Electric grids - Can they withstand electromobility?

Author:  Philipp Nobis, Sebastian Fischhaber, Forschungsstelle für Energiewirtschaft e.V. (As of February 2015) The penetration of electric vehicles (BEVs) expected in the future, as well as the increased use of distributed generation plants, pose new challenges for the distribution grid. The uncontrolled charging of a large number of electric vehicles may not only generate additional load peaks, but also increase existing peaks, which may lead to voltage maintenance problems or excessive equipment loads in low-voltage networks.

Disturbance of distribution grids due to electric mobility

In the project "Smart Grid - Basis of an Electromobile Future", the grid repercussions due to electric vehicles and the expansion of renewable energies on the low-voltage grid were investigated. Fig. 1 shows the average charging power per vehicle of a simulated grid area with 30% BEV penetration as a function of charging power. It can be seen that the peak load of the electric vehicle fleet occurs between 5 and 8 pm, overlapping with the existing peak load. As the charging power increases, the peak load increases by 39% compared to a charging power of 3.5 kW [2]. Higher charging powers than 10 kW cause a smaller change in peak load than in the range between 3.5 kW and 6 kW. This is due to a lower simultaneity of charging at higher charging powers, as a result of shorter charging times. Furthermore, it can be seen that the load profile is shifted forward by about 2 h with increasing charging power.

Fig.2 shows the effects of increased BEV penetration on the simulated local grid transformer in the form of annual duration curves. It can be seen that BEV penetration has a high impact on the annual peak power. The vehicle charging power also has a high influence. Compared to the case without BEVs, for the same BEV penetration of 10%, a higher charging power of 14 kW causes a three times higher change in peak load - compared to 3.5 kW. A positive effect of electric vehicles is the increase in self-consumption in the grid area, which is evident from the shift in negative power in the continuous line.

Germany-wide energy demand of electromobility

In the project "Merit Order of Energy Storage 2030", among other things.considered, among other things, how high the total charging power and energy of all electric vehicles in Germany will be in 2030.

In the following (Fig. 3), the annual load curve of all electric cars is shown for the year 2030 with 3.3 million vehicles. The load curve shows a volatile pattern with a maximum at about 1.5 GW. The electric vehicles have an annual energy demand of about 3.6 TWh. Compared to the total German consumption of 550 TWh/a with an average power of about 70 GW, the demand of electric vehicles is to be considered low. The consumption of the vehicles and thus also the amount of energy charged depend strongly on the temperature and are thus subject to seasonal influences.

It can be seen that the load peaks in summer are significantly lower than in fall and winter.

In Fig. 4, the load curve of two days of an average week in 2030 is shown in color as an example (week 40). The bars show the hourly energy consumption of the fleet. The color coding of the bars indicates the minimum time by which the energy of the respective hour could be shifted without affecting the amount of charging energy. For example, the dark blue portion of the load curve shows the portion of the charging energy that could be shifted by at least 12 h, the light blue portion shows the portion that could be shifted by at least 6 h, etc.

It can be seen that a large portion of the charged energy has a high potential for flexibility. The reason for this is the charging behavior of the users, who mostly charge the vehicle battery after the last trip of the day and only use it again the next morning. On average, this means that charging processes can be postponed by about 11 hours. Consequently, negative grid feedback could be significantly reduced by intelligent charging control, for example by shifting peak loads to off-peak periods.

Summary

At present, electric vehicles do not lead to equipment overloads or voltage band violations in German low-voltage grids. However, with high penetration of electromobility, overloads in weakly developed LV grids may be imminent. Charging controls can shift charging operations to off-peak periods or periods of high PV yield. When shifting charging, however, it should be noted that widespread charging controls can also lead to high concurrency, which in turn can have feedback effects on any load peaks.

In the overall German context, the required power for electric vehicles in 2030 is not particularly high. At about 1 GW (annual maximum 1.5 GW), it amounts to only 1.4% of the overall German load of 60-80 GW. However, it is high enough to be able to offer secondary services - such as control power provision - in 2030.