Ungrounded power TR in grounded network 3

[mirilovic]

Our contractor wants to install power transformer 110/20/6kV, Y,yn9,d11 in a substation. Primary (110kV) wounding of TR is not grounded. 110kV network is directly grounded.

Whats are the consequncies of connecting ungrouded primary in grounded network?

 

[slavag]

Hi.
Actually isn't problem, but you must check with your utilities. Point is, a ground fault level in this area.
For example only now, we installed two trafos 45/63MVA Y is ungrounded in 170kV grounded network. Reason is , installation is near ( 200-300m) to four big GSU ( 400 MVA) trafos.
Good Luck.
Slava

 

[Marmite]

Make sure the transformer is specified correctly as the primary insulation may be graded towards the star point if its designed for a grounded system.
Regards
Marmite

 

[mirilovic]

If i got this clear. Ground fault on one phase couses overvoltages in other two phases and in isolated star point. So I need higher insulation level on TR and on switchgear?

 

[slavag]

Hi Mirilovic .
Star point is isolated, is true. But grid is directly grounded.
You haven't overvoltages in case of ground fault. Of course if you haven't some islanding mode or your trafo is GSU.
Best Regards.
Slava

 

[rempman]

In the design of the transformer and substation was there a fault current study done? Where do you want the zero sequence currents to go. Typically protection engineers like to see the zero sequence currents return to the source transformers of the system, not to through the load transformers. By not grounding the Star Point of a Wye connected windings or using a delta connected system you eliminate the zero sequence path.