Voltage unbalance in Australian power-quality checks
How voltage unbalance uses comparable three-phase voltage readings, average deviation and an entered threshold basis.
What Voltage Unbalance Means
Voltage unbalance describes how far comparable three-phase voltage readings differ from their average. In Australian power-quality work it is commonly reviewed around 400 V line-to-line readings or 230 V phase-to-neutral readings, but those two measurement sets should not be mixed.
The result belongs to one measurement point and one time or loading condition. It is not the same as phase balancing, motor voltage dip, cable voltage drop or harmonic THD.
Formula Fields
The average-deviation worksheet uses these relationships:
Vavg = (V1 + V2 + V3) / 3.
maximum deviation = largest absolute difference between a measured voltage and Vavg.
unbalance_% = maximum deviation / Vavg x 100.
For line-to-line mode, keep Vab, Vbc and Vca together. For phase-to-neutral mode, keep Van, Vbn and Vcn together. The threshold basis must come from the project, equipment manufacturer, DNSP requirement, power-quality report or engineering review.
| Field | Use it for | Why it matters |
|---|---|---|
| Measurement set | `Vab`, `Vbc`, `Vca` or `Van`, `Vbn`, `Vcn`. | Mixed sets make the percentage meaningless. |
| Measurement point | MSB, distribution board, motor terminals or generator output. | Unbalance is location-specific. |
| Loading condition | Time, load state and instrument or report source. | Voltage spread can change with operating condition. |
| Threshold basis | Project, DNSP, report or manufacturer criterion. | The calculator does not create a universal limit. |
Worked Measurement Example
A main switchboard record has Vab = 400 V, Vbc = 397 V and Vca = 405 V.
Vavg = (400 + 397 + 405) / 3 = 400.67 V.
The maximum deviation is 405 - 400.67 = 4.33 V, so unbalance_% = 4.33 / 400.67 x 100 = 1.08%.
| Item | Example value | Reading |
|---|---|---|
| Measurement set | Line-to-line `Vab`, `Vbc`, `Vca`. | Keep it separate from 230 V phase-to-neutral data. |
| Highest and lowest | `Vca` highest at 405 V, `Vbc` lowest at 397 V. | Voltage spread is 8 V. |
| Result | 1.08% average-deviation unbalance. | Compare only with the entered threshold basis. |
| Context | Main switchboard, loaded condition, instrument report. | The value should stay tied to the record source. |
Related Power-Quality Terms
Voltage unbalance is not a phase-balancing worksheet. Phase balancing usually looks at load distribution. Voltage unbalance looks at measured voltage spread. It is also different from motor voltage dip, which is a starting or event issue, and harmonic THD, which uses harmonic magnitudes.
Some engineering reports may use negative-sequence VUF or other methods. Keep that method label visible if the source is not the average-deviation calculator.
Next checks
- Use voltage unbalance when the three comparable voltage readings and threshold basis are ready.
- Use the formula chart when the average, maximum deviation and percentage relationship needs review.
- Use the power-quality measurement table to keep measurement point, loading condition and instrument context visible.
Boundaries
- This page does not set a universal allowable unbalance.
- It does not diagnose root cause or replace a power-quality investigation.
- DNSP requirements, equipment manufacturer data, report criteria, measurement conditions and qualified review remain controlling inputs.