Environmental Engineering Reference
In-Depth Information
path between the ESS and the M/G and driveline. In the power electronics, dc
current from the ESS is converted to variable frequency and voltage ac currents for
injection into the M/G through the process of high frequency switching. In order to
process kW of electric power and not overstress the internal components it is
necessary that parasitic inductances and stray lead inductances, all of which cause
voltage steps on their associated switching devices and all connected devices, be
absolutely minimized. The voltage step occurring when currents are switched at
inverter switching speeds can be very high. For example, suppose a typical current
transition having a slew rate of 260 A/
m
s at the switch must flow through 100 nH of
stray inductance, including inductance of the power module itself, the bus bar and
finally the link capacitor. This current transition will result in a voltage step on the
silicon metallization of
V
step
¼
L
stray
di
ð
t
Þ
dt
ð
4
:
36
Þ
which computes out to 26.7 V of additional voltage stress. In a 42 V systemwith 80 V
rated semiconductor switches, this amount of voltage overshoot could not be
tolerated. The solution is low inductance bus bars [38-40]. Table 4.22 lists the
common types of bus bars and the inductance of individual conductors.
For example, a laminar bus bar system consists of positive and negative bars
that are 6
00
wide, 0.125
00
thick, and separated by Nomex sheets having a thickness of
0.125
00
. The complete bus bar system is approximately 28
00
in length. On inserting
these dimensions into the expression for inductance, the value returned is 209 nH.
The conductor area,
W
t
, is estimated for the rated current,
I
in A
dc
, based on the
empirical expression given as (4.37):
W
t
¼
0
:
0645
p
I
½
1
þ
0
:
5
ð
N
1
Þ ð
mm
2
Þ
ð
4
:
37
Þ
where
N
is the number of conductors in the bus bar assembly, in this case
N
=2.In
the earlier example where the dc link current was 242 A
dc
, the required bus bar area
would be 49 mm
2
with an aspect ratio
W
/
t
~ 48:1. Therefore, the end item bus bar
would be approximately t = 1 mm thick each conductor and W = 49 mm wide. The
rated current density in the conductors would be 4.9 A/mm
2
.
Because of skin effect in the bus bar conductors, there will be a significant ac
resistance component to the bus bar so that current flows at the surface and does not
penetrate the full bar depth. This has required the calculation of an equivalent dc
current from which the bus bar is re-sized. For the example cited, the nominal dc
current of 242 A must be increased in proportion to the known frequency compo-
nents of current. High frequency ac currents flow on only one side of each bus bar
conductor, the side facing the dielectric, and from this side only one skin depth into
the bar. The following expressions quantify the process.
