On MPC-based Strategies for Optimal Voltage References in DC Microgrids

Modern power systems are characterized by low inertia and fast voltage dynamics due to the increase of sources connecting via power electronics and the removal of large traditional thermal generators. Power electronics are commonly equipped with fast controllers that are able to reach a desired voltage setpoint within seconds. In this paper, we propose and compare two approaches using Model Predictive Control (MPC) to compute optimal voltage references for the power electronic devices in order to minimize the losses in a DC microgrid: i) a traditional setpoint-tracking MPC which receives a previously computed optimal setpoint; ii) an economic MPC which does not require a priori computed setpoints. We show that the economic MPC outperforms the setpoint-tracking MPC in simulations with the CIGRE benchmark system when multiple load disturbances occur. Some insights and discussions related to the stability of the closed-loop system using its dissipativity properties are highlighted for both approaches.

I. Introduction

In the last years, DC microgrids (MGs) have become technically feasible due to the recent advances in semiconductor converter technology [1]. The problem of maintaining constant voltage levels in DC MGs under varying load conditions is well studied in literature and it is called primary control. On the one hand, droop-based methods [2], [3] are widely-used decentralized approaches and exhibit favorable properties such as (limited) power-sharing. Several improvements such as nonlinear, adaptive or dead-band droop have also been proposed, as summarized in [4]. However, these methods show load-dependent voltage deviation and steady-state voltage offsets, which need to be compensated by a higher level control. On the other hand, passivity-based controllers tackling the shortcomings of droop-based approaches have been proposed recently [5], [6]. These regulators achieve an offset-free regulation of a given voltage reference and exhibit advantageous plug-and-play properties for distributed generation units (DGUs) while guaranteeing overall asymptotic stability via passivity. Similar stability properties can only be achieved with droop-based methods by simplifying the system models with questionable assumptions and approximations [3]. However, these passivity-based controllers necessitate a secondary control to achieve power-sharing or coordination.