Stability Analysis of DC Distribution Systems with Droop-Based Charge Sharing on Energy Storage Devices

Abstract

Direct current (DC) distribution systems and DC microgrids are becoming a reliable and efficient alternative energy system, compatible with the DC nature of most of the distributed energy resources (DERs), storage devices and loads. The challenging problem of redesigning an autonomous DC-grid system in view of using energy storage devices to balance the power produced and absorbed, by applying simple decentralized controllers on the electronic power interfaces, is investigated in this paper. To this end, a complete nonlinear DC-grid model has been deployed that includes different DC-DERs, two controlled parallel battery branches, and different varying DC loads. Since many loads in modern distribution systems are connected through power converters, both constant power loads and simple resistive loads are considered in parallel. Within this system, suitable cascaded controllers on the DC/DC power converter interfaces to the battery branches are proposed, in a manner that ensures stability and charge sharing between the two branches at the desired ratio. To achieve this task, inner-loop current controllers are combined with outer-loop voltage, droop-based controllers. The proportional-integral (PI) inner-loop current controllers include damping terms and are fully independent from the system parameters. The controller scheme is incorporated into the system model and a globally valid nonlinear stability analysis is conducted; this differs from small-signal linear methods that are valid only for specific systems, usually via eigenvalue investigations. In the present study, under the virtual cost of applying advanced Lyapunov techniques on the entire nonlinear system, a rigorous analysis is formulated to prove stability and convergence to the desired operation, regardless of the particular system characteristics. The theoretical results are evaluated by detailed simulations, with the system performance being very satisfactory.

Investigation of the Impact of Large-Scale Integration of Electric Vehicles for a Swedish Distribution Network

“…Energy storage is one the most promising proposed techniques that is mostly combined nowadays to AC/DC microgrids because offers several alternatives ranging from simple energy storage services to voltage stability [39] and energy management solutions [40]. In addition, optimal charging controllers for coordinated charging of EVs [41], decentralized control schemes in order to enhance AC/DC microgrids stability [42], as well as artificial intelligent methods (AI) such as machine learning (ML) open new horizons for the power systems management [43].…”

Stability of DC Distribution Systems: An Algebraic Derivation

“…This approach does not provide rules for stability and often provides little (mathematical) insight into the origin of the (in)stability. Thirdly, an analysis based on Lyapunov methods can be conducted [28][29][30][31][32]. However, these are not easily applied and require a suitable construction of the Lyapunov storage function.…”

Robust optimisation‐based order reduction and stability analysis of autonomous DC microgrid with consideration of non‐linearity

“…The duty cycle ratios obtained as the normalised droop output pulses for PV (Δd) and fuel cell (d fc ) are in (13) and (14). The duty cycle ratios obtained as the normalised droop output pulses for PV (Δd) and fuel cell (d fc ) are in (13) and (14).…”

“…A mathematical model of storage batteries connected to bidirectional DC/DC converter 14 for each of the sources, as in Figure 4, is derived from Kirchhoff's current and voltage laws given in (4) and (5).…”

“…Similar equations are formed for fuel cell-converter droop dynamics with restoration terms as δi 2 and δv 2 and corresponding states μ fc and η fc . The duty cycle ratios obtained as the normalised droop output pulses for PV (Δd) and fuel cell (d fc ) are in (13) and (14).…”