IEEE Electrification Magazine - June 2016 - 61

in communication with all of the breakers. In this
article, two other control approaches are discussed:
local breaker control and paired breaker control. The
merits of all three control strategies are presented
here and some simulation results are shown in the
following section.

Central Control

inverter is isolated, the steady-state current through the
bidirectional breakers should reverse. Initially, the SCRs in
bidirectional breakers are not gated, so the breakers will
switch off as the current falls to zero. It is then the
responsibility of the local control to identify that the
breakers turned off due to a current reversal and not due
to a fault. This is one example of a
case where the control must gate the
SCRs to switch the breakers back on.
In this case, the local control
would be monitoring the output and
input currents of the breakers. If the
control observes that the input and
output current fell to zero without
exceeding a threshold set for fault
currents, that would indicate that
there is no shunt fault at the breaker output, and the breaker must be
switched back on. If this control
strategy is employed and the fault occurs at location 6
as described in the aforementioned example, the bidirectional breakers will be sent gate signals and will
switch back on, allowing them to conduct current in
the reverse direction. The advantage of this method is
that it does not require an elaborate communication
infrastructure. It would be much easier to expand the
system if each dc breaker had its own independent
local control.
There are some disadvantages associated with this
control strategy. If the fault is located behind the breaker, the control would misconstrue that it is a case similar
to the example above and would attempt to switch on
the device. For example, consider a fault at the output of
one of the generators (G). The breaker at that terminal
will not see a high current at its output, so the control
would gate the SCRs of the breaker. The SCRs will still
isolate a shunt-resistive fault because it blocks negative
current, but this is risky; if the fault impedance has high
inductive element, the fault current might resonate and
still interfere with the system. Another disadvantage of
this control scheme is that it will not isolate all fault
locations. If the fault is at the input end of the breaker,
the local control embedded in the breaker will not be
able to distinguish it from the case where the breaker
must be switched back on. All of these scenarios are
summarized in Table 1.

It would be much
easier to expand the
system if each dc
breaker had its
own independent
local control.

With central control, all the breakers
receive gate signals from a central
processor through a communication
protocol. At the same time, all the
breakers send a signal corresponding
to their input and output current to
the central control. The input current
is compared to a small threshold to
indicate whether the breaker is
switched on or off. The output current
is compared to a large threshold
value that indicates whether the breaker experienced a
fault at its output. Using this information from all breakers, the central control determines the fault location in the
grid and ensures that only the minimum required number
of breakers are switched off to isolate the fault. The central
processor also provides gating signals for start conditions
and use of the breakers as dc switches.
Central control has been simulated in previous work and
shown to perform well. The advantage of central control is
that it can locate and isolate a fault at any location in the
grid. Its processing requirement is not excessive, but the disadvantage of central control is the required communication
infrastructure. For example, if a universal asynchronous
receiver/transmitter is used as a communication protocol,
each breaker must decode an additional device and encode
the data into the proper format.

Local Breaker Control
In ac systems, differential protection is one of the most
common schemes employed for bus protection. For this
scheme, the output current is compared to the input
current. Under normal conditions, the input and output
currents are similar, but when a fault occurs, either the
input or output current changes rapidly, and the relay
uses that as an indication of a fault, signaling the breaker to open.
Each of the breakers can be monitored using the same
principle. The one important difference is that the dc
breaker does not require a relay to switch off; however,
under certain conditions, it may need to switch back on
when there is no fault. For example, suppose the inverter
load in Figure 4 is much greater than the dc/dc converter
loads combined. In that case, the steady-state current
flow through the bidirectional breakers would be from
nodes 9 to 5. Now, imagine that a fault at node 6 turns off
the breaker at node 6 to isolate the fault. This would also
isolate the inverter load from the system. Once the

Paired Breaker Control
It is possible for some breakers to operate with the
same gate signals. For example, consider the two
breakers on line 4. Either both the breakers conduct
the same current or neither of them conducts a current; it is not possible for one of them to be conducting
and the other to be open because they share the same
path. Therefore, it is possible for these two breakers to
share the same gate signal. The breakers that will be
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