IEEE Electrification Magazine - June 2015 - 44

The breaker voltage
has to be
considerably higher
than the actual dc
voltage to cause a
negative voltage at
the inductances.

will rise quickly and to high values
until the dc switch opens. Again,
additional energy has to be dissipated-this time in the dc switch.
The generator continues to feed
into the system until the current is
reduced to zero.
xx
It should be noted that Figure 3(a)
and (b) is valid for the diode bridge
and IGBT converter (independent
of the number of levels used). The
diode-bridge structure and its
implication [see Figure 1(a) and (b)]
is straightforward: Even after
blocking all IGBTs, the generator feeds into the short
circuit via the diodes.
xx
Figure 3(a) and (b) also describes the options available
for MMC with two-quadrant modules (2Q-MMC). This is
less obvious because the structure seen in Figure 1(c)
hides this property. However, the two-quadrant subfigure clearly depicts an uncontrolled diode path in each
module-no matter how many modules are stacked
per arm of the MMC, the diode bridge structure is
inherent to 2Q-MMC converters. Even blocking all
IGBTs in all MMC modules will not prevent the generator from feeding into the short circuit via the diodes.
xx
Figure 3(c) illustrates the use of a thyristor-based converter [as, for example, in the case of conventional
high-voltage dc (C-HVDC) transmission lines]. It is also
valid for an MMC with four-quadrant modules
(4Q-MMC). It is assumed that the energy transfer to the
dc ship grid is stopped by the converter control as soon
as the short-circuit condition is detected. Only the
energy stored in the cable (and, eventually, a dc-side
filter) has to be dissipated in the short circuit, reducing
the short-circuit reaction considerably and limiting
damage as much as possible. There is no relevant overload for the converter valves.
xx
Basically, Figure 3(d) seems identical to Figure 3(c).
However, here the control of these converters is used
to reverse the flow of energy, decreasing the dc current as quickly as possible. The energy is fed into the
generator, speeding it up. As the generator and its
prime mover suffer from loss of load, this is perhaps
not the best option because the generator speed
might increase too much. This aspect depends strongly on the inertia of generator and prime mover.
xx
Figure 3(e) refers to 4Q-MMC only because only this
converter type offers controllable energy storage
capability within its module capacitors also in case
of a dc-side short circuit, similar as in Figure 3(c)
and (d). It can be controlled such that energy from
the dc side is not transferred to ac but into the module capacitors. The capacitor voltages will increase,
but as the energy of the dc side will be low, this
increase should be within the usual operation

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

limits. The energy on the dc side is
particularly low in this case because
with optimal dimensioning of the
4Q-MMC, neither relevant filters nor
additional slope-limiting reactors on
the dc side are needed.
One major aspect should be
stressed: There is a very considerable
difference between the options (a) and
(b) compared to the options (c)-(e).
Options (a) and (b) put considerable
stress on many components and cause
a high amount of energy to be dissipated because energy flow from the generator into the fault is not stopped at once. Options (c)-(e)
stop the energy flow on the generator side very quickly and
reduce the energy to be dissipated or stored considerably.
In addition to short-circuit considerations, the impact
of the chosen technology on the dc ship grid has to be
taken into account. Thyristor-based converters usually
employ current-based grid operation, the main smoothing
element being an inductor. Moreover, the number of voltage levels and the switching frequency are low, leading to
relatively high harmonics, which have to be mitigated by
an additional filter.
With regard to all these aspects, MMCs are a promising solution for medium-voltage dc local and especially
ship grids. MMCs in two- and four-quadrant configurations offer multilevel characteristics with reduced or
quasi-eliminated filtering and optional redundancy.
Although they represent a relatively new technology,
they have been in industrial on-shore HVDC use for a
few years. Reliability and proof of concept can be regarded as given. In the case of dc-transmission application,
currently, the two-quadrant layout, including converter
protection by additional thyristors relieving the lower
diode and emergency turn-off via ac circuit breakers, is
employed [3]. Cables with a low risk of short circuit make
this a technically sound solution. In the case of ship grids
with a higher risk of cable fault and high requirements
for fast fault clearance and power restoration, the fourquadrant layout appears to be advantageous.

Measurement-Based Verification of 4Q-MMC
Short-Circuit Mitigation
The test setup is shown in Figure 4. The MMC shown in
detail in Figure 1(c) is fed via a transformer from a usual
laboratory ac grid, replacing the shipboard generator. It
connects to a grid loaded by a dc load via a small inductor
(0.24 mH). No relevant capacitive smoothing is included
on the dc side of the MMC. Directly behind the small
inductor, a switch allows for a hard short circuit. The following measurement results document the reaction of the
MMC based on the selected quantities.
The applied control scheme is described in [4]. The
measured results match the simulation results generated

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015