Abstract
Protection of transformers, as one of the most expensive equipment in the power system, against lightning overvoltage impulses is a vital task. This paper, for the first time so far, investigates the effects of a filtered spark gap on the protection level of transformers against lightning overvoltage impulses. The filter is an inductor that is placed in series with the transformer and before the spark gap aiming to reduce the voltage at the connection point of the spark gap, and hence, enhancing the protection level of the transformer under lightning overvoltages. The experimental laboratory tests are accomplished on a 400 kVA, 22/0.4 kV, Delta-Star ( Δ − Y ) connection type transformer under 110 kV, and 125 kV overvoltage impulses, whereas the size of the spark gap is set to 80 mm and two inductors of 35 μ H and 119 μ H are considered. In order to perform a more in-depth analysis, a model that works reasonably close to the empirical case is developed in the EMTP-RV software. An optimization algorithm is used to determine the sensitive parameters of the double-exponential function, which is used to reproduce the applied laboratory lightning impulse voltages in the EMTP-RV environment. Moreover, the transformer is modeled according to the Cigre Guidelines (Working Group 02 of Study Committee 33). The behavior of the spark gap is simulated as close as the practical situation using the disruptive effect method. The preciseness of the simulated filtered spark gap model is verified by comparing the results of the simulated model in the EMTP-RV with the results of experimental tests. After verifying the model, different sizes of inductors are studied in the EMTP-RV environment to investigate whether larger or smaller inductors provide better protection for the transformer under lightning conditions. A comparison is performed among the conventional spark gap, surge arrester, and the filtered spark gap to provide a better analysis of the potential of the proposed device. The results indicate that proper sizing of the inductor, within an effective range, slightly enhances the protection level of the transformer.
Highlights
Electrical equipment and, the equipment that is operating in open areas are subjected to more natural hazards such as lightning phenomena
110 kV impulse voltage shows that in both cases the inductor resulted in increasing the applied voltage, the 125 kV applied impulse activated the spark gap to protect the transformer against the dangerous condition
16, and the case without any inductor, in Figure 12, shows that this inductor resulted in an increase voltage, which confirms the ineffectiveness of installing large inductors in reducing the amplitude of of about 20% in the flashover voltage, which confirms the ineffectiveness of installing large inductors lightning overvoltages
Summary
Electrical equipment and, the equipment that is operating in open areas are subjected to more natural hazards such as lightning phenomena. The application of surge arresters is regarded as the most appropriate method for the protection of power transformers, where these devices may decrease the probability of flashover per year due to both the direct and indirect lightning phenomena [19]. This work, for the first time so far, investigates the effects of a filtered spark gap on the mitigation of lightning overvoltages over distribution transformers both in the laboratory and in software. The primary contribution of this work is proposing a novel, inexpensive configuration, namely filtered spark gap, to reduce the amplitude of lightning overvoltage on the high voltage terminals of the transformer and improving the effectiveness of the conventional spark gap on protecting the power system equipment.
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