DC-Side Harmonic Currents Calculation and DC-Loop Resonance Analysis for an LCC-MMC Hybrid HVDC Transmission System

For an LCC-MMC hybrid HVDC transmission system with reverse current blocking diodes, this paper presents a method for dc-side harmonic current calculation and dc-loop impedance calculation. First, in the calculation of dc-side harmonic currents, the line-commutated converter at the rectifier side is replaced by the three-pulse harmonic voltage sources; the modular multilevel converter is represented by an equivalent passive circuit based on the linearization theory; an improved calculating method for the coupled line model is introduced into calculating the admittance matrix of the dc transmission lines to improve the computational efficiency; and then this paper describes the complete procedures of the proposed method. Second, based on the dc-side equivalent models mentioned before and the nodal voltage analysis method, this paper presents an analytical method for the calculation of dc-loop impedance according to the definition of dc-loop impedance Finally, the PSCAD/EMTDC simulation verifications have been carried out based on a 1500 MW/+500 kV MMC-HVDC system, a 3000-MW/+/- 500-kV LCC-MMC hybrid HVDC system, and its dc network. The simulation results and the analytical results coincide with each other, and the effectiveness of the proposed methods is proved.