Difference between Primary protection and Secondary protection

[DrDrreeeaaa]

Hi Guys

Friend and I are having a bit of a disagreement on the difference between Primary protection and secondary protection.

He thinks it refers to the difference between primary protection (i.e. feeder breakers) and secondary protection (i.e. transformer backup protection). It could also refer to primary as transmission and secondary as distribution?

Any thoughts? Cheers

 

[mgtrp]

It probably depends on who you're talking to.

Some people will install fully redundant protection relays, and call one "Primary" and the other "Secondary", with little difference in terms of function or speed of clearing faults.

Some people consider "Primary" to be the fast-acting main protection (e.g., transformer or generator differential) and "Secondary" to be the slower backup protection (e.g., timed overcurrent on the same equipment).

I've not heard primary as being transmission and secondary as being distribution, but wouldn't be surprised if it was used this way somewhere.

 

[ScottyUK]

Over here duplicate systems are frequently referred to as 1^st main and 2^nd main.

I would suggest you get hold of the single line diagram and have a look at the relay functions associated with primary and secondary protections. That way you don't rely on our speculation.

 

[DiscoP]

As others have said you can have a number of answers to this.

To give an example, in some of the older subs in our network, the primary protection means the Busbar protection that operates the no.1 trip coil, the secondary operates the number 2 (from a separate battery and CTs). The two schemes are identical in every other way. On the feeders and transformers we call the schemes #1 and #2. It is only Busbar protection that gets this distinction.

The idea was to not get bus section nos and prot scheme numbers mixed up, but most people these days think it makes everything more confusing so it is no longer used.

When a new person joins us and we visit one of these subs, I normally ask what they think primary and secondary means on these panels. Not surprisingly nobody has guessed correctly!

In saying that, most answers given make perfect sense.

The moral of this long winded post is you are probably both right, and you probably won't get anything definitive to help your argument from this forum !

 

[DrDrreeeaaa]

Thanks guys