IEEE Electrification Magazine - June 2016 - 60

D1

is

S1

C

dc
Source

N2

R

is

io

N1
S2

io

dc
Load

Fault

D2

dc Breaker/Switch

Figure 3. A bidirectional, coupled-inductor dc breaker.

Figure 1. The coupled-inductor dc circuit breaker.

is (A)

8
4

G

1

A

C

4

D

5

G

6

-

~

E

100 ┬Ás

0
7

3

io (A)

100
50

G

2

B

J

11

F
K

9

0

-

-

--

Figure 4. The dc microgrid test system.

As can be seen,
central control
provides the desired
result for all of the
fault locations,
without exception.

bidirectional SCRs. SCRs are also
added in the negative rail, which
was seen as necessary in the
microgrid system to prevent circulatory currents. To use the breaker
as a dc switch, current flow direction should be known. If I 1 , as
shown in Figure 3, is positive, then
S 1 should be gated. This will discharge the capacitor on the secondary winding of the transformer
such that the primary current will
force I1 to zero, thereby switching off the SCRs in the
primary conduction path. If I2 is positive, S2 must be
gated to use the breaker as a dc switch.

notional DC Microgrid
Figure 4 shows the dc microgrid that will be studied
herein. The generators (G) are standard dc power supplies with droop control for power sharing. Coupledinductor breakers are placed to protect the sources,
lines, and loads. Two bidirectional coupled breakers
(breakers E and F) are placed on one line that can conduct power in both directions. The line impedance is
modeled by small a inductance and resistance. Node 3
represents the bus, with both the sources feeding into
it. Nodes 6, 8, and 10 are terminal nodes leading to
I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2016

8
10

I

Figure 2. The measured source and load currents.

60

H

loads. An inverter load is connected
to node 6, while nodes 8 and 10 feed
dc/dc converter loads. In this
notional dc microgrid, the dc/dc
converters are of the boost topology.

Control Options

The dc breaker proposed in this article is autonomous in that it does not
need to detect the fault to switch off.
The fault response is completely
automatic in the absence of gate signals for the SCRs; however, in the dc microgrid configuration shown in Figure 4, there may be certain cases where
external control will be required, such as:
xx
for maintenance or repair, it might be required to divert
power from one branch to another (this is an example
of where the breakers will be used as a dc switch)
xx
for loads with a high starting current, where some
control will be required to gate the SCRs until the system reaches steady state
xx
for reconfiguring the system such that the direction of
current through a bidirectional breaker has to be changed
xx
for integrating a branch or load back into the system
after a fault has been removed.
In a previous work with z-source dc breakers, the proposed control was a central control processor constantly



Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016

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