Symmetrical Components

Developed by Charles Fortescue in 1918, symmetrical components transform the complex problem of unbalanced three-phase analysis into three independent single-phase calculations. The positive-sequence component represents the normal balanced three-phase system rotating in the forward direction. The negative-sequence component represents a balanced system rotating in the reverse direction — it appears during unbalanced faults and causes heating in motors. The zero-sequence component represents in-phase currents in all three phases simultaneously — it flows in earth faults and returns through the neutral or earth path. For symmetrical (three-phase) faults, only the positive-sequence network is needed. For single-phase-to-earth faults (the most common type), all three sequence networks are connected in series. For phase-to-phase faults, positive and negative sequence networks are connected in parallel. The zero-sequence impedance is significantly different from positive-sequence impedance for transformers (depending on winding configuration — delta, star, zigzag) and cables (depending on sheath earthing). IEC 60909-0 uses symmetrical components extensively for calculating various fault types and provides rules for determining sequence impedances of system components. Understanding symmetrical components is essential for protection engineers designing earth fault and negative-sequence protection schemes.