Parallelling Transformers at utility transmission system

[hmchi]

There have been a lot of ink on warnings about parallelling power transformers at distribution voltages or at Power User substations. People usually are cautioned against it due to increased fault current and the possible circulating currents when the two transformers mismatch on voltage and impedance.

However, at 230kv, 345kv, and 500kv, utilities parallel transformers at different ends of the transmission lines all the time [The transmission circuits are mostly closed loop circuits].

Are the utilities doing anything to address the circulating current problems ? if so, what ? They do not seem to try to match transformers, some could be purchased 10 years ago and new one added in a new substation and connected in a closed loop.

Just idle curiosity …

 

[alehman]

Where transformers are in parallel, the impedances of the lines helps with circulating currents. For closely coupled transformers, paralleling balancers with load tap changers can be used. Line drop compensators can be used to balance the load on parallel lines.

http://www.cdnova.com/Portals/0/Link 1 - App11- Introduction to Parallel Control.pdf