Metering 4 wire 240 3ph open delta

[mclark]

The newer solid state POCO polyphase meter ( Landis&Gyr kbase S4 series AXS4e) reads the 3 phases as Phase A 0deg Phase B 180deg and Phase C (high leg) 270deg for Phase Voltages, then reads phase currents with phase angles corresponding - for example Ia 37deg Ib 215deg Ic 305deg yielding somewhere approx 0.8xx power factor each phase in my opinion. from Cos(37-0) Cos (215-180) Cos (305-270).

But how do we sum these to obtain the actual billed kWh? Why the orthogonal angles rather than 120deg?

Although this small general service does not have demand nor power fastor charges - we are trying to ballance the loads, apply power factor correction if needed to sub panels, and minimized billable kWH. The idea of investing in power factor and balance being that a closer to unity balanced system is easier to drive by a small sized standy generator even if the POCO does not charge power factor penalty.

In trying to manage the loading and relate the work we are doing to actual billable kWh we are reading the meter several times a day and logging that data - I am just having a hard time relating our "in the field" true rms power probe readings to the accumulated digitially integrated meter readings. Can convert the vars to watts with the Cos(angles) as above but how does that L&G meter vector sum 0deg, 180deg and 270deg for billable kWh?!

 

[erickench]

The reason why you are not reading a 120 degree displacement is because your service has a high leg and the meter is measuring from the midpoint of the AB winding. Phase A is the reference so it's angle is 0 or 360 degrees. Phase C is the high leg so there is 360 - 90 = 270 or -90 degree displacement. Phase B is opposite from phase A as measured from the midpoint and is therefore 180 degrees displaced.

 

[erickench]

Use the cosines for each phase to determine the power factor and then apply the formula for each phase:

Power = VxIxPF

Then you add these three values to obtain the 3-phase power.

 

[mclark]

Thanks! That is what I was doing but it seemed odd to simply add 3 phase legs to get total 3 phase power.

I guess this orthogonal metering method handles imbalances better, by treating every leg separately. I wish I could apply a balanced 3 phase load and then a single phase load to each leg to verify, don't know if I can kill the power to the facility to play those experiments!

I am still not sure how the meter is doing this as the neutral is not monitored for current in this open delta configuration, just the 3 corner points of the delta. So if the meter Voltage is referenced Phase A to neutral but all the currents are measured line to line it seems like an imbalance in Phase C would cause current to neutral that was not measured. {Phase A and B are the CT xformer like single phase 240 and C is the high leg and is actually on the outside leg of the meter even though it is the middle phase of the 0 120 240 phase angle schematic.}

So how does the meter measure I C-A and I C-B and I A-B and then digitally determine I C-neutral for the 270deg leg?

We created an excell spreadsheet to calculate each leg and then sum the 3 vars and watts. It just seems odd to use straight addition of 3 legs to get total power.

Thanks for you assistance!

 

[erickench]

The neutral is the midpoint in the AB phase. There would have to be a meter lead connected to this midpoint.

 

[mivey]

The newer solid state POCO polyphase meter ( Landis&Gyr kbase S4 series AXS4e) reads the 3 phases as Phase A 0deg Phase B 180deg and Phase C (high leg) 270deg for Phase Voltages, then reads phase currents with phase angles corresponding

That is the way the old meters did it too.

But how do we sum these to obtain the actual billed kWh?

See Eric's #3

Why the orthogonal angles rather than 120deg?

Because of the 4th wire (the neutral). Using a common point allows the reduction of metering devices without a loss in accuracy (Blondel's Theorem).

In trying to manage the loading and relate the work we are doing to actual billable kWh we are reading the meter several times a day and logging that data - I am just having a hard time relating our "in the field" true rms power probe readings to the accumulated digitially integrated meter readings. Can convert the vars to watts with the Cos(angles) as above but how does that L&G meter vector sum 0deg, 180deg and 270deg for billable kWh?!

It is metering the system as a combination of a 120/240 volt single-phase service and a
208 volt (high-leg) single-phase service.

Thanks! That is what I was doing but it seemed odd to simply add 3 phase legs to get total 3 phase power.

Power consumed is power consumed and you can add individual readings to get the total consumption.

I guess this orthogonal metering method handles imbalances better, by treating every leg separately.

Any valid meter method would accomplish the same thing.

I am still not sure how the meter is doing this as the neutral is not monitored for current in this open delta configuration, just the 3 corner points of the delta.

By Blondel's Theorem:

If energy is supplied to any system of conductors through N wires, the total power in the system is given by the algebraic sum of the readings of N wattmeters, so arranged that each of the N wires contains one current coil, the corresponding voltage coil being connected between that wire and some common point. If this common point is on one of the N wires, the measurement may be made by the use of N-1 wattmeters.

It just seems odd to use straight addition of 3 legs to get total power.

Why? You would not think twice about measuring the gallons flowing through three pipes to get the total gallons delivered.