IEEE Electrification Magazine - June 2015 - 45

Qdes
Pdes

ac Grid or
Generator

iDC

dc Grid

0.24 mH

uDC

Figure 4. A schematic diagram of the short-circuit scenario used in

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Current (A)

Voltage (V)

the test setup.

vDC, p, KS3

iDC, p, KS4

-50

10
20
Time (ms)

-100
40

Figure 5. The dc voltage and current during the short circuit.

Current (A)

by the simulation tool, which is used for the simulationbased comparison between 2QS-MMC and 4QS-MMC performed later [5].
The fully controlled short-circuit mitigation is proven
by adding a reactive power command to the ac-side reference values. With the occurrence of the short circuit, the
active power has to drop to zero quickly (because a direct
short circuit will not consume much energy). Mainly, the
losses in the converter will lead to a low remaining active
ac-side current. The reactive current component should,
however, remain unchanged.
Figure 5 displays the reaction of the dc voltage and current to the short circuit. Induced by the occurrence of the
short circuit, the dc voltage drops rapidly. Consequently, the
dc current rises fast and is limited by the subordinated control loop. This subtransient regime lasts for about 10 ms.
Subsequently, in the transient and steady-state regime, the
asset characteristic detects the short circuit and, consequently, reduces the dc current to zero. Now, the switches
can be opened at zero current without effort (switching dc
under full current would require large and expensive
switchgear). Having removed the short circuit, the converter
could re-erect the dc grid and energy transfer. The reaction
can be seen in the dc voltage, which drops below zero for a
short time to reduce the dc-side current.
It should be noted that the dc current itself is not limited because it is irrelevant to the power-electronic converters making up the MMC. Instead, the arm currents are
limited because the converters that have to be protected
are located in the arms of the MMC (Figure 6).
The ac-side currents pass over from mixed active-reactive to purely reactive currents within approximately 20 ms
after the short circuit has occurred. Neither the short circuit
itself nor the limiting of the arm currents within the MMC
influences the functionality of the MMC and its control on
the ac side. The converter remains in controlled operation
and establishes the desired ac-side (reactive) currents.
The sudden change in the dc-side voltage and power is
compensated very well by the control. However, the reaction
on the ac side is still not instantaneous. Consequently, the
difference in energy between ac and dc has to be accommodated. In case of an MMC, the module capacitors can be used
as temporary energy storage elements due to the dimensioning of these capacitors with regard to the single-phase
working principle (Figure 7). These voltages will reach their
new stationary conditions after a few periods, but distinctly
later than the other quantities already discussed. The exact
value of the module capacitor voltage is important, but not
as important as the external quantities of the MMC.
These detailed measurement results show that the
internal behavior of an MMC is very complex. It also
depends on the actual state of operation in which a certain event (e.g., a fault) occurs. As a consequence, a full
assessment of MMC behavior must be based on a series of
scenarios-in simulation or in measurements. The worstcase scenarios are hard to envision in advance because

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iap, 0

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iap, 1

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iap, 2

ian, 1

ian, 2

ian, 0

Figure 6. The arm currents during the short circuit.

the interaction of the ac- and dc-current components with
the circulating currents in the arms of the MMC, altogether combined with the state of the fluctuating voltages of
the module capacitors, is very complex.

Simulation-Based Analysis of
DC Ship Grid Short Circuit
In the following, two converter topologies are compared
with regard to their mitigation of the dc ship grid short

IEEE Electrific ation Magazine / j une 2 0 1 5

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