Abstract
Hybrid modular multilevel converter (MMC) topology, employing both half-bridge (HB) and full-bridge (FB) submodules (SMs), has dc fault current limiting capacity. In order to add the capacity of reactive power support to the ac grid during dc fault, higher proportion of FB-SMs are required which results in increase of cost and losses. This work attempts to minimize the proportion of the FB-SMs without compromising the performance. The major obstacle faced is the tendency of MMC arm currents to become unipolar with reduction in the dc voltage. This phenomenon affects the cyclic charge/discharge of the HB-SMs in the MMC which depends on the polarity of the arm currents. A method is proposed in this work where a second harmonic circulating current (SHCC) is injected in each leg of the MMC during the reduced dc voltage conditions. This ensures that the arm currents have both the polarities at any dc voltage level. Hence, the unnecessary increase of FB-SMs can be avoided. The method is tested through simulations performed on PSCAD /EMTDC software platform.
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