Effects of variable grid fees on distribution grids with optimized bidirectional battery electric vehicles

In the future, electric vehicles will lead to an additional load on the distribution grids and an increased need for grid expansion 1. Especially if the great flexibility of bidirectional electric vehicles is used for market-based trading in the case of vehicle-to-grid use cases (V2G) 2. Grid status-dependent, variable grid fees can be one way to use the flexibility of bidirectional electric vehicles for better grid integration of electromobility. This is being investigated in the “Bidirectional charging management” (BDL) project at the FfE among others 3.

A grid state-dependent variable grid fee could result in 24 % less grids being overloaded. This is shown by a comparison of a mixed scenario for the year 2040 with and without variable grid fees for the most effective parameter combination, which was determined within the scope of this study. For the mixed scenario (MS), it was assumed that approx. 30 % of the electric vehicles charge bidirectionally in a price-optimised manner. Of the bidirectional electric vehicles, approx. half participate in the V2G use case and therefore react to variable grid fees in combination with variable market prices. The other half increases self-consumption in buildings (V2H) in combination with PV systems. Demand-led direct charging is assumed for approx. 70 % of the electric vehicles. The grid load was determined with the distribution grid simulation model GridSim in combination with an optimisation model at building level. The variable grid fees were calculated based on a transformer load forecast for each low-voltage grid and raised by one level each based on two defined transformer utilisation limits.

Decisive parameters for the calculation of the grid fees, such as the forecast quality, the transformer utilisation limit and the grid fee spread, were examined within the scope of a sensitivity analysis. As a result, it can be seen in Figure 1 that the quality of the forecast has the greatest influence on the grid relief. However, a larger spread of the grid fees and a higher transformer utilisation limit also further reduce the share of overloaded grids. In a final sensitivity, it was assumed that all electric vehicles are price-optimised based on variable grid fees. Although this leads to the most relieved grids (36 % relieved), it is also considered unrealistic.


Impact on overloaded grids by variable grid fees
Figure 1: In the considered sample of 189 grids in the "mixed scenario" (MS), in which electric vehicles (EVs) are also the cause of grid congestion, the influence of load forecast quality, grid spread, transformer utilisation limit and share of participating EVs was examined.

The fact that most grids cannot be fully relieved is due to several effects. On the one hand, in grids with a high, long-lasting base load, which is mainly caused by inflexible heat pumps, grid fees are high for several days at a time and thus lose their grid-serving incentive compared to the market price. On the other hand, the grid fee spreads can also be completely overlaid by market spreads, so that they are no longer significant. Furthermore, line congestion and voltage violations contribute to grid congestion, but are not included in the calculation of the variable grid fees.

Overall, the model shown for variable grid fees has already enabled a significant share of flexible loads to be activated to serve the grid. The inclusion of additional future consumers, such as heat pumps, in the variable grid charge system can strengthen this effect and will be investigated further.

This paper was presented at the Grid Service Markets Symposium in Lucerne, which took place from 04-05.07.2022.