Assessment and Modernization of Relay Protection Systems for Auxiliary Transformers at Thermal Power Plant No. 4: Bachelor Thesis

Abstract

This thesis presents an engineering assessment and modernization proposal for the relay protection system of auxiliary transformer 2BT at Thermal Power Plant No. 4 in Mongolia. The reliable operation of the auxiliary power supply system is essential for maintaining the continuous and safe operation of major plant auxiliaries, including boiler feed pumps, draft fans, circulating pumps, and other motor-driven equipment required for unit operation. The study evaluates the technical condition of the existing 25 MVA, 10.5/6.3/6.3 kV auxiliary transformer 2BT and its associated relay protection system. The assessment includes connected load analysis, transformer loading verification, short-circuit current calculation, review of the existing protection functions, selection of a suitable replacement transformer, and calculation of relay protection settings for the proposed modernization scheme. The load calculation was performed using the demand-factor method, while the short-circuit current calculation was carried out using the per-unit method. The calculated apparent load of transformer 2BT is 26.25 MVA, which exceeds the rated capacity of the existing 25 MVA transformer. This indicates insufficient loading reserve under the considered operating condition and justifies the replacement of the existing unit with a higher-rated transformer. Based on technical compatibility and capacity requirements, the ТРДНС-32000/10 У1 transformer with a rated power of 32 MVA is selected as the preferred replacement option. The calculated three-phase short-circuit currents are 26.07 kA at the 10.5 kV side and 15.08 kA at the 6.3 kV auxiliary bus section. The existing electromechanical relay protection devices are technologically outdated because they offer limited setting flexibility, lack self-diagnostics, lack disturbance recording, and have restricted communication capability. Therefore, the SEL-387A microprocessor-based transformer protection relay is proposed for modernization. This relay provides three-winding differential protection, backup overcurrent protection, harmonic restraint, thermal overload protection, breaker failure protection, event recording, and communication functions. The economic evaluation shows an estimated annual saving of 33,000,000 MNT and a payback period of approximately 9.4 years. The results demonstrate that the proposed transformer replacement and relay protection modernization will improve the reliability, selectivity, sensitivity, and maintainability of the auxiliary power supply system at TPP-4. Assessment and Modernization of Relay Protection Systems for Auxiliary Transformers at TPP-4