IEEE Electrification Magazine - December 2013 - 45

After a bus fault, a
converter can protect
the system by either
actively limiting the
fault current or by
turning off the
converter switches.

the faulted part of the system is isolated. By using the measurement of the
equivalent resistance at the terminals
of both power converters and segmentizing contactors, we can distinguish
short-circuit faults so that the system
can be protected from damage. above
all, the measurement of resistance
allows the contactors to discriminate
whether to open so that they can isolate the faulted section of the system.
the simulation results demonstrate that the fault detection method
using time-to-trip curves as a function of apparent circuit resistance properly coordinates
the feeding power converters with the contactors and efficiently protects the system against short-circuit faults. in
the example, the fault was detected and isolated within
10-20 ms, and the distribution bus was re-energized within 40-60 ms. the coordination of converters and contactors does not rely on communication; therefore, only local
information is needed. for systems with a short distribution bus and a high nominal current, the consequent low
resistance of the cable sections of the bus increases the
probability for the device to trip out of the desired range of
time. the same effect happens for a distribution bus with
sections of irregular length. this means that in certain
conditions, the contactors are not able to isolate the
smallest portion of the distribution bus after a fault and
one or more healthy sections of the system are disconnected. in these situations, this detection method can be
combined with differential current measurements that
can provide more accurate fault detection with the help of
communication between protection devices. in this way,
the resistance-time detection method can provide backup
protection in case of communication failure.
in conclusion, new fault protection strategies are
essential to the adoption of dc power distribution systems,
especially for electric ships. new approaches that capitalize on the current-limiting capabilities of electronic power
converters respond to this need and open the door to new
challenges and opportunities in power systems.

For Further reading
n. doerry, "transitioning technology to naval ships," science technol. found. future naval fleets, special
rep. 306: naval engineering in the 21st century, 2011.
m. e. Baran and n. r. mahajan, "dc distribution for industrial systems: opportunities and challenges," IEEE Trans. Ind.

Applicat., vol. 39, no. 6, nov./dec. 2003,
pp. 1596-1601.
J. g. ciezki and r. W. ashton, "selection and stability issues associated
with a navy shipboard dc zonal electric distribution system," IEEE Trans.
Power Deliv., vol. 15, no. 2, apr. 2000,
pp. 665-669.
h. li, W. li, m. luo, a. monti, and f.
ponci, "design of smart mvdc power
grid protection," IEEE Trans. Instrum.
Measure., vol. 60, no. 9, sept. 2011,
pp. 3035-3046.
p. cairoli, i. Kondratiev, and r. a.
dougal, "coordinated control of the bus tie switches and
power supply converters for fault protection in dc
microgrids," IEEE Trans. Power Electron., vol. 28, no. 4,
apr. 2013, pp. 2037-2047.
p. cairoli, K. lentijo, and r. a. dougal, "coordination
between supply power converters and contactors for fault
protection in multi-terminal mvdc distribution systems,"
in Proc. IEEE Electric Ship Technology Symp. (ESTS 2013),
apr. 22-24, 2013, arlington, va, pp. 493-499.

biographies
P. Cairoli (pietro.cairoli@gmail.com) earned his dr.eng. and
m.s. degrees in electrical engineering in 2010 from the
politecnico di milano, milan, italy. he earned his ph.d.
degree in electrical engineering from the University of
south carolina in 2013. he currently works in the department of electrical engineering at the University of south
carolina in the power and energy systems group on dc
distribution system analysis and protection, simulation
models, power electronics, and embedded control for
power management and protection.
R.A. Dougal (dougal@cec.sc.edu) earned his ph.d. degree
in electrical engineering from texas tech University, lubbock, in 1983. he is currently the chair of the electrical engineering department at the University of south carolina,
columbia, where he also leads the power and energy systems group. he is a director of the electric ship r&d consortium, which is developing electric power technologies
for the next generation of electric ships, codirector of the
national science foundation industry/University cooperative research center for grid-connected advanced power
electronic systems, and leads the development of the virtualtestBed-a computational environment for simulationbased design and virtual prototyping of dynamic,
multidisciplinary systems.

IEEE Electrific ation Magazine / d ec em be r 2 0 1 3

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