IEEE Electrification Magazine - September 2013 - 33

Overview and applications.

throughout the ship, while loads connected to the faulted bus are temporarily switched to an opposite redundant dc bus during fault isolation. Present techniques fold back the voltage on the faulted bus using
an upstream phase-controlled rectifier (PCR) to force currents to zero
while an electromechanical switch isolates the faulted part of the bus.
However, such systems are slow in response and jeopardize power
continuity. The application of SSPDs to dc ZEDS has received considerable attention because of their fast response speed.

Distribution and protection
Within an electrical Zone
Within an electrical zone and between sources and loads, there is an
additional challenge presented by the current-limiting nature of SSPDs.
Because the SSPD actively limits the current during the fault-isolation
process, the current is the same upstream and downstream from the
fault location. Therefore, conventional approaches to fault protection
and coordination-which rely on voltage droop to ensure that protective
devices closest to the fault trip off first-will not work. Instead, the local
intelligence and speed of the SSPDs must be fully used to guarantee predictability in the response of the protection scheme to short circuits, no
matter where in the system they may occur.

system Overview
Figure 1 shows a hypothetical shipboard dc system that includes
aspects of both ZEDS protection and protection within electrical zones.
The system shown in Figure 1 achieves both bus segmentation and isolation of faults from the bus for fault
management. Using SSPDs, this system implements
fault protection between electrical zones by using the
SSPDs to quickly drive fault currents to zero or low levels and fold back the affected dc distribution bus (if
necessary) instead of allowing the PCRs to perform
that function. This enables electromechanical switches
(S1A, S2A, S2, S3, S3A) to open, isolating the fault from
the rest of the system. When faults occur internal to a
zone, cascaded SSPDs in combination with electromechanical switches (S2B, S2C) perform coordinated protective functions with conventional circuit breakers
only being used for protection of single downstream
loads or load panels with low current ratings.
As suggested above, two fault-isolation paradigms
need to be considered.
The first is one where a fault occurs on the dc bus
itself. In this scenario, the SSPDs on the bus (i.e., SSPD
1A and SSPD 2, if the fault occurs on the bus in Zone 2)
in Figure 1 will limit fault current, assuming that shore
power is the only source. A fault-location algorithm
(such as the one described in [3]) determines the location of the fault and which of the electromechanical
switches needs to be opened (i.e., S1A and S2). Meanwhile, as long as the fault is on this bus, the voltage supplied
to any loads connected to the bus is nearly zero, and many of the loads
will lose power continuity. Critical portions of the electrical zones will
be fed by dc/dc converters (CONV) or inverters (INV) whose inputs are
diode auctioneered with the opposite bus (according to the architecture described in [2]). Because the SSPD limits the fault current to safe
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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2013