IEEE Electrification Magazine - March 2015 - 34

DSP Control Board
Full-Bridge Inverter

Dual Active Bridge
HF Transformer

Figure 10. The EV bidirectional charger prototype.

53.2

4.5

52.2

1.5

51.2

-1.5

50.2

-4.5

49.2

-7.5

48.2
50

Time (s)

100

-10.5
125

Frequency with P-f Droop
Frequency Without P-f Droop
Active Power with P-f Droop
Active Power Without P-f Droop
Figure 11. The frequency and EV active power response to the
microgrid islanding.

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

Active Power (kW)

Frequency (Hz)

and software modules, promoting an active and intelligent
management of electric grids in scenarios characterized by
a progressive integration of renewables and EVs. A distinct
feature of this laboratory relies on the operation of a
reduced-scale power system, comprising the integration of
both commercially available solutions and in-house developed prototypes. The laboratory constitutes the physical
space that integrates both equipment and software
modules, allowing an individual and fully integrated development and testing of concepts, algorithms, and communication solutions. These are envisioned to support the operation of future power systems. Simultaneously, such infrastructure provides the necessary conditions for the
development of new concepts and solutions that will be
later transferred to the industry.
The EV charger functionalities previously described,
providing frequency support in isolated power systems,
were tested in a bidirectional charger prototype specifically developed. The power electronic converter is composed

of half-bridge assemblies, including insulated-gate bipolar
junction switches and hybrid gate drivers as well as passive components such as protection devices, voltage and
current sensors, and control hardware. The inverter control algorithms run in a stand-alone digital signal processor, which allows the inverter to operate autonomously.
Figure 10 shows the EV bidirectional charger prototype
that was implemented. The charger can be divided into
two stages with independent control schemes: a grid-tied
full-bridge inverter that controls the power flow between
the dc bus and the grid and a dual active bridge that regulates the current flowing from/to the batteries and assures
the galvanic isolation (through the high-frequency transformer) between the grid and a battery pack. The fullbridge inverter regulates the dc bus voltage to ensure
adequate supply to the dual active bridge input stage. The
inverter is controlled as an active power source with a
limit of ±3,680 W and unitary power factor using a proportional-integrative controller implemented in a synchronous reference frame.
To emulate the operation of an autonomous power system, SMA Sunny Islands 5048 battery inverters (3 × 5 kW,
230/400 V each) are used. These inverters are mainly used
for the electrification of remote areas, being able to operate autonomously in isolated systems. The experiment
conducted in the laboratory consisted in the parallel operation of a load, the SMA sunny island inverters, and the EV
charger prototype. A step load connection was then
performed to evaluate the frequency response as well as
the EV charger prototype power response. The main
results are depicted in Figure 11. When the EV P-f control
is not present, the frequency drops to 48.3 Hz. When the
EV P-f control is active, the EV reverses the power flow and
transiently operates in the vehicle-to-grid mode. The participation of the EVs reduces the total load, consequently
reducing the minimum frequency reached (48.8 Hz). These
results provide evidence of the benefit resulting from the
participation of EVs in islanded system frequency regulation. The active participation of the EV charger clearly contributes to reduce the frequency excursions following
power imbalances in the system.

conclusions
This article presents the main challenges and identifies
possible solutions related to the operation of remote, isolated power systems with a large integration of renewable generation, taking as example the real case of the
Portuguese Atlantic islands. Several studies are detailed
based on the implementation of new control solutions
devised to mitigate stability problems that may occur,
particularly in terms of frequency response. The development of new control strategies, as well as the use of new
grid technologies, is an important step toward an efficient and secure operation of isolated or autonomous
power systems. The role of energy storage, either in the
form of stationary storage, such as flywheels or in the

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