Energy and grid feedback
Kiel University of Applied Sciences: Institute for Electrical Power Engineering, Prof. Dr.-Ing. Hans-Jürgen Hinrichs
Based on the analysis and evaluation of the resulting grid feedback from the FESH-eHighway system, the effect of the system on the public electricity grid was assessed and a grid expansion strategy for a possible extension of the eHighway was subsequently developed.
Background
The eHighway system was supplied with energy via the public electricity grid, with overhead line hybrid trucks drawing electrical energy in the form of direct current via the overhead line. This overhead line was fed by so-called rectifier substations, which convert the energy from the public grid from alternating current to direct current. When the alternating current is rectified, grid perturbations occur that can affect the stability of the public electrical grid due to voltage dips, harmonics and flicker. The analysis and evaluation of the grid repercussions of the eHighway system has provided information on the influence of the rectifier substations on the grid.
The overhead power supply concept of the eHighway system is based on technologies used in streetcars and trolleybuses. The use of rectifier technology ensures a stable and reliable energy supply. Due to the geographical conditions in Schleswig-Holstein, where there is a high proportion of renewable energy systems that feed in via converter technology, it was investigated which technology (rectifier or converter) is better suited to grids with a high proportion of renewable feed-in. The evaluation of the grid repercussions and the determination of the energy efficiency of both technologies formed the basis of this investigation.
A corresponding grid expansion strategy is also required for the possible expansion of such an eHighway system. This includes identifying the necessary grid connection points and determining the associated connection costs. In cooperation with the local grid operator, the route between Hamburg and Lübeck was examined for possible connection points. Based on this, the grid connection costs for the integration of the rectifier substations were determined.
Analysis and evaluation of the outgoing network reactions of the substations
To record and validate current and voltage curves, measurements were carried out at three selected measuring points in the area of the substations and the grid connection point, both at medium voltage level and at DC voltage level. High-precision voltage and current transformers were used to collect the measurement data.
Network quality and network stability
The analysis of the open-circuit measurement (actual condition recording) and the evaluation of the power quality indicators has shown that the emission limits in the medium-voltage grid are well below the permissible limits. The power quality indicators, which include parameters such as voltage quality, frequency stability and load flow, are used to describe the status and quality of the grid. These indicators are essential for monitoring the performance and reliability of the grid and ensuring that it meets regulatory requirements.
Technology comparison between rectifier and converter for use in grids with high renewable energy content
The study compared the outgoing grid perturbations of both substations (rectifiers and converters) in order to compare the technology. Various load scenarios were run through in order to record and evaluate grid-specific effects (system services) of the medium-voltage grid in addition to the voltage behavior.
In addition, energy balances of various load scenarios from long-term measurements for both technologies (rectifier and converter) were created.
In addition to the technical comparison, the extent to which the converter dynamically supports the grid and can provide short-term power reserves to compensate for grid fluctuations was investigated.
- 30 feet steel container HC
- Inverter cabinets
- Measuring device cabinet
- Medium voltage switchgear
- 30 kV transformer
- Cooling system
- Block foundations
- Grounding system
- Main grounding rail
Development of a network expansion strategy
An analysis of the connection options for direct current substations to the public electricity grid between Hamburg and Lübeck was intended to provide initial indications of which sections might be suitable for closer consideration of a system expansion. Among other things, the grid connection costs were determined in relation to the suitable grid connection points and the voltage level of the grid operator.
A 34 km stretch between Hamburg and Lübeck was considered for the development of a grid expansion strategy:
- Identification of suitable expansion areas with associated grid connection points including voltage level
- Determination of the required cable length in relation to the grid connection of the extension line along public service roads
- Determination of the grid connection indicators for the identification of possible grid connection performance
- Recording of the grid connection costs (reference values) in relation to the specified grid connection points of the grid operator. The grid connection infrastructure costs for the respective line sections were calculated taking into account different variants of the connection levels (voltage level, 20 kV, 30 kV and 110 kV).
Investigations have shown that 26 km of the total 34 km route are suitable for an overhead line extension on the network side without having to overcome any major challenges. It should be noted here that an overhead line system such as the eHighway does not necessarily have to be expanded to cover the entire area, provided that there are sections of the route whose supply can be ensured from the public power grid via nearby towns if necessary.
Closer look at an exemplary section
A suitable mains connection is available, for example. in the village of Meddewade. This would be suitable for supplying power to a 5.2 km section of overhead line between Bad Oldesloe and Klein Barkau. Further prerequisites for such a system expansion are the construction of a grid connection point (NAP) and the laying of a medium-voltage line (between the village of Meddewade and the NAP as well as between the DC substations). The laying of the medium-voltage line of a total length of 3.8 km would be sufficient for such an exemplary section.
