Questions

“Because of space constraints, we can only install two current transformers (CTs) to monitor a three-phase circuit. Is there a correction factor we can use to compensate for only monitoring two of the three phases?”

“What if we only monitor one of the three phases?”

Answer

For balanced three-phase, four-wire (wye) circuits, each CT will measure exactly one-third of the total current. So if you are measuring two of the three phases, you would multiply your results by 1.5 to scale the reading to the correct value. If you only measure one phase, you would multiply by 3 to scale the reading to the correct value.

Limitations

There are a few different ways a three-phase circuit can be imbalanced that can cause reduced accuracy with this approach:

  • The load may be imbalanced. Three-phase motors are generally well balanced, but other loads may not be. If your load is actually multiple loads (like monitoring the three-phase service to the floor of a building), then there is a high likelihood of imbalance.
  • The voltages from neutral (or ground) to each phase may be imbalanced. There is always a small amount of imbalance, but there can be more imbalance depending on the service and other loads. For example, if one phase voltage is 1.0% higher than the other phases and you are not monitoring the one phase that is high, your power readings would be 0.5% low.
  • In rare cases, one phase voltage may be grounded (called “grounded delta” or “grounded leg”). In this case, the WattNode meter will measure zero power on the grounded phase, so the easy solution is to monitor the other two phases and eliminate the correction factor of 1.5. In this case, you must monitor both of the active (non-grounded) phases for accurate results.

Recommendations

If possible, you should use a hand-held power analyzer or multimeter (DMM) to verify that the load is reasonably well balanced. With the power analyzer, you can measure the power on each phase and compare. With the DMM, you can check the phase-to-neutral or phase-to-ground voltages to ensure they are very similar. If you have a meter with a current clamp, you can also check the current in each phase to verify that they are well balanced.

Extra Notes

Doesn’t Blondel’s Theorem allow the use of two CTs to monitor a three-phase three-wire (delta) circuit?

Yes, it means it is possible to design a meter with only two elements (and only two current transformers) to monitor a three-wire delta circuit. But it does not mean all meters can take advantage of this. In order to utilize Blondel’s Theorem, one of the three phases must be used as a reference point, so that the other two phases are measured relative to this reference point.

The architecture of WNB and WNC series WattNode meters only allows for ground or neutral as the reference points, not one of the voltage phases. Therefore, Blondel’s Theorem cannot be applied to this series of WattNode meters to allow the use of two CTs for three-wire ungrounded delta circuits. As noted above, if your load is balanced, you may be able to use just one CT and multiply readings by 3. Or use two CTs and multiply the readings by 1.5.

However, in applications using potential transformers (PTs), the secondary of the PT can be wire to provide a reference point. Therefore, in this application, the WNB and WNC series meters can be used with only two CTs. See Figure 3: Monitoring a Delta Circuit on the Using Potential Transformers page.

The WND series meters can measure 3-phase 3-wire delta, 4-wire delta, and corner grounded delta services using only two CTs.

See Also