Impact of Large 132/33 kV Power Transformer on Distance Protection in Bangladesh

To meet the rapid load increasing in Bangladesh, the government incorporates large power transformers (132/33kV, 80/120MVA) to serve the demand. Due to their low equivalent impedance, it significantly impacts the distance relays zone reach settings, especially in zone-3. This paper presents the effect of large power transformer on zone-3 reach settings, protection zone overlaps, the adjoining long line outside of the protection zone, and their resulting consequences. This paper proposes a systematic approach for overcoming the distance relays zone setting reach problem, ensuring zone overlapping on distance protection relays for the adjoining long line, and incorporating large power transformers into distance protection zone with proper relay coordination. For simulation, the DIgSILENT PowerFactory 2021 software is used. The short-circuit calculation is performed using the IEC 60909 method. For illustrating impedance seen by the distance relay in different fault conditions and locations, the 7SA522 line distance relay of SIEMENS is utilized. Simulations results (R-X diagram) show the effectiveness of the proposed approach, which can save the transformer from firing and minimize its damages in Bangladesh.