Analysis of DC Harmonics Using the Three-Pulse Model for the Intermountain Power Project HVDC Transmission

The harmonic analysis of the dc-side of an HVDC line transmission requires realistic models of the converters, the dc line, and other relevant equipment. These models must include all important paths for harmonic current, and appropriate sources of harmonic voltage generation. The "classical" converter modeling technique described by Kimbark (1) and others uses the voltage waveform produced by a 6-pulse bridge as the basis for the calculations. Fourier analysis of this waveform produces primarily harmonic voltages of order (6n) (n integer), with harmonics of order (12n) dominant for 12-pulse operation. Discrepancies have been observed between the measured harmonic interference levels and the values which were calculated using classical techniques for several recent HVDC transmissions, including the Intermountain Power Project (IPP) HVDC Transmission. The discrepancies for IPP were manifested in two ways: First, the measurements indicated a much larger ground mode current flowing on the electrode line portions of the HVDC line than the original calculations had predicted. Second, the spectrum of the harmonic currents in the field measurements indicated dominant harmonic currents of order [3(2n + 1)]. For these reasons, a new model of the converter bridge which takes into account the major stray capacitances of the converter (the "Three-Pulse" model) has been developed, and is described in detail in (2). In this paper, comparisons between the classical and three-pulse calculations are presented for IPP. An example of calculated induced voltage (mV/km) versus distance along the IPP dc line is presented in Fig. 1.