Environmental Engineering Reference
In-Depth Information
multilevel inverters. Although not entirely relevant to vehicle propulsion systems
because of relatively low voltage (LV), there are instances where multilevel
(matrix) converters have found application in the
<
600 V
dc
range. Trentin
et al
. [8]
describe a two-stage matrix converter for aircraft electric actuators for flight con-
trol surfaces. Modern aircraft rely on 230 V
ac
, 3-phase power, but at variable fre-
quency of 360-800 Hz. A matrix converter adapts a regenerative 3-phase electric
actuator directly to the mains power.
At medium voltage (2.3-13.8 kV), the multilevel converter excels and is often
the power electronic converter of choice. Wu [9] provides a comprehensive treat-
ment of multilevel converters including two-level VSI, cascaded H-bridge inverter,
diode-clamped multilevel inverter, neutral point clamped multilevel inverter and
current source inverter (CSI). Table 6.3 lists the power processing level versus
voltage typical of medium voltage (MV).
Table 6.3 Power levels for medium voltage by distribution level voltage
Voltage (kV)
2.3
3.3
4.16
6.6
11
13.8
Power (MW)
0.4
1
2
4
10
40
Power device
IGBT, BIGT
BIGT
SCR, GTO
GTO, GCT
The CSI is suited to MV ac motor drives because it offers inherent short-circuit
protection and lower stress waveforms. The load commutated converter is one
example and is suited for applications to 100 MW. The CSI is also very amenable
to MV because of low capacitor rating requirements.
Li
et al
. [10] discuss a CSI active power filter as means to reduce dc link filter
capacitor ratings in a hybrid electric vehicle (HEV) inverter. In this field, there are
two methods to reduce dc link harmonic currents: (1) improve the modulation
strategy and (2) use advanced filtering methods that admit film capacitors. Lu and
Peng [11] developed a theory to calculate the optimum dc link capacitor rating and
a method to calculate the link capacitor current and voltage ripple. The dc link
capacitance is related to the shunt current ampere-seconds magnitude divided by
the dc link voltage ripple,
e
U
d
, where
e
is a ripple factor.
One of the pressing needs in hybrid propulsion systems today (2010) is
requirement for in-depth understanding of the effects of dc/ac inverter and dc/dc
converter ripple currents on capacitor and battery service life and ageing. For
example, the inverter input ripple current is not entirely suppressed by the dc link
filter and some fraction will be present at the dc ESS, whether this is a battery, an
ultra-capacitor, or a combination of the two. On this same topic, a second need is
for better understanding of the impact peak power and peak currents in motoring
and regenerative modes will have on dc link capacitor, battery and/or ultra-
capacitor components. Not to mention the undesirable effects such high frequency
ripples will have on ESS cell management electronics, fault sensing and commu-
nications channels.
