Abstract

Introduction

Settings Description

Settings Comparison and Consequences

Proposed Methodology

Result and Discussion

Conclusion

References

Abstract

     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.

Introduction

     The power system of Bangladesh is expanding rapidly and getting complex day by day. The load of the grid substation is also increasing with time due to the government’s hundred percentage electrification target. The government made power system master planning in 2010 and revised it in 2016 [1]. The maximum generation of Bangladesh significantly changed after 2009. In the year 2009-10, the maximum power generation was 4606 MW, and in the year 2018-19, the maximum power generation was 12,893 MW, as shown in Fig. 1 [2]. Due to the pandemic situation (Covid-19), the 2019-2020 generation was near about the same as 2018-2019. Due to the rapidly increasing demand, power evacuation from generation to distribution has become a challenge. A vast infrastructure needs to develop for power evacuation throughout the country. The government has already taken initiatives to establish the infrastructure. Many projects are ongoing at the generation, transmission, and distribution levels. Some projects are finished, and some projects will be completed soon. Due to the rapidly increasing demand, the power sector of Bangladesh passed through a transition period.

Conclusion

     Large power transformers are greatly impacted on distance relay zone reaches. The zone-3 reach of the PGCB method is conservative, which does not cover the next long line in most cases. Therefore, if the proposed philosophy can be adapted in distance protection, it will provide backup protection for the adjoining long line and transformers. The effectiveness of the proposed methodology is shown by quadrilateral impedance diagram of SIEMENS line distance relay (7SA522) using the DIgSILENT PowerFactory 2021 software at different fault conditions at 132kV and 33kV voltage levels. This paper illustrated how distance zone overlapping was missing due to the PGCB settings philosophy and how the proposed methodology effectively ensures the zone overlapping and secures the tripping with maintaining proper relay coordination at 33kV and 132kV voltage levels. This paper also explains the leading cause of the increasing damages of failure transformers in Kushtia and Mymensingh and how damages can be minimized. It is revealed that the proposed methodology can provide fast fault clearing time in case of the circuit breaker or DC failure with proper selectivity, which will significantly reduce the damage of equipment and quick restoration of power systems. In the future, the research will extend for formula estimation of critical length at 33kV level where 33kV and 132kV relays need to coordinate for different transformer conditions and formulating chart for critical 33kV length in Bangladesh perspective.