Environmental Engineering Reference
In-Depth Information
performance is set by inverter switching frequency, controller loop execution times,
current or voltage sensor bandwidth and all associated lag times.
Recall that the inverter modulation index,
m
i
, is defined as the ratio of the
reference vector to the maximum value of a square wave. The limiting values of
the modulation index for sine-triangle PWM and SVPWM are listed in (6.25) for
convenience, where the scaling is relative to a square wave:
p
2
¼
0
:
787
3
m
i
PWM
¼
2
ð
6
:
25
Þ
p ¼
1
:
15
m
i
Squarewave
¼
1
m
i
SVPWM
¼
Table 6.4 summarizes the salient aspects of the various PWM techniques used
as algorithms to control power electronic inverters. It can be seen from the com-
ments in Table 6.4 that space vector modulators provide the best overall perfor-
mance. In certain applications, the alternative modulators prove very useful.
Hysteresis current regulators are simple to implement in the ac drive system sta-
tionary reference frame and produce very fast dynamic performance, but at the
expense of requiring high speed power switching elements since the operating
frequency of such regulators can be very high. The hysteresis band setting effec-
tively determines the operating frequency of such regulators. This system is used
principally in low power drives but is now replaced by synchronous frame current
regulators as more and more ac drives become fully digital.
Predictive controllers, on the other hand, are useful in very high power inver-
ters such as GTO, based on which the switching frequency is constrained to be
quite low. These systems need high speed DSP controllers and accurate mathe-
matical models of the machine in order to predict the current vector trajectory and
decide a priori on the next switching state to set the inverter switches to.
6.7 dc/dc converters
The need for, and importance of, dc/dc converters in HEVs has been one of the
most important attributes and continues to be an area of intense development.
Figure 6.18 illustrates a typical electrical and control architecture for a hybrid
propulsion system consisting of a single M/G and dc/dc converter to support the
vehicle with 14 V PowerNet from the HV traction ESS.
In Figure 6.18, the dc/dc converter shown must interface the HV battery to the
vehicle electrical distribution system voltage, nominally 14 V and standard 12.6 V
lead-acid battery. The 14 V converter output must be rated at least 1.5-2.0 kW to
support all LV requirements of the vehicle, such as external and internal lights,
electronic control units for engine, transmission, safety and security systems plus
electric drive ancillaries such as EPS, pumps, fans and compressors. In some
