Surge Arrester Rating for Delta Tertiary

[lkincer]

I'm working on the specification for a 161/69/13.2 kV, 36/48/60 MVA, autotransformer with a loaded delta tertiary and a zig-zag grounding transformer downstream of the tertiary. The HV and LV windings are wye connected and solidly grounded. I'm specifying the tertiary winding with a 110 kV BIL and an arrester rating of 10 kV duty cycle or 8.4 kV MCOV. I've seen some utilities use an arrester rated 15 kV or 18 kV duty cycle on a similar unit. Has anyone worked on a similar design? Thanks for your help.

 

[magoo2]

If you didn't have the grounding transformer, you'd use the 15 kV or 18 kV duty cycle rating.

With the grounding transformer, a 10 kV rating can be used.

 

[odlanor]

10kV is enough if grounding transformer is sized for effectivelly grounded 13.2 kV system

 

[jghrist]

Could the tertiary be energized without the grounding transformer connected? If so, use the higher rating.