A transportation system based on electric vehicles affects the load on the power grid. How much new infrastructure that is needed largely depends on when in time EV charging is introduced. Here, research shows that tariff design could play a significant role on the peak power exerted on the low-voltage system.
Therese Lundblad, PhD student at Chalmers, has investigated how electric vehicle charging affects the local grid. The research has been conducted in the SEC project “V2G to manage variations in the electricity system – environmental performance”.
Enabling large-scale analysis of distribution grids
Within the project a model called REGAL, Reference Electricity Grid Analysis, has been developed. REGAL is a synthetic representation of Sweden’s low-voltage grid, and it enables large-scale analysis of distribution grids even when the real grid data is unavailable.
“My colleagues and I have used open data on the number of electric cars, residents, and types of housing in Sweden. Then we have been fortunate to be given access to some confidential data for calibration. By using the calibration data, we have generated estimates for the transformer capacity, cable lengths and structure for each square in a grid with cells of one square kilometre covering all of the residential Swedish low-voltage grid. This does not mean that we can look at individual squares and be totally accurate, but it gives rather good estimates when considering larger areas”, says Therese Lundblad.
Large variations in peak power
By using REGAL and logged data from just under 200 electric vehicles, Therese has built an optimization model that minimizes the charging cost with different price signals, thereby giving the charging patterns that different price signals incentivize.
“We have studied how electric vehicles charge optimally from various cost structures and different charging scenarios in relation to grid capacity and influencing factors. With this information, it’s then possible to develop strategies to avoid overloading the local grid”, says Therese.
Therese shows in her doctoral thesis, based on simulated cases with a fully electrified vehicle fleet, that tariffs have a large impact on the peak power exerted on the low-voltage grid. If consumers pay the running energy price without any power tariffs applied at all, there is a 73% increase. Dynamic power tariffs, where the price for using the electricity grid is linked to the level of power demand on the local grid, give a 30% increase. Tariffs that are active all hours of the day only increase the power peaks with 10%. However, this is also the option with the lowest flexibility.
“It’s rather obvious that if everyone is charging at the same time, problems will occur in the local grid. There is a lot of potential to even out the peaks with electric vehicles since they are usually parked for longer than needed for charging. However, it is also problematic if everyone were to optimise based on the same low spot price, which will only move the problem to another time. The spot price only reflects the balance between production and electricity need in a larger area, not in the local grid. So, there can be a trade-off between following low electricity spot prices and lowering the loading on the low-voltage grid”, says Therese.
Substantial impact on necessary transformer upgrades
A fully electrified transport system will require a reinforced charging infrastructure on some level regardless of tariffs. How much, and to what cost, is largely dependent on how strained the grid becomes and how flexible we want the system to be. Therese has done estimations on the necessary upgrades for Sweden’s approximately 200,000 transformers in the Swedish low-voltage grid. “If there are no control mechanisms at all, the modelling shows that we need to upgrade about 40,000 transformers due to EV charging compared to only 400 if we use the grid’s capacity optimally. So, charging behaviour has a substantial impact.”
Therese finished with the two main conclusions: “The situation in the price area does not correlate to the local grid, so it is not optimal if everyone is guided by this. It’s necessary to consider local conditions. Then I just want to underline that how we manage EV charging will have a large impact on how much it’s necessary to strengthen the power grid.”
About Therese Lundblad
“I grew up in the city of Gävle and studied energy technology at KTH Royal Institute of Technology. Before I started as a PhD student, I worked at the Swedish Transport Administration. The reason that I wanted to do a PhD, besides my curiosity to learn, was that in my previous work there wasn’t really time to dig deep into the questions that interested me. The PhD studies have really taught me a lot about research and to interpret studies and what can be done with them.”
Download the thesis Energy Infrastructure for Road Transport Electrification
