On Stability of Sustainable Power Systems: Network Fault Response of Transmission Systems with Very High Penetration of Distributed Generation

Power systems are undergoing a historic structural and technological transformation. The increase of distributed generation (DG), recently mostly wind power park modules (WPPMs) and photovoltaic power park modules (PVPPMs), is already changing the way power systems are structured and operated. Distribution systems are turning from ‘passive’ into ‘active’ parts of the system (ADS). This structural and technological transformation influences the power system’s network fault response and stability.
This thesis investigates the network fault response of transmission and distribution systems with very high penetration of distributed generation. The impact of DG on transient stability, large disturbance voltage stability, and frequency stability is analysed. The analysis is limited to symmetrical, three-phase network faults only but extended to adequately represent all voltage levels, including DG connected at low voltage (LV). Requirements for the response of DG to network faults are defined in grid connection requirement (GCR).
The massive insertion of DG into distribution systems (DSs) leads to new challenges like the regular occurrence of reverse power flow (RPF) from the distribution to transmission level etc. Further, DG is more likely to be exposed to fault-induced delayed voltage recovery (FIDVR) events than transmission and sub-transmission connected generating facilities. The overall objective of this thesis is, therefore, to critically review the necessity and the specification of current and proposed grid connection requirements with regard to the network fault response of transmission systems with very high penetration of distributed generation and to propose changes to the specification where needed.
The scientific contributions of this thesis are (1) a proposed comprehensive methodology of aggregation of DGs and dynamic equivalencing to derive highly accurate dynamic equivalent models of ADSs that considers the composition of ADS with DG classes in terms of their technology type and grid code performance (performance legacy); (2) a case study demonstrating that nowadays undervoltage protection schemes for small- and medium-scale DG connected in LV distribution networks may become a risk for power system stability; (3) identification of minimum requirements and improvement of existing grid connection requirements for the network fault response of DG to maintain power system stability.