Metering CT or Protection CT

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mbrooke

mbrooke

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When current is required for instrument indicating amp meters, do you guys use metering CTs or protection CTs? Whats the difference?
 
The monetary considerations will dictate how much your accuracy will be. For a revenue meter measuring 700MW output will require a more accurate set of CTs (class 0.5) than a CT measuring your motor amps(class 3)!
 
topgone said:
The monetary considerations will dictate how much your accuracy will be. For a revenue meter measuring 700MW output will require a more accurate set of CTs (class 0.5) than a CT measuring your motor amps(class 3)!
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2000 amp 480 volts.
 
metering cts unless it is for protection. metering cts are accurate at low and normal levels but will saturate at fault current levels.

since we want good relay reaction at fault levels, we sacrifice low-level accuracy in protection cts to reduce saturation at fault levels.

hv station cts will usually have both metering and protection windings, and usually multiples of each.

for load currents, opt for accuracy at normal levels. for fault currents, opt for accuracy at abnormally high levels.

generally speaking, metering is targeting good accuracy up to 4X normal current (20 amps) and protection is targeting good accuracy up to 20X (100 amps) normal.
 
mbrooke said:
Why 600:5 or 800:5 instead of 2000:5?
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Because it gives you better accuracy at low currents. If the rating factor is 4X, a 600:5 CT is good up to 2400:20 and the 800:5 is good up to 3200:20.

For example. A CT with 600:5, 0.3 accuracy, RF = 4.0. This gives 0.3% accuracy from 600 to 2400 (4X) amps. It gives 0.6% accuracy from 60 (10%) to 600 amps with 60 amps as the minimum reliable reading. A 2000:5 would only give 0.6% accuracy from 200 to 2000 amps and 200 amps is the minimum reliable reading. A 800:5 would give 0.6% accuracy from 80 to 800 amps and 0.3% accuracy from 800 to 3200 amps and 80 amps would be the minimum reliable reading.

Another example. An Encompass 0.3 accuracy, 600:5 CT with an RF of 0.4 - 4.0. Now you have 4%-40% and 40%-400% ranges instead of the 10%-100% and 100%-400% you get with a standard 4X CT. So for 240 to 2400 amps you get 0.3% accuracy and you get 0.6% accuracy from 24 to 240 amps. This CT RF type gives even better accuracy at low currents and 24 amps is the minimum reliable reading and so much better than 200 amps.

Another example. A standard 0.15 accuracy, 600:5 CT with an RF of 4.0. Now you have 5%-100% and 100%-400% ranges instead of the 10%-100% and 100%-400% you get with a standard 4X CT. So for 600 to 2400 amps you get 0.15% accuracy and you get 0.3% accuracy from 30 to 600 amps. 30 amps is the minimum reliable reading.

I don't like to push the max rating so at 600:5 and 2000 amps you will be at 3.33X which is fine. If ambient is high or the RF is 3, then I would use 800:5 and would be at 2.5X at 2000 amps. Suppose the RF is 4.0 at 30C but we operate up to 55C. Then it will have RF = sqrt(4.0^2 * (85 - 55) / 55) = 2.95 at 55C.

For what it is worth, indicating CTs usually only need 0.6% or 1.2% accuracy. Metering uses 0.3% unless you want high accuracy and then you might use 0.15% CTs.

Also note that high ratio CTs like at HV substations may have the RF at less than 100%.

For relaying, we get 10% accuracy at 20X normal current as long as we stay within the burden rating so we don't saturate the windings. We usually use C400 or C800 rated CTs.

So for a C400 CT we can have a maximum burden of R = 400 / (20 * 5) = 4 ohms to maintain 10% accuracy. Our burden would be much less so our accuracy would be better than 10%. If the burden is higher, we might go to C800. For higher accuracy we might use a 2.5C400 so we get 2.5% accuracy instead of the standard 10%.

The 2.5% or 10% is not close to the 0.3% needed for metering but allows us to have good relay response when we have a fault and currents are high. The metering CT might only reach 100%-400% normal but the relay CT can let us see up to 2000% normal current.

It just depends on what you are doing with the output.
 
mbrooke said:
Alright... how common are 20 amp meters?
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Meters not so much but often microprocessor protection and metering relays have a “range” of 0-5 A but many accurately read up to 20 A. After that many use a software method to overcome saturation issues (WDM) so protection CTs up to 20X are no longer necessary. Also their electronic inputs have little to no burden.

