High Frequency Stability Constraints Based MMC Controller Design Using NSGA-III Algorithm

The occurrence of high frequency resonances (HFRs) has been frequently observed in several MMC-HVDC projects. To avoid these HFRs, the controller design of an MMC must satisfy two requirements: 1) The controller should remain stable while in the high frequency range, and 2) MMC impedance should not possess a negative real part in the high frequency range. So far, majority of the related studies on MMC controller design have been unable to address these requirements precisely. This paper first describes the simplified high frequency MMC impedance model developed indigenously by the authors. Subsequently, the driving mechanism of two kinds of HFRs is revealed using the said developed model, including: 1) MMC controller instability, and 2) Interaction instability between MMC and AC cables. Furthermore, the mathematical expressions outlining the controller stability constraint and positive damping constraint are proposed. Additionally, NSGA-III based multi-objective optimization algorithm is adopted, to identify the region most suitable for satisfying the proposed stability constraints under the MMC controller parameters. The proposed controller design method is capable of effectively evading the HFRs triggered by the incompatible MMC. The detailed time-domain simulations generated using PSCAD/EMTDC software validate the proposed designed method and endorse the improved results.