Environmental Engineering Reference
In-Depth Information
the bar. When the current magnitudes are known at the major frequencies of
interest and bus bar dc resistance is known, the equivalent dc current becomes
s
X
R
ac
ð
f
k
Þ
R
dc
I
dc eq
¼
I
ac
ð
f
k
Þ
ð
A
dc
Þ
ð
4
:
39
Þ
k
Suppose the bus bar conducts ac currents of 250 A
rms
at 12 kHz and 80 A
rms
at
90 kHz in addition to the dc average current of 242 A
dc
. The equivalent dc current
will then be over 600 A
dc_equiv
. With this newfound knowledge, the bus bar
structure must be modified to accommodate this higher current loading to meet its
thermal environment specifications. The new size is
W
t
= 121 mm
2
or a bar of cross
section 1.59 mm by 76 mm, where the aspect ratio of
W
/
t
has been used.
The frequency components of the ac currents in the bus bar are determined by
monitoring the link capacitor ripple currents and using either a spectrum analyser to
identify the major frequency components and magnitudes or by doing a Fourier
transform on the current time series. In either case the impact is on the proper sizing
and selection of the bus bar. This also gives further impetus to proper selection and
sizing of the dc link ripple capacitors discussed earlier.
4.5.3 High voltage disconnect
Supporting systems in the energy storage area of a hybrid vehicle include battery
disconnect, battery state of health monitoring, energy management and climate
control. Figure 4.47 illustrates a 42 V PowerNet mild hybrid battery system and its
interface to the VSC through its dedicated battery management module (BMM). At
42 V it is unlikely that cell management would be required. For higher voltage, and
for advanced battery modules, it will be necessary to also include a cell manage-
ment system consisting of some means to stabilize individual cell charge. Forms of
cell management include charge transfer devices or resistive dividers. In practice,
high voltage NiMH batteries will have a cell management unit for strings of six to
ten cells. In lithium ion, it may be necessary to have cell management for every
three to five cells. Electrical protection is provided by fuse and contactor arrange-
ments. For high voltage batteries, it is also necessary to interlock the internal cell
stack(s) from the case terminals or connector. Generally, a plug is used that opens
the cell pack from the terminals and disengages the contactor in the process (dis-
connect before break).
4.5.4 Power distribution centres
Modern automobiles, and luxury brands in particular, have evolved to multi-zone
electrical distribution centres. In this zonal architecture, the local branch circuits
are fed from power distribution centres in much the same manner that apartment
complexes have a single service entrance but distribution panels for each individual
apartment. The Jaguar XK8, for example, uses three zone power distribution,
under-hood, passenger compartment and trunk (boot). The trunk power distribution
centre is important in high end vehicles because so much electronic content is being
