China is the frontrunner of the global plug-in electric vehicle (PEV) market. The rapid and widespread uptake of PEVs and fast charging technologies will have a significant impact on power generation and distribution systems.

However, PEVs could potentially not only draw power from the grid as a load, they could also act as storage systems. By leveraging storage capacity of PEVs, their impact on the grid system can be minimized through Vehicle-Grid Integration (VGI) measures such as vehicle to grid (V2G) and managed charging.

In this study, we have adopted a quantitative model to quantify the impact that unmanaged PEV charging will have on the grid system and to what extent this impact can be minimized by VGI measures. Using the Monte Carlo Simulation, the study analyzes PEVs’ impact on the city level and distribution level. Combining Monte Carlo Simulation and Linear Programming, the model further investigates whether VGI measures are sufficient to mitigate PEVs’ impacts on the distribution level.

Results based on this model in Beijing and Suzhou reveal that on the city level, the impact of PEVs on the grid varies greatly depending on the level of PEV stocks and the adoption of ultra-fast charging:

  • In the Business-as-usual (BAU) Scenario, cities witness a moderate pace of PEV growth (PEV stock within 3 million by 2035 or 4.5 million by 2050) with the majority of its PEVs relying on slow charging, increases in the peak load between 2035 and 2050 will range between 1-5%. This increased demand could be possibly accommodated by the existing generation and transmission capacities.
  • In the High-impact Scenario, cities experience an accelerated pace of PEV growth (PEV stock larger than 3 million by 2035 or 4.5 million by 2050) with the majority of PEVs relying on fast charging, EV’s charging will result in nearly 12% increases in the peak load in 2035 and 2050, possibly overstressing the generation and transmission systems.  

On the distribution scale, the impact of unmanaged charging is even greater. When electrification of private vehicles exceeds 50%, the majority of transformers in residential neighborhoods risk being overloaded. With the exception of bus or freight depots, which require their own dedicated transformers, areas affected by unmanaged charging of PEVs and that may require transformer capacity expansion include commercial and office complexes with fast chargers. This last-mile grid challenge will constrain the deployment of charging infrastructure and potentially backfire on PEVs promotion.

Furthermore, the results show that VGI measures such as managed charging and vehicle discharging to grid (vehicle-to-grid, V2G) can effectively eliminate the needs for distribution capacity expansion. In contrast, simply increasing capacity to satisfy the ‘unreasonable’ demand of unmanaged charging by PEVs will increase cost and may very easily face restrictions.

Between two VGI measures, V2G is most effective in shaving the local peak loads. V2G can use a small number of PEVs to shave peak loads, which is even more ideal than managed charging. Within managed charging measures, controlled charging is more ideal to optimize PEVs’ charging loads. While the time-of-use (TOU) utility tariff may be useful to reduce peak loads, its impact is limited because it may create another load spike at the beginning of the off-peak period.

The study concludes that with the increased use of PEVs, grid operators and governments should plan in advance and implement measures that mitigate the impact of PEVs on the grid. Although V2G is most effective, its technical, regulatory, and public acceptance barriers prohibit recent large rollout. In the near term, controlled charging is the most viable and necessary approach to curb PEVs’ impacts on local transformers and defer relevant investments. The benefits of controlled charging on the distribution transformers mean that efforts should be taken to overcome obstacles such as business models and technical standards to ensure scalability. In the mid- to long-term, as technical and regulatory safeguards of V2G improve, small-scale V2G pilots would be possible for certain use cases (such as urban logistic freights providing peak shaving or frequency regulation) with demonstrated commercial values. The regulatory and technological safeguards to enable VGI measures will be discussed in the next report of the series.