# Charging Schedule for Load Peak Minimization on Large-Scale Electric Bus Depots
## 1 Introduction  
The city of Hamburg is determined to reduce its CO\({}_{2}\) emission due to public transportation. In accordance with this goal, the city has decided to allow purchases of exclusively emission free buses from the year 2020. The two public transportation companies in the city, Hamburger Hochbahn AG and Verkehrspetriebe Hamburg-Holstein (VHH) have already started electrifying their bus fleets and building the necessary charging infrastructure. Both companies have decided to use the centralized depot concept. Hochbahn plans eight large-scale depots in the city, with the smallest one operating 50 buses and the biggest one operating 240 buses. Other charging concepts such as opportunity charging have their advantages but they have shown to be less effective in the particular case of the City of Hamburg. The buses in Hamburg have circular routes with very short stops at the bus stations (1-2 min). This is not enough to charge the buses effectively; especially taking into consideration that very often two or more buses arrive at one bus station in the same time. In such a scenario, the charging organization poses a big problem. Additional issue is the charging infrastructure for opportunity charging due to the lack of space at the bus stations, especially in the inner city. For these reasons, the public transportation companies in Hamburg chose to implement the central depot-charging concept. Nevertheless, the centralized depot concept also has several disadvantages. The integration of such large-scale electric bus depots into the local distribution grid can lead to problems. Load profile on one bus depot is highly dependent on the charging schedule of the buses and therefore unevenly distributed. In the periods when multiplebuses charge simultaneously, there will be load peaks. Depending on the number of buses operated at the depot, the additional load for the grid can be significant. If this load exceeds the reserves in the local substation, new infrastructure and investments are necessary. The local grid is not the only one affected by the charging schedule. The electrical infrastructure at the depot needs to be designed to operate with the highest expected loads. Uneven load profile with short periods of extremely high loads, followed by long periods of lower demand, leads to an overdesigned equipment and bigger losses within the depot itself. To avoid the problems mentioned above it is necessary to optimize the charging schedule of buses and minimize the load on the bus depot. Numerous researchers already studied the scheduling of electrical vehicles and its impact on the electrical grid [1; 2; 3; 4; 5; 6]. At the same time there are only a few studies investigating scheduling for electric bus fleets. Even less researchers focused on large-scale bus fleets and the corresponding charging infrastructure. [PERSON] et al. analyze different charging concepts for bus fleets: flash, opportunity, and depot charging [7]. They investigate the operational feasibility and the grid impact of these three concepts. [PERSON] et al. focus on power quality issues of fast charging station using the real data from the electric bus fleet in Gothenburg [8]. [PERSON] et al. analyze the impact of electrical vehicles (including electrical buses) on the substation reserves in the city of Hamburg [9]. These studies analyze the potential issues when it comes to the impact on the grid but they do not suggest solutions in a form of a charging schedule. There are several proposed optimization methods for electrical bus scheduling for bus fleets using the battery exchange concept [10; 11; 12]. [PERSON] et al. propose various optimization methods for fast charging [13]. [PERSON] et al. propose a new charging strategy with the fast charging during the day and regular charging at night [14]. They optimized the charging sequences on one fast charging station in the city of Beijing. [PERSON] et al. analyze scheduling bus fleets based on flash wireless charging concept [15].