At present, our country power quality national standard was chiefly stipulated five quotas: frequency, harmonic and voltage deviation, three-phase unbalance and the voltage fluctuation and flicker.

The article introduces voltage unbalance of neutral non-grounding system, analyzes the cause of voltage unbalance. It is valuable on fault analysis in practice.

Larger system voltage unbalance degree causes larger capacitor voltage pulsation and negative sequence current.