Grid-Forming AC/DC Converters for Meshed DC Networks via Dual-Measurement Frequency Droop

Abstract

This paper proposes and analyzes a dual-measurement frequency-droop (DMFD) controller for meshed MVDC networks. Using only two local DC measurements, bus-voltage deviation and a filtered DC-port current or power deviation, the controller enables grid-forming operation at both AC and DC ports across the mesh. Although the DMFD regulates neither the DC voltage nor the DC current directly and acts only on the terminal frequency, it is shown that AC-side synchronization closes an effective DC droop at every synchronized terminal, so that the steady operating point of the mesh exists and is unique. Constant frequency deviations at a terminal and single-terminal islanding enter the same DC equations as equivalent current offsets, with the island frequency fixed by the island power balance, allowing node voltages and branch currents to be obtained from one linear system. The steady-state model yields design-use envelopes to map the droop ratios and the net DC-injection levels into bounds on DC-voltage deviation and terminal currents. A small-signal dynamic model is developed and used to generate eigenvalue loci and damping-ratio charts that guide controller-gain selection. Averaged EMT simulations on a four-terminal ring corroborate the analysis: in regimes where PLL-based grid-following control is known to lose damping, the DMFD controller maintains stable operation and follows the predicted steady-state characteristics.