IEEE Electrification Magazine - September 2016 - 11

the H-bridges as the converter can still remain in operThree-Phase,
Single-Phase,
ation with only open-circuit faults.
Three-Level,
Three-Level,
Another topology is based on a single-phase cascaded
NPC Bridges
NPC Bridges
g
g
H-bridge converter and has multiple dc links, as shown in
Figure 11. This topology is essentially formed by multiple
back-to-back converter modules with a three-phase side
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in parallel and a single-phase side in series.
V
The multiple-dc-link converter has a better output
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waveform and better fault tolerance than the back-toback converter. Because all of the single-phase bridges are
in series, the output waveform is formed by several levels,
dc-Link Capacitors
drastically reducing the total harmonic distortion of the
and Second
Harmonic Filter
overhead voltage and the need for filters. In the case of a
fault of one of the H-bridges, the faulty module can be
Figure 10. A high-power back-to-back converter with a single dc link.
bypassed, maintaining operations if the remaining modNPC: neutral point clamped.
ules can cope with the increase in their individual voltage. If enough modules are used, the single-phase
Local dc-Link with Second
transformer can be potentially omitted, reducing the cost
Back-to-Back
Harmonic Filter
Submodule
of the converter.
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The modular multilevel converter (MMC) is a topology
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that was first used for high-voltage dc transmission lines.
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It is formed by arms, where a half-bridge of full-bridge
submodules with a flying capacitor are connected in
series. Each arm has a reactor to smooth the current
waveforms. The greatest benefit of this topology is its
modularity and ability to work with high voltages. If n
modules are connected in series, the effective switching
frequency for each arm is n times higher than the device's
switching frequency. This topology is similar to the multiple-dc-link converter, with the key difference that both
Figure 11. A multiple-dc-link converter layout.
the three- and single-phase sides have multilevel waveforms. The number of modules is chosen to give the
desired rms voltage of the overhead line. For example, a
15-kV overhead line voltage (i.e., approximately 21 kV)
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could be obtained with seven modules per arm with a flying capacitor voltage of 3.25 kV using 6.5-kV IGBTs.
Line
Overhead L
V
Two MMC configurations are suitable for railway static
W
converters: direct ac/ac (Figure 12) and indirect ac/dc/ac
(Figure 13). The direct converter uses full-bridge modules
that have positive and negative output voltages. Under
normal operations, the converter circulates power
Figure 12. A direct ac/ac MMC for a 15-kV, 16.67-Hz substation.
between the three arms to compensate for the pulsating
power on the overhead line side.
The circulated power has two frequencies, the sum and the difference frequency between the
three- and single-phase sides. This
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imposes a limitation on the direct
Overhead
Overhe
three-phase-to-one-phase conLine
V
verter as it requires a frequency
separation. If the input and output
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frequencies are the same, no energy
can be circulated between the converter arms. This converter is well
suited for low-frequency railways
because the circulating-power Figure 13. An indirect ac/dc/ac MMC for a 25-kV, 50-Hz substation.
IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 6

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016