Converter-Dominated Grids: Control and Protection Concepts for Ancillary Services of the Future

Abstract:
Because of the change in the future energy system to a more decentralized bidirectional power flow, the power system gets more complex. As conventional power plants continue to decline and the responsibility shifts towards renewable energy sources, decentralized energy resources (DER) will assume a more crucial role in providing ancillary services (AS), to ensure a reliable and safe electric grid. Current controls and protection systems have to be adapted. The first focus is on the provision of AS in the future energy system. Two of the four AS will be specified, the voltage and the frequency stability. Different algorithms will be presented and validated. In the low voltage level, a reinforcement learning controller is developed to provide voltage stability from the grid connection point of the converter. For the frequency stability different controllers are developed. For the control power a two staged controller will provide active power to ensure a stable frequency. Due to the decreasing inertia caused by the reduction of synchronous generators, a grid-forming converter is developed. This control concept, based on a virtual synchronous generator, enables the converter to provide inertia. The various controllers are tested in both software and hardware. After that, the second focus lies on analysing the performance of conventional protection systems in converter-dominated grids. It is shown that especially distance protection systems are negatively influenced by intermediate infeed. Then, it will be evaluated how the converters contribute to AS provision during faults. Next, new developments of both protection systems and converter controls are shown. It is stated that the converter can play an active role in the protection system environment to support the functionality of conventional protection systems. Finally, a methodology together with a testbed is presented for experimental validation of both the findings and developed concepts with a simplified and laboratory-scale testbed of a medium-voltage grid.