Electronic Energy Meter Readings

How does your meter work? Does it look at the maximum current current of all 3 phases? Does it only monitor one phase and assume the others have the same current? And does it assume a constant power factor? Without knowing more about the meter design it's hard to answer. If the designer made any of these assumtions the meter could be working "according to design" but not "according to how a customer would be expecting it to work".

Yes, this can happen. It is a somewhat common occurrence in instrumentation and metering.

I see 2 likely possibilities:

1) You are using the wrong type of meter for the type of service connection.

If the service load is wye (star) connected, you must use a 3-element meter, which monitors the voltage and current in each of the 3 phases. For a delta service load, a 2-1/2 element meter, which monitors 2 currents and all 3 voltages, is acceptable.

In a delta connection, measuring any 2 of the 3 phase currents provides complete monitoring. The return path for the 3rd phase must be through one of the measured phases, and is measured by the the current elements in those phases.

In a wye connection, some of the phase current may return via the neutral conductor, and never be recorded unless your meter has a measuring element in every phase.

2) Your voltage sensing uses open delta potential transformers.

An open delta connection uses 2 transformers to provide a 3-phase voltage signal. Loss of one of the HV phases feeding the open delta results in the loss of 2/3 of the voltage signal. The recorded energy usage would be significantly below the actual values.

Your query is missing some important details, however some definite assumptions can be made from what you have related.

First, in the case you suggested where only 1 of the 3 phases is present, then being used by the customer to power all of his load. For Wye metered power, the metering would be accurate since the meter elements measure voltage from that element to neutral, this is assuming that the neutral is being used and not an impromptu connection to ground, in which case the meter would not work at all.

Another possibility is that there would be no metering display since the electronics are powered up by only one of the 3 phases, and if that phase happens to be out, there is no power to the electronics.

In any case, the metering would be correct with a 3 element meter Wye or Delta if there is any metering at all.

With other forms or element arrangements, the possibility exists to UNDER meter the load, but not to OVER meter the usage. You did indicate that the usage measured was much higher when no legs were missing, so this latter is probably your situation.

A qualified meter testman will be able to definitively calculate the actual usage regardless of which situation you have or what the metering error might be during this condition, since there will be specific relations that can be calculated between the measured amount and the actual amount once the determination of the details of the error are known.

We could probably do that here if we had specifics about the meter, voltage, form, which legs are out, how the load was re-wired, and the usage data before, during and after the occurrence.

Does your power authority provide meter testing by trained metering technicians?