IEEE Power & Energy Magazine - May/June 2015 - 84

The term "resilience" means the ability to prepare for
and adapt to changing conditions and to withstand
and recover rapidly from disruptions.

LV grid controller is used. Furthermore, small home automation systems are installed at participating households for communication and local optimization, and a customer's sites are
also linked via broadband Internet connection (fiber coaxial
hybrid) for communication with inverters, smart home appliances, and electric vehicle charging. The controller is allowed
to send commands to the inverters to request reactive power
consumption or generation, or control loads (in Koestendorf
only for charging of electric vehicles or other smart home
appliances), implementing a cooperative voltage correction
that has proved higher efficacy. In case of a communications
failure, all inverters continue working independently.
The solution may be improved taking advantage of
smart metering deployment. Within the demo measurement,
results of voltage levels (5 min rms/interval ~ 1 min, accuracy: ~0.5%) are sent from smart meters to the local voltage
controller to apply localized corrections while taking into
account the state of each segment of each LV line.
As a complement, a classical solution of medium voltage
(MV) grids, such as voltage transformers with the capability to modify the transformation ratio using an on-load tap
changer (OLTC), has been implemented at the LV level in
SSs. The voltage controller will sense the voltage levels in
the lines and operate the OLTC to obtain the correct voltage
profile for all feeders. So far, the local control at the OLTC is
compounded by the voltage-level measurements from peripheral parts of the feeders. It has the possibility to modify the
voltage in a range of ±5% of the nominal value. As the OLTC
can only step up or down the level at all nodes synchronously,
the performance of this control is limited due to different
voltage levels in the branches and phases even at one node.
Finally, in certain problematic and long LV lines, a point
in the middle has been selected, and an autotransformer has
been installed. The autotransformer is composed of three
sets of small transformers in series with the line to correct
the voltage in small increments (1.5% and 4.5%) using the
possibility to combine them in a series resulting in a total
range of ±6%.
As a representative example, Figure 2 shows the characteristic of voltage control from measurements at the inverters
and the OLTC voltage control in one feeder of Austrian LV
demo projects. The results demonstrate the impact of reactive
power in real operation, reducing the voltage level 1.7% and
a combined automatically optimized setup of compounded
control at the SS and dynamic reactive power characteristic
Q(U) at the inverters up to 3.5%. Related to the 3% of voltage
84

ieee power & energy magazine

band intended to for voltage rise in LV grids (reference), the
hosting capacity is increased by 116%, which implies a much
higher resiliency of the system.
The Austrian approach is very efficient but requires the
client acceptance to "collaborate" with their PV inverters
managing reactive power to contribute to the grid stability.
In Corredor del Henares, Spain, a static synchronous compensator has been installed to test an alternative approach
for correcting voltage problems on LV and MV networks.
The advantage is that DSOs can freely operate it to produce
or consume reactive power when needed, but the clear drawback is its cost.
Regarding MV grids, a supervisory control and data
acquisition approach has been tested in Austria. The automatic voltage control concept of DG DemoNet and ZUQDE
is based on the distributed system-state estimator and the
volt/var control (VVC). VVC performs dynamic optimization of the network combining OLTCs, capacitors, and
reactive power of DG. The first objective of the automation
is to keep the voltage in all distribution network bus bars
within the limits. Additionally, different objectives like loss
minimization or demand reduction can be combined. Furthermore, reactive power exchange with the transmission
network is also controlled. Generators are provided with
a primary reactive power control, while transformers have
OLTC control.
In the Italian Isernia project, voltage regulation on
the MV network is achieved by controlling the reactive
power injected into the network from a few PV customers
of 4-5 MVA and a 1 MVA/0.5 MWh Li-ion battery storage facility. Similar to the Austrian demonstration projects
(Koestendorf, Eberstalzell, and DG DemoNet), the solution
can operate autonomously. Voltage levels are monitored at
the connection point by the inverter, and the reactive power
is controlled locally or in a coordinated mode where the set
points are determined by a centralized control algorithm in
the distribution management system.
The storage facility in Isernia is also used for other
purposes like optimizing the local energy management
of a combination of renewable generation with electric
vehicle charging by means of an energy storage system
and, if needed, offer ancillary services to the distribution
network. It may also be used for the management of MV
lines, peak shavings, and load profiling and will be able to
replace the charging system or receive energy directly from
the PV plant.
may/june 2015



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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