PS Class CT Knee point voltage. 2

[sakaran51]

Hi,
The CT requirements for the electrical system are to be specified in the initial detail engineering stage itself for any project.For differential protection for equipment like Generator,Transformer,Bus etc.,the required knee point voltage based on system fault current and relay manufacturer recommendation is to be determined and mentioned in PS Class CT spec.But the Rct value is not known and not readily available from the CT manufacturer at that stage and the knee point voltage is calculated mostly on assumed value of Rct, which may not be achievable by the CT manufacturer.
I would like to know from the forum members how to sort out this issue?
Thanks.

 

[scottf]

Technically, a bushing CT is a 600V class CT, where the bushing provides the insulation. The CT itself sits at ground potential. Of course, if can be applied on apparatus of any voltage rating. Perhaps that's what Veritas meant.

With a bushing CT, clearly the primary turns is going to be 1 for sure.

My only point is folks need to be aware that not all CTs have 1 primary turn. Someone using a rule-of-thumb like that without thinking could make a serious mistake...for instance on a live-tank breaker with free-standing CTs having multiple primary turns.

 

[sakaran51]

In the case of dead tank CT, live tank CT and transformer/breaker bushing CT only single primary turn is feasible,I think.Even in indoor cicuit breakers mostly it is single turn bar primary type.

 

[scottf]

sakaran51-

For bushing CTs, yes, only 1 primary turn is possible.

For other types of MV and HV CTs, like post-type, hair-pin CT, etc...multiple primaries are possible and very common.

 

[veritas]

Okay, let's look at a recent example I had:

145kV (PROT) 2400/2000/1400/800/1A 0.2PX 320 R1.6 ON 400/1

My understanding is that the manufacturer is stating that on the 400/1 tap, Rct = 1.6ohms. Now admittedly this translates to 4mohms per turn (my 5mohms/turn is more conservative when it to calcs involving burden). Now if what scottf is saying that there could be 2 primary turns then it would be 800/2. This means that either the 1.6ohms is for only 1 primary turn and applicable to all taps (i.e. Rct varies but always with reference to 1 primary turn so on 800/1 tap it is 3.2ohms)or there are 2 primary turns applicable to all ratios and so the 1.6ohms is for 800 turns.

This means for many of the CT's I've looked at where I consistently get around 5mohms/turn that they are all based on 2 primary turns (or even more).

I contacted a CT manufacturer here. She said that by far the most common is 1 primary turn. Multi-primary turns are more common for very low ratio CT's so as to generate the desired mmf - which makes sense to me. But they don't manufacture HV post type CT's - mostly window type for swbds or bushings which I've always seen to have 1 primary turn.

I will contact the HV post type CT manufacturer to get their comment as well.

Technically, a bushing CT is a 600V class CT, where the bushing provides the insulation. The CT itself sits at ground potential. Of course, if can be applied on apparatus of any voltage rating. Perhaps that's what Veritas meant.

Correct.

scottf - point taken - maybe I've been to lax and just assumed 1 primary turn. Best to make sure where possible even consulting the CT test results if possible. Interesting discussion this and I'm certainly learning a thing or two.

 

[scottf]

veritas-

As stated above, a 2400A rated CT is likely 1 turn. Multiple primary turns would normally start as you get below 2000A continuous. For reference, I work for an international instrument transformer manufacturer with experience from 600V to 800kV CTs. My HV and MV experience is with post-type/bar-type designs. I'm not as familiar with all of the details on dead-tank/hair-pin style that is more common in Asia.