Environmental Engineering Reference
In-Depth Information
6.6 Comparison of PWM techniques
In the field of hybrid propulsion systems, it is advantageous to have an under-
standing of the full gamut of power electronic control techniques. This section
presents some of the more important modulation techniques, the major ones have
been covered in the preceding sections, but there are some variations that should be
kept in mind. In this section the reader is presented with a high level summary of
the pros and cons of these competing techniques [12].
The point has already been made that the aim of alternative PWM modulation
techniques is to improve the overall drive system performance by operating at lowest
possible losses while making optimum use of the available power supply. On the
hybrid vehicle, this means making the most use of an on-board ESS that is not stiff
and tends to droop significantly when under load. This is typical of all traction sys-
tems and a point that must be considered in any hybrid propulsion system design.
In his investigation of PWM techniques, Holtz [12] has laid out a performance
criteria based on (1) current harmonics, (2) harmonic spectrum, (3) torque harmo-
nics, (4) switching frequency, (5) polarity consistency rule and (6) dynamic per-
formance. All of these criteria can be seen to be very important for the hybrid
propulsion system designer. Current harmonics give rise to additional iron and
copper losses in the machine and power inverter, particularly in the dc link capa-
citors, and the harmonic spectrum that may be sparse or dense with frequency
components at various relative amplitudes. Holtz goes on to propose a figure of
merit for modulation techniques based on the product of spectral amplitudes and
switching frequency as a means of comparison. Torque harmonics are not only
obvious to the hybrid propulsion system designer and typically a strong function of
the electric machine topology, but also dependent on inverter current harmonic
injection. Switching frequency is important because the fidelity of the load currents
increases linearly with switching frequency, with an accompanying reduction of
current ripple. However, switching frequency is strongly dependent on available
semiconductor device technology. IGBTs in the form of ultra-thin wafer processing
(~85 m m to even 65 m m in the laboratory and rated at 600 V) are capable of
switching rated current at up to 80 kHz. This is far too high for motor drives, but
shows their capability. Higher voltage and current power electronic devices such as
GTOs for very high power (e.g. in MW) are capable of switching at 500 Hz to
perhaps 700 Hz. Switching frequency also gives rise to (EMI) concerns if not
properly treated. Inverter packages with low inductance bus bars, or laminated bus
bar and output phase terminations, provide the lowest EMI structures. Additional
filtering for common mode and transverse mode noise are possible with filters in the
output lines. Polarity consistency rule is useful for grading those PWM techniques
that select voltage vectors which do not have the same polarity as the reference
voltage. Recall that the sine-triangle PWM was able to select voltage vectors that
were not in the same sector as the reference. SVPWM is the best because no more
than two active and two null vectors are used at any one time. Dynamic performance
is a metric for inner-loop performance or current regulator bandwidth. Dynamic
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