cyriousn
Electrical
- Jul 19, 2017
- 27
From reviewing some responses on other threads from forum user PRC and also reviewing some modeling data I have access to, it appears that the impedance between two secondary windings of a 3 winding transformer is roughly 0.95 the summation of the impedances between the primary and each of the secondary windings. Does that sound right and is there any way to calculate this value instead of using a rule of thumb?
The reason I ask is we are looking at trying to model a three winding transformer in the hopes of reducing the fault current on the secondary side to a reasonable level below 40kA since that is the limit for commercially available 34.5kv gear. Transformer is currently a two winding 210/280/350MVA 345kv Wye-G / 34.5kv Delta with Z1 of 8.5%. The low impedance seems to be the issue along with the fact that the project is located near a generating facility only a few buses down on the 345kv network.
If we move towards a 3 winding transformer with 8.5% between the primary and each of the secondary windings and we use the 0.95 rule of thumb for the impedance between the two secondary windings this yields 16.15% impedance which drops the fault current down to 38.1kA for my scenario. This is still probably too close to the 40kA limit which means we can also try and increase the impedance of the transformer to within a limited that the Independent System Operator will allow without dinging the project for a Material Change.
Thanks in advance.
The reason I ask is we are looking at trying to model a three winding transformer in the hopes of reducing the fault current on the secondary side to a reasonable level below 40kA since that is the limit for commercially available 34.5kv gear. Transformer is currently a two winding 210/280/350MVA 345kv Wye-G / 34.5kv Delta with Z1 of 8.5%. The low impedance seems to be the issue along with the fact that the project is located near a generating facility only a few buses down on the 345kv network.
If we move towards a 3 winding transformer with 8.5% between the primary and each of the secondary windings and we use the 0.95 rule of thumb for the impedance between the two secondary windings this yields 16.15% impedance which drops the fault current down to 38.1kA for my scenario. This is still probably too close to the 40kA limit which means we can also try and increase the impedance of the transformer to within a limited that the Independent System Operator will allow without dinging the project for a Material Change.
Thanks in advance.