Robust Control Design of Converter-Interfaced Resources in Hybrid Electricity Grids under Consideration of Harmonic Stability

• Applicant: Professor Dr.-Ing. Marco Liserre, Ph.D.
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 359982322

The project “Formal stability assessment of hybrid distribution grids based on the correct modeling of the effect of synchronization of the power electronics interfaces” carried out by the University of Kiel and EPFL within the DFG SPP 1984 Phase 1 has focused on the accurate modelling of both static and dynamic phenomena of the synchronization of converter-interfaced distributed generation with particular reference to the generation and propagation of harmonics into the grid. The main achievement of the static analysis has been the development of a general algorithm capable to perform a harmonic power-flow study of polyphase grids with converter-interfaced resources, which uses generic models of the power-system components. In contrast to state-of-the-art approaches, the proposed method explicitly accounts for coordinate transformations between the models of the different components (i.e., grid, power hardware, and controller software). Therefore, the system model is fully modular, that is its components can be described in any suitable frame of reference and can be either grid-forming or grid-following converters. The main achievements of the dynamic analysis have been the accurate modelling of PLLs and studies on power- and self-synchronization techniques. Topics like DC-voltage stability, the impact of communication on the system performance, the design of centralized controllers and their validation have partly been investigated and do represent the central research topics during Phase 2 of the priority program.In particular, the goal of Phase 2 is to investigate interactions of AC and DC controls, which requires to include DC subsystems into the modelling framework developed in Phase 1, and to extend the considered grid-synchronization techniques. It is planned to develop a scalable state-space model that includes the interaction between the synchronization and external control loops (e.g., DC-link and AC-grid voltages) to accurately study the stability issues in the outer-loop time-scale while being easily scalable to multi-converter grids. This model will be the basis of a design framework for a global controller, and will allow to identify the key controller parameters which will need to be tuned in order to ensure stability.Using these methods, the following research questions will be addressed:How can the AC and DC controls and the grid synchronization be optimally designed to achieve robust stability against harmonics?How can communication delays be properly characterized (e.g., minimum/mean/maximum value, jitter, etc.) and what is their impact on the harmonic stability of hybrid power systems?Which control scheme is most robust against harmonic instability in view of grid synchronization and communication: centralized, decentralized, or distributed control?Which are the key controller parameters which need to be tuned to achieve robust performance?

• DFG Programme: Priority Programmes
• Subproject of: SPP 1984: Hybrid and multimodal energy systems: System theoretical methods for the transformation and operation of complex networks
• International Connection: Switzerland
• Major Instrumentation: Communication Network Emulator
• Instrumentation Group: 2720 Impedanz- und Dämpfungsmeßgeräte, Frequenzgangmeßgeräte, Netzwerkanalysatoren
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partner: Professor Dr.-Ing. Mario Paolone, Ph.D.