Summing CTs for load flow/metering
Summing CTs for load flow/metering
(OP)
Anyone have a general idea how to sum the currents of breaker CTs from many lines (say 10 138kv lines) for the purpose of calculating total MVA for load flow?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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Summing CTs for load flow/metering
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RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
I can get you a scan of the summation CT type of scheme from work tomorrow if its any use / interest? I think the benefit of the multiple wound primaries is that it allows for correction of differing CT primaries to a common base. Direct paralleling only works if all the CTs are of equal ratio.
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
@stevenal, had not thought of that, I do need the current down the meter-able levels. My prior concern was fault current however. The primary L-G fault currents are around the 40,000 amp range.
@davidbeach, makes sense and thank you. :)
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
I don't have precise details of the summation CTs themselves unfortunately but my understanding is that each summation CT primary is specific to the CT connected to it, such that the primary for a 400A CT creates four times the amount of flux in the summation core that the primary for a 100A CT does for equal primary current.
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RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
One of my former employers used to use summation CTs for metering applications, although as the equipment was LV, changing them was nowhere near as challenging as dealing with dead tank equipment. They had 4 primary inputs, all at the same ratio, and as a result all the primary side CTs had to be the same ratio. They also restricted the number of primaries thus decisions on number of feeders and so on were (partially) influenced by the summation CTs. I've heard of just paralleling CTs into equipment without using summation CTs, but never seen it done.
I'd not be feeding protection class CTs into them, rather I'd use metering CTs as inputs. Custom primaries like ScottyUK has asserted would work, but make any changes or upgrades more painful.
Its much easier to either use dedicated metering (which was done on one or two sites), or use whatever data is available from the relay, and then collate that data for total load flow. Of course, if the relays aren't capable of providing the data, then that's a different story. Whether it would be cheaper to upgrade the relays or change the CTs is debatable.
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
https://library.e.abb.com/public/ec0a90a6b482e53d8...
The other option is a live tank with separate CT but I am unfamiliar with my options on that side.
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
RE: Summing CTs for load flow/metering
A few points to note:
- in the IEEE world, it is not uncommon for bushing CTs to have both a protection and metering accuracy rating.
- if the application is indicating metering and not revenue metering, a protection class CT will work just fine. In reality, it would probably work fine for revenue metering, you just wouldn't have the ratings and test results to back it up like you would need for revenue metering. If using for metering and if you have a multi-ratio design, try to pick the most appropriate ratio for the expected current level. Use as low of a ratio as you can and let the rating factor be used to cover the maximum continuous current expected.
- if you need to order a breaker with more CTs, take a look at lowering the protection class rating of the protection CTs. Just about everyone specifies C800 ratings still, but that is normally over-kill for most applications that use electronic relays. With everything else held constant, a C400 class CT is roughly half to 3/5 the height of a C800 class CT.
RE: Summing CTs for load flow/metering
But if you need to do it via the CT;s and if the CT's are all the same ratio, then simply pass the secondary of each through the window of a 50:5 CT and use that to feed the power meter.
RE: Summing CTs for load flow/metering