Primary vs Secondary OLTC

[comalcoeng]

What are the deciding factors or rationale between deciding on whether the On-Load Tap-Changer (OLTC) is on the primary or secondary side? As I understand there are a number of considerations:-

* voltage rating - hence if on primary side OLTC is placed toward the neutral of the winding, but a lower voltage rated unit can be used on the secondary.
* current rating - obviously there is lower current on the primary side and a smaller unit can be used.

Also physically, the primary winding on a tranny is usually furthest from the core for insulation reasons, but if the secondary has the OLTC then it must (usually) have its tapped winding outermost in relation to the core.

I guess I am just a bit confused as I have seen both methods used (primary and secondary OLTC on different units of similar rating) and am just unsure as to why you would specify either way. If on the primary is there something to do with maintaining constant flux (V/f ratio?).

The particular application is 220kV/33kV nominal 110MVA with taps to give secondary voltage between say 28kV and 38kV.

Sorry if this question has been asked before, I did search but only came across a thread re reactor vs resistive OLTCs.

Thanks in advance.

 

[jbartos]

Suggestion: Transformer OLTCs tend to be more frequently located on the primary side since the currents are normally lower on the higher voltage side, which is normally primary side. See

http://www.dupont.com/nomex/Mobile_transf_pauwels.pdf

 

[jbartos]

Suggestion: The OLTCs may be on the transformer secondary sides that usually have lower voltage and the higher current. Visit

http://www.dynamicratings.com/dynamicratings_faq.htm

for the OLTC being a primary or secondary side option.