IEEE Power & Energy Magazine - March/April 2020 - 39

00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23

Ampere/Phase

The analysis of the data obtained in the monitoring proThe maximum registered overload during an outage of
another line in the region was 1,700 A/phase (+34.5% over cess points to some important characteristics of the physical
the nominal rating), very close to the allowed conditional phenomena that deserve attention, since they involve safety
capacity of 1,706 A/phase. The measured temperature of considerations. The general assumption that high-generation
conductors was approximately 50 °C, due to the extra cool- periods are necessarily associated with high wind velocities
ing with wind speeds higher than the 1-m/s criterion. This must be properly verified in practice. Only this will ensure
extra transmission capacity has allowed more than 2,200 MWh that the conductor cooling will be sufficient, leading to
of renewable energy to be supplied to the system that other- higher line capacity in general.
However, the dynamics of the process have important excepwise would be wasted.
Based on the good initial results of these efforts, additional tions. The most important of these is when a rapid increase in
equipment was installed. An instrument for current and tem- wind speed at the generation plant, typically the arrival of a
perature real-time monitoring and two weather stations were headwind, causes rapid growth of the generated power at the
brought into operation, thus making it possible to obtain infor- wind farm. This increases the current in the line almost instanmation from two locations close to the line terminals. The new taneously, while the wind will arrive over the length of the line
acquired data allowed for more accurate and reliable informa- in time gaps up to 20-30 min. In this interval, the heating of
tion and analysis. The data confirmed that the weather condi- the conductors can be considerable and undesired.
The inverse situation, where the wind suddenly drops
tions were similar along the entire line length and also included
the actual temperature of the conductors. Also, it was clear that down, will keep the current up for some time, due to turbine
when the current increased due to higher wind generation, the inertia, and the conductors will generate considerable internal
wind at the conductor's height also increased to some extent. heating while rapidly losing some of their cooling capacity.
Thus, the cooling of conductors was better, and the conductor However, this type of event will have a much shorter duration,
temperatures did not increase as much as predicted using the on the order of a few minutes, since wind turbines do not have
high inertia.
standard rating methodology.
An additional increase of the transmission-line capacity
As another important precaution, the types of rating in--
was then evaluated. The new allowed limit, designated as creases presented here cannot be generally assumed. Ambiconditional capacity level 2, was 1,860 A/phase or 930 A/ ent conditions may vary significantly over longer lines and
subconductor. A few exceptions to higher ratings were noted irregular terrains, causing different conductor temperatures at
during delays between the current increase and the higher speed different points along the line.
winds at the conductors' height. This caused the temperaThere are two very important conclusions resulting from
ture of the conductors to increase at a high rate, which was the analyses developed during this article. The first is that the
an issue of concern. A very careful analysis of available data combination of modeling, analysis, and real-time monitoring
showed that, in some cases, the conductor temperature was very of line conductors and ambient conditions has the potential
close to the safety design temperature limit.
to increase current limits of transmission lines connecting
After the definition of the conditional capacity level 2 limit and the
installation of more wind genera2,000
tors, there was an increase in windpower generation, and the line
1,800
operated frequently at the higher
1,110-MWh
limit. The temperature of the con1,600
Effective
ductors never violated the temperaIncrease
1,400
ture design and, hence, maintained
safe distances to ground.
1,200
Operating at higher currents in the
line allowed the transmission of more
1,000
energy to the national network and
avoided the undesirable curtailment
800
of renewable generation. Figure 10
600
shows the results of a specific day
when it was possible to generate and
Hour
deliver an additional 1,100 MWh.
Design Current: 1,262 A
Conditional 01: 1,706 A (+34.9%)
Between October 2014 and August
Conditional 02: 1,860 A (+47.0%)
Measured Current
2015, the average daily increase was
approximately 1,000 MWh, with the
best day up to 1,600 MWh.
figure 10. A snapshot of current measurement from 1 May 2015.
march/april 2020

ieee power & energy magazine

IEEE Power & Energy Magazine - March/April 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2020