[Paper Review] A scalable line-independent design algorithm for voltage and frequency control in AC islanded microgrids

Michele Tucci, Giancarlo Ferrari‐Trecate|arXiv (Cornell University)|Mar 7, 2017

Microgrid Control and Optimization9 references3 citations

TL;DR

This paper presents a scalable, line-independent decentralized control algorithm for voltage and frequency regulation in AC islanded microgrids, enabling Plug-and-Play operations without retuning existing controllers when new units are added. The method ensures global stability via a Lyapunov-based design using only local DGU parameters, with stabilizing controllers always computable via LMI optimization regardless of electrical parameters.

ABSTRACT

We propose a decentralized control synthesis procedure for stabilizing voltage and frequency in AC Islanded microGrids (ImGs) composed of Distributed Generation Units (DGUs) and loads interconnected through power lines. The presented approach enables Plug-and-Play (PnP) operations, meaning that DGUs can be added or removed without compromising the overall ImG stability. The main feature of our approach is that the proposed design algorithm is line-independent. This implies that (i) the synthesis of each local controller requires only the parameters of the corresponding DGU and not the model of power lines connecting neighboring DGUs, and (ii) whenever a new DGU is plugged in, DGUs physically coupled with it do not have to retune their regulators because of the new power line connected to them. Moreover, we formally prove that stabilizing local controllers can be always computed, independently of the electrical parameters. Theoretical results are validated by simulating in PSCAD the behavior of a 10-DGUs ImG.

Motivation & Objective

• To develop a decentralized control synthesis method for voltage and frequency stability in AC islanded microgrids (ImGs) that supports Plug-and-Play (PnP) operations.
• To eliminate dependency on power line parameters in local controller design, ensuring that adding or removing DGUs does not require retuning neighboring controllers.
• To guarantee global stability of the ImG under arbitrary electrical parameters, without requiring bounds on coupling parameters or global tuning constraints.
• To enable the use of identical regulators for DGUs with the same electrical parameters, allowing pre-computation and off-line deployment.
• To formalize a stability proof using a separable Lyapunov function and LaSalle’s invariance principle, ensuring robustness and feasibility.

Proposed method

• The control design uses a reparametrization of local controllers and a Lyapunov function structured around the admittance matrix of the electrical network, which has a Laplacian structure.
• Stability is proven by showing the Lyapunov function decreases along trajectories and applying LaSalle’s invariance principle to identify the largest invariant set.
• Local controllers are synthesized via Linear Matrix Inequality (LMI) optimization, using only the parameters of the individual DGU and a scalar global parameter.
• The method ensures line-independence: neighboring DGUs do not need to update their regulators when new lines are added or removed.
• The design is modular and scalable, with identical DGUs receiving the same controller, computable off-line once.
• The approach avoids reliance on precise knowledge of line parameters or bounds on coupling, unlike prior PnP methods.

Experimental results

Research questions

• RQ1 Can a decentralized control algorithm for AC islanded microgrids be designed to be independent of power line parameters while ensuring global stability?
• RQ2 Does the proposed method allow Plug-and-Play operation without requiring retuning of existing controllers when new DGUs are added or removed?
• RQ3 Can stabilizing local controllers always be computed regardless of the electrical parameters of the DGUs and network topology?
• RQ4 Is it possible to achieve global stability using a separable Lyapunov function and LaSalle’s invariance principle in a decentralized control framework?
• RQ5 Can the control design be made modular such that identical DGUs use the same controller, computed off-line?

Key findings

• The proposed control algorithm is line-independent: local controller synthesis depends only on the DGU’s own parameters, not on connected line models.
• Plug-and-Play operations are fully supported: adding or removing a DGU does not require retuning any existing controller, even if it connects to neighbors.
• Stabilizing controllers can always be computed via LMI optimization, regardless of the electrical parameters of the DGUs or network.
• The stability proof is based on a separable Lyapunov function and LaSalle’s invariance principle, ensuring convergence to the origin under the proposed control law.
• The method removes the need for a small global tuning parameter, unlike prior PnP designs, enhancing robustness and design flexibility.
• Simulation results in PSCAD validate the approach on a 10-DGU microgrid, confirming stability and performance under dynamic plug-in/out events.