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Hi there,
Greetings from India.
We receive power at 33KV (+/-10%), 50 Hz (+/-3%) from the solidly earthed system of the local utility company, which we step down to 6.6 KV through a 12.5 MVA, 33KV/ 6.6KV, 50Hz, 10.435% Z, Dyn11, Transformer. The secondary Star point is grounded through a 9 Ohm NGR. The 6.6 KV bus feeds two cage induction motors (3.25 MW and 2.35 MW) and a 2.5 MVA, 6.6KV/ 0.415V, Dyn11 transformer for the plant utilities. The systems are designed for symmetrical S.C currents of 26 KA (3-second) at 33KV and 6.6 KV bus and 50 KA (1-second) on the 415 V bus (neutral solidly earthed).
The 33 KV side of the 12.5 MVA Transformer has a set of 250/1 A, 5P10 CTs for OCEF relays (51/ 51N - earth fault element residually connected). In addition, it's also provided with a Differential protection relay. The three PS class CTs for this protection are as follows:
33 KV side: 250/1A (CT secondaries connected in Star). RCT and RL values may be taken as roughly 4 Ohm and 0.75 Ohm respectively
6.6 KV side: 1250/1A (CT secondaries connected in Star), followed by 1/0.577A Interposing CTs, with secondaries connected in Delta (this arrangement had to be made since direct 1250/0.577A CTs were not readily available in the market). RCT values may be taken as roughly 6 Ohm for the main and 4 Ohm for the interposing CTs.
From the above, could someone please help me with computation of the VK (knee-point voltage) and Imag at VK/2 for the 33 KV and 6.6 KV Differential protection CTs (including the interposing CTs)? In addition to Differential protection, the 6.6 KV side also has attracted armature type (with a stabilizing resistance in series) Restricted E/F relay (often called partial differential E/F scheme), with three line and neutral CTs (1250/1A, class PS) connected in parallel. Can you help me with the VK and the Imag at VK/2 for this also? The RCT values can be taken as the one-way control cable lead resistance (RL) values for all the cases can be taken as 0.7 Ohm max. I shall really appreciate your input.
Regards,
Tilak Ghoshal
Greetings from India.
We receive power at 33KV (+/-10%), 50 Hz (+/-3%) from the solidly earthed system of the local utility company, which we step down to 6.6 KV through a 12.5 MVA, 33KV/ 6.6KV, 50Hz, 10.435% Z, Dyn11, Transformer. The secondary Star point is grounded through a 9 Ohm NGR. The 6.6 KV bus feeds two cage induction motors (3.25 MW and 2.35 MW) and a 2.5 MVA, 6.6KV/ 0.415V, Dyn11 transformer for the plant utilities. The systems are designed for symmetrical S.C currents of 26 KA (3-second) at 33KV and 6.6 KV bus and 50 KA (1-second) on the 415 V bus (neutral solidly earthed).
The 33 KV side of the 12.5 MVA Transformer has a set of 250/1 A, 5P10 CTs for OCEF relays (51/ 51N - earth fault element residually connected). In addition, it's also provided with a Differential protection relay. The three PS class CTs for this protection are as follows:
33 KV side: 250/1A (CT secondaries connected in Star). RCT and RL values may be taken as roughly 4 Ohm and 0.75 Ohm respectively
6.6 KV side: 1250/1A (CT secondaries connected in Star), followed by 1/0.577A Interposing CTs, with secondaries connected in Delta (this arrangement had to be made since direct 1250/0.577A CTs were not readily available in the market). RCT values may be taken as roughly 6 Ohm for the main and 4 Ohm for the interposing CTs.
From the above, could someone please help me with computation of the VK (knee-point voltage) and Imag at VK/2 for the 33 KV and 6.6 KV Differential protection CTs (including the interposing CTs)? In addition to Differential protection, the 6.6 KV side also has attracted armature type (with a stabilizing resistance in series) Restricted E/F relay (often called partial differential E/F scheme), with three line and neutral CTs (1250/1A, class PS) connected in parallel. Can you help me with the VK and the Imag at VK/2 for this also? The RCT values can be taken as the one-way control cable lead resistance (RL) values for all the cases can be taken as 0.7 Ohm max. I shall really appreciate your input.
Regards,
Tilak Ghoshal
