Operating Region of Modular Multilevel Converter for HVDC with Controlled Second-Order Harmonic Circulating Current: Elaborating P-Q Capability

The operating region of the modular multilevel converter (MMC) is limited by the maximum-allowable voltage ripple of cells, which is associated with the arm energy variation. Injecting the controlled second-order harmonic component into the circulating current can reduce energy variation of the arm and, thus, extend the operating region of MMC. Care must be taken, however, that the second-order circulating current affects the arm current, and inappropriately injected harmonic current may cause an undesirable arm current polarity change. Therefore, this paper first investigates and proposes the maximum-allowable second-order circulating current for extending the operating region reliably and efficiently. The active and reactive power capability of MMC is further elaborated and illustrated in line with the maximum excess energy capability for two cases, running with: 1) the original operating region and 2) the extended operating region. Finally, the P-Q diagrams for contingent operating conditions with faulty or disabled submodules are presented. Understanding the unique shape and change of P-Q capability with credible submodule failure contingency should be crucial for planning MMC-HVDC lines, and determining the energy requirements and the level of redundancy in submodules properly in order to achieve the envisioned benefits of the new HVDC. The efficacy and accuracy of the research findings are validated for the MMC-HVDC system using PSCAD/EMTDC.