CT Polarity Question

[nuckythompson]

Hi all, if I had 3 CT's (one per phase) and I reversed the polarity on one of these CT's, how would that affect the system. For example, if the CT outputs went into a Watt transducer which uses voltage and current.

Thanks

 

[jim dungar]

Watts are watts, regardless of the angular relationship between current and voltage.
Any calculation depending on leading vs lagging PF will be impacted.
From my experience, this condition can exist for decades without being noticed in the real world.

 

[nuckythompson]

Watts are watts, regardless of the angular relationship between current and voltage.
Any calculation depending on leading vs lagging PF will be impacted.
From my experience, this condition can exist for decades without being noticed in the real world.

I am confused. Watt meter takes in 3 phases of current and voltage. If one CT is reverse polarity, will that not affect the current input to the watt meter for that phase?

 

[jaggedben]

In practice, the equipment will likely use a single set of voltage references for all three CTs, so the result will be a negative watt consumption reading for the leg with the reversed CT.

 

[nuckythompson]

In practice, the equipment will likely use a single set of voltage references for all three CTs, so the result will be a negative watt consumption reading for the leg with the reversed CT.

Thank you very much for your response. I am trying to understand why it is negative. Is it like subtractive polarity? It is AC not DC.

 

[jaggedben]

If the directions (mathematical signs) of the voltage and current don't match, the product will be a negative number.

There are four ways this can happen:
1) CT wrong direction around conductor
2) CT leads swapped on their terminals
3) voltage reference wires swapped on their terminals
4) actual reverse power flow, like from a solar inverter

An even number of such reversals will cancel out and result in a correct reading. Accidentally swapping voltage reference terminations will affect all CT readings. In a three-phase system there are obviously more permutations for how things can go wrong.

 

[jaggedben]

Slight correction on the even number of reversals...
If one of the reversals is actual reverse power flow, then the reading will be positive (consuming), but incorrect.

 

[jim dungar]

I am confused. Watt meter takes in 3 phases of current and voltage. If one CT is reverse polarity, will that not affect the current input to the watt meter for that phase?

I have often seen meters wired incorrectly that have been in service for decades without any one noticing.

It really depends on your metering equipment what results you see. A lot of KW metering is direction independent partly because of duplicating the ratchet mechanism on old utility kWHr meters. Newer electronic meters and onsite generation will likely mean impropet CT installation will be noted earlier.