IEEE Electrification Magazine - March 2015 - 32

∆f (Hz)

0.5
0.25
0
-0.25
-0.5
-0.75
-1
-1.25
-1.5

Bus 1, with VSPH
Bus 2, with VSPH

5
t (s)
(a)

Minimum Value
Reached by ∆f/∆f (Hz/s)

Bus 1, Without VSPH
Bus 2, Without VSPH

Bus 1,
Without VSPH

Bus 2,
Without VSPH

-5
-10
-15
-20
(b)

Figure 7. The dynamic simulation results of a three-phase short-circuit in a specific 60-kV bus-bar for a severe operating scenario in Madeira Island.
(a) Time evolution of frequency deviation in two important bus-bars for load-shedding activation and (b) minimum value reached by the rate of frequency
change in two important bus-bars for load-shedding activation.

studies that show the impacts of EVs if integrated in the
power system as conventional loads. The main conclusion is
that some controllability degree must be introduced in the
EV-charging process to mitigate many problems that may
arise from EV integration. In papers by Nies et al. and Lopes
et al., advanced control strategies are presented enabling a
better usage of existing grid resources while postponing
investment in grid reinforcement.
Therefore, this usage of EV resources
available in the grid must consist of a
synergetic pathway for both EV owners
and system operators.
As previously mentioned, isolated
systems such as in small islands are
very weak grids with small inertia.
This is too often the bottleneck for the
possible expansion of renewables such
as wind and solar since these technologies have a highly variable behavior
and do not provide power frequency
control (or inertia) to the system. Large
load/generation imbalances may be
caused by resource fluctuations, jeopardizing the quality of service and the
continuity of supply. Therefore, the
presence of a sophisticated management structure for future EV integration should be an enabler for further
solar and wind power integration in island systems.
In terms of dynamic operation of an isolated system,
EV flexibility is particularly interesting for primary
frequency control, especially when highly variable
renewable generation sources are available but their
integration is limited because of security constraints.
The flexibility provided by EVs connected to the grid
can be exploited for primary frequency control purposes in an isolated power system. This is achieved by including a P-f droop control strategy in the implementation of

the EV coupling inverter. This allows the EV to actively
adapt the power exchanged with the grid based on the
local frequency measurement. As shown in Figure 8, for
frequencies around the nominal value (in this case,
50 Hz), the EV will charge the battery at a predefined
charging rate. If a disturbance occurs and the frequency
drops below the deadband minimum frequency, the EV
reduces its power consumption,
thus reducing the load of the system. If the system frequency overpasses the deadband maximum frequency, the EV can also increase its
power consumption. For large disturbances, causing the frequency to
go below the zero-crossing frequency (f0), the EV starts to inject power
into the grid [vehicle-to-grid (V2G)
functionality].
The participation of the EV in the
frequency regulation mechanisms of
the islanded power system is limited
to a predefined frequency range. When
the frequency becomes out of this
range, the EV will inject/absorb a fixed
power, which can also be defined. The
definition of the EV control parameters
will depend on the EV charger characteristics and on the willingness of EV
owners to participate in such services. These parameters
may differ from grid to grid and can be adapted by grid
operators to promote adequate coordination with the available frequency regulation mechanisms (such as loadshedding schemes and availability of energy storage devices and their state of charge).

In all situations
where frequency
stability problems
were detected
without the control
action of VSPH units,
the activation of this
control was able to
eliminate the
frequency security
problem.

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

Flores Island Study Case
The current operational practices in Flores Island recommend the use of at least one diesel unit to perform load

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