IEEE Power & Energy Magazine - May/June 2017 - 34
OLTCs that use a few local
field measurements to control
the voltage in the feeders (widearea control, WAC) are effecHC Increase
tive in only 48% of the feeders.
Average
Relative Standard
A centralized solution that comValue [%] Deviation
bines OLTC and DG control
produces good results in only
67.00
0.57
20% of the feeders. In the rest of
64.53
0.67
cases, as additional generation
62.99
1.38
is added, many feeders become
thermally constrained, and this
53.36
1.27
voltage-control solution is ineffective. Potentially more expen20.02
0.19
sive solutions like curtailment or
classical grid reinforcement are
HC Increase
applicable everywhere.
Average
Relative Standard
System performance is deValue [%] Deviation
pendent on many factors. Ex37.75
2.87
perience shows that a centralized solution using coordinated
37.13
1.62
OLTC and DRESs allows for
34.81
3.10
the highest amounts of HC.
Coordinated operation can be
29.72
3.53
achieved using a distribution
13.89
2.25
state estimator or an OPF to
10.83
0.39
establish the best settings at
each time. On average, the introduction of an OPF triples the
HC of a VVC system and almost doubles the effectiveness of a
WAC and VVC solution.
The combined effect of OLTCs used at primary and secondary substations is also an interesting case. Utilities around
the world use high-voltage (HV)/MV transformers equipped
with OLTC to control voltage levels [Figure 5(a)]. This has
some limitations when some of the MV feeders have contrasting voltage issues. The combined use of OLTC at both
HV/MV and MV/LV transformers can provide much more
flexibility for managing voltages, increasing HC up to 179%
in our experiments.
An alternative approach is inserting autotransformers in
the middle of the problematic MV lines [Figure 5(b), the
MV side] to correct these contrasting voltage levels. Then
the flexibility can be doubled at the MV level to absorb the
LV variations due to generators and loads. When LV lines of
a certain secondary substation present contrasting voltage
levels, introducing autotransformers [Figure 5(b), the LV
side] in some LV lines also increases the flexibility of the
whole system. The problem with this and similar ideas is the
cost of the new elements, but the potential to increase HC
and the effectiveness of existing assets is enormous.
Flexibility can be also obtained from third parties [Figure 5(c)]
such as generators and consumers. Active demand techniques
or energy storage installed at customers are some of the possibilities to regulate the loads in the lines. In some countries,
table 1. The HC increase resulting from applying
different SG solutions in the studied networks.
Centralized SG Solutions
Demo
MV centralized voltage control with ...
Germany
Field measurements-OLTC
Spain
OLTC + STATCOM control
Austria
Field measurements-OLTC + DG reactive
power control
Austria
Field measurements-OLTC control
Spain
STATCOM control
Distributed SG Solutions
Demo
MV distributed voltage control with...
Germany
AVR
France
DG reactive power control (droop Q-V control)
France
DG reactive power control [fixed tan(phi)]
Italy
OLTC + DG reactive power control
Greece
DG curtailment + DG reactive power control
Greece
DG curtailment
1.0
0.20
0.8
0.16
0.6
0.12
0.4
0.08
0.2
0.04
0.0
pdf
cdf
voltage-control solutions appear to be the most appropriate given
the large proportion of affected feeders. However, it is still necessary to test the applicability of each solution on real networks.
In our study, the network selection from different countries
showed that voltage-control solutions based on a local control
of reactive power from the DG inverters (volt-var control, VVC)
can successfully increase the HC in 60% of the feeders.
10 20 30 40 50 60 70 80 90 100
% Loading
figure 4. The probability distribution function (pdf) of
distribution grids from one Austrian DSO in function of
the loading reserve. The voltage-constrained feeders are
in blue, loading-constrained feeders are in red, and both
voltage- and loading-constrained feeders are in magenta.
The cumulated density function (cdf) of these grids in
function of the loading reserve is shown in black.
34
ieee power & energy magazine
may/june 2017
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