IEEE Power & Energy Magazine - March/April 2020 - 38

ieee power & energy magazine

t (°C)

I (A)

variable thermal ratings allowed
increases of up to 50% in wind ge--
Generation
Load Center
ETD
NTT
Collector
neration capacity and virtually eliminated the restriction of the power
Overloaded
generated by the wind farms conNational
Postponed
Network
nected to this line.
The maximum power generation in these wind power plants
always occurs with higher winds
(between 8 and 12 m/s) at the height
level of the wind turbines. Because
of the favorable profile of terrain
figure 8. A schematic diagram of 230-kV wind-plant feeders to the Estremoz II
along
the line route, it is expected
(ETD) and Natal III (NTT) substations.
that the wind speeds reaching the
line conductors will be consider-
reinforcement was delayed for 18 months, leaving the old ably higher than the conservative design criteria of 1 m/s.
transmission lines as the only option for connecting the gen- Thus, the current capacity in the line is likely higher than the
eration plant to the main transmission system for a consid- values considered in the worst-case criteria and would not
erable period. Transmission-system studies and load-flow violate the maximum permissible temperature of the conducanalyses showed that a 16-km-long, 230-kV line was a bot- tor and minimum safety distance to ground of this design.
tleneck between large collector substations and the national For example, for a 3-m/s wind speed, the line can transmit
network. A short-term increase of this line capacity was 738 MVA, 47% higher than the original static line-rating cristrongly desired.
teria, without violating safety clearance and design temperaThe postponement of the transmission reinforcement via ture requirements.
a second circuit of the 230-kV Extremoz II-Natal III and the
This hypothesis had to be corroborated by field measureimminent increase of wind generation injected into Extremoz ments and correlations between the different variables of intercould overload the first circuit; this could be relieved only est. Real-time measurements of current and temperature of the
by significantly restricting generation, as shown in Figure 8. conductors were performed. Consideration of a higher wind
The line's thermal static rating was 502 MVA for a twin velocity with the other meteorological variables (solar radiation,
bundle with two all aluminum alloy conductor Flint ambient temperature) unchanged was used to determine a DLR.
740-kcmil conductors. This constant rating was calculated
The time plots of Figure 9 indicate a correlation between
using an assumed wind speed of 1 m/s perpendicular to the line. two monitored variables: conductor current (I pd) and temperaBy allowing the line's thermal rating to vary with wind ture for the duration of a day. Under these conditions, the realspeed, it was shown that the line rating was correlated with time monitoring reveals a possible latent increase in capacity
the wind-turbine generation level. As a result, the use of corresponding to the 15 °C difference between the temperature predicted by the standard rating methodology (Testim) and
the measured temperature (Tpd), which could be explored to
increase power transfer.
The analysis of these initial monitoring data led to the
45
450
consideration
of allowing line transfers up to 680 MVA,
40
400
equivalent to 1,706 A/phase or 853 A for each conductor
35
350
in the bundle under high wind generation conditions, while
30
300
maintaining a safe, conservative approach. This increased
25
250
value was designated as conditional capacity and rep20
200
resented a 35% uprate relative to the nominal capacity of
631 A per subconductor in the bundle, calculated with the
15
150
original design criteria and used for long-term applications
10
100
0 2 4 6 8 10 12 14 16 18 20 22
(steady-state operation of the network).
(Hour)
As an evaluation of this initial effort, between December
Ipd (A)
Tpd (°C)
Testimated (°C)
2014 and mid-January 2015, line current values exceeded
the original nominal rating for 13 days. During this period,
since the wind speeds were favorable, the conductor temfigure 9. The current and temperature daily profile: blue
peratures
under higher currents did not reach the design temindicates the current, red represents the measured conducperature
of
61 °C, defined for calculations of allowable sags
tor temperature, and orange shows the estimated conducand safe distances during the monitoring period.
tor temperature according to the design methodology.
march/april 2020

IEEE Power & Energy Magazine - March/April 2020

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