Designfaktoren für den stabilen Inselnetzbetrieb in NS-/MS-Netzen

Willenberg, Dominik; Monti, Antonello (Thesis advisor); Raisz, David (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

In the event of supply interruptions to the superordinate grid, the increasing number of distributed generation (DG) units can be used to continue supplying consumers on site until the grid has been restored via the transmission grid. Current concepts focus on dominant grid-forming units, e.g. hydropower plants. The use of a single existing DG unit as a non-dominant grid-forming unit enables transferability to a larger area of the existing grid infrastructure. However, this islanded micro grid operation poses challenges with regard to the dynamics in the event of unforeseen power changes. In addition, the generally low availability of detailed grid data makes it difficult to determine suitable existing low and medium voltage grids for islanded micro grid operation. In this work, design factors for the stable operation of temporary low and medium voltage islanded micro grids are defined, with the help of which the suitability of existing grid areas for islanded grid operation can be determined on the basis of accessible data, e.g. generation capacity, consumption and grid connection technology. For this purpose, first, generic plant and control models for existing DG units are validated on the basis of laboratory tests. The models used and further developed within the scope of this work represent the relevant dynamic component behaviour with power changes sufficiently accurately. Thus, the approach proves the applicability of generic models for an analysis and evaluation of the dynamics of stable islanded micro grid operation during power changes. Due to the lower inertia of rotating DG units, the grid frequency is the critical grid parameter in terms of stability. In combination with the active power frequency control of existing, grid-following DG units, a reduction of the maximum permissible power change by up to 38 % can result due to control interactions. The use of innovative concepts (synthetic inertia) in wind turbines enables an increase in the maximum permissible power change by up to 47 %. When using grid-forming battery inverters, the grid frequency and stable operation are influenced to a negligible extent as long as the power of grid-following DG units does not exceed that of the grid-forming unit. Based on the studies, an analytical approximation of the maximum frequency deviation of rotating grid-forming units is derived. The maximum frequency deviation is mainly determined by the power change, the nominal power, the inertia time constant and a machine or control time constant. As a result of the studies conducted, the nominal power of the grid-forming and grid-following DG units, the associated control and prime mover technology, and the maximum power change are defined as design factors. A share of grid-following, inverter-coupled DG units of 68 % or higher with a rotating grid-forming unit can lead to unstable islanded micro grid operation.

Institutions

• E.ON Energy Research Center [080052]
• Chair of Active Energy Distribution Grids [614010]
• Institute for Automation of Complex Power Systems [616310]