IEEE Electrification Magazine - June 2016 - 69

Future Trends
There are many challenges to providing fault protection in both ac
and dc microgrids. In traditional
ac distribution systems, protective device coordination during
faults is achieved by choosing
appropriate circuit-breaker current-
time characteristics under clear
design guidelines and without
intercomponent communication.
The protection coordination strategy of typical SSCB technologies is
based mostly on a cluster of networked and centrally controlled
SSCBs and relies heavily on intercomponent communication,
ensuring coordination of the fault
detection and clearing among multiple devices. The proposed normally on JFET approach shows
promise in achieving coordination
in radial systems through simple
tuning strategies and without

Operation Capacity
of Power Electronic
Converters

103

10
Electromechanical
Circuit Breakers
Time (s)

which is roughly ten times faster
than other SSCBs reported in the
literature and 10,000 times faster
than mechanical circuit breakers.
Figure  4 provides a comparison of
time-current characteristics among
different circuit-breaker technologies, with the thermal-electrical
limit of power electronic converters
plotted. It can be seen that the new
ultrafast SSCBs are well within the
overload capacity-tolerance window of power electronic converters.
The ultrafast SSCB can limit the
peak fault current to a much lower
level and dramatically reduce the
magnetic and thermal stress on distribution components (e.g., cables
and bus bars), leading to a safer and
less expensive overall system. The
short duration of fault events
results in minimal line-voltage disturbances to the other loads on the
common distribution bus. The ability of other loads in the microgrid to
ride through a fault without losing
power provides improved system
performance without a major hardware investment.

10-1

Thermal-
Electrical Limit of
Power Electronic
Converters

10-3

10-5
Current SSCB Technology
Ultrafast SSCB Technology

10-7
0.1

1
10
Current (× Normal Rating)

102

Figure 4. A comparison of time-current characteristics among different circuit-breaker
technologies and the thermal-electrical limit of power electronic converters.

intercommunicatiIn  this regard, a oneThe proposed normally
ons. However, any
on-one supply-load
on JFET approach
SSCB approach will
arrangement, with
shows promise in
require additional
each power converter
achieving coordination
complexity and capiconnected to a dediin radial systems.
tal when it comes to
cated load, is the pregalvanic isolation of
ferred architecture,
the fault, which is
as shown in Figure 6.
t y p i c a l ly a c c o m p l i s h e d w i t h
A  large power source can be
mechanical contactors that open
designed as a cluster of smaller modafter the SSCB has driven the curular power converters, with each
rent to zero. This fact and the risks
associated with achieving device
coordination in radial systems that
look like ac distribution lead to the
Centrally
Autonomous
Controlled
conclusion that alternative breakerSSCBs
SSCBs
less solutions should be explored.
Circuit
It may be beneficial to take full
Protection
advantage of the inherent powerStrategy
limiting and self-shutdown capabiliProtective
Autonomous
Power
ty of the power converters in the
Hybrid CBs
Converters
microgrid, as shown in Figure 5. This
would help to eliminate hardware
Figure 5. The circuit protection strategy
costs and power losses associated
that combines a diverse set of circuit-breaker
with additional circuit breakers.
technologies. CBs: circuit breakers.
IEEE Electrific ation Magazine / J UNE 2 0 1 6

69



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