IEEE Power & Energy Magazine - May/June 2021 - 23

limitation strategies deployed in the DER controller. More
specifically, the priority given to the active or reactive component by the DER controller affects the short circuit current power factor. The overall current I CB7 sensed by CB7
is higher if the current I CB4 provided by the upstream network has a similar angle to the fault current I PV . Figure 3
demonstrates how the power factor of the short circuit current contribution from PE-interfaced generation units and
the inductive (X) to resistive (R) ratio of the lines affect the
total fault current magnitude.
All in all, the representation of PE-interfaced units in
short circuit studies is not straightforward. Unlike the models universally adopted for synchronous generators, the
contribution of PE-interfaced units is affected by operating
conditions, grid code requirements (e.g., fault ride through
and current contribution in voltage sags), and the control
algorithms deployed to meet these requirements. Incompatibility between the applied protection scheme and the fault
ride-through requirements can pose a significant threat to
the microgrid stability as demonstrated in the " Fault RideThrough and Protection System Coordination " section.
Last but not least, the grounding issue becomes an emerging issue in microgrid protection. The ground connection
provided by the main grid, i.e., the grounded neutral of the
power transformer, is lost during the islanded operation of
the microgrid, resulting in ineffective ground fault protection. Therefore, provisions for an alternative ground connection should be made. This can be achieved by installing a
grounding transformer at the main bus of the microgrid [bus
2 in Figure 3(a)], which is connected to the network only in
case of the loss of mains (i.e., when CB1 or CB2 is open)
by an appropriate interlocking scheme between the transformer's circuit breakers (i.e., CB1 and CB2) and grounding
transformer circuit breaker (i.e., CB20).

Fault Ride-Through and Protection
System Coordination
A major concern in microgrid operation is the impact of
topology changes on short circuit currents and the coordination of fault ride-through requirements with the protection
system. The latter can be crucial for the overall system stability, especially for microgrids in permanent islanded mode
(off-grid microgrids). Off-grid microgrids serve physical
islands or geographically remote areas that are isolated from
the interconnected distribution system. The high production
costs based on diesel units and the potential renewable energy
sources (RESs) frequently available offer attractive incentives for their transformation to off-grid microgrid systems.
The configuration of a typical islanded microgrid with
high RES (primarily PVs and wind generators) penetration
is presented in Figure 4(a). To achieve high RES penetration, battery storage units are required. All of these units
(i.e., the RESs and storage) are interfaced by the PEs. In
traditional island systems, diesel generators are responsible
for fault current contribution, and the protective devices are
set accordingly to achieve fast fault clearing times. Typical
existing protection schemes consist of relays on the feeders
(controlling CB4 and CB12) and fuses on the lateral lines.
The increasing penetration of PE-interfaced units, however, significantly reduces the available fault currents.
In Figure 4(b), the current characteristics of the protective
devices as well as the short circuit current contribution of
the battery unit and the diesel generator of the island system
of Figure 4(a) are presented. It is seen that when the diesel
generators are not in operation, the typical relay settings and
fuse time-current characteristics lead to a very slow protection response. Moreover, changes in the topology of the
islanded microgrid can significantly affect the fault current
levels due to the different behavior of the units.

IQ: Reactive Current
IN: Nominal Current
∆V: Voltage Deviation
VN : Nominal Voltage

100

Voltage (%)

90
80
Greek
Noninterconnected
Islands
Germany

60
40
20
15

-50%

IQ
IN

-10%
-10%

∆V
VN

-100%

0
0

0.15

0.5

1.5
Time (s)
(a)

2.5
(b)

figure 2. (a) Typical fault ride-through characteristics and (b) the wind turbine dynamic reactive current contribution
during a fault.
may/june 2021

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IEEE Power & Energy Magazine - May/June 2021

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