Spenningsunbalance

Spenningsunbalance, also known as voltage unbalance, describes the extent to which the three phase voltages in a polyphase electrical system differ in magnitude and/or phase angle. In a perfectly balanced three-phase system, Va, Vb, Vc are equal in magnitude and 120 degrees apart.

Voltage unbalance is typically expressed as the voltage unbalance factor (VUF) or as a percentage of nominal voltage. A common approach defines VUF as the largest deviation of a phase voltage from the average of the three, divided by the nominal voltage, times 100. Let Va, Vb, Vc be phase-to-neutral voltages; Vavg = (Va+Vb+Vc)/3; VUF = max(|Va - Vavg|, |Vb - Vavg|, |Vc - Vavg|) / Vnom × 100%. Voltage unbalance can also be analyzed with symmetrical components: a significant negative-sequence component indicates stronger unbalance, while a dominant positive-sequence component indicates near-balance.

Causes of spenningsunbalance include uneven loading of phases, long feeders or conductors with impedance imbalances, single-phasing due to line faults or switching, transformer connections and aging, and inadequate distribution design. Effects on equipment can include increased current and heating in motors, reduced efficiency and torque pulsations, premature insulation wear, nuisance tripping of protection, and higher neutral currents in wye-connected systems.

Measurement is performed with three-phase power quality meters at the service or equipment terminals, and analysis often uses sequence components or VUF to quantify the unbalance. Standards and guidelines generally emphasize keeping unbalance low for sensitive equipment, with tighter limits for critical loads.

Mitigation strategies focus on balancing loads across phases, upgrading conductors or feeders, using phase-balancing transformers, and ensuring proper installation so that line impedances are similar.