IEEE Power Electronics Magazine Compendium - March 2018 - 101

transformerless solution
Complex multiwinding transformers can mitigate harmonics through phase shifting in modular converters.
However, an isolation transformer can represent 30-50%
of electric drive system size and 50-70% of the system's
weight. The comparison of the average drive system
space and weight with and without a transformer is
shown in Figure 9. In addition to soaring raw material
costs for the transformer itself, an isolation transformer
adds to total drive costs with extra cabling, air conditioning to cool the transformer, engineering time, concrete
pad construction for outdoor transformers, and overall
installation [32].
Issues such as cost, space, weight, and platform balance can be improved significantly with fewer transformers for an offshore platform. To control pump motors on
downhole wells, each drive requires a multiton transformer. Offshore platforms typically require 20-MV drives
(sometimes up to 40 or more) [32]. A significantly large
cooling system is required in a tropical environment,
because a 1,000-kVA transformer generates up to 2 kW
of heat energy. The transformerless solution provides
substantial energy savings [33]. This feature is also
important in applications such as utility distribution
systems and high-voltage vehicle drives [34]. Solutions
are desired that reduce CM voltage, produce sinusoidal
output waveform, and limit dv/dt in a transformerless
MV drive.
Even though transformers protect the motor from CM
voltage, the high-level CM voltage stress on the motor
is imposed on the transformer and cable insulation. To
withstand the CM voltage stress, specific transformer
and cable insulation is required. But in transformerless

Percentage

100
75
50
25
0

Space

Weight

FIG 9 A comparison of average drive system space and weight
with and without a transformer.

solutions, MV variable speed drives require very good
insulation on the motor side to sustain high CM voltage
stress. Hence, the integrated CM dc choke is used to block
the CM voltage and reduce motor neutral-to-ground voltage. The structure of the integrated choke and its connection diagram in transformerless motor drives is shown in
Figure 10 [35].
Transformerless MV motor drives usually require a
high level of motor insulation to overcome CM voltage
stress or require the use of an additional inductor with
approximately the same impedance as the transformer
to be replaced [36]. The use of shunt active hybrid filters
solves this problem [24]. In Figure 11, a calculation of FIT
rates of the considered transformerless converter topologies is given and compared to a conventional traction drive
system using a low-frequency transformer [37].

With Transformer
Without Transformer

B1
B2

va
vb

ac/dc

dc/ac

id +ic /2
A1

id -ic /2

vc
B1

Induction
Motor
c

Integrated Choke

FIG 10 The structure of the integrated choke and its connection diagram in single-word motor drives.

June 2016

z IEEE PowEr ElEctronIcs MagazInE

101

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018