Which brings up the point. You can buy a nice Shark 100 meter that fits in a round meter hole or a Schneider power meter that read all three amps and volts. That costs under $200. Pay more and it estimates ground current, logs amps, kilowatts, kvars, kilowatt-hours, and in the more expensive versions does harmonics, Ethernet or WiFi, and can even be used as a “poor mans” protection relay for about the cost of a decent panel meter without the selector switch. I don’t see where the old meters and selector make financial sense anymore.
 
paulengr said:
Meters not so much but often microprocessor protection and metering relays have a “range” of 0-5 A but many accurately read up to 20 A. After that many use a software method to overcome saturation issues (WDM) so protection CTs up to 20X are no longer necessary. Also their electronic inputs have little to no burden.

Which brings up the point. You can buy a nice Shark 100 meter that fits in a round meter hole or a Schneider power meter that read all three amps and volts. That costs under $200. Pay more and it estimates ground current, logs amps, kilowatts, kvars, kilowatt-hours, and in the more expensive versions does harmonics, Ethernet or WiFi, and can even be used as a “poor mans” protection relay for about the cost of a decent panel meter without the selector switch. I don’t see where the old meters and selector make financial sense anymore.
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Right. But I like old fashioned. Its just who I am.
 
mivey said:
Because it gives you better accuracy at low currents. If the rating factor is 4X, a 600:5 CT is good up to 2400:20 and the 800:5 is good up to 3200:20.

For example. A CT with 600:5, 0.3 accuracy, RF = 4.0. This gives 0.3% accuracy from 600 to 2400 (4X) amps. It gives 0.6% accuracy from 60 (10%) to 600 amps with 60 amps as the minimum reliable reading. A 2000:5 would only give 0.6% accuracy from 200 to 2000 amps and 200 amps is the minimum reliable reading. A 800:5 would give 0.6% accuracy from 80 to 800 amps and 0.3% accuracy from 800 to 3200 amps and 80 amps would be the minimum reliable reading.

Another example. An Encompass 0.3 accuracy, 600:5 CT with an RF of 0.4 - 4.0. Now you have 4%-40% and 40%-400% ranges instead of the 10%-100% and 100%-400% you get with a standard 4X CT. So for 240 to 2400 amps you get 0.3% accuracy and you get 0.6% accuracy from 24 to 240 amps. This CT RF type gives even better accuracy at low currents and 24 amps is the minimum reliable reading and so much better than 200 amps.

Another example. A standard 0.15 accuracy, 600:5 CT with an RF of 4.0. Now you have 5%-100% and 100%-400% ranges instead of the 10%-100% and 100%-400% you get with a standard 4X CT. So for 600 to 2400 amps you get 0.15% accuracy and you get 0.3% accuracy from 30 to 600 amps. 30 amps is the minimum reliable reading.

I don't like to push the max rating so at 600:5 and 2000 amps you will be at 3.33X which is fine. If ambient is high or the RF is 3, then I would use 800:5 and would be at 2.5X at 2000 amps. Suppose the RF is 4.0 at 30C but we operate up to 55C. Then it will have RF = sqrt(4.0^2 * (85 - 55) / 55) = 2.95 at 55C.

For what it is worth, indicating CTs usually only need 0.6% or 1.2% accuracy. Metering uses 0.3% unless you want high accuracy and then you might use 0.15% CTs.

Also note that high ratio CTs like at HV substations may have the RF at less than 100%.

For relaying, we get 10% accuracy at 20X normal current as long as we stay within the burden rating so we don't saturate the windings. We usually use C400 or C800 rated CTs.

So for a C400 CT we can have a maximum burden of R = 400 / (20 * 5) = 4 ohms to maintain 10% accuracy. Our burden would be much less so our accuracy would be better than 10%. If the burden is higher, we might go to C800. For higher accuracy we might use a 2.5C400 so we get 2.5% accuracy instead of the standard 10%.

The 2.5% or 10% is not close to the 0.3% needed for metering but allows us to have good relay response when we have a fault and currents are high. The metering CT might only reach 100%-400% normal but the relay CT can let us see up to 2000% normal current.

It just depends on what you are doing with the output.
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What is RF? I'm sure it obvious- but I'm juggling a few mental concepts here.

I want to thank you again for this reply. Its worth more than it looks. :)
 
RF or TRF, thermal rating factor, the capability of CTs to accurately read the current even at RF times its listed current rating w/out thermal damage.
 
20 amp instrument rated meters are common. If your metering 480 volts your most likely going to need a few potential transformers also. Most meters now are multi form. 9s or 8s depending on wye or delta.
 
Electricmo said:
20 amp instrument rated meters are common. If your metering 480 volts your most likely going to need a few potential transformers also. Most meters now are multi form. 9s or 8s depending on wye or delta.
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I to clear up confusion I just want to be able to read the amps on each phase, plus any zero sequence current.
 
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