Optimal Steady-State Secondary Control of MT-HVdc Grids with Reduced Communications

TL;DR

This paper introduces a centralized secondary control method for multi-terminal HVdc grids that optimizes steady-state operation with reduced communication, ensuring convergence to optimal voltage setpoints.

Contribution

It presents a novel primal-dual based secondary control with a communication triggering mechanism for HVdc grids, reducing communication needs while achieving optimal steady-state operation.

Findings

The control converges to the optimal steady state in simulations.

Reduces communication traffic between control units and converters.

Effective in minimizing current and reducing losses in case studies.

Abstract

In this paper, we propose a centralized secondary control for the real-time steady-state optimization of multi-terminal HVdc grids under voltage and current limits. First, we present the dynamic models of the grid components, including the modular multilevel converter (MMC) stations and their different control layers. We also derive the quasi-static input-output model of the system, which is suitable for the steady-state control design. Second, we formulate a general optimization problem using this quasi-static model and find the Karush-Kuhn-Tucker optimality conditions of its solutions. Third, we propose a secondary control based on primal-dual dynamics to adjust the voltage setpoints of the dispatchable MMCs, with which the system asymptotically converges to a steady state that satisfies these optimality conditions. Fourth, we provide a communication triggering mechanism to reduce the communication traffic between the secondary control unit and the MMC stations. Finally, we verify our proposal for different case studies by adapting it to an offshore multi-terminal HVdc grid composed of heterogeneous MMC stations simulated in the MATLAB/Simulink environment. The problems of proportional current minimization and loss reduction are two special case studies.