Paulo F. Obase

Analysis of lightning-caused distribution transformer failures

This paper investigates the effects of direct strokes to medium-voltage (MV) lines by analyzing the surges at the primary and secondary sides of a single-phase distribution transformer installed in a typical rural network of the state of Rio Grande do Sul, located in the South of Brazil. The distribution transformers of AES Sul, the electric utility, present a high failure rate and a significant number of the failures are attributed to lightning. The transformers are in general protected by surge arresters at the MV terminals and in a few cases also at the low-voltage (LV) side. Different distances between the MV arrester and the transformer as well as various values of ground resistance are considered in the analysis.

Diagnostic of silicon carbide surge arresters using leakage current measurements

Nowadays, the zinc oxide surge arresters (ZnO) are widely used in power systems, however, a large number of silicon carbide surge arresters (SiC) are still in service in the utilities. On the other hand, it is not possible to replace all SiC surge arresters in a short time period, being necessary to review the maintenance program taking into account the surge arresters that are more degraded. In this context, a research project was established between the University of São Paulo and the electrical utility CTEEP, aiming the investigation of its SiC surge arresters. This work shows that the leakage current measurement, a common diagnostic method for the ZnO surge arresters, can provide useful information related to the condition of the SiC surge arresters. Analysis of amplitude and distortion of the leakage current, also considering thermovision measurements, result in better evaluation of the SiC surge arresters.

Lightning-caused transformer failures in distribution systems

This paper presents the main results of an investigation conducted with the aim of reducing to an acceptable level the lightning-caused distribution transformer failure rate in a region in the South of Brazil. The region, in the border of the State of Rio Grande do Sul and Argentina, is characterized by a high lightning activity. The analysis of failed transformers and field results shows that the installation of surge arresters at the transformer LV side does not drastically change the failure rate. Computer simulations corroborate this result and confirm that failures are mostly associated with surges coming from the MV side. Therefore, the recommendations are mainly related to the surge arrester installation procedures at the primary side. A discussion is provided on the reasons for the apparent discrepancy with regard to the conclusions of similar investigations carried out in Australia, Norway, and the USA, according to which the installation of secondary arresters leads to substantial reduction in transformer failure rates.

The effect of the distance between transformer and MV arresters on the surges transferred to the LV side

The western part of the state of Rio Grande do Sul, in South Brazil, is characterized by high ground flash densities. In the period 2003-2011, lightning overvoltages accounted for about 47 % of the total number of distribution transformer failures observed in the service area of the power company AES Sul. This paper presents the results of an investigation on the influence of the distance between transformer and medium voltage arresters on the surges transferred to the secondary side. The analysis, performed through simulations using the Alternative Transients Program (ATP), shows that in general higher voltages are produced by subsequent strokes. Although the amplitudes of the voltages transferred to the secondary side tend to increase as the surge arrester approaches the transformer, the distance between these equipment should be as short as possible.

Analysis of the effect of lightning on the energized shield wire line technology

This paper examines the effect of lightning overvoltages resulting from direct strikes to the two SWL (energized shield wire line technology) systems located in the State of Rondônia, Brazil, with the aim of understanding how lightning affects the SWL system. The analysis covers the periods from 1996 to 2000 (Jaru SWL) and from 1997 to 2007 (Itapuã do Oeste SWL). Field measurements were carried out regarding both soil resistivity and tower footing resistance, as well as laboratory tests to determine the positive and negative critical impulse flashover voltage (CFO) of insulator strings. The influence of soil ionization is also considered.

Diagnostic of silicon carbide surge arresters

The gapped silicon carbide (SiC) surge arresters are being removed from the operating system of the utilities, however, a large number are still installed in the power system. Therefore, it is very important to give priority to SiC surge arresters that are more degraded in order to replace them by the zinc oxide (ZnO) ones. This work shows that the leakage current measurement, normally used as diagnostic technique for the ZnO surge arresters, can also provide important information concerning the condition of the SiC surge arresters. Results from the laboratory tests and substation measurements are presented.

Metodología de Evaluación de los Pararrayos de Carburo de Silicio

Resumen Este trabajo presenta los resultados de una investigacion cuyo objetivo fue el diagnostico de los pararrayos de carburo de silicio. En el laboratorio fueron realizados ensayos en pararrayos, siendo determinados los valores de tension disruptiva en frecuencia industrial y de impulso atmosferico. Esos resultados fueron comparados con valores de la corriente de fuga, radiointerferencia y termovision, obtenidos en mediciones en los mismos pararrayos. Se observo que valores mas elevados de la corriente de fuga, en terminos de amplitud y componente de tercer armonico, estaban asociados con una mayor degradacion en esos pararrayos. Con base en esos resultados, se concluyo que la medicion de la corriente de fuga puede proveer informaciones importantes sobre el estado de esos pararrayos. Mediciones del comportamiento de la corriente de fuga a lo largo de los anos en las suestaciones pueden ayudar a identificar los pararrayos mas degradados. Palabras clave : carburo de silicio, corriente de fuga, pararrayos, subestacion