Storks and the transmission of electrical energy may appear to be completely unrelated topics. However, in Portugal, the nesting habits of these large birds have had a detrimental impact on the reliability of the Rede Eléctrica Nacional (REN; Lisboa, Portugal) transmission system.

For centuries, Portugal has been visited by the white stork (Ciconia ciconia), which builds its nests atop tall, available structures. Thousands of years ago, trees were the bird's habitat of choice, but when the bell towers of churches started to appear, storks sought these as alternative sites for their heavyweight nests. After adapting to the religious landscape, storks saw other opportunities in the works of man, whose abandoned industrial chimneys provided the birds with another desirable choice.

With the advent of high-voltage (HV) towers, the birds opted to nest in these tall structures, which offered numerous advantages. First, many were located close to feeding grounds and were remote from the threat of humans. Furthermore, unlike bell towers, these structures were noiseless. The number of storks using HV towers for nest sites rapidly increased, and their impact on system performance became a matter of concern.

The Portuguese utility REN, which is responsible for the operation and control of the 6543-km (4090-mile) transmission system operated at the voltage levels of 400 kV, 220 kV and 150 kV, identified several circuits that were subject to high fault rates. REN's investigation showed a correlation between the presence of stork nests on the towers and the number of line faults. Thus, in 1993, the utility added a specific data category — incident caused by stork — to its network incidents statistics. Because the stork is listed as a “vulnerable species,” REN embarked on a Strategy for Stork Nests Management program, which has succeeded in reducing the number of circuit outages, even though the number of nests built on transmission towers has increased six fold. (Fig. 1).

Fault Causes Attributable to Storks

The stork is a large bird that stands 1 m (3.3 ft) high, has a wingspan of 2 m (6.6 ft), and feeds on large insects, eels and other fish, crustaceans, small mammals and birds. While storks prefer wetlands, marshes and rice fields where food is ample, they sometimes nest near cultivated fields. They migrate over the large distance between Europe and sub-Saharan Africa in advance of the March/April breeding season. (However, following a decline in their population, this protected species has recovered and now some storks no longer migrate.)

The storks are protected by the Convention of Bonn on Migratory Species, the Convention of Bern on European Wildlife and Natural Habitats, and the Birds Directive of the European Union. Since destroying the birds and/or nests was not an option, REN faced a major dilemma.

The main cause of fault outages because of stork nests on transmission line towers is the bird excrement, which pollutes the insulator strings to a point where they lose their insulation characteristics. Even without touching the insulators, the excrement “jet stream” may also create a zone of low dielectric resistance within the strong electrical field in the insulator vicinity, triggering an electric arc along the string. In addition, faults can arise from the sticks, the raw material from which the nests are constructed, which can fall near the insulators, causing a line fault. Hence, the position of the nest on the transmission tower pylon is the most critical issue concerning the impact in network reliability. Figure 2 shows the nest locations that are commonly built on transmission towers.

REN's Strategy for Stork Nests Management

Through its Strategy for Stork Nests Management initiative, REN sought to improve system reliability while protecting the nesting habitats of the storks. This initiative required the utility to:

• Propose, take and report preventive and corrective actions and seek permission from the National Environmental Authorities.

• Cooperate with nongovernmental environmental organizations.

• Install devices designed to inhibit nest buildup at bad locations on towers, to facilitate nest buildup at good locations in pylons and to afford protection to insulators.

• Relocate nests after the breeding season.

To inhibit nests from being sited at bad locations on towers, REN began using nylon strings, a practice that was abandoned after a few years at the 400-kV pylons. These strings broke, contributing to the occurrence of line faults. However, they are still in use and being mounted on 220-kV and 150-kV pylons.

Figure 3 shows the current inhibiting, facilitating and protecting devices used at REN. The inhibiting device, which looks similar to an anemometer, comprises a vertical metal bar with a rotor on top. Three horizontal bars, with a 120-degree separation terminated with a half-sphere, are designed to rotate in the wind and thus discourage the storks from nesting near this device. The facilitating device is a square metal platform that measures 80 by 80 cm (31 sq inches). Initially installed near the ground-wire position on towers, these platforms can be placed at additional locations later, as required. Finally, the protection device is a simple metal plate installed above the insulator that provides a shield (Fig. 4).

