Swapping Polarity on a CT - Metering Impact?

L

Lauren

Member
Location
NH
Occupation
Engineer
I have a project where we installed a digital revenue meter (SEL-735) on a service entrance breaker, connected to a set of PTs (wye connected) and CTs (also wye connected). The power flow is from the utility to the load (no generation). The meter is currently reading positive kW. However, the customer is asking for the meter to read negative kW. There is no option to program this change in this specific meter, so a wiring change will be needed.

I requested our contractor to swap the wiring directly on the meter to reverse the polarity of the CTs. However, I've received some pushback, stating that the power factor will be off due to this swap, and essentially putting the current 180 degrees apart from the voltage.

I believe the swap will put both KVAR and KW into a negative quadrant, and thus creating an accurate PF, but I'd like some additional opinion if there is something that I'm missing in this analysis?
 
Lauren said:
I requested our contractor to swap the wiring directly on the meter to reverse the polarity of the CTs. However, I've received some pushback, stating that the power factor will be off due to this swap, and essentially putting the current 180 degrees apart from the voltage.

I believe the swap will put both KVAR and KW into a negative quadrant, and thus creating an accurate PF, but I'd like some additional opinion if there is something that I'm missing in this analysis?
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If you are already getting the opposite KW than you calculate/expect, the PF is already off.
 
I assume that the reason the customer cares is that they're dragging the readings into a BMS/EMS/etc. Since positive KW is correct, they ought to change it inside their logging system; changing it at the hardware to be incorrect is a bit of a hack job.

ETA- I assume you've checked with SEL.
 
zbang said:
I assume that the reason the customer cares is that they're dragging the readings into a BMS/EMS/etc. Since positive KW is correct, they ought to change it inside their logging system; changing it at the hardware to be incorrect is a bit of a hack job.
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It's kind of a complicated situation (the utility is technically the customer here), but yes, they'll be bringing it into a monitoring SCADA system. Since it's a revenue meter, I don't think it's easy to just swap the reading at SCADA, which is why they're asking for it to be swapped at the meter itself.

I understand this is a bit unconventional, but why is it incorrect or a hack job to swap it at the meter? It seems like you should be able to design a system reading KW in either direction, since it's really just a reference point. When I design a generation bus, I'll wire each of the generator meters to read one direction, and all the loads to read opposite. I'm not sure how this is much different?
 
zbang said:
From the OP, it sounds like the meter is connected and reading correctly (that is, consuming power from the PoCo and reading positive consumption).
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From the OP, it sounds like the metering was installed for consumption but the customer wants it for production.
Either the SEL configuration needs to be changed or the CTs need to be 'flipped'.
 
jim dungar said:
From the OP, it sounds like the metering was installed for consumption but the customer wants it for production.
Either the SEL configuration needs to be changed or the CTs need to be 'flipped'.
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Essentially yes. This was designed to read as a consumption. But since it's actually the utility monitoring this meter, they want it to read opposite (I've seen this a decent amount where the end customer and the utility meters are wired opposite, in order to read opposite directions).

SEL tech support says it can't be done in the software...so sounds like "flipping" the CTs is the correct action to take. Thanks!
 
LarryFine said:
Am I correct that you can simply swap each CT's wires and not have to physically flip them?
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My understanding is that it will have the same result. My preference would be to swap the secondary wiring right at the CT, but that will require a full shutdown. So, we will probably swap the secondary wiring at the first shorting block.
 
zbang said:
Or the SCADA software needs to change. Since apparently it can't flip the sign of a value in software where that's dead easy to do, we're flipping the CTs to make up for the SCADA deficiency, and that is a hack. Might be a hack that works, but IMHO it's still a hack.
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Except flipping the CT, or its leads, also lets the SEL relay 'see the world as it is'. In the future, some programmer working on the SCADA system is not going to say 'I think I've found the problem, some fool put a negative sign here'.

I would also notate or update the as installed drawings to show the swap.
 
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