When fitting sub-meters to monitor energy usage, measuring three-phase voltage is crucial to calculate true kWh accurately. Energy usage is determined by the formula:
Why Voltage Monitoring Matters
Voltage Fluctuations
Voltage in real systems varies due to changes in load demand, distribution conditions, or equipment operation. Assuming a fixed voltage, like 400V, introduces errors.
The potential error is calculated as:
For a 5% fluctuation (e.g., actual voltage of 380V instead of 400V), this results in a 5% error in kWh calculation.
Variable Power Factor
Power factor (PF) changes based on load type. Assuming a fixed PF (e.g., 0.9) leads to inaccuracies, especially for systems with motors or non-linear loads.
The error is given by:
For an actual PF of 0.7 instead of 0.9, the error is 22.2%.
Combined Impact
When both voltage and power factor assumptions are incorrect, the combined error can exceed 25%, significantly impacting billing, energy management, and compliance.
Conclusion
Monitoring three-phase voltage ensures accurate kWh measurement by capturing real-time variations in voltage and power factor. These measurements safeguard against costly errors, ensure fairness in billing, and support effective energy management.
For the most precise results, always opt for sub-meters capable of true voltage, current, and power factor measurement.