IEEE Power & Energy Magazine - May/June 2015 - 83
figure 2. Measured voltage levels and active and reactive power in one feeder of the rural low-voltage grid in Eberstalzell, demonstrating different solutions under test.
5,000
4,000
3,000
2,000
1,000
0
-1,000
210 220 230 240 250
5,000
4,000
3,000
2,000
1,000
0
-1,000
210 220 230 240 250
210 220 230 240 250
(c)
(a)
-4,000
-2,000
2,000
0
5,000
4,000
3,000
2,000
1,000
0
-1,000
Umax = 241 V
-6,000
210 220 230 240 250
-4,000
Umax = 249 V
-6,000
210 220 230 240 250
-4,000
-2,000
2,000
0
6,000
4,000
Umax = 245 V
-6,000
210 220 230 240 250
-4,000
-2,000
2,000
0
6,000
4,000
Umax = 243 V
-6,000
210 220 230 240 250
-2,000
2,000
0
6,000
4,000
Compounded Control
Local Control Q (U )-Dynamic
Local Control-Fixed Setpoint
Local Control Q (U )-Static
Compounded Control
Local Control Q (U )-Static
Solutions Under Test
6,000
4,000
5,000
4,000
3,000
2,000
1,000
0
-1,000
210 220 230 240 250
(b)
Reactive Power
Active Power
Secondary Substation:
PV-Inverter:
Reactive Power
Active Power
Reference
Reactive Power
Active Power
Active Power
may/june 2015
Reactive Power
of DG systems, like roof-top installations
of photovoltaic (PV)systems, operate synchronously. In cases of high penetration,
the total voltage rise exceeds the limit of
25% (11.5 V). Typically, local demand
compensates for the voltage rise at least
partly. But as load is almost uncontrollable
for grid operators, the problem of violating voltage limits (especially overvoltage)
is usually encountered before the security
of supply issues resulting from increased
power flows (overcurrent) and exceeding
thermal constraints.
Various novel approaches have been
developed to manage this issue at different
voltage levels. An interesting experience at
LV levels has been implemented in Koestendorf and Eberstalzell, Austria. (See Figure
1.) At both locations, there is a substantial
amount of DG connected to the LV network
in the form of residential PV panels. It was
agreed upon with the local residents to create a large-sized laboratory in the field to
test grid behavior and the novel approaches
being implemented for voltage control. The
first step was to install an inverter, at a consumer's PV panels, that is able to sense the
voltage and to react autonomously following a predefined curve to inject or consume
reactive power, hence locally modifying the
voltage level. The type of inverter used in
the project performs voltage measurement
with an accuracy of about 1%.
The capacity to correct voltage grid
deviations by reactive power consumption
is limited, but the solution can be spread
throughout the lines, impacting near the
distributed sources, without a requirement
for telecommunications to coordinate the
operation. The local voltage control by reactive power can be strongly influenced (typically up to 2%) in case of the relatively high
impedance of lines (e.g., 25 m, 2.5 mm2,
and 0.36 Ω) and single-phase inverters (e.g.,
3 kW, 13 A) between the relevant point of
common coupling and the connector of the
inverter.
A second step in this implementation is
connecting the inverters to a telecommunications network. In Eberstalzell, Austria, this
was done via power line communications,
and the controller is implemented locally
on a small industrial computer placed at a
secondary substation (SS). In Koestendorf,
a virtual server on the network for the smart
ieee power & energy magazine
83
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