3 Phase Utility Power Metering

I don't disagree you can measure this way, but wouldn't this be accurate only if all 3 phases are balanced?

I just went and looked at a system meter on a rotary drum dryer at our plant. The entire process is down currently A Current 6 Amps, B Current 0 Amps, C Current 6 Amps. Voltage AB 477, BC 477, AC 477 PF .712 Lag. So while this system is off their is a large imbalance in the load mainly from lighting and small loads.

If power company is reading power on a 3 phase system and only has CT's on A and C Lines then this is going inaccurately report power. Since they are only using (2) CT's and reading the A and C current. Then they will be charging me almost 3 more KW per hour than I am actually using. if this system stayed down 24 hours with only lighting loads this would be 72KW per day being overbilled from just a small area of our plant.

We are a large factory, and we typically run 3 phase loads. But we have tons on non 3 phase loads as well, lighting, ac systems, computers, wall outlets etc. When your facility takes up 54 acres this could add up quite a bit. It would be nice to say over the years electrical peeps kept this as balanced as possible.. But to average guy adding a power panel to feed single phase loads I don't even think they consider this.

I don't disagree you can measure this way, but wouldn't this be accurate only if all 3 phases are balanced?

No, they do not need to be balanced. Of course, there should be no neutral or some of the current has another way in and out.

"Two Wattmeter Method can be employed to measure the power in a 3 phase, three wire star or delta connected the balanced or unbalanced load."

https://circuitglobe.com/two-wattmeter-method-of-power-measurement.html

Simple algebra proof of 2 Wattmeter method:

3 Wattmeter: P = V₁I₁ + V₂I₂ + V₃I₃

For a no-neutral system: I₁ + I₂ + I₃ = 0, or I₃ = - I₁ - I₂

P = V₁I₁ + V₂I₂ + V₃(-I₁ - I₂)

P = (V₁-V₃)I₁ + (V₂-V₃)I₂ (2 Wattmeter method)

I'm wondering if your billing difference is just a difference between real kilowatts and real and reactive power measured in kVA? Adding some reaction correction might save you money.

We have the best PF correction you can hope to have. Our largest power consumers are (2) 3000HP Synchronous Ball Mills. These run at basically Unity PF. Running these two guys keeps our plant power factor generally in the range of 96% for the month

All of our internal metering takes into account Real, Reactive Power.

What I can say is the power company after running tests on metering felt there could possibly be a problem in way they were billing so they have agreed to and have installed another meter on pole before our existing meter. So they could monitor any differences they see between the two. Only issue they forgot to pull CT jumpers on new CT's and the are still shorted so now I have to wait till 6/2 for them to power down again and pull jumpers.

This may seem elementary, but you say the bills are excessive. Can you actually read the utility meters yourselves, to check billed kWh against metered kWh? Have you seen bill yourself to see if there are reactive load or unbalanced load additions?

As I see it, misconnection would cause a watthourmeter to read low but with a KVArh meter could read your unity p.f. watt load high.

Ensure the power company checks the phasing & polarity of all connections in the path to the meters aginst meter connection diagram, preferably by injection test.

Interesting problem, but we're missing a piece of the puzzle. That piece is the a comparison between the actual kWhrs billed vs. the sum of all your internal metering points. There are a couple of possibilities that might explain the discrepancies, and knowing the actual/billed readings will provide more information than just the 350,000 delta.

You haven't stated your location, but the fact that the utility has installed a second metering point ahead of yours indicates that they seem to be willing to help you track down the problem.

Regarding the two-wattmeter method, you can see the math here.

For Unbalanced load, preferable to have 3 Current coil element in the meter.

Similarly 3 Potential coil in the instrument.

Some times instrument will have 3 coils but connection made only from 2 CTs

Similarly connections to the meter might have inter changed like S1 and S2.

Also if current coil c1 is connected phase 1 then potential coil P1 should be connected to phase 1 only

. These suggestion may look odd but some times we miss out these.