IEEE Electrification Magazine - March 2014 - 60

The controllability of a
singular dc bus voltage also
results in the controllability
of tie-line current flows
toward other dc buses.
hydrogen fuel cell vehicles has still not matured enough,
and although it offers good efficiency and practically
zero emissions, the challenges related to hydrogen production and its storage within the vehicle keep restraining its market penetration.
therefore, heVs that use batteries only for power leveling
by maximizing the fuel economy of the associated ice and
using concepts such as regenerative braking, throttle actuation, and power steering, among others, have emerged as
the best alternative to conventional vehicles. the electrical
distribution of heVs is conceptually similar to those that
characterize consumer electronic devices and is shown in
Figure 4. it can be seen that the output terminals of the car
battery are extended to form the main electrical bus that
has all the other electronic and conventional loads directly
connected to it.
since it is very likely that the electrical consumption in
modern cars will reach several tens of kw in the near
future, an increase of the distribution voltage is inevitable
for having high power-distribution efficiency and restraining the wiring weight. in that sense, higher nominal voltages are used in the popular heV models, i.e., 201.6 V in
the toyota Prius or 355.2 V in the chevrolet Volt. By deploying power electronic converters, buses with other voltages
can be realized, including the classical 14-V bus, which
may serve for electrical supply of standard loads such as

window lifters, consumer electronic chargers, or ventilation systems.
another important prerequisite for the rapid increase in
the number of plug-in heVs on the roads is the
construction of advanced eV charging infrastructure. a
highly desirable feature of eV charging stations is the ability
to provide a recharging procedure that is as similar as possible to conventional petrol stations, as seen from the perspective of the vehicle user. the main obstacle in achieving
this capability is the limitation of power extraction from
conventional ac plugs of up to 10 kw, which makes the
recharging process very slow and, hence, unattractive for
public locations. as a solution to this problem, fast charging
directly from the station's dc link has emerged as a viable
alternative. however, since it implies power extraction of up
to 50 kw, there is a legitimate concern about the adverse
impact of large fleets of charging stations on utility mains.
therefore, from our standpoint, it will be mandatory to use
some kind of dedicated ess, at least for short-term powerleveling purposes. a diagram of an eV fast charging station
formed around a common dc link that uses a flywheel ess
for the power-balancing task is shown in Figure 5. a flywheel is selected here since it provides very high power
density and incomparably longer cycle-life than electrochemical batteries and hence suits this kind of application
much better, despite a higher initial cost.

HEV
DSO
Commands
va, b, c

Local Grid
Control

ia, b, c
Grid

ac−dc

iBAT

dc−dc

vBAT
FastCharge
Controller

vdc

dc
Link

Grid
Converter

Figure 5. An EV fast-charging station can use a flywheel ESS for power balancing.

I E E E E l e c t r i f i c ati o n M agaz ine / MARCH 2014

vdc

Flywheel
Control

va, b, c
ia, b, c

dc−ac
Flywheel
Converter

Electric
Machine

Flywheel

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2014