Ref protn ct sample calculation
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This document summarizes the sample calculation for selecting relay protection current transformers (CTs) for a 23,000 KVA transformer with a voltage ratio of 66/11kV. It details the transformer ratings and parameters, calculates fault currents, secondary lead resistance, and voltage drops to determine the CT ratio, knee point voltage, and excitation current needed to properly protect the transformer.
Ref protn ct sample calculation
- 1. Transformer rating (S) = 23000 KVA Transformer voltage ratio = 66/ 11 KV System voltage (V) = 66.000 KV Full load current (IL) = (S / Sqrt(3) * V) = (3150 / Sqrt(3) * 0.690) = 201.1978 A CT Primary Current (IP) selected = 200 A CT Secondary Current (IS) = 1 A Transformer percentage impedance (%Z) = 8 % Primary Fault current (If) = (IL / %Z) = 201.1978/0.08 = 2398.067 A Secondary Fault Current (Isf) = (If x Is/Ip) = (32946.62 * 5 / 3000) = 11.99 A CT Resistance (Rct) (assumed) = 4 ohm Max. distance from trafo. CT to REF relay = 20 m Resistance of 2.5 Sq.mm cable (from cable catalogue) = 7.41 ohm / km Therefore, Secondary Lead resistance (RL) = (20/1000) * 7.41 = 0.15 ohm Voltage (Vs) = (Isf x (Rct + RL)) = (54.91 x (4 + 0.15)) = 49.74 V Knee Point Voltage (Vk) = 2 * Vs = (2 x 227.78) = 99.48 V Selected Parameters for the CT CT ratio 200/1 A Accuracy class PS Knee point voltage Vk >= 500 V Excitation current Im <= 30 mA 66/11 KV LUKENYA SUBSTATION REF PROTECTION CT - SAMPLE CALCULATION FOR 23000KVA, 66/11KV TRANSFORMER REF Protn CT - Sample calculation AFANDA RODGERS