A Three-Pulse Model of D.c. Side Harmonic Flow in HVDC Systems

This paper presents a new model for the analysis of d.c. side harmonic current flow in HVDC systems. In recent years field measurements of d.c. side harmonic currents and induced voltages on parallel test lines have indicated the presence of triplen order harmonics of high magnitude. Such harmonics are not predicted by a conventional analysis of d.c. harmonics. An increase in the level of characteristic 12-pulse harmonics above the calculated values has also been noted. The discrepancy between calculated and measured values of harmonic current is attributed to the omission from the classical model of the ground path formed by the various stray capacitances of the converter transformers, bushings, and buswork. The new model proposed by the authors includes this stray capacitance. As it is essential to model correctly the harmonic driving voltages between the converter terminals and the stray capacitances, each 12-pulse converter is represented in the new model as four three-pulse harmonic voltage sources connected in series, as shown in Figure 1. The paper derives the harmonic voltages for each threepulse source and justifies the representation of stray capacitances and the internal impedance of the converter. A detailed analysis is given of the flow of harmonic currents of both odd and even triplen orders, in balanced and unbalanced bipolar and monopolar operation.