous model of an induction machine is presented. The discrete time-variant equations are derived from a partial discretization of the continuous state equation. The discrete model can be used with advantage in simulation and real-time control applications. A discrete time-variant reduced-order flux observer based on the discrete model is proposed. Computer simulation results are shown to compare the proposed discrete model with that obtained from the Euler method and the exact continuous model. >

H/sub /spl infin// control for a variable-speed adjustable-pitch wind energy conversion system

This paper presents a robust control design for a variable-speed adjustable-pitch wind energy conversion system with a uncertain multivariable controller is designed to turbine speed to extract maximum power from the wind and to allow pitch adjustment to power regulation and to use low speed wind. A change in the operating point of the plant was simulated for a nominal model and for a disturbed one showing the robustness of the controlled system.

Power harvesting from shield wires for monitoring apparatus on overhead transmission lines

The apparatus installed on overhead power transmission lines for security or monitoring purposes are usually powered by photovoltaic (PV) cells. In this paper, we investigate a reliable and low maintenance alternative to the use of PV cells. The currents in the phase cables induce AC currents in the overhead shield wire. Current transformers, installed on this wire, can supply power to devices on the top of the towers. The simulations with the ATP program and field measurements show the feasibility of this powering method.

Two-port wideband models of a single-phase distribution transformer with center-tapped secondary

This paper proposes a pair of two-port wideband models for a 10 kVA 7.967 kV/240-120 V single-phase transformer with center-tapped secondary used in distribution lines in Brazil. The models are valid from 10 Hz to few MHz and are derived from measurements of the admittance matrix and the voltage ratio between the high-voltage and low-voltage transformer terminals. Two different conditions are assumed for deriving the models for the transformer. One assumes that one of the low-voltage terminals is short-circuited, while the other assumes that the same terminal is left open-circuited. Both models are validated by means of comparisons with experimental data. It is shown that voltages transferred from the high-voltage side to the low-voltage side of the transformer reach higher amplitudes for the condition where one of the low-voltage terminals is left open-circuited.

Detection and location of direct lightning strokes to overhead power transmission lines by measuring currents from shield wires

This paper proposes a new method for detection and location of lightning strokes to overhead power transmission lines. The method uses sensors distributed along the transmission line, placed on the top of the towers, to detect and locate the stroke by measuring the stroke currents flowing from shield wires to the towers. When a lightning strikes the shield wire, the stroke currents flow from the wire to ground through the towers. If the lightning strikes the phase cable, the stroke currents induces currents in the shield wires which flow to ground through the towers. Each sensor measures the current and send the rate of change and the peak value to a computer for location processing and evaluation of the stroke severity.

Power transformer modeling based on wide band impedance and admittance measurements

This paper evaluates the transient behavior of a single-phase, two-winding, 127/12 V, 12 VA signal transformer according to two different high-frequency models. The first model is based on the direct measurement of the transformers impedance matrix, while the second model consists in a combination of the admittance matrix and voltage transfer measurements. Both are alternatives to obtain a model with more information on the transformers behavior at frequencies ranging from 10 Hz to 2 MHz. The admittance matrix resulting from the inversion of the measured impedance matrix (in the first model) and the modified admittance matrix (in the second model) are approximated by rational functions via the vector fitting technique, which leads to two equivalent RLC models that are stable and ready to be used in EMTP-like programs. The obtained models are first compared in the frequency domain with measurements carried out at the transformer terminals. Then, in order to validate the developed models in the time domain, comparisons between the theoretical and measured results of the transferred voltages from the primary to the secondary windings of the transformer are made. These comparisons consider distinct load conditions and the application of impulse waves with different time characteristics.

Fault location on overhead power transmission lines by measuring currents from shield wires

In this paper, we propose a new method to locate faults on overhead power transmission lines. Line-to-ground or line-to-line faults induce transient currents in the shield wire, which flow partially to ground through the towers. In the towers closest to the fault position, the transient currents are greater and this behavior can be used to locate the fault. The method uses sensors distributed along the line, placed on the top of every tower to measure the peak currents of the transients. The sensors send these values to a computer for location processing. The method can locate faults with precision in a wide range of impedances.

Current Controller for Autocommissioned Vector-Controlled Induction Motor Drives

A current controller composed of a current regulator associated to a parameter adaptation scheme is proposed. The current regulator considers the inherent coupled structure of the induction machine. The adaptive block is coupled to this regulator in order to carry out a high performance current control with low sensitivity to parameter variations. It is shown that this controller can also be used to estimate the machine saturation characteristic and the rotor time constant as part of an indirect vector control autocommissioning procedure. A discrete time approach is utilized for the design considering microprocessor implementation.A current controller composed of a current regulator associated to a parameter adaptation scheme is proposed. The current regulator considers the inherent coupled structure of the induction machine. The adaptive block is coupled to this regulator in order to carry out a high performance current control with low sensitivity to parameter variations. It is shown that this controller can also be used to estimate the machine saturation characteristic and the rotor time constant as part of an indirect vector control autocommissioning procedure. A discrete time approach is utilized for the design considering microprocessor implementation.

Preliminary study of the effect of dispersive ground parameters on transferred lightning surges

This paper presents a preliminary study on the effect of dispersive ground parameters on surges transferred from medium-voltage to low-voltage distribution lines located in rural areas. In this study, a direct lightning strike is assumed at different points of a distribution line terminated at a 10 kVA 7.967 kV/240-120 V single-phase transformer with center-tapped secondary. The distribution line is modeled using the JMarti model modified to include the variation with frequency of the ground resistivity and permittivity. The transformer is represented as a two-port model valid in a wide frequency range. Insulation breakdown is modeled using the disruptive effect method and for simplicity the grounding impedances are represented as simple resistances. Voltages transferred to the transformer secondary are calculated for different conditions. It is shown that considering dispersive ground parameters does not have a significant influence on phase-to-neutral voltages transferred through the transformer to its low-voltage side in the investigated cases.

Metal-oxide surge arrester's leakage current analysis and thermography

Metal-Oxide arresters are used to protect substations equipment against surges. Because their constructive properties, a small current, called leakage current, flows through them in normal operation condition. The resistive part of this current is a good indicator of the arresters degradation state and is also responsible for its heating. This paper discusses the use of thermography and some methods that measure the leakage current for autonomous monitoring of metal-oxide surge arresters. Environment factors that contribute for its surface temperature and, therefore, affect thermography results are evaluated and discussed. In addition, a simple and direct current measurement method is used to obtain the resistive current and show its relation with thermography.