Single-system differential cutoff of power transformers of rural power networks constructed from aperiodic magnetizing inrush current

Abstract

At the present moment, the possibility of a distribution transformer suffering heavy damage because of short circuit current in its tank is still very high. Thus, it is very important to determine any coil damage inside the tank of a transformer with the highest possible speed and sufficient sensitivity. The problem of magnetizing inrush current must be solved to set up a system of protection that meets all these requirements. With digital protection devices, this problem is solved mostly by using second harmonics and a high level of cutoff operation current. All this may affect the system of protection negatively. At the same time, there is an opportunity to sustain very efficient and not overly complicated systems of protection with an appropriate level of current tripping in a distributied electrical network with low short circuit currents, such as rural power networks may be. The possibility of creating simple and effective (as well as reliable) differential cutoff is considered in this paper. The elaborated differential cutoff is more sensitive to the two- or threephase short circuits. The scheme of the differential cutoff is shown with a numerical example, which improves its efficiency. There are also oscillograms of two regimes. The first oscillogram is the regime of inrush current, and the second one is the regime of short circuit current. The elaborated device is blocked in the regime of magnetizing inrush current, and works in the short circuit regime, which disconnects the damaged transformer from the network.