Point of PT for Under voltage relay in ATS

[PAP]

Dear all
Anyone please explain about the connection point of PT( to power system) for undervoltage realy in automatic transfer system that have 2incomer and 1 bustie.
For my system relay 27 (undervoltage) and 27I (Instantaneous UV) are connected to PT that connect to the point of before main incomer 1 and 2 ,but for 27R (Residual voltage for bustie closing permission when ATS operate) are connected to the PT that connected to the bus bar. This system also have manual transfer with synchronizing check (no-interruption transfer).
Thanks in advance for your kind.

 

[crisps]

Pap,

You're saying your PT (VT) is connected on the primary side of the incoming breaker?, well this is correct in ATS systems. If say you have a non recovering undervoltage fault on incomer 1 and the Incoming breaker trips, the design is that the bus tie closes. Correct?. How do you transfer back to a healthy system once incoming voltages are back to normal again??, If the PT was connected to the secondary (or downstream of the breaker)the voltage would be fine and in sync becuase its the supply off incomer 2. You could close up and have a big problem if the two incoming supplies are out of sync - if your VT's on the downstream of the incoming breakers you'd never know.
Because of check sync you need to see the primary voltage coming in, if this is restored and within the allowable sync angle then you can close quite happily, if it isn't then the UV relay would inhibit the close on the incoming breaker until that incoming supply is ok.

Crisps