TX PROTECTION

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