Final answer:
To select suitable CT ratios and plug settings for a radial system with relay points A, B, and C, consider the maximum load currents and the relay specification ranges. example CT ratios could be 400/5 for point A, 250/5 for point B, and 150/5 for point C. optimal plug settings might be 180% for A, 105% for B, and 65% for C, ensuring proper coordination and functionality.
Step-by-step explanation:
When considering a radial system with three relay points A, B, and C, it is vital to calculate suitable CT ratios and plug settings. With the maximum load currents at A, B, and C specified as 360 A, 210 A, and 130 A respectively, one approach is to select CT ratios ensuring that the relay operates within its optimal range. CT ratios are typically selected based on the maximum load current and the expected fault current levels. For a high load current like 360 A at point A, a higher ratio CT such as 400/5 could be appropriate, allowing the relay to measure current without being overloaded and providing better accuracy. Points B and C, with lower maximum currents, could have CT ratios like 250/5 and 150/5 respectively.
Plug settings depend on system protection coordination, which would take into account the setting range of 50% to 200%. To ensure correct grading and coordination, relay A could have a plug setting of 180%, B could be set at 105% considering its maximum load of 210 A and the CT ratio, and C at 65% accounting for its maximum load current and CT ratio.