IEEE Power & Energy Magazine - September/October 2015 - 43

nodes, which show the direction of active
and reactive power flows (blue and purple
arrows, respectively), and arrow thickness,
which varies according to the angle difference between the nodes. The phaseangle differences between the nodes are
also explicitly marked. Together with the
geographic display, other visualization
and control displays can be shown, for
example, plots of voltage magnitude evaluation or results of the oscillatory analysis
function. Other displays contain configuration data such as visualization options
and alarm threshold settings. Specific
alarms are automatically triggered in case
of threshold violations, e.g., in the case of
high/low voltage magnitude.

Dynamic Line
Thermal Monitoring
The third local application tested is the realtime monitoring of the line temperature.
Indeed, utilizing online voltage and current
phasors measurements by PMUs, transmission line parameters and temperature can be
estimated. For the calculation of the temperature, taking into account the high sensitivity with which this varies with the electrical
measurements, it is necessary to use a Kal- figure 2. The Terna control room WAM man-machine interface (MMI).
man filter since a direct calculation could be
affected too much by the noise of measures.
So the Kalman filter is adopted to estimate the transmission line with ENTSO-E policies, periodical joint tests of restoration
positive sequence parameters. The average line temperature is (Figure 6), energizing long paths with a total length mode of
then calculated based on the estimated series resistance. The line more than 1,000 km on overhead lines at no-load conditions.
During the last practical tests, due to long transmission paths
model used is classical PI model, which is reported in Figure 3.
Figure 4 shows the logic calculation tree of line tempera- with nonoptimal phase symmetrization, very significant voltage
ture. Applying this technique to congested lines in case of profile differences were observed. WAM systems overcome this
high wind production, the wind itself is also able to produce challenge simply based on the related measurement principle.
a very strong cooling effect against the static current limit. Figure 7 shows a recording of a 2013 test that underlines this,
Figure 5 shows the effect of applying this procedure to a real as the asymmetry on the lines is evident. In addition, during real
line (year 2013); it is worth noting that the curtailment of restoration, the exact knowledge of an angle difference between
different electrical islands is very important to guarantee the sucwind plants due to overload is drastically reduced.
cess of re-meshing the system.

Power System Restoration Support Tool
A very promising application of PMUs is during restoration
tests and, if needed, during emergency restoration procedures;
in fact, WAM systems are, in principle, completely independent from supervisory control and data acquisition (SCADA)
and energy management system applications and could be
immediately available to the control room during an emergency situation. In addition, it must be considered that, during
very large disturbance or restoration maneuvers, the electrical
quantities can fluctuate largely outside the typical range where
the control systems are designed to give exact information to
the operators. Terna and Swissgrid performed, in agreement
september/october 2015

Í1

É1

PMU1

Ýπ

Í12
Žπ

PMU2 Í2

Ýπ

É2

figure 3. An electrical pi-scheme of an overhead line.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2015