IEEE Power & Energy Magazine - May/June 2015 - 40

50.4

Frequency (Hz)

50.2
50.0
49.8
49.6
49.4
te = 3.0 s
te = 4.0 s
te = 5.0 s

49.2
49.0
48.8

0

20

40

60
80
Time (s)

100

120

figure 6. The impacts of communications delays.

local and central control to overcome potential uncertainties
in the communication systems.

Microgrid-Aided Restoration
The occurrence of general blackouts, resulting either from
natural disasters or intrinsic events related to power system
operation, is a rare event but has severe economic and social
impacts, as a consequence of the long restoration times
resulting from complex procedures followed by system
operators. In such a catastrophic scenario, the microgrid
operational flexibility and local generation capabilities can
be coordinated to provide local restoration capabilities. The
microgrid restoration procedure (black start, BS) is triggered
by the MGCC when a general or local blackout occurs or
when the MV network is not able to restore microgrid operation after a predefined time interval. Similar to conventional
power systems, the microgrid restoration procedure can be
technically regarded a sequence of actions to be checked and
performed by microgrid LCs (and MCs) in coordination
with the MGCC. However, the procedure should be fully

50.4

Frequency (Hz)

50.2
50.0
49.8
49.6
49.4

Ideal
Average
Maximum Deviation
Standard Deviation

49.2
49.0
48.8

0

20

40

80
60
Time (s)

100

figure 7. The impacts of communications losses.
40

ieee power & energy magazine

120

automated without requiring the intervention of distribution
network operators.
To perform service restoration at the microgrid level, it is
necessary that the microgrid includes the following:
✔ communicating their generation availability and operational status to the MGCC
✔ LV switches to disconnect the microgrid feeders, loads,
and microsources in case a generalized fault occurs
✔ communication infrastructure powered by dedicated
auxiliary power units to ensure the communication
between the MGCC and the local controllers.
✔ adequate protection equipment to protect microsources and the LV grid from the fault currents and to isolate the faulted area. Since the BS procedure involves
a step-by-step connection of microsources to the LV
grid, the short-circuit power at the point where protection devices are installed will change. Thus, under
such protection strategy, it is assumed that, during the
restoration procedure, the MGCC has the ability to
change protection devices settings to efficiently detect
and isolate microgid faults.
After a general blackout, the MGCC will trigger the BS
procedure, being possible to organize the overall procedure
in the following sequence of events:
✔ Microgrid status determination. The MGCC evaluates the network status both upstream and downstream:
*	 Upstream network status: The MGCC only actives
the restoration procedure if there is not any alternative to reconnect to the main grid. Before activating
the procedure, the MGCC waits for a confirmation
of a local or general blackout occurrence from the
distribution system operator to eliminate the possibility of an interconnection switch malfunction or
possible reconnection to the main grid through MV
network reconfiguration.
*	 Downstream network status: The MGCC evaluates the LV network status, analyzing switch status
and alarms, to check the existence of local faults or
equipment failures. At this stage, the MGCC also
evaluates the generation and active load resources to
ensure the successful microgrid service restoration.
Historical data resulting from the microgrid operation can provide information about the priority loads
to be restored.
✔ Microgrid preparation to start the restoration
procedure. The MGCC should send a signal to the
local controllers (MC, LC) to ensure the disconnection of loads and microsources from the LV network.
Then microsources with local BS capability can be
restarted and used to power some local loads. This
procedure ensures that the microsources with storage
capacity providing back power to their local loads are
not energizing larger parts of the LV network.
✔ Microgrid energization, connecting the microgrid
central storage unit at the MV/LV substation and
may/june 2015



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2015

IEEE Power & Energy Magazine - May/June 2015 - Cover1
IEEE Power & Energy Magazine - May/June 2015 - Cover2
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IEEE Power & Energy Magazine - May/June 2015 - Cover3
IEEE Power & Energy Magazine - May/June 2015 - Cover4
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