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Implementation of electric vehicles and vehicle to grid
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An electric vehicle (EV) is modeled as a battery
(see :ref:`battery_behavior`) with randomly drawn SOC level
from a uniform distribution.
The functionality of the *EVPark* class is to aggregate the EVs
at a bus into an EV park for each bus in the system.
The amount of EVs on a bus can be specified by the user. 
An EV park is modeled to build up an aggregated battery solution
with the available EVs specified on each bus in the system.
The EV park contains information on the amount of EVs in the
park and the SOC level on each EV. This information is used
to make the EV park where the available power in the EV park
is aggregated based on the EVs and the SOC level of the EVs in the park.

The user can specify if vehicle-to-grid should be activated
or not for the EVs. If vehicle-to-grid is activated, the EV park
can charge and discharge the cars equal to a battery. If vehicle
to grid is not activated, only charging the EVs in the EV park is possible.