Q9Power System - I
Question
Q.9. What do you mean by symmetrical and unsymmetrical spacing?
Answer
Symmetrical spacing means the three conductors of a transmission line are placed at the vertices of an equilateral triangle (equal spacing between all pairs), giving equal and balanced inductance/capacitance per phase, while unsymmetrical spacing has unequal conductor spacing, causing unbalanced inductance/capacitance unless transposed.
In symmetrical spacing, the three phase conductors are arranged at the corners of an equilateral triangle, so the distance between every pair of conductors is identical (Dab = Dbc = Dca); this results in equal self and mutual inductance/capacitance for all three phases, giving an inherently balanced line without requiring conductor transposition. In unsymmetrical (asymmetrical) spacing, commonly used in flat or vertical horizontal configurations for practical tower design reasons, the spacing between conductor pairs differs (Dab ≠ Dbc ≠ Dca), which causes unequal inductance and capacitance in each phase; this imbalance is normally corrected by transposing the conductors (cyclically interchanging their positions at regular intervals along the line) so that each conductor occupies each position for an equal length, making the average inductance/capacitance per phase equal, using the geometric mean distance (GMD) in place of Dab, Dbc, Dca in the standard formulas.