CTs in Switchgears 1

[DanielElias]

Dear sirs
Most of american switchgears has as standard one CT per phase (Total:3) in order to use it for protective and measurement, while most of european switchgears have 02 CTs per phase (total:6)for the same use (protective &measurement). ¿What is the technical advantage to use one one CT per phase? ¿Is there any American standard that explain something about this subject?
Thanks

 

[dpc]

There is no standard limiting the number of CTs. Most US switchgear can accommodate at least two sets of CTs on the bus side and the load side of the breaker, for a total of four sets.

A lot of switchgear only has one set of CTs because that is less expensive. With modern digital relaying and monitoring equipment, the CT burden is so low that one set of CTs is perfectly adequate. In the old days, with electro-mechanical devices, there was an advantage in using a separate set of CTs for relaying and another set for metering.

It is still difficult to obtain high metering accuracy in the toroidal CTs typically used on switchgear breakers.

 

[busbar]

I don't believe that's a very accurate statement regarding ANSI medium-voltage metalclad assemblies. The North American IEEE C37{.20}-series switchgear standards do not seem to support the assertion.

Stock {unmodified} insulated throats as part of primary disconnects {six per drawout beaker} readily permit two 600V-class CTs each, allowing 4 per phase and 12 per breaker cell. In any case, I am not sure what purpose providing more CTs than needed for the application at hand would serve.

 

[DanielElias]

Probably I didn't explain you very well, but this is my case.Most minning companies in my country use Cutler Hammer Switchgears, when the customer need an Overcurrent protection and measurement the switchgears mostly has one CTs per phase. On the other hand I've seen some utilities that have ABB switchgers that use 02 CTs per phase for the same porpose (Overcurrent protection and measurement). That's the case I'm trying to know the reason about that difference. I understand clearly that the design of CH Switchgear can use not only one CT per phase because it depends of the application and other protection required (ex. relay 87B)

 

[dpc]

The design you describe (one CT for overcurrent relaying and meter) is very common. The main advantage is that it is the simplest and cheapest way to do it. Unless you need extreme accuracy in metering and are concerned about burden on the CT, there probably isn't much wrong with this design, if you accept the limitations and lack of spare CTs.

 

[jghrist]

It is common European practice to use separate metering CTs with a low saturation level. The idea is to limit the current in the meter in case of a fault.

It is common American practice to use one set of (relaying accuracy) CTs for both metering and relaying. I have never heard of a meter being damaged by fault current with this arrangement. The accuracy is not revenue class, but in most cases, this level of accuracy is not needed.

 

[alpha75]

there are cts that have both relaying and metering class (e.g., C100, 0.3B1.8 or IEC rated 30va 5P20 protection class and 15 va CL0.5 metering class). some are approved for revenue metering.

with today's electronic relays and meters having low burdens, high relaying accuracy/burden are not that required.

 

[DanielElias]

Thanks to you gentlemen for your useful answers.

 

[peterb]

You may also want a second set of CTs for busbar differential protection, although these would typically be located on the line side of the breaker, while the overcurrent protection CTs would typically be on the bus side of the breaker. In general, I agree that separate protection/metering CTs would not be common on metalclad breaker circuits.