ABB current transformers 1

[rmiell]

In 2004 we built a primary metering station on a pole for a newly built RO plant. Recently we retired this setup, to install smaller CTs. The orginal cts (ABB KON-11, 100:5), when opened on the ground, were each shown to have the 2 secondary terminals shorted together, with the factory link. This link must have been in place from the get-go. Metering tech on board at the time of installation has since retired. A check on the history of this site has shown a hefty consumption and demand, in line with the installed equipment.

My question is this. Is it possible for these cts to provide the information to the metering? If it could still provide infomation to the metering, would it be correct? If not correct, how far off would it be?

When a meter tech "shorts out" the cts for servicing the meter, what exactly is happening? One side of the secondary leads is connected to ground?

Thanks for helping me get a handle on this.

Rick Miell

 

[jraef]

When a CT is "open circuited" meaning not connected to a load such as a meter, the voltage potential across the terminals rises to a level commensurate with the CT ratio, in other words very dangerous levels. To avoid this when servicing equipment then, technicians place a shorting bar or use shorting contacts across the secondary leads. This allows the secondary current to flow at it's normal rate, opposing the current in the primary (source) and everything is normal. If the shorting bars were left in place, there would be no output going to whatever the CT was feeding.

That fact will not, however, stop the average Power Company from sending you a bill, they will just estimate it. that usually tends to work out in their favor, so making sure you have a proper metering setup is in a user's best interest.


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[rmiell]

jraef, the metering has been working, with no estimation happening. As far as how correct it is remains to be seen. Any idea how it could still have been causing a reading in the meter?

Rick Miell

 

[scottf]

Rmiell-

If the metering has been working (i.e. having roughly the correct current input), then the CTs weren't shorted in-service. I suspect the secondary was shorted prior to the secondary leads being disconnected, as is standard practice.

For medium voltage CTs, the normal design for a single-ratio unit (e.g. 100:5A) is to have a terminal between the X1 and X2 terminals that is electrically connected to the base plate (grounded) about which the shorting device rotates. I know that's how it is with my company's design and I'm 99% sure the same holds for the ABB design.

 

[apowerengr]

If the secondary isolation links were shorting the CT terminals and you connected a low-burden revenue meter the current would divide based on the ratio of impedances of the parallel paths. No way to know what percentage of the load the meter was registering unless you measure the impedance of the parallel paths from the point the metering wiring for the current circuits was connected to the CT secondaries. Even then its just a good estimate.

I'd expect you were metering 10-30% of the actual load.

 

[rmiell]

scottf, the line crew that took the cts down are the ones who noticed the connection, and brought it to my attention.

The ABB unit has 2 terminals for the secondary conductors, with a stud in between these two terminals. The stud had a rotatable bar that can only rotate when the holding nut is loosened. There is a 3rd terminal down lower in the box, which is marked gnd, which was grounded during installation.

My question remains, if the ct was shorted in place, would the meter still register a load, and what percentage of the load whould show up? 33%? 50%? 66%? or ????

Thanks to all how have answered.

Rick Miell

 

[rmiell]

apowerengr... I was posting to scottf when your post came in. Thanks for this info. I take it from what you are saying that it was possible the meter could have shown usage, tho in some reduced percentage of actual load.

Thanks for this.

Rick Miell

 

[GTstartup]

Are you sure it was shorted and not just a multi ratio CT with some part of the winding shorted?

 

[rmiell]

GT, I believe that it is not a multi-tap type of CT. There was only the 2 terminals for the secondary, marked X1 and X2.

Rick Miell

 

[davidbeach]

Partially shorted multi-tap would also not properly represent the primary current in the metering equipment. (Not that it would matter here since it wasn't multi-tap.)

 

[scottf]

Rmiel-

Was the meter pulled prior to the CTs coming down? Could a meter tech (not sure if you're in meter group or not) have shorted the secondary prior to your crew's arrival?

As was said above, some current could have registered at the meter if the CT was shorted and it would a function of the current division of the different impedances.

You could calculate the impedance of the meter circuit by adding the lead resistance and the meter impedance....this is likely to be dominated by the leads.

You could measure the resistance of the shorting device and allow a bit for contact resistance...of you would have that for the secondary meter leads as well.

I think you'll find that when you do the estimation, you'll come up with a value in the range of 1-2% of the current could make it to the meter.

Since the CTs have been replaced, you must have some sense of the metered load. How does it compare with past data?

 

[rmiell]

Scottf, the cts were taken down at the same time the meter was taken out of service, by the same crew.

This was just done yesterday, so in a few more days we will have enough of a history to see just what the differences are, if any.

Thanks again for all the replies.

Rick Miell

 

[waross]

I don't suppose that you are fortunate enough to have an electronic KWHr meter with history enabled?
I was involved with an installation where the CTs were hung on cable loops. One CT was turned upside down. Rain water found it's way into the plastic cover over the connections. The history on the KWHr meter showed that most of the current was shunted through the water and very little current passed through the meter for that phase. My memory is a little shaky but as I remember about 10% of the current passed through the meter and 90% was shunted by the water. With the shorting link in position I would expect less than 1% of the current to reach the meter. I suspect that you are not being told the whole story. There may be a misunderstanding or honest mistake or someone may be 'pulling your chain". I advise looking wise and refraining from comments or statements for a few days until you get the full story. Stay cool and wait for more information.
By the way, there are models of CTs where it is not possible to install the plastic cover over the connections when the shorting link is in the shorting position.


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

 

[scottf]

waross-

Your comment on not being able to install the covers with the terminals shorted is really only applicable to 600V CTs and not MV CTs as we have here.

It's correct that on most 600V CT designs, the terminal covers can only be installed where the terminals are accessible to outside wires when the units are not in the shorted position.

 

[waross]

For reference, the units that were shorted by water were probably 500:5 but the secondary current rating would be the same in both instances.
It is good practice to put the shorting link in place on a CT before disconnecting the secondary wires. That is the first thing that many old* meter techs will do when working on a CT.
* As in;
"I never wanted to be the smartest man on the crew. I always wanted to be the oldest man on the crew!"

I strongly believe that the first man up the pole put the links in before he started to disconnect the secondaries. Possibly someone less experienced saw the links and commented. Rather than explain, someone decided to string him along (not a good way to promote safety) and things got out of hand.

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