IEEE Electrification Magazine - June 2015 - 47

current until t = 0.66 s (detection of
short). With the dc short detected, the
maximum output voltage is applied to
quickly extract energy from the system,
reducing the currents to zero. Afterward,
currents are kept at zero, finishing the
whole reaction within an extremely
short period of time. In the case of the
2Q-MMC, even the blocking of all controlled switches (IGBT modules) cannot
control the current-it rises to more
than three times the current seen in the
4Q-MMC case, leading to high thermal
stress for the diodes. After opening the
ac-side switches, the currents return relatively slowly to zero
because of the free-wheeling characteristic caused by the
diodes of the 2Q-MMC. The energy remaining in the inductors
is dissipated into the cable and the diodes of the MMC
because the short is supposed to be ideal. Mitigating the fault
takes about one second in total.
It should be noted that after fault mitigation, the 2Q-MMC
is completely switched off, with its power-electronic devices
heated up by the high short-circuit current. Even if the defective segment of the cable can be separated (optimally by automatic disconnectors switching under no load), re-establishing
the dc grid voltage is time consuming. In the case of the
4Q-MMC, the converter is still in controlled "hot-standby" after

fault mitigation. Hardly any extra heat
was generated. After separating the
defective cable segment, normal operation can be restored immediately.
An additional advantage of the
4Q-MMC topology (not simulated here)
is the soft startup of the dc ship grid. In
the case of the 2Q-MMC, closing the generator-side ac switch lets the dc grid voltage rise immediately to about half the
nominal voltage, caused by the inherent
diode structure. If the short circuit still
exists, a high short-circuit current will be
the consequence. In the case of the
4Q-MMC, the current control limits the current during startup,
detecting a short circuit at low current and shutting down the
system again immediately.
With all these aspects in favor of 4Q-MMC technology,
the drawbacks (which of course also exist) shall not be
concealed. On the one hand, losses are higher by a factor
of perhaps 1.6, strongly depending on the point of operation and the voltage level selected for the modules. On the
other hand, module electronics have to start working
autonomously at half the value of the initial capacitor
voltage of the modules of a 2Q-MMC.

Thyristor-based
converters usually
employ currentbased grid operation,
the main smoothing
element being an
inductor.

Pmot

2
0
-2

Pgen

-4
-6
0.50

Pgen

Pmot

-8
0.55

0.60

0.65

0.70

0.75

t/s
Figure 10. The ac-side power of the two 4-QS MMC in the case of a
dc-side short circuit; positive: converter to ac.

15
U/kV; I/kA

iDC, mot
iDC, gen

iDC, mot

vDC, gen

-5
0.55

0.60

0.65

0.70

iDC, mot
iDC, gen

iDC, gen
vDC, mot

-10
0.50

vDC, mot
vDC, gen

The principle of operation of the MMC and the reason for
the difference in behavior of 2Q-MMC and 4Q-MMC can
best be illustrated by the MMC arm voltages. The following
figures show the MMC arm voltage of the module stack p1
[see Figure 1(c)]. Figures 13 and 14 show the time function
for the 2Q-MMC and 4Q-MMC arm voltage, respectively.
In normal operation, both voltages are identical. The
expected 50-Hz oscillation with its dc offset given by the dclink voltage is seen. At the moment of the dc short circuit,
the 2Q-MMC voltage is quickly reduced to zero. The 4Q-MMC
offers more options: The voltage immediately drops to the
controlled current-limiting voltage, and the 50-Hz oscillation
is now centered at a slightly positive mean value for 10 ms.
After 10 ms, the dc short is detected and a short negative
voltage spike reduces the dc current to zero. After this, the

U/kV; I/kA

P/MW

Comparison of MMC Arm Voltages

vDC, mot
vDC, gen

vDC, gen v
DC,, mot
vDC, mott

iDC, mot

iDC, gen

-5
-10
0.50

0.75

Figure 11. The voltage and current of the two 2-QS MMC in the case
of a dc-side short circuit.

0.55

0.60

0.65

0.70

0.75

t/s

Figure 12. The voltage and current of the two 4-QS MMC in the case
of a dc-side short circuit.

IEEE Electrific ation Magazine / j une 2 0 1 5

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