IEEE Electrification Magazine - March 2015 - 31

time-domain simulation, for different operating scenarios
and for each one of the previously described disturbances.
Security improvement was evaluated by comparing the
obtained transient behavior of the power system with and
without the frequency control action of VSPH units. The
most relevant obtained results are described next.
Figure 6 presents the evolution of the system frequency
deviation (Tf ) and the rate of frequency change (Tf/Tt)
obtained from simulating the frequent renewable power
variation disturbance for a severe operating scenario.
These results show that VSPH units were able to significantly improve the transient behavior of the system frequency. Particularly, this control action was able to
eliminate the considerable large frequency excursions that violate the
security criteria for frequent disturbances. None of the obtained behavior, with and without the VSPH
control action, is severe enough to
provoke load-shedding activation.
Figure 7 reports a situation where
the VSPH action was able to avoid
automatic load-shedding activation. It
presents the time evolution of the frequency deviation (Tf ) in two important bus-bars for load-shedding activation. For clarifying the analysis, load shedding was not
included in the simulations. The disturbance was a threephase short-circuit, simulated at 1 s in a specific 60-kV busbar, with a duration of 150 ms. During the fault, the voltage
drops in several parts of the system led to the disconnection
of some wind parks (including a wind park with fault-ridethrough capabilities) and to the disconnection of the oldest
pumped hydro units. In this situation, the available spinning
reserve in conventional thermal and hydro generation was
enough to compensate the obtained power loss but was not
fast enough to alone avoid load-shedding activation.
As a general result of this study, it was possible to
conclude that in all situations where frequency stability

problems were detected without the control action of VSPH
units (only two situations are described in this article due to
size issues), the activation of this control was able to significantly improve the transient behavior of system frequency
and, therewith, eliminate the frequency security problem.

Benefits of Introducing EVs in Flores Island
Flores Island is one of the islands in the Western group of
Azores. It has an area of about 140 km², with a length of
under 17 km2 and a width of over 12 km. Its total population is below 4,000 inhabitants.
Flores Island has a 15-kV distribution network with two substations,
each located in one of the two existing
generation plants. The generation system is composed of four diesel generators with the nominal power of
625 kVA, two wind turbines with nominal power of 330 kVA, and four hydro
units (three of 370 kVA and one with
740 kVA). In 2009, the annual peak
load was 2,200 kW and the valley load
was 750 kW. In this network, the most
critical operation period is the load
valley, where the extremely low load
leads to a generation dispatch composed of a rather small number of
generation units. Therefore, frequency stability concerns
may be expectable, especially in good wind resource conditions. In fact, the island could explore more of its endogenous resources, but operational constraints prevent it. There
is a great expansion potential for wind power as well as
hydro generation, which so far is concentrated only on a single site, with others readily available for exploration.
The potential for renewables integration in Flores Island
allows considering the electrification of the transportation
sector. This will play a major role in reducing carbon emissions from the energy sector. But the integration of EVs in
islanded systems can be both a great opportunity and a
threat to the local power system operation. There are several

The full-bridge
inverter regulates
the dc bus voltage
to ensure adequate
supply to the dual
active bridge
input stage.

1.25
∆f/∆t (Hz/s)

∆f (Hz)

0.75
0.25
-0.25
-0.75
-1.25

50 60
t (s)
(a)

100

1
0.75
0.5
0.25
0
-0.25
-0.5
-.0.75
-1

Without VSPH

50 60
t (s)
(b)

100

With VSPH

Figure 6. The dynamic simulation results of frequent variations in renewable power for a severe operating scenario in Madeira Island: (a) the
time evolution of system frequency deviation and (b) the time evolution of the rate of frequency change.
IEEE Electrific ation Magazine / March 2 0 1 5

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2015