35 / 60
Figure 2). For example, Figure 3 shows the
configuration of a high power VFD, designed
and built with three power modules, each
employing eight medium voltage rated
injection enhanced gate transistors (IEGTs); a
total of 24 silicon devices.
In order to deliver the same power,
another VFD topology, such as a cascade
H-bridge, would use 42 power modules
connected in series with four low voltage
insulated gate bipolar transistors (IGBTs),
each totalling 192 IGBTs. Fewer IEGTs results
in a more reliable VFD, as there are fewer
silicon devices to fail and hence less
complexity. An example of assured quality
during the design phase, leading to reliable
equipment, is the use of press-pack IEGTs
(PPI), as illustrated in Figure 4
7
and Figure 5.
In the PPI, in addition to the reduced number
of devices, the full contact between IEGTs makes them less
vulnerable to thermal fatigue, and a hermetically sealed
structure provides high resistance to the effects of
environmental contaminants, such as moisture, humidity and
dust. If necessary, the PPI can be water-cooled for greater heat
transfer.
Many electrical connections between the semiconductor
devices and the heat sink are housed inside the PPI and the
IEGT stack, thereby eliminating many cables and connectors in
the VFD. This quality, intrinsic to the design, delivers many
desirable characteristics. It creates ease of access for
maintenance and inspections, reduces the inspection points
for loose connections, and reduces the problems caused by
weak electrical contacts. The quick and user-friendly
changeover of a stack-based power module restores
production with as little downtime as possible.
VFD reliability is expressed in terms of the mean time
between failure (MTBF). For water-cooled VFDs that are
commonly applied to LNG compressors, the MTBF can, in
some cases, exceed 38 years.
A VFD is a system of components connected in series, thus
the failure of one component will result in the failure of the
VFD. Since each component can fail independently of another
component, the reliability of the VFD can be represented by
Table 1.
Maintenance costs of a gas turbine and a motor-VFD system
Gas turbine
3
Estimated
costs
Electric motor-VFD Estimated
costs
Minor
maintenance
Every 4000 –
8000 hr (6 – 10 days
downtime)
US$23 000 –
US$38 400
Every 8670 hr
(excluding motor,
24 hr downtime)
US$5760
Major
overhaul
Every 20 000 –
30 000 hr (30 days
downtime)
US$115 200
every3–4years Not required
US$0
Reliability
(mean time
between
failure)
6 – 13 months
–
38 years
–
Repair time
(mean time
to repair)
0.5 – 3 days
US$11 520
30 min.
US$120
Notes: US$240 000 per hour of labour; 16 hours per day; no material; no travel and living; no overtime; one
man.
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