Asymmetric Power Distribution Analysis of Delta-MMC RPC

AC-electrified railway substations fed through a Scott-T transformer present two single-phase catenary feeders that are 90 degrees apart in phase. Trains operate independently on the two feeders, producing continuous load unbalance and harmonic distortion on the three-phase utility side. This article analyzes a delta-connected modular multilevel converter (MMC) operated as a railway static power conditioner (RPC) for this Scott-T fed system. The principal contribution is a closed-form asymmetric functional decomposition theorem: under the FBD-based compensation reference and the zero-circulating-current design constraint, each arm current decomposes into a linear combination of the M-seat and T-seat harmonic load currents with distinctly asymmetric weights. A corollary of the theorem, the Scott phase law, shows that the 90-degree seat displacement of the Scott-T transformer rotates every odd load-harmonic order to an interseat phase of plus or minus 90 degrees when the two seat rectifiers operate in the same conduction state, pinning the ab-arm share of each odd harmonic at exactly one sixth on this ideal locus, independent of the seat harmonic amplitude ratio, and leaving the bc- and ca-arms to carry the remaining five sixths; on this locus the ab-arm is the strictly smallest harmonic carrier. A seven-point time-domain sweep of the seat-load unbalance confirms the predicted share structure: ab-arm shares of 21 to 22 percent against the one-sixth law value at near-nominal loading, where the measured inter-seat phase lies a few degrees off the ideal locus, and a systematic redistribution, captured by the closed form with the measured load phasors, when the rectifier conduction states differ. A four-block integrated control architecture is also proposed and verified. In PLECS simulation, the proposed control reduces the per-phase source-side current total harmonic distortion from about 28, 24, and 40 percent to the 1 to 2 percent range on each phase. A mini-scale laboratory prototype confirms, at two operating points, that the ab-arm carries the smallest cell-capacitor ripple of the three arms, by a factor of 7 to 11.