IEEE Electrification - September 2020 - 34

The dc bus system
significantly reduces
the complexity of
the IEP system
because the
essential,
intermediate dc
voltage of each ac
drive is unified and
connected as the
common dc bus.

carefully set by the bandwidth of
the hysteresis controller, as seen in
Figure 4(b), to ensure the power
semiconductors in converter are
still within their safe operating
areas. The clamped current will
inevitably cause under voltage at
the low-voltage ac bus of the hotel
loads during the fault-clearance
interval. The short circuit fault discrimination and clearance are prioritized and the under-voltage trip
should not override the short circuit fault clearance. Therefore, the
under-voltage protection threshold
needs to be lifted or modified
when the hotel load supply converter engages the short-circuit
fault ride-through procedure.

Connection to New Energy
Storage and Sources
The dc bus offers friendly interconnection to dc supplies
and loads because only dc-dc converters are required
rather than dc-dc and dc-ac converters for the ac bus
system, as shown in Figure 5. This technological
advancement is starting to show merits for increasing
deployment of onboard solid-state energy storages, such
as batteries and supercapacitors, for meeting the
demands of higher efficiency and lower emission. In the
future, other dc loads, such as electronic equipment, and
dc supplies, such as solar power generation, can be
directly fed to the dc bus. The direct dc-fed network for
electronic loads improves uninterrupted-power-supply
response time and increases system-level reliability and
efficiency, and this network has been successfully pioneered in other emerging applications such as data-center power supplies.

Protection

Due to lower system impedance from
the absence of transformers and generator impedance at the dc bus, shortcircuit fault current in the dc bus
system has a much higher rate than in
the ac system. Power converters are
prone to overcurrent for even a very
short time because the silicon-based
power electronic devices exhibit a
much smaller thermal time constant
and more sensitivity to excess temperatures, so the converter needs to be
cutoff before the fault current reaches
to the very high value. The fast
increase rate of dc fault current re--
quires the fault protection mechanism
to complete disconnection much faster
than its ac counterpart. The absence of
natural zero cross of the dc current
introduces more challenges to disconnecting the fault current than in the ac counterpart
because the arc from breaking a dc current is difficult to
suppress and contain.
Instead of using circuit breakers (CBs), the state-ofthe-art solution of breaking dc short circuit fault current
is to use ultrafast fuse (as known as the power electronic
fuse), which can disconnect a faulty circuit between 300
and 600 µs, subject to the fault current value. Fast fuses
are connected at the dc terminals of any converters and
bus tie connected to the dc bus, as shown in Figure 2.
The ultrafast fuse is a sacrifice solution and the fuse
must be replaced once protection occurs, meaning longer down time than CBs is inevitable after a fault due to
fuse replacement.
To have a quicker recovery time from a fault, reusable
and automatic solid-state switches (SSS) are emerging
as a high-performance solution. The SSS incorporates

ac Bus dc Load/Supply Connection

dc Bus dc Load/Supply Connection

ac Bus Switchboard

ac-dc Converter
Transformer

Energy Storage

Line Reactor

Figure 5. A comparison of the energy storage connection for the ac and dc bus systems.

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

dc-dc Converter

Stora
Energy Storage

IEEE Electrification - September 2020

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