Analytical Evaluation of Existing ANSI 50/51 Overcurrent Relay Settings for Coal Mill 2B Motor at STG-BB Power Plant, PT Pupuk Sriwidjaja Palembang

Abstract

This study presents an analytical evaluation of the existing ANSI 50/51 overcurrent relay settings for the Coal Mill 2B motor at the STG-BB power plant of PT Pupuk Sriwidjaja Palembang. Direct trip testing was not conducted because the motor is part of an operating industrial installation. The evaluation combines a review of the existing relay settings, calculation of the manufacturer-defined Long Time Inverse characteristic, comparison with the reported motor starting current and starting time, and analysis of ten days of recorded operating current, voltage, and active power. The ANSI 51 pickup is 136 A, equivalent to approximately 1.03 times the 132 A motor nameplate current, with TMS = 0.025. The reported high-set ANSI 50 stage is 1,227.6 A, equivalent to 9.3 times the motor nameplate current, with a definite delay of 0.01 s. Using the MiCOM P225 Long Time Inverse characteristic, the calculated operating time at the reported starting current of 985.91 A is approximately 0.48 s, which is close to the upper end of the reported 0.3–0.5 s starting time. Therefore, starting-current ride-through cannot be conclusively verified from the available data. During the ten observation days, the recorded running current ranged from 76.93 A to 95.80 A, with an average of approximately 85.69 A, equivalent to 64.9% of the 132 A nameplate current. The observed current remained 40.20–59.07 A below the 136 A pickup. These results provide an analytical assessment of the existing relay settings under the observed operating conditions, but do not demonstrate complete selectivity, protection coordination, fault sensitivity, or overall relay reliability.