KJvR
Electrical
- Feb 20, 2006
- 56
I would appreciate any comment on the type of CT to be used for a Z-transformer (NEC / Earthing Transformer) protection scheme. The Earthing Transformer is connected to a generator bus and therefore needs separate winding protection. I will use delta connected CTs to prevent zero sequence currents going to the earthing transformer O/C protection relay (as recommended by most literature). However, I have seen an existing installation using a Class PX CT while our experienced Protection Engineer believe a Class 5P is O.K. My fear is that with the O/C protection set at a very low current (10%) of the CT nominal current, the unbalanced CTs might cause spill currents in the Delta during a single phase to earth fault and cause a false O/C trip of the earthing transformer protection. I can not find any literature giving a CT requirement on this arrangement. Is a Class PX ct a requirement or will a Class 5P ct be adequite. References to literature will also be appreciated.