IEEE Electrification Magazine - March 2015 - 28

bus-bar or line section disconnection, but also frequent and
short-term typical power fluctuations from the renewable
power sources. The expected major renewable power loss
disturbance was considered to be the sudden disconnection
of all the PV power production (i.e., a 2-MW power loss). To
define the most typical severe values for the frequent and
short-term renewable power variations, some wind and PV
power production time series were analyzed for this power
system, having a resolution in the range of seconds. From
this analysis, a 20% and 50% change within 5 s was considered for the frequent power variations of wind and PV,
resulting in the time evolutions presented in Figure 3.

FESS Sizing for Major Renewable Power Losses
This power system was considered to lose security if,
after a major power loss, the transient frequency drop
provokes an automatic load shedding. Under this definition, the security criterion was that the maximum value
of transient frequency drop, obtained from a power loss
simulation, must not decay more than 1.3 Hz.
To find the minimum FESS Pn value that ensures the system security in the case of the major expected renewable
power loss, the dynamic simulation of this disturbance was
performed without an FESS and also by considering the
control action of an FESS with alternative Pn values. A

Pg (MW)
Pn

FESS Sizing for Frequent Renewable Power Variations
Line Slope = -1/R

∆f (Hz)
FDB
-Pn

(MW)

Figure 2. The FESS frequency droop characteristic.

2
1.5
1
0.5
0

0

20

40

60

80 100 120 140 160 180 200
Time (s)

Total PV Power Production
Total Wind Power Production
Figure 3. The frequent renewable power variation disturbance considered for FESS sizing.

28

typical value of 1/Pn Hz/MW was considered for the R
parameter. The simulation results are presented in
Figure 4. In these simulations, the loss of power was considered to appear at 10 s.
These results comprise the evolution of the system frequency deviation (Tf ) and of the FESS stored energy (E) for
all of the sizing alternatives. The results without an FESS
present a transient frequency drop with a maximum value
around 2 Hz. This result reveals that, although having in the
analyzed scenario enough diesel spinning reserve to accommodate a 2-MW power loss, this loss of renewable power,
without an FESS, may jeopardize the power system security
by provoking large transient frequency drops. The obtained
results also show that, from the tested situations, a 1-MW
FESS has the minimum required Pn value to avoid violating
the previously described security criterion.
By examining the results obtained with a higher Pn
value, it is interesting to realize that, for the sake of system security, it is not recommended to oversize the FESS
Pn value. In fact, although providing a better frequency
performance in the first postdisturbance seconds, the 1.5and 2-MW FESS suffer from a faster draining out of their
stored energy, leading to an early end of the FESS control
action. This Pn oversize may deteriorate the postdisturbance behavior by presenting transient frequency deviations with increased magnitude (like the ones obtained
with a 2-MW FESS). The same effect of early draining out
of the FESS stored energy was observed for an undersizing
of the R parameter value.

I E E E E l e c t r i f i c ati o n M agaz ine / March 2015

The security criterion regarding frequent renewable power
variations was that system frequency deviations should
not systematically violate the range of ±0.3 Hz. Under this
criterion, the capability of the 15-MJ/1-MW FESS sized for a
major expected power loss was evaluated to guard security for frequent renewable power variations (those presented in Figure 3). This disturbance was simulated without an
FESS and also considering the control action of a 1- or
1.5-MW FESS, having a 1/Pn Hz/MW value for the
R parameter. The results obtained are shown in Figure 5.
The results without an FESS yield transient frequency
variations that systematically reach +0.5 Hz and −0.6 Hz
and, therefore, violate the security criterion defined for
frequent renewable variations. On the other hand, the
15-MJ/1-MW FESS was, in the dynamic simulation, able to
guard the system security for frequent renewable
variations since the obtained transient frequency variations are, after the first postdisturbance seconds, stabilized within ±0.3 Hz. These results also show that, before
this disturbance, the required stored energy in the FESS
must be around 12 MJ.
Once again, the results obtained with a 15-MJ/1.5-MW FESS
exemplify the problems that may arise from oversizing the
FESS Pn value. In fact, although showing to provide the best
frequency performance in the first 100 disturbed seconds, the



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