Voltage Control of DC Islanded Microgrids: Scalable Decentralised L1 Adaptive Controllers

Abstract

Voltage stability is a critical feature of an efficiently operating power distribution system such as a DC islanded microgrid. Large-scale autonomous power systems can be defined by heterogeneous elements, uncertainty and changing conditions. This paper proposes a novel scalable decentralised control scheme at the primary level of the typical hierarchical control architecture of DC islanded microgrids with arbitrary topology. Local state-feedback L1 adaptive controllers are retrofitted to existing baseline voltage controllers of DC-DC boost converters, which interface distributed generation units with loads. Furthermore, local controller synthesis is modular as it only requires approximate information about the line parameters that couple neighbouring units. The performance of the proposed architecture is evaluated using a heterogeneous DC islanded-microgrid that consists of 6 DC-DC boost converters configured in a radial and meshed topology. The use of L1 adaptive controllers achieves fast and robust microgrid voltage stability in the presence of plug-and-play operations, unknown load and voltage reference changes, and unmodelled dynamics. Finally, sufficient conditions for global stability of the overall system are provided.

Decentralised L1 Adaptive Primary Controllers and Distributed Consensus-Based Secondary Control for DC Microgrids with Constant-Power Loads

“…, and (i, j) ∈ N = {1, ..., n} denotes the set of DGUs, and j ∈ N i = {1, ..., n − 1} denotes the neighbour set of DGU i. From [28], the matrices of (5) for the boost converter are defined as,…”

“…Centralised L 1 AC architectures have been notably successful in safety-critical applications including; sub-scale NASA aircraft auto-pilots [34], manned aircraft [35], and unmanned water/aerial vehicles [36,37]. In addition to our decentralised [28] and distributed [29] L 1 AC architectures, decentralised schemes were formulated in [38,39], and were implemented to augment aircraft baseline controllers [40]. Unlike [34][35][36][37][38][39][40], we consider unmatched interconnections and provide robustness to constant disturbances via integral action.…”

“…Subsequently, the term is reintroduced when GAS conditions are provided. Moreover, as in [28], the integrator provides adequate robustness to unmatched constant disturbances, and therefore ĩl can be represented as,…”

“…Each DGU provides power to a 380 V DC bus which is loaded with a 5 kW linear resistive load and a 3.8 kW non-linear closed-loop controlled DC motor. Each load is interfaced to the bus via buck converters, where the bus voltage is stepped down to 48 V. System parameters are detailed in Table I of [28].…”

“…In [28], we developed a scalable decentralised L 1 adaptive control (L 1 AC) architecture to augment the primary voltage controllers of DGUs. Steady-state robustness guarantees are provided in the presence of arbitrary topology, PnP operations, uncertain couplings and unknown load changes.…”