Stability Analysis of Converter Control Strategies for Power Electronics-Dominated Power Systems
The electric power system, whose well-established structure consolidated over decades of studies is composed of large centralized generating units, transmission systems, and distributed loads, is currently experiencing a significant transformation, posing new challenges for its safe operation in the near future. The increasing amount of grid-connected power electronics-based converters associated with renewable energy sources, is reducing the amount of energy produced by means of conventional generating units, generally represented by large synchronous machines (SMs) directly connected to the grid. As a consequence, declining system inertia, as well as reduced fault currents affecting short-circuit level and retained voltage under fault conditions, are expected. This has caused concerns among system operators (SOs) worldwide about the stability of the future power system, triggering discussions in different countries about the need for new converter control strategies, which would allow safe system operation under the expected grid configuration. In this scenario, the concept of ”grid-forming (GFM) converters” has been recently proposed as a possible solution allowing high-penetration of power electronics-based generation. Initially introduced in the context of microgrids, the concept of GFM converters needs to be reviewed for applications in wide interconnected systems. Indeed, at the present time, a well-established formulation is still missing in the literature, and several committees worldwide are currently working on a definition for identifying the characteristics of such converters. Due to the initial concern of SOs related to declining system inertia, the concept of GFM converters has been often associated with the idea of virtual inertia, and namely the emulation of a synthetic inertial response by means of a power electronics-based converter. Yet, this is only one aspect related to the increase of power electronics-based generation, and the concept of a GFM converter includes other features, which, however, need to be properly specified in order to provide clear guidelines for manufacturers aiming to the development of suitable converter control strategies. This thesis addresses the topic of GFM converters from a control perspective, and aims to characterize potential features, as well as the relevant issues related to this technology. First, the characteristics of a GFM converter are identified according to an extensive literature overview, so that by reviewing international practice on this technology, a general formulation for a GFM converter control structure is identified. Particular emphasis is given to the synchronization principle adopted by the converter which, contrary to state-of-the-art grid-connected converters adopting a dedicated unit for grid synchronization purposes, is generally achieved in a GFM converter by reproducing the power-synchronization mechanism of a SM. An extensive small-signal stability analysis is performed in order to identify the implications of the identified converter behaviour on converter stability, as well as the effects due to the interactions between converters operating nearby. Finally, potential issues related to the implementation of a GFM converter are highlighted, and possible solutions are proposed, whose effectiveness is validated by means of hardware-in-the-loop (HIL) simulations, as well as experimental tests in a laboratory environment, by adopting power-HIL (PHIL) test benches.
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