Transformer 51 Relay Protection Setting 1

[ThePunisher]

We have a 10/13.33MVA, 25-4.16kV, step down transformer. The ONAF primary current is 308A at 25kV.

This transformer is protected by a 51 relay connected to a 3 x 600/5A phase CTs at its primary circuit breaker. The existing setting was set way above its ONAF: 1.2 x 600 = 720A (pickup).

I am wondering as to why it was set this way and has been there for many years. In my opinion, the pickup should be no greater than the ONAF rating (maximum kVA). The transformer is installed here in Alberta, Canada and it is for industrial applications.

Although ANSI allows 125~150% of its kVA rating, I believe this may only be applicable to accomodate cyclic emergency loading...as we normally see in utility applications. For industrial applications where loads are continuous..this may be an exemption.

Any comments?

 

[dpc]

125% of the maximum primary current is a reasonable setting for the pickup. You still have to consider upstream and downstream coordination, transformer damage curve, transformer inrush and other factors.

Transformers can generally handle short-term overloads.

David Castor

http://www.cvoes.com