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PS Class CT Knee point voltage.
2

PS Class CT Knee point voltage.

PS Class CT Knee point voltage.

(OP)

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.

RE: PS Class CT Knee point voltage.

First of all what CT standard do you specify your CT's to?

RE: PS Class CT Knee point voltage.

(OP)
Standard followed for CT Spec.will be IS Standard which is line with IEC only.

RE: PS Class CT Knee point voltage.

Hello,

The rct value is needed to determine the total burden that the CT will have to support.
What you can do is asking the supplier (or some other CT manufacturers) to provide you the whole data of CT's design under IEC standard
and they would provide:
CT Ratio (the one you are taking into account)
PS Class
Rct
Vknee
In this way, you will have information available in order to be able to compare your calculations

However, based on experience, I can tell you that leads resistance is typically the most demanding factor
to take it into account to your calculation.

Besides that, it is important you to know the kind of relay to be used, currently digital ones are almost the rule, however
in some cases, if the relay were of the olds ones, the burden is very , very high.

Best Regards

RE: PS Class CT Knee point voltage.

The best is to state the Vkp as a function of a variable Rct, since your required Vkp will change based on the Rct of the CT.

That also gives the CT manufacturer the most flexibility and could end up saving you money over specifying an arbitrary Rct value and forcing the manufacturer into that figure.

RE: PS Class CT Knee point voltage.

(OP)
Yes Scottf.That is the only way to go about when the CT secondary resistance is not known.
Thanks everybody!

RE: PS Class CT Knee point voltage.

No problem with CT mnf.
You can calculated Ukp according to protective terminal CT requirements, calculate current loop and ask CT mnf for approx. Rct <= of needed value. Mnf send to you some value, you recalculated parameters and if needed ask mnf again for other parameters, few times ping pong with mnf. and you found optimal data.

RE: PS Class CT Knee point voltage.

So, as I read these various CT specifying threads, pretty much all coming out of IEC land, I have to ask - do people seriously try to specify the "optimal" CT for every application?

I'm much more familiar with generally "over specifying" and then not needing to reevaluate the CTs for every system configuration change. It seems that "optimizing" results in lots of different CTs in use and the risk that a breaker can't easily be reassigned. Heck, we issue a blanket spec for breakers for the next several years, complete with CTs, long before the use of each and every breaker is determined. But experience has shown that we can get acceptable CT performance in pretty much every application with the same CT specification. Sure there are places that we might get by with a lesser CT, but only at the cost of a life time of reduced flexibility.

RE: PS Class CT Knee point voltage.

it's depend on: :)

Utilities is usually used same oversizing CT's for decades, for lines.

but, for special application, gas, steam turbine, step up, step down, metering, revenue metering, CT must be calculated every time.

RE: PS Class CT Knee point voltage.

sakaran51 - since you mentioned diff protection I assume you are dealing with a Class X CT as per IEC 60044.1? To take Scottf's suggestion a step further, Ekp may be expressed as follows:

Ekp = A*(Rct + B)

A = KSSC*Ktd
KSSC = rated symmetrical short-circuit current factor = If/CTR where If is the symmetrical fault current
Ktd = rated transient dimensioning factor (most often taken as X/R + 1. I sometimes reduce this as it is the most onerous)

B = total external loop resistance = leads + relay resistance

You will find more detail regarding the above in IEC 60044.6

RE: PS Class CT Knee point voltage.

Would like to add following as well:

For high impedance differential applications such as REF, high Z buszone, etc. a value for Rct is required when calculating the stability voltage. Rct can then be approximated using 5mohms/turn for a 1A CT and 2.5mohms/turn for a 5A CT. I would recommend that this be discussed with the CT manufacturer as the calcs may need to be revisited and fine-tuned if required.

RE: PS Class CT Knee point voltage.

(OP)
Hi veritas,

"Rct can then be approximated using 5mohms/turn for a 1A CT and 2.5mohms/turn for a 5A CT. I would recommend that this be discussed with the CT manufacturer as the calcs may need to be revisited and fine-tuned if required."

That is exactly the point.What I want to know is whether that approximation is reasonably holds good to start with? Can it be applied for all the CTs? BTW,the value 5mohms/turn and 2.5 mohms/turn for 1A and 5A CT respectively, are actually 500 milli ohms(0.5 ohm) and 250 milli ohms(0.25 ohm)?

RE: PS Class CT Knee point voltage.

I have done this calculation for many Class X CT's throughout the years and I can assure you 5milli ohms/turn is indeed a very good rule of thumb (in my experience that is). So for high Z applications I use this to specify Rct and cannot recall it ever being a problem.

RE: PS Class CT Knee point voltage.

(OP)
Sorry Veritas,
I made a mistake in a hurry.You are correct.I should have mentioned that it is 0.5 ohm and 0.25 ohm for 100 turns for 1A and 5A CTs respectively.

RE: PS Class CT Knee point voltage.

Just a point of clarification, all of the references to an approximation of ohms/turn seems to be assuming a window-type CT, i.e. where the number of primary turns is known to be 1 turn. In medium-voltage and high-voltage applications, you would not be able to know the exact number of primary turns used in the CT design without asking the manufacturer.

For example, a 100:5A, 15 kV CT might have 4 primary turns, so there would be 80 turns on the secondary and not 20 (if you assumed 1 primary turns).

RE: PS Class CT Knee point voltage.

scottf - the CT's I deal with typically are from 11kV to 500kV. Yes, most are window type with a single bar primary. Does it really matter if there is an extra turn here or there since my rule of thumb is based on declared performance specifications from the manufacturer.

RE: PS Class CT Knee point voltage.

If it is a primary extra turn or two it does matter. A lot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PS Class CT Knee point voltage.

I was not talking about extra primary but secondary turns.

RE: PS Class CT Knee point voltage.

Yes, the primary turns matter a lot.

For example, a CT with an 800:1A ratio.

1 turn primary = 800 secondary turns
2 turn primary = 1600 secondary turns
etc...

Point being is that if you use an ohms/turn rule of thumb, you need to be sure of the number of turns.

Veritas...unless we're mixing terms, there is no such thing as a 500kV window-type CT. Once you get above 46 kV or so, they are going to be exclusively post-type/live tank (where the cores are at the top and the tank is at HV) or hair-pin-type/dead-tank, where the cores are at the bottom and the tank is at ground potential.

For ratios above 2000:1 or 5A, you can normally be pretty confident that the CT will have a 1-turn primary. For ratios below that, you have no way of knowing without talking to the manufacturer or looking at the schematic of the specific CT...and normally a schematic will not show the exact number of primary turns.

You said you weren't talking about extra primary turns but extra secondary turns. I'm not sure I follow that comment. If the ratio is set, then the relation of primary turns to secondary turns is set. There are no real "extra" turns.

RE: PS Class CT Knee point voltage.

Wouldn't a bushing CT be considered a "window-type" CT? I guess, though, it wouldn't really be a "500kV" CT even if applied on the bushing of a 500kV breaker or a 500kV transformer winding.

RE: PS Class CT Knee point voltage.

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.



RE: PS Class CT Knee point voltage.

(OP)
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.

RE: PS Class CT Knee point voltage.

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.

RE: PS Class CT Knee point voltage.

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.





RE: PS Class CT Knee point voltage.

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.

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