IEEE Power & Energy Magazine - May/June 2015 - 39

may/june 2015

w
wmax

ΔP

w0
w
wmin

Pmin

0

PVSI

Pmax

P

figure 4. The active power-frequency droop characteristic.

variation or the loss of control information, that needs to be
accounted for to ensure the resilience of microgrid control
systems. The severity of communications uncertainty is
more relevant in demanding control applications like frequency control in islanding operating conditions, where a
fast and coordinated action is mandatory in combining central and local control schemes. In Figure 6, the frequency
variation in a microgrid resulting from different communication delays (jitter) of the set points exchanged with the
MGCC is shown. The ideal response, represented with the
full line, can have small variations as in the case where an
average 2-s delay is considered or a more noticeable effect
when the average delay is higher.
In Figure 7, the loss of frequency control set points on
top of delay variation is depicted. Although the system frequency is able to recover, despite higher requests to the local
control to immediately sustain the frequency value, with
higher data losses the system is very likely to collapse. These
examples show the importance of a communication infrastructure for demanding applications, like frequency control
and the need to ensure the necessary coordination between

50.2
50
Frequency (Hz)

controllers (LCs), microsource controllers (MCs) and electric vehicles (EVs) controllers (VCs).
Two basic problems need to be addressed in the operation of the microgrid: voltage control and frequency/loadgeneration balance. When in grid-connected mode, voltage
control becomes the key issue and can be performed through
a combined utilization of central decision that include control of MV/LV on load tap changing and control of the active
power of the microgeneration units. Local control is affected
through the use of a power/voltage (P/V) droop control
solution. When in islanding mode, frequency control is the
main concern.
Microgrids shifted to islanded mode require some form of
energy buffering to ensure initial energy balance. The necessary energy storage can be provided by flywheels, supercapacitors, or batteries (static or mobile when associated with
electric vehicles) and connected through appropriate power
electronic interfaces. Microgeneration units and active loads
can contribute to balance the system by responding locally
using a droop control approach, as shown in Figure 4.
Figure 5 shows an example of recorded microgrid frequency response from a laboratory scale microgrid during
islanding considering three cases, a base case without load
shedding, case 1 considering a single load disconnection,
and case 2 considering the shedding of two loads, followed
by the reconnection of one of them to evidence the load following capability during autonomous operating conditions.
During islanding, the microgrid frequency will not reach
its nominal value by the primary reaction of the droop controlled DER. The secondary control will dispatch controllable microsources to correct the frequency deviation. In
general, secondary load-frequency control strategies can
be identified: one implemented locally at each controllable
microsources and another centralized and mastered centrally
by the MGCC. Local secondary frequency control is added
to the active power control of microsources to determine the
new active power reference to compensate the power injected
by the storage units. The main advantage of local secondary
frequency control is that it only relies on local measurement
to define the new reference power. However, the active power
response of the microsources will also depend on the controller parameters. Centralized secondary control determines the
new microsources set points based on the overall state of the
microgrid. Controllable load shedding also plays an important role as an emergency functionality to aid frequency restoration to its nominal value after microgrid islanding.
The participation of the different elements of a microgrid
controllable portfolio is based on a supporting communications infrastructure that ensures the exchange of control set
points. Despite the fact that several technologies are available to ensure the necessary connectivity within microgrid
control schemes, it will involve the interconnection of several
data networks, where quality of service might not always be
appropriate. Nevertheless, there are still uncertainties associated with the communications systems, namely the delay

49.8
49.6
49.4
Islanded
Base Case
Case 1
Case 2

49.2
49
48.8
60

70

80

90
100
Time (s)

110

120

figure 5. Microgrid frequency during islanding (recordings
were made at INESC's microgrid lab).
ieee power & energy magazine

39



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
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