IEEE Electrification Magazine - June 2015 - 42

The current control
limits the dc current
immediately,
protecting the
converter and
maintaining
controlled operation.

Filter

With regard to dc short circuits, a
second aspect is of relevance: Will
the source (generator) feed into the
short circuit or not? In the case of
the thyristor bridge, the firing angle
of the thyristors can be controlled
such that the dc-side current of the
thyristor bridge is quickly reduced
to zero. In principle, energy flow
could even be reversed by applying
negative voltage, but with regard to
the connected generating unit, this
option seems not viable: In addition
to the loss of load, accelerating
torque would be generated by feeding energy into the generator. However, no danger for
the converter or grid results from this topology.
In the case of a diode bridge topology, as shown in
Figure 1(b) (whether or not IGBTs are used), the generator
will inevitably feed into the short circuit as long as it generates voltage. The current is limited mainly by the relevant
impedances of the generator. Switches on the ac or dc side
are needed to clear the fault. The switching characteristics
and diode surge current capability have to match-otherwise, the diodes will be destroyed.
The dc-side switches are readily available up to a rated
voltage of about 3 kV, resulting from the requirement of dc
traction equipment. Higher voltages (and also large currents) can, at this moment, only be handled by additional

uDC

(b)

(a)

uDC

Two-Quadrant

uDC

Four-Quadrant

(c)
Figure 1. The basic topologies for feeding a dc grid from an ac source:
(a) a controllable rectifier, (b) a self-commutated converter (uncontrolled rectifier with additional switches), and (c) MMC: two- or four-quadrant modules.

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

power electronics, increasing losses
and decreasing reliability. A parallel
structure of a mechanical switch for
loss reduction and a power-electronic
switch for fast turn-off might be used.
However, the switching time might be
increased considerably because, at
first, the current has to be commutated from the mechanical switch to the
power electronics, then the mechanical switch can readily be opened, and
after that, the power electronics can
switch off [2].
Reducing the dc current requires a
countervoltage-the switch has to
deliver a distinctly higher voltage during turn-off than the
rated value of the dc ship grid voltage, making the rating
and size even more troubling. The demand for a higher
turn-off voltage results from inductances within the dc
grid. These can be intentionally placed within the filters or
as a slope limiter, but cable inductance also adds to the
effect. The following simplified explanation disregards the
resistive effects, which are usually low because of high-efficiency energy transport.
While the short circuit occurs, the full dc voltage acts on
the inductances, quickly increasing the current (see Figure 3).
Once the breaker voltage reaches the dc voltage, the dc grid
current remains constant but is not yet reduced. The breaker
voltage has to be considerably higher than the actual dc voltage to cause a negative voltage at the inductances, which
reduce the current to zero. Once the current reaches zero
again, the dc breaker voltage becomes equal to the dc grid
voltage at zero current. Please note that the instantaneous
power definition and the resulting energy transfer holds true
for breakers: The integral of the product of the voltage and
the current at the dc breaker during fault mitigation is equal
to the energy, which has to be dissipated within the breaker.
With regard to effort and size, the power-electronics part
of the switch would be designed for adiabatic switching-
practically, it is not feasible to instantly remove the heat
generated within the semiconductors during turn-off. Time
is needed to cool down the switch, limiting the number of
switching actions possible in a short period of time.
Obviously, a diode bridge topology is not easy to handle
in short-circuit situations. Verification of these aspects can
also be found in [2].
Resulting from these basic technical and topological
aspects, five main scenarios of short-circuit reaction can
be discerned with regard to turn-off energy flow as depicted in Figure 2(a)-(e):
xx
Figure 2(a) shows the reaction in the case of an ac switch
clearing the fault. The energy of the system is dissipated
in the fault and in the ac switch. As long as current is
flowing, the generator continues to feed energy into the
system. In this way, the short-circuit current is either
increased further or supported until the ac switch

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