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A novel hybrid HVDC grid configuration consisting of line commutated converters (LCCs) and voltage source converters (VSCs) is investigated in this paper. The proposed hybrid HVDC grid can be used to deliver power to intermediate urban/remote load centers or collect power from remote/renewable generation at sites along a long LCC transmission line. An electromagnetic transients (EMT) simulation model of a four terminal HVDC grid was developed together with master controls required to maintain the desired power flow. Two possible operating modes were investigated. In the first mode, two VSCs tap into an LCC HVDC link. In the second mode, two VSCs share the surplus transmission capacity of an LCC HVDC line for point-to-point power transfer. Control strategies to start-up the converter stations, energize the grid, and controls to achieve the desired power flow between converter stations are proposed. With the proposed controls and energization sequence, smooth operation of the whole grid could be achieved for both modes of operation.
Naushath et al. (Sun,) studied this question.