Decentralized Control of DC Microgrids: Introducing the State-of-Grid Concept: How the Principles of Communicating Vessels Could Be Used in DC Microgrid Control
Authors: Veit Hagenmeyer
Extended Abstract:
Modern energy systems are increasingly shaped by renewable generation, battery energy storage, electric mobility, and the electrification of industrial and residential loads. Many of these technologies are inherently based on direct current, which makes DC microgrids a promising architecture for future energy distribution. However, the reliable and scalable operation of DC and hybrid AC/DC microgrids requires effective coordination among distributed storage units, renewable sources, loads, and interlink converters. Conventional approaches often rely on centralized control, supervisory communication, or predefined signaling schemes, which can increase cost, complexity, communication requirements, and vulnerability to failures.
The article introduces the State-of-Grid concept as a decentralized and communication-free control principle for DC microgrids. The main idea is inspired by the physical principle of communicating vessels. In such a system, fluid levels naturally equalize through pressure differences without the need for external coordination. Analogously, in the proposed control concept, the state of charge of each battery is mapped to a small deviation of its local voltage reference. A battery with a higher state of charge slightly increases its voltage reference, while a battery with a lower state of charge slightly reduces it. As a result, energy naturally flows from more highly charged units to less charged units through the electrical network, leading to autonomous state-of-charge balancing and power sharing.
This mechanism allows each converter to react only to locally measurable quantities, while the common DC bus voltage becomes an aggregated indicator of the overall energy state of the microgrid. The concept therefore avoids explicit data exchange between units and does not require centralized dispatch or complex communication infrastructure. In addition to DC-side balancing, the article shows how the same principle can be extended to hybrid AC/DC systems by mapping the State-of-Grid to small frequency deviations on the AC side. This enables coordinated power exchange and state-of-charge balancing across DC and AC domains.
The article further discusses the mathematical basis of the concept, including the mapping between state of charge, voltage references, and the aggregated State-of-Grid. It also presents experimental validation using the Smart2DC laboratory infrastructure at the Karlsruhe Institute of Technology. The experiments demonstrate autonomous balancing under different operating conditions, including changing photovoltaic generation, varying load demand, unequal battery capacities, asymmetric line resistances, and hybrid AC/DC operation with storage units on both sides of the interlink converter. The results show that the State-of-Grid concept can maintain stable voltage and frequency behavior, achieve dynamic power sharing, and balance storage units without centralized communication.
Overall, the State-of-Grid concept provides a simple, scalable, and resilient approach for decentralized microgrid control. By embedding energy-state information directly into electrical variables such as voltage and frequency, the method allows distributed energy resources to coordinate in a natural and self-organized way. This makes it particularly relevant for future DC distribution systems, residential and industrial microgrids, hybrid AC/DC infrastructures, and converter-dominated energy systems.
Additional information
The work is part of ongoing research at the Karlsruhe Institute of Technology on decentralized control methods for DC and hybrid AC/DC microgrids. The experimental validation was carried out using the Smart2DC microgrid laboratory, which provides a flexible platform for investigating DC distribution, power electronic converters, battery energy storage systems, photovoltaic integration, controllable loads, and hybrid AC/DC coupling. The broader research objective is to develop communication-free, scalable, and experimentally validated control concepts for future converter-dominated energy systems.
For the representative figure, we suggest using a figure that illustrates the analogy between communicating vessels and the electrical State-of-Grid concept, since this visual directly conveys the main idea of the article.
represents PV power, an outlet valve corresponds to a dc load, and a pump symbolizes the ac
grid connection