IEEE Power & Energy Magazine - July/August 2021 - 22

accordingly. This concept of " diversity " is fundamental to
the efficient design and operation of electricity networks.
Figure 5 shows that the majority of the EV charging load
occurs in the evening and reaches its peak at approximately
from 7 p.m. to 8 p.m., depending on the vehicle type. This is
due to the cumulative effect of charging sessions " stacking
up " as more drivers plug in during the early evening. The
load declines after the peak as the charging sessions end.
CrowdCharge and GreenFlux were given profiles of the
available capacity for each half-hour of the day (separate profiles
for weekdays and weekends). These capacity profiles
were set to be representative of an 11-kV feeder and the level
of demand management that would be necessary if 30% of the
vehicles were EVs. Smart charging was used to ensure that the
demand from each group of chargers did not exceed the available
capacity. The trial results showed that demand management
was required regularly when the winter capacity profiles
were active, with more than 75% of participants experiencing
smart charging during trial 1.
The additional loading predicted by Figure 5 and the
need for management during the trial indicates that there is
likely to be a need for flexibility in EV charging. The next
two sections look at this in more detail.
Flexibility in Charging
The previous section discussed the need (from a distribution
company's point of view) to limit the increase in the peak load
caused by EV charging at home. The following sections use
data from Electric Nation to demonstrate to what extent drivers
can be flexible with their home charging and how this can
be exploited using smart charging.
Is There Flexibility Available?
In this context, flexible charging means that the overall charging
speed could be reduced without inconveniencing the driver. Flexibility
was calculated for each charging event by calculating the
100
90
80
70
60
50
40
30
20
10
percentage of the plug-in duration (the time when the vehicle
was connected to the charger) when the vehicle was actively
charging. Subtracting this quantity from 100 gives the degree
of flexibility.
Consider the following two scenarios.
1) The car charges for the entire duration of the plug in (flexibility
= 0%). If the charging speed were reduced, the
state of charge of the vehicle when it was unplugged would
be lower than if it had charged at its maximum rate.
2) The car charges for 1 h but is plugged in for 4 h (flexibility
= 75%). The charging speed could be reduced
but the vehicle would still be fully charged when it
was unplugged because there was time available to
make up for the slower charging.
If the majority of charge events had low flexibility, then
using smart charging to reduce charging speeds in the event
of network constraints could inconvenience drivers. Their
vehicles would be in a lower state of charge than they had
expected when they unplugged. If, however, flexibility is
generally high, then there is scope to reduce the charging
speeds without impacting drivers.
Figure 6 depicts the distribution of the flexibility results
for 62,198 weekday charge events. For each hour, the blue
line shows the median flexibility for plug-in events in the
hour. The edges of the shaded area indicate the interquartile
range for each plug-in hour.
As an example, look at weekday charge events where the
vehicle was plugged in between 5 p.m. and 5:59 p.m. Of the
charging events,
✔ 75% have a flexibility of more than 45%
✔ 50% have a flexibility of more than 79%, indicating
that they are charging for only 21% of the time that
they are plugged in
✔ 25% have a flexibility of more than 87%.
Charging flexibility reaches a peak in the evening, which
Interquartile Range
Median
is advantageous because it aligns with the times when smart
charging may become necessary.
High flexibility indicates that it
should be possible to reduce the
charging speed without impacting
customers.
Is It Possible
Plug-In Hour
figure 6. Flexibility across different plug-in hours (weekdays).
22
ieee power & energy magazine
to Exploit Flexibility?
Electric Nation successfully
deployed smart charging to more
than 500 participants for 18
months, proving the technical feasibility
of using smart charging to
exploit flexibility in vehicle charging.
The customer acceptability of
this is explored in later sections of
this article.
The charging events were analyzed
to detect when the vehicle
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Flexibility (%)
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12 p.m.
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IEEE Power & Energy Magazine - July/August 2021

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