Comparison of switching surges and basic lightning impulse surges at transformer in MV cable grids

Övrigt konferensbidrag, 2009

The electric transients generated during the switching of a breaker placed near the transformer in a system with a substantial amount of cables can have a very short rise time and can reach very high magnitudes. In practice, the magnitude of the voltage surges is usually limited using the surge arresters while the rise time of the surge to which the apparatus is exposed, remains unchanged. In this paper, the characteristic of the switching transients generated in the MV cable grids are analyzed. For this purpose, cases with both dry-type and oil insulated transformers are studied. These cases are typical for a MV industrial grid and a wind park (WP) collection grid. Furthermore, the case with and without surge arrester protection is analyzed in order to observe the impact of the surge arresters on the voltage surges. Simulations show that the magnitude of the voltage surges exceeds the basic lightning impulse insulation level (BIL) defined by present standards. Furthermore, the rise time of the voltage surges is much shorter compared to the rise time of the lightning impulse especially in the case of the dry-type transformers and the WP collection grid. When surge arresters are used, the magnitude of the surges is limited to the BIL but the rise times of the surges can be twenty five times shorter compared to the BIL. Only when surge arresters are used with other fast transient mitigation equipment such as RC protection or surge capacitor protection, the magnitude of very fast voltage surges are brought down.