M.L.B. Martinez

Estimation of the influence of direct strokes on the lightning performance of overhead distribution lines

When assessing the lightning performance of distribution lines, major attention is in general devoted to the induced voltages from nearby strikes rather than to direct events hitting the line, as the latter are always expected to cause a fault in the line. This paper proposes a Monte Carlo procedure capable to make it possible the evaluation of the overvoltage probability due to both indirect and direct strokes. The results are presented making reference to specific nodes of medium voltage overhead distribution networks, namely the locations of MV/LV transformers connections. The problem is of interest when a major upgrade of surge arresters is required, for instance when the neutral grounding practice needs to be changed (e.g. from grounded to resonant).

Protection systems against lightning-originated overvoltages in resonant grounded power distribution systems

A joint research project between AES Sul, the Federal University of Itajubá and the University of Bologna aims at defining the required protections against lightning-originated overvoltages in a resonant grounded distribution system in order to achieve the adequate lightning performance level with minimum cost. The paper aims at illustrating the effectiveness of the presence of a surge arrester or an arcing-horn gap and its protective zone by means of simulations obtained with a computer code for the calculation of lightning induced voltages that takes also into account the steady-state voltage. The results of a statistical analysis for a single multiconductor line are eventually presented and discussed.

Insulation coordination for a 23 kV medium voltage distribution

The insulation coordination performance of distribution lines may be associated to two events: the direct lightning strokes and the lightning induced overvoltages both of them are attached to the local flash density to ground. Of course there are particular aspects that must be considered, as for instance, the line route, the shielding degree offered by tall structures, trees and many other ones. This paper deals with the possibility of considering flat and perfect conductively soil, associated to the use of the Monte Carlo technique. However, some simplifications were considered. The first simplification is to consider one-conductor line. The insulation withstanding was obtained from the original field three-phase system. The insulating system CFO voltage and its attached standard deviation were obtained from a complete up and down testing procedure, which must take into account the impulse voltage rate of rise. The statistical insulation withstanding of the system considered a symmetric Weibull distribution.

A methodology to predict the lightning insulation strength for distribution transformers by applications of reduced lightning standard impulse voltages

The lightning withstanding of transformers are based on a set of impulse voltage applications with 1.2/50 μs shape, in accordance with international standards. The standard lightning impulse test is a destructive test, in which the transformer insulation strength is predicted by comparisons between reduced and rated levels for full and chopped impulses. In this paper a different method named “progressive impulse test” is proposed to provide estimation with respect to the voltage level in which evidences of insulation failure appears. Modifications on oscillation frequency of current waveforms may indicate sparks or short-circuits between turns, layers or between windings and the ground. By comparisons among these current waveforms it is possible to predict the insulation performance, as well as manufacturing quality process without the necessity to submit the transformer to all lightning impulse sequence test.

Inspection of Pin Insulators in Distribution Medium Voltage Lines & Outage Reductions

This paper shows a complementary study aiming at verifying the influence of the weather conditions on the effectiveness of a previous set of techniques applied to define the operational condition of porcelain pin insulators installed in medium voltage distribution lines. The research method, as discussed in a previous paper [1], deals basically with a preliminary rough inspection with an electromagnetic noises detector and a final close inspection with an ultrasonic noises detector. During the last inspection the temperature, the wind speed and the relative humidity of the air were also recorded to populate a data base with the insulators main data. Once detected a single noisy insulator all insulators installed in the inspected pole were removed and substituted by equivalent post insulators. These insulators were further classified, tested and a comparison of field and laboratory results was used to verify the influence of the weather conditions on the noise level that defines the boundary limit of damaged & no - damaged insulators. The laboratory tests were carried out at the High-Voltage Laboratory of Federal University of Itajuba.

Implementation of a Simulator for the Most Commonly Found Industrial Motor Loads Based on Pulse-width Modulation Inverters and Torque Estimator

Abstract The aim of this article is to present the results of the implementation of a simple generic load simulator using pulse-width modulation inverters. The implementation of this load simulator took place at Universidade Federal de Itajubás electrical drives laboratory. Different types of load can be simulated to evaluate the performance of electric motors in electrical drives. The implemented system shows satisfactory results, even at low speeds. The system arrangement utilizes two pulse-width modulation inverters (one for the motor and the other for the generator), a PC microcomputer, and a data acquisition board installed in a free slot of the microcomputer. A tachogenerator is also used to provide an analog speed signal as well as LabVIEW data acquisition (Hungary) for filtering speed and torque signals for better visualization and interpretation of the results. The torque signal is originated from the motor pulse-width modulation inverter torque transducer.

Distribution transformer impulse test — comments

The objective of the impulse tests on distribution transformers is to evaluate the winding insulation performance against lightning. Therefore, the acquired wave shapes must not present failure evidences. The key point on these cases is that some measurement equipments rarely show failure signals, mainly signals due to high frequency oscillation, considering as perfect a failed transformer. This paper discusses some mathematical model and testing results that shows that some high frequency oscillations are related to turn-to-turn failures.

AES Sul's experiences using series compensation on medium-voltage distribution system

This work relates the experience of AES Sul on implementing series compensation to solve voltage drop problems on feeders. First, measures from a system, which present about 300% of compensation, will be showed. During the energizing, that feeder presented significant voltage oscillation. It occurred on every test was made due to the coupled between series capacitor and three-phase induction motors used in pumps on rice farms. The analyze of that situation showed significant voltage instability what makes impossible its work. After the analyze of damp insertion on electromagnetic transient programs, it was decided to install this series capacitor in another feeder. On that other feeder, the results were good, obtaining an elevated compensation level and a well voltage behavior. These results are also presented on this work.

Characterization of induced surges impulses and their generation in laboratory

Lightning surges are one of the main causes of failures in transmission lines and equipments. Power equipments are usually designed to withstand voltages related to direct lightning strikes, but, for distribution lines, usually there is a natural shielding against direct strikes due to nearby tall structures. In this case, induced overvoltages caused by lightning strikes in the proximities of the line are more frequent and can be a preponderant aspect for the line design. This paper presents a study for the characterization (peak values, front and tail time) of induced surges, generation and evaluation of their effects on 13.8 kV equipments. This characterization is made considering a massive set of induced overvoltages obtained by means of stochastic methods, electromagnetic coupling simulations and statistical analysis. The results were used to design components for the impulse generator of Federal University of Itajubá, so that the induced surges could be generated for equipment testing.

Distribution reliability analysis program

The objective of this paper is to present a computer program developed for Windows to compute reliability indices for different customers and sets of customers supplied by the same distribution feeder. This program deals with information obtained directly from utility databases to create equivalent network modeling and carry out contingency analytical simulations. The systems average failure rates must be informed by the user, based on historical utility data, manufacturer test data or typical values. The program enables one to compute penalties and annual costs of energy interruptions and compare the obtained indices with the reliability index targets set by the national regulatory agency (ANEEL) for the different sets of customers and distribution feeders under analysis. The program allows to analyze the influence the variation of the system rates and also modifications of the topology of the network and, thus, to carry out a sensitive study to verify the effect of some different protection equipment and modifications in the topology of the systems, justifying investments that can improve the reliability, the power quality, and reduce costs of energy interruption and other related costs.