Is KW really KW?

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11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
A new facility in the area will be served by the POCO and metered by them on the primary. Three CTs and three PTs, owned by the POCO.
Before the year is up the primary voltage from the POCO will convert from 12.4/7.2Y to 25/14.4Y. The thinking is that at the start the meter set will need dual voltage PTs and the customer that is providing his privately owned transformer will need a dual voltage high side on his transformer. Customer doesn't have a choice in this situation if he wants power before the primary voltage conversion.

I am thinking on the metering side that the 14.4 PTs could be installed from the start, never change when the primary line converts and still be able to accurately measure the customer's usage usage from the metering set. The CT's do not require a voltage and would be installed with an insulation rating for the higher voltage. In the short run while the line is 12.4, The PTs would produce only 60 volts as compared to the normal 120 because of the fixed turns ratio in the PTs. My math shows the net KW from volts X amps to equal at the metering set before and after the voltage conversion. This does not figure in PF but now my head hurts and I am going to stop thinking about it for a while.
What else should I be looking at? I am not the meter guy and delivery time for the dual voltage PTs is the only reason I am thinking like this.

Thanks
 
You could try the manufacturer of the PTs and see what they say. It seems like it should work. If they are being used by a protection relay you would have to change the settings when the voltage increases but if its just a meter just set the PT ratio and it should be good.
 
jim dungar said:
The VTs may not like continuous 50% undervoltage voltage condition.
Click to expand...

iceworm said:
I'm not familiar with the failure mode. What might it be?
Click to expand...

Jim - I'm definitely interested. This comes up occasionally - 14.4/120, 120:1, usually get connected open delta, but sometimes connected wye on 13.8. What should we be watching for?

ice
 
150328-1225 EDT

Any iron core transformer can be operated at any voltage below its rated voltage without damage. In some cases one might want a voltage somewhat in the range of its rating to keep the transformer warm to avoid moisture penetration. At any voltage below the transformer's rating the voltage stresses are lower, and the residual power dissipation is lower.

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WELL.....SINCE EVERYONE IS ENTITLED TO MY OPINION....The meter manufacturers have accuracy requirements regarding revenue metering. Those requirements assume a voltage within a certain range. They are VERY unlikely to certify those meters to accurately measure KWh at less than the rated voltage. Since the POCO depends on the meters for billing accuracy, they are likely, as well, to be less than enthusiastic about meters operating at other than rated voltages. It may not be technically a problem as much as an accountability and accuracy problem. We, as utilities, are required to test meters for accuracy. That test assumes certain rated voltage and current values.
 
I don't know enough about revenue metering to say whether it would like being driven below nameplate voltage.

For what it's worth, it's pretty common to see X:120V PTs driven at 69V-N when hooked up in a 3? wye, and they perform just fine with no loss of accuracy.
 
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