An Improved Dual Grid-Forming MMC with Port-Hamiltonian Structure

The operation of converters interconnecting ac and dc grids in future inertia-free and weak networks poses significant challenges. Modular multilevel converters applying dual grid-forming controls are able to effectively synchronize to weak grids and control the dc voltage levels simultaneously. This paper proposes an enhanced dual grid-forming strategy based on an equivalent voltage representing the converter’s internal energy. The converter’s control variables have been redesigned in order to improve system stability margins while at the same time preserving the macroscopic port-Hamiltonian structure of the converter. The correct operation of the converter for different grid inertia constants has been proved through small-signal and time-domain simulations, including inertia-free conditions. Moreover, the system is stable at both very weak and strong grid conditions, with short circuit ratios from 1 to 30.