HOW MANY METERS NEEDED?

[EEAOC]

I have situation where I have a switchgear fed by four 750 kva transformers from the Utility. Since switch gear is nearly 40 years old with many loads that are unknown.
I have requested a consultant to meter the current loading on switchgear in order to ensure that the switch gear can take any additional.

I felt that in order to measure the current loading of this SW I need only one meter by which I can take reading of the main bus from which all the loads are fed. However I was told by a higher authority, that I need to use three more meters to measure the actual loading on this SW. This is some thing, I am not sure I agree as I feel that only one meter is adequate to measure the actual loading on the SW. I assume since the SW is fed by four network transformers, I was told I need 4 meters to measure the loading!

In my opinion no matter how many transformers feed the SW, finally every thing ends up in the main bus, so you need only one meter to measure the loading on this SW.

If anyone thinks otherwise please let me know why. Thank you in advance

 

[wareagle]

From your post it sounds like you have 4 inputs to the switch. If there is a place that you can connect Ct's that will measure all of the inputs at the same time you only install one set of metering equipment. If the transformer
conductors are connected to a common bus, it seems like it would be difficult to meter all 4 with one set of Ct's but I do not know how the physical connections are made.

 

[jghrist]

Everything ends up on the main bus, but the load splits in different directions. There is no single point on the main bus that carries all of the load.

 

[waross]

Do you have one switch only fed from the bus bar or is there more than one switch? Do the transformers feed one end of the bus and the load tap off the other end, or are the transformer connections spaced along the length of the bus?
You must provide us with a little more information please.
Thank you
respectfully

 

[LionelHutz]

I think what waross is alluding to is that the most likely case is you having 4 transformers in parallel feeding into switches for each transformer with these switches feeding one buss. If this is the case, then you may want to measure each transformer load to ensure there is not an overload on one of the transformers or it's secondary switch.

 

[DanDel]

If you have four transformers as sources (T1, T2, T3, T4)and multiple loads (L1, L2, L3, L4), configured as shown, and can get the monitoring CTs on the buses between the 4 transformers and the multiple loads, then you can measure the total load with one instrument, since that section of bus carries all of the load current.


T1 T2 T3 T4
| | | |
| | | |
| | | |
| | | |
--------------------------------------------------
| | | |
| | | |
| | | |
| | | |
L1 L2 L3 L4


If, however, you have a configuration as shown here, you will need 4 instruments, since there is no one place on the bus where all of the load current will pass through.

T1 T2 T3 T4
| | | |
| | | |
| | | |
| | | |
---------------
| | | |
| | | |
| | | |
| | | |
L1 L2 L3 L4

 

[BJC]

The temporary meters your consultant are most likly to use split core or "window" CTs. IT would require some physcial rework ( and a shutdown) to put the CTs on the high side of the transfromers.
You might get by with one set of CTs on the high side but it depends on the physical configuration of the installation.
In theory you can do it with one set on the secondary. In practice you may not be able to get all the scondaries though one set of CTs.
From the diagram above I would vote for four CTs and use somthing like these.

http://www.dentinstruments.com/CurrentTransformers.htm

I have used the flexible current sensors and there have been places where it's hard to get them around all the secondary conductors of a 2,500 Kva transformer.
I don't think you have a theory problem, it's a practical problem.