Zero Sequence CT

[tulum]

Hello,

Can anyone enlighten me on how to size a zero sequence ct for a multilin 750 relay.

The manual is not real clear and it suggests a 1 to 5000A primary with a 1 or 5A secondary.

The system will be resistance grounded with a 10A NGR...does this matter? Since it is wrapped around the three phase conductors are you not just looking for any current?

Regards,
TULUM

 

[dpc]

Multilin has a special CT they use for the sensitive ground CT input, or at least they used to. The HGF3 and HGF5. The ratio is 50:.025, as I recall. I think it's the same CT used on the 469 motor relay.

Otherwise you can use a standard CT, but at 10A max current, it will need to be 50/5 or something like that.

 

[oldfieldguy]

For the SR750, I see a 50:5 CT typically applied to zero sequence function. The setpoint in the relay allows a wide variation, but the 50:5 is typical.

Pickup settings are typically 0.2 x CT.

old field guy

 

[tulum]

Thanks guys...

50:5A seems to be consistant with other suggestions I have received.

This is a 1200A feed and their will be multiple runs of 500kcmil... all the 50:5A CT's I have seen have very small windows... has anyone had experience with larger CT's... I am having trouble sourcing one at such a low ratio.. 200:5 seems to be the smallest ratio with a large window?

Regards,
TULUM

 

[swn1]

You have to select a zero-sequence CT that won't saturate under worst-case GF current situations. Since the available GF current depends on how the power system is grounded, this requires application-specific engineering.

 

[whycliffrussell]

1. Calculate the available gnd fault current (that is where the NGR comes into play...it limits the gnd fault current).
Do a double line to gnd and line to gnd short circuit calculation. If you don't want to do the entire calculation you can use the line to gnd voltage and NGR impedance as a quick calc...I=Vline-gnd/NGR

2. Select a CT that is in the range of the gnd fault current. CT's are non-linear near the upper and lower current values...thefore current values in these ranges will not be accurate.

3. Check that the CT you use is compatable with the multilin spec. See

http://www.geindustrial.com/products/manuals

for the appropriate manual.

 

[dpc]

The OP said the ground fault current is limited to 10A.

I would contact GE. They now own the Intstrument Transformer Co. and also make the special HGF flux summation CT I mentioned previously. 50/5 is a common ratio for flux summation CTs.

http://www.gecatalogs.com/content/offline/buylog/17_BL.pdf

 

[rbulsara]

I presume you have (3) phase CTs as well. If so and this is a new system (not buit or on site yet) why bother with a ZSCT on a resistance grounded system? You know that the GF current is limited to 10A. I would only use a 50/5 or 25/5 CT around the neutral conductor as shown in figure 3-12 of the manual and use sensitive ground inputs. It will be a single CT, smaller, simpler and less expensive, easier to install.

Or better yet just use the residual connection method, you won't need any ground CT's. Phase CTs will do the job.

Remeber you have to order SR-750 with desired (correct) CT secondary, 5A or 1A but can't have both. All CTs connected to a SR-750 have to have identical secondary current rating.

 

[dpc]

rbusara wrote:

"Remeber you have to order SR-750 with desired (correct) CT secondary, 5A or 1A but can't have both. All CTs connected to a SR-750 have to have identical secondary current rating."

Actually, I think the phase, sensitive ground and zero sequence CT inputs can be specified independently on the SR-750. At least that is way it used to be. I haven't spec'd one in a while.

 

[rbulsara]

dpc:

You are partly correct and I was partly wrong.

On page 5-17 of the manual (for current sensing setup)that "when the relay is ordered, the phase, ground and sensitive ground inputs must be specified as 1A or 5A."

The ordering information on page 2-6 does give you options for either 1A or 5A, phase, zero seq and senstive ground CT secondary, but they are not independent of the SR-750. You still have to order it correctly or order the CT's accordingly.

 

[tulum]

Thanks folks,

I have contacted Multiliin and I am waiting for a response.

Rbulsara... there is already GF protection on the NGR of the transformer upstream of this breaker. It is the mining standard to have both the NGR monitor/GF on the sourece transformer and additional GF on the feeders...

Thanks folks..

 

[rbulsara]

tulum:

OK. It was not clear to me that you are talking about ZSCT on 'feeders. It makes sensen now. I 'assumed' you are talking about the main breaker only.

However, it would be challanging to achieve GFP coordination between mains and the feeders with just 10A of GF current. Or may be you are not planning to trip the breaker on GF, that is when 10A NGR are normally chosen.

 

[arcflash99]

We have installed larger CT's when dealing with multiple/ parallel conductors. Our's were supplied from the manufacturer (Powell Industries). They tand to be twice the size of a "normal" ZSCT.