Phase unbalance

Phase unbalance means the voltages or currents of a three-phase system are not equal in magnitude, or the angles between the phases are not equal. Meters express it as an unbalance factor: the negative or zero sequence component divided by the positive sequence component, given as a percentage. This is based on symmetrical components. Unbalance is common on distribution feeders because many loads and generators are connected single-phase, so studies of high PV penetration need detailed unbalanced line models. Utilities set limits on unbalance. One service rulebook requires customer load to stay within 10 percent between phases, and another says voltage unbalance caused by a generator's unbalanced currents must not exceed 2%. If one phase opens (single-phasing), the currents become unbalanced and motors can overheat and fail. Inverters often trip on voltage or current unbalance, so check for phase loss when you see one of these faults. Meter diagnostics that flag voltage imbalance can point to a lost phase voltage, a blown VT fuse, a wrong VT ratio or a wiring error. In the field, have the phase conductors and neutral measured and the wiring rechecked. On three-phase microinverter branches, keep unused connectors to a minimum. Some inverters also have phase balance or apparent power unbalance limits that you can set.

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• sentence 2: support check: partial - Ratio and percent are stated, but 'meters' (Acuview software) and 'based on symmetrical components' are only implied via sequence components.
• sentence 3: support check: partial - Passage says unbalance is common due to single-phase connections and detailed line models are needed, but 'many loads and generators' and high-PV studies mostly fit; unbalanced model need is stated generally.
• sentence 5: support check: partial - Single-phasing effects and phase loss check are stated, but 'inverters often trip on voltage or current unbalance' is only loosely supported by threshold settings and one fault. -->