Attenuation of transient overvoltages in the low voltage network due to chopped lightning impulse tests

Summary

The paper presents a case-study of failure and damages on electronic equipment when overvoltage transients is produced by tests with chopped impulse voltages or during disruptive discharge voltage tests took place at the High Voltage Laboratory of UNIFEI – Federal University of Itajubá, Brazil. In order to prevent other damages, studies have been carried out to detect the source of the problem. The studies were divided in two main steps to get a better understanding about the overvoltages, their source and finally the correct techniques to mitigate them. All measurements were taken by applying the standard lightning impulse shape (1.2x50 ?s) on polymeric insulator for 36.2 kV medium voltage networks. For this purpose, it has been used an Impulse Voltage Generator (Marx Generator) installed at the High Voltage Laboratory at UNIFEI. The voltage level applied during the measurements was set at 240 kV. Preliminary measurements were performed during the chopped impulse tests using an oscilloscope connected in the phases and grounding connections of the low voltage network, where the peak-to-peak overvoltages were measured. In the meantime, the program ATPDraw® was used to model the impulse generator, the test sample and the grounding mesh impedance at high frequencies to determine the amount of current flowing to the grounding system during the disruptive discharges. Afterwards, the second hypothesis considered is about electromagnetic interference. A set of measurements were performed using conductor disconnected from the AC source in different layouts, near and far from the impulse generator. The results were significantly different and higher amplitudes were detected closer to the impulse generator. With the identification of the cause of transient voltages, some techniques and approaches for mitigating the problem were tested and evaluated, such as: surge protection devices (SPD’s), ferrite cores, shielding, filters and insulated transformers.

Additional informations

Authors

PEREIRA, JOSEPH, DUFFEAU, BROWN

Keywords

Impulses Voltages, Transient Overvoltages, Damaged Electronic Devices, Attenuation.