IEEE Power Electronics Magazine Compendium - March 2018 - 98

40
30
20
10
0

500 1,000 1,500 2,000 2,500
Frequency (Hz)
3L-NPC
4L-FLC

120

Converter
Semiconductor Losses (kW)

50

Converter
Semiconductor Losses (kW)

Converter
Semiconductor Losses (kW)

60

100

3L-FLC
5L-SC2LHB
(a)

80
60
40
20
0

500 1,000 1,500 2,000 2,500
Frequency (Hz)
3L-NPC
4L-FLC

3L-FLC
7L-SC2LHB
(b)

150

100

50

0

500 1,000 1,500 2,000 2,500
Frequency (Hz)
3L-NPC
4L-FLC

3L-FLC
9L-SC2LHB
(c)

FIG 5 NPC, FLC, and series-connected converter semiconductor losses versus switching frequency at (a) 2.3 kV, (b) 3.3 kV, and
(c) 4.16 KV.

may result in failure of the motor winding insulation due
These problems can be resolved at the expense of addito partial discharges. Furthermore, such rapid voltage
tional effort and cost. The operating cost can be reduced
transition induces rotor shaft voltages that cause current
with the minimization of switching losses. This also
flow in the shaft bearing, which finally leads to motor
enables reducing the cooling requirements. Hence, the
bearing failure [16]. The switching pattern of the power
cost and size of the drive are reduced. The switching
switches affects the wave reflection value, which is prolosses of MV semiconductor devices contribute the major
duced by the mismatch between the
portion of the total device losses.
cable and the inverter and the motor
Hence, a reduction in switching frewave impedances. The motor cable
quency allows increasing the maxiThe high switching
works as a transmission line where
mum output power. On the other
the voltage pulses will travel very
hand, harmonic distortion at the line
frequency of power
fast, up to 150-200 m/µs [15]. When
and motor side increases with the
devices results in high
the pulses take more than half the
decrease of switching frequency [13].
rise time to move from the inverter
A comparison of losses in a
dv/dt at the rising and
to the motor, a full wave reflection
three-level neutral point clamped
falling edges of the
occurs. For that worst case, the wave
(3L-NPC) converter, a three-level
reflections will double the voltage on
flying capacitor (3L-FLC), a fourinverter output
the motor terminals at each switchlevel flying capacitor (4L-FLC), and
waveform.
ing transient. The critical cable
a nine-level series-connected Hlength for 500 V/µs is in the 100-m
bridge (9L-SCHB) is reported in [14],
range and for 10,000 V/µs in the 5-m
[15]. Figure 5 shows switching losses
range [1]. The wave reflection coefficient C is dependent
as a function of frequency at different classes of output
voltage (2.3 kV, 3.3 kV, and 4.16 kV) of various types of
on the ratio between motor and cable wave inductances
MIs [15]. It can be observed that in all types of convertC = (Z motor - Z cable) / (Z motor - Z cable). Nevertheless, cable
ers, the losses increase with an increase in the switchdiameter (Z cable) is around 80-180 X , which is much
ing frequency and with the voltage. The smallest losses
smaller than motor wave impedance, which is around
are found in the 9L-SC2HB MI. At 2.3 kV, losses in the
2-0.4 kX [17].
3L-NPC and SC2LHB MIs are almost the same, but at
The high dv/dt also causes EMI on the cables
4.16 kV, losses in the 3L-NPC MI are almost double those
between the inverter and the motor. The expensive
of the SC2LHB [15].
shielded cables are used to avoid these effects; nevertheless, the electromagnetic emission may affect the
operation of nearby installed electronic equipment. In
Motor side challenges
the inverter, the dv/dt still depends on the switching
characteristics of the power devices, and it could still
High dv/dt and Wave Reflection
be problematic if no output filter is used. To get guarThe high switching frequency of power devices results in
anteed low THD in both motor and line ends, passive
high dv/dt at the rising and falling edges of the inverter
filters are commonly employed. The high value of the
output waveform. Such a high rate of change of voltage

98

IEEE PowEr ElEctronIcs MagazInE

z	June 2016
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018
