Transformer Protection 1

[sky2020]

I have questions about primary and secondary transformer protection.

Primary transformer protection is protecting the mechanical and damage curve of the transformer. It should not respond to secondary transformer faults. It should allow inrush current. Primary transformer protection does not protect the transformer from overloads.

Secondary transformer protection is protecting the transformer overload and secondary cable.

Is there a problem if the primary transformer protection miscoordinates with the secondary transformer protection?

Do you agree? Do you have any reference?

 

[RRaghunath]

"Is there a problem if the primary transformer protection miscoordinates with the secondary transformer protection?" - Only that primary protection also may operate for secondary faults. There is no worry from load point of view, as the load dropped is same whether the protection on secondary operates or that on primary.
I have seen the practice of some clients who don't provide any protection on secondary side of transformer.

 

[cranky108]

What book did this come from? And what size transformer are you talking about?
For a smaller transformer, there may only be fuses, so is that primary, or secondary protection?

 

[sky2020]

The reference is IEEE Std 242 & C37.91

 

[bacon4life]

Practices also vary by size and whether the transformer is covered by the NESC or the NEC. Utility service transformers typical only have primary fuses. Utility substation transformers often have differential protection, which relegates high side overcurrent to a backup role. Protection on utility transformers may be sized to allow for very extreme overloads, with the actual management of loading being based operators monitoring transformer loading.

 

[crshears]

Protection on utility transformers may be sized to allow for very extreme overloads, with the actual management of loading being based on operators monitoring transformer loading.

Quite true; beyond their continuous rating, "my" utility's transformers are almost universally provided with limited time ratings for periods of ten days, two hours, fifteen minutes, and five minutes, but with the proviso that any pre-loading of the transformer beyond continuous, no matter how short the interval, immediately invalidates the use of any LTR.

As far as as relaying goes, they typically as a bare minimum have fully duplicated differential protection; a usually single gas pressure wave trip, occasionally but not often duplicated; and two per unit overload protection, meaning 200% of continuous, with no intentional time delay, sometimes duplicated, sometimes not.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]