IEEE Electrification Magazine - June 2017 - 60

Magnitude (dB)

powered by the DERs, including
inverter-interfaced and rotating-
based DERs. In the grid-connected
Positive/Negative/
vabc
Directional
Zerooperation mode of a microgrid, be-
Sequence Estimation
Information
cause the short-circuit capacities of
Positive/Negative/
iabc
the upstream system and the DERs
Zeroare significantly different, the fault
Sequence Estimation
current is mainly contributed by the
Current
upstream distribution system via
vabc
Positive/Negative/
Protection
Current
Zerothe PCC, whereas in the islanded
Threshold
Sequence Estimation
operation mode of a microgrid,
Tripping
Voltage
because no upstream distribution
iabc
Positive/Negative/
Signals
Protection
Voltage
system is connected, the fault cur-
ZeroThreshold
Sequence Estimation
rent is solely contributed by the
I 2t
DERs. The fault current level is
iabc
Positive/Negative/
Protection
I 2t
much lower than that in the grid-
ZeroThreshold
Sequence Estimation
connected operation mode because
of the limited fault current capaci-
Complementary
Enhanced
ties of inverter interfaces, especially
Communication
Protective Device
for the islanded microgrids with a
high penetration level of inverter-
interfaced DERs. Hence, an adaptive
Figure 5. The configuration of the enhanced protective devices used in microgrids.
setting of a protective device should
be used to make it suitable for both
grid-connected and islanded operation modes.
For the remote microgrids, as mentioned previously,
io
L
L
the settings of the protective relays should be selected on
Inverter
DER
the basis of the types of DERs in the system. Meanwhile, it
Interface
+
should be adaptive to the topology changes and capable of
C
vo
sensing bidirectional fault current. On the basis of the pre-
-
ceding technical requirements, an enhanced protective
(a)
device can be developed, which integrates the directional
information of fault current and the current/voltage/I2t
thresholds to successfully identify the faults. A comple-
100
mentary communication network also can be used to
Resonance Peak
assist in the detection of fault current direction. A typical
50
configuration of the enhanced protective device is shown
in Figure 5.
0

enhanced Stability: effective
Damping in the Weak Grid Context

-50

Phase (°)

90
45
0
-45
-90
1.6

1.8

2

2.2

2.4

2.6

2.8

Frequency (kHz)
(b)

Figure 6. A frequency domain analysis of an LCL filter. (a) A typical
circuit configuration of an LCL filter. (b) A Bode plot of the transfer
function from the inverter output voltage to the output current at the
PCC side.

60

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2017

In contrast to conventional electric grids, remote micro-
grids are commonly regarded as weak grids. This assess-
ment is based on the relatively low short-circuit capacity
at the PCC. Because a remote microgrid is usually sup-
ported by backup diesel generators, which can be con-
nected at the PCC, its short-circuit capacity is much
lower than that of conventional large-scale electric grids.
In other words, the interconnected impedance at the
PCC of a remote microgrid can be larger than that of tra-
ditional power systems, which induces a weak grid con-
dition. The weak grid characteristics may cause
additional stability issues because multiple DERs and
loads are coupled to the common backup generators via
relatively large impedance. Each interface inverter is
connected to an output filter comprising an inductor



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