The increased penetration of Distributed Energy Resources (DER) is challenging the entire architecture of conventional electrical power system. Microgrid paradigm, featuring higher flexibility and reliability, becomes an attractive candidate for the future power grid. In this paper, an overview of microgrid configurations is given. Then, possible structure options and control methods of DER units are presented, which is followed by the descriptions of system controls and power management strategies for AC microgrids. Finally, future trends of microgrids are discussed pointing out how this concept can be a key to achieve a more intelligent and flexible power system.

A Review of Power Electronics Based Microgrids

Fundamentals of power systems modelling in the presence of converter-interfaced generation

The challenges of achieving a 100% renewable electricity system in the United States

Performance assessment of grid-forming and grid-following converter-interfaced battery energy storage systems on frequency regulation in low-inertia power grids

To counteract a potential reduction in grid stability caused by a rapidly growing share of intermittent renewable energy sources within our electrical grids, large scale deployment of energy storage will become indispensable. Pumped hydro storage is widely regarded as the most cost-effective option for this. However, its application is traditionally limited to certain topographic features. Expanding its operating range to low-head scenarios could unlock the potential of widespread deployment in regions where so far it has not yet been feasible. This review aims at giving a multi-disciplinary insight on technologies that are applicable for low-head (2-30 m) pumped hydro storage, in terms of design, grid integration, control, and modelling. A general overview and the historical development of pumped hydro storage are presented and trends for further innovation and a shift towards application in low-head scenarios are identified. Key drivers for future deployment and the technological and economic challenges to do so are discussed. Based on these challenges, technologies in the field of pumped hydro storage are reviewed and specifically analysed regarding their fitness for low-head application. This is done for pump and turbine design and configuration, electric machines and control, as well as modelling. Further aspects regarding grid integration are discussed. Among conventional machines, it is found that, for high-flow low-head application, axial flow pump-turbines with variable speed drives are the most suitable. Machines such as Archimedes screws, counter-rotating and rotary positive displacement reversible pump-turbines have potential to emerge as innovative solutions. Coupled axial flux permanent magnet synchronous motor-generators are the most promising electric machines. To ensure grid stability, grid-forming control alongside bulk energy storage with capabilities of providing synthetic inertia next to other ancillary services are required. 

Low-head pumped hydro storage: A review of applicable technologies for design, grid integration, control and modelling

In this paper, different control approaches for grid-forming inverters are discussed and compared with the grid-forming properties of synchronous machines. Grid-forming inverters are able to operate AC grids with or without rotating machines. In the past, they have been successfully deployed in inverter dominated island grids or in uninterruptable power supply (UPS) systems. It is expected that with increasing shares of inverter-based electrical power generation, grid-forming inverters will also become relevant for interconnected power systems. In contrast to conventional current-controlled inverters, grid-forming inverters do not immediately follow the grid voltage. They form voltage phasors that have an inertial behavior. In consequence, they can inherently deliver momentary reserve and increase power grid resilience.

Overview on Grid-Forming Inverter Control Methods

In order to ensure reliable power supply and stable grid operation in future, predominantly distributed power grids, a trend to increase the grid-forming capability of distributed converters has been proposed [1]. Besides grid-forming control methods, an appropriate current limiting is required which, on the one hand, protects the converter against undesired over-currents for all present scenarios (e.g. fault ride through (FRT), significant frequency and phase angle fluctuations) and, on the other hand, does not affect normal operation. This paper presents a current limiting method for grid-forming power converters based on limiting the provided voltage phasor of the grid-forming control in a defined area with respect to the measured output voltage phasor. The permissible area is derived from the internal impedance and in addition allows the prioritization of different quantities (e.g. active and reactive power). In a laboratory test setup, the current limiting method is verified for symmetrical LVRT, phase angle jump as well as defined frequency changes.

Voltage Phasor Based Current Limiting for Grid-Forming Converters

This paper demonstrates the potential of an enhanced grid-forming control for grid-forming converters, to optimize the interconnection process of black started microgrids (MG) after a global blackout. A point of common coupling switch (PCC switch) is proposed, which does not require any higher-level control entities (e.g. MG-controller) since the switching condition only considers phase angle differences, which enables the interconnection of unintended MGs. The verification of the grid-forming control for enhanced interconnection process with overlaid grid is performed by the presented simulation study.

Peter Unruh

Papers

Overview on Grid-Forming Inverter Control Methods

Voltage Phasor Based Current Limiting for Grid-Forming Converters

Grid-Forming Control for Enhanced Microgrid Interconnection