Reactive power grid adequacy studies for distribution grids with high distributed generation

The study aims to develop practicable grid planning procedures for an advanced reactive power management at the transmission-distribution (T-D) interface by making use of controllable reactive power sources at the distribution level, like distributed energy resources (DER) and reactive power compensators. In Germany, also the high voltage level is mainly considered as a part of distribution grid, and the T-D interface is determined at the interface between high voltage and extra high voltage level. A predefined reactive power range at the T-D interface is considered and the term reactive power grid adequacy is subsequently used, to describe the compliance level of the distribution level with the predefined reactive power range at the T-D interface. For this reason, the applicability of common methods in (active power) grid adequacy studies on research questions in reactive power grid adequacy studies are analyzed and discussed. The reactive power grid adequacy study is performed for a real German distribution grid section with very high distributed generation. In a first step, two different assessment approaches are developed, with a technical assessment, based on detailed annual power flow simulations, and a theoretical assessment, based on an analytic assessment of coincident measurement data at the T-D interface and the distributed generation. In a next step, the active and reactive power transfers at the T-D interface and the underlying substations are analyzed and relevant impact factors, correlations and dependencies are identified. Based on this assessment, the reactive power demand of the underlying distribution grid is characterized and an estimate on the future reactive power demand at the distribution level is provided. In a next step, transparent and practicable performance indicators for DER reactive power support at the T-D interface are developed and applied. The proposed performance indicator can be used to identify interesting DER types and DER reactive power capability characteristics for an advanced reactive power management at the T-D interface. Finally, a techno-economic assessment tool for the additional reactive power compensation demand at the distribution level is developed. The tool bases on a detailed assessment of the annual duration curve of the reactive power transfer at the T-D interface with or without DER reactive power support and enables a comprehensive sensitivity analysis of different cost and simulation assumptions. For the investigated case study, a relevant DER reactive power support potential at the T-D interface is determined and DER reactive power support could relevantly reduce the additional reactive power compensation demand at the distribution level. Nevertheless, the effectiveness of DER reactive power support is highly sensitive on the applied case study and depends among other things especially on the applied requirements at the T-D interface. The developed planning approaches can be adjusted for different requirements at the T-D interface and are applicable for different case study areas.