Figure 5 shows a facilitating device and the inhibiting device installed on the tower crossarm adjacent to conductor termination position.

The wind device is most effective on towers positioned at windy locations because it does not move unless sufficient breeze exists. Amazingly, some storks are so reluctant to give up their favorite nest position that they place nest sticks under the wind device to stop its rotation (Fig. 6).

The last component of REN's strategy is the relocation of nests. Before relocating nests, however, careful consideration must be given to their size and weight. A completed nest may weigh more than 150 kg (330 lb) and moving it to an acceptable position on the tower is difficult. To encourage the relocation, not destruction, of nests to new platforms, environmental organizations count the existing nests. Data confirms that the storks accept approximately 75% of relocated nests.

Result of Strategy for Stork Nests Management

Table 1 illustrates the Strategy for Stork Nests Management program activity. For the period 1998 to 2003, it shows a quantitative assessment of REN's activity, detailing for each device the number that have been installed. As stated previously, nylon strings have been withdrawn from 400-kV pylons, and a new device composed of metal strings is being introduced.

Table 2 shows the recorded stork nest positions on the 14,700 towers that support REN's 6543-km (4090-m) transmission system. Note that of the 1005 nests recorded for 2003, less than 7% of the tower population has been used by storks for nesting purposes. In actuality, this figure is smaller, because whenever a tower is selected for nesting by a stork, the probability of other storks using the same tower increases. Hence, many towers carry more than one nest. Although the geographical distribution of nests is variable, there is a concentration of them in areas adjacent to feeding grounds.

The effectiveness of the measures taken by REN is evident from the annual fault rate statistics attributable to stork nests (Table 3).

The statistics show that despite a sevenfold increase in the number of nests on towers during the period 1993 to 2003, the number of faults attributable to storks has fallen from 102 to 72, or around 30%. (This reduction has been hampered by an increase in the last year. The causes are being investigated, and there is conjecture that they are due to a lack of protective measures on lines that are going to be dismantled.)

Similarly, faults caused by stork nests has decreased from 0.74 to 0.07. The costs associated with this nest management program are mainly labor related, as the devices used are simple in design and use inexpensive materials. Furthermore, as the installation work on nest control is coordinated with other work required on the towers, the total annual budget has been contained to around euro 100,000, a modest investment compared to the vast improvement in the transmission system reliability.

José Amarante dos Santos received his BSEE degree from the Instituto Superior Técnico in Lisbon in 1971 and has been working in the Portuguese electrical utilities since May 1976. During his career, he has worked on SCADA software within telecontrol projects associated with hydro power plants and National Dispatch. Currently, Amarante is chief of the National Dispatch at Rede Eléctrica Nacional.
amarantedossantos@ren.com

Table 1. Statistics showing the number of devices installed by REN from 1998 to 2003.

	Nylon Strings	Inhibition Wind Rotors	Metal Strings	Protection Metal Plates	Facilitation Platforms	Relocation Transferred Nests
At July 1998	250	97		500	143
Jan. 1998-Jan. 1999	-77	352		151	105
Jan. 1999- Jan. 2000	0	210		195	115
Jan. 2000- Jan. 2001	-69	288	76	123	61
Jan. 2001- Jan. 2002	-8	355	30	172	140	66
Jan. 2002-Jan. 2003	28	365	25	126	141	60
At Jan. 2003	124	1667	460	1250	550

Table 2. Recorded stork nests, 1998 to 2002.

	Nest Locations at Pylons
	On Platforms	Over Insulators	In Passage Zone	Others	Total
July 1998	195	153	82	72	502
July 1999	274	115	85	115	589
July 2000	414	92	65	141	712
July 2001	472	96	146	100	814
July 2002	568	84	140	103	895
July 2003	643	94	168	100	1005

Table 3. Annual fault rates attributable to stork nests.

	1993	1994	1995	1996	1997	1998	1999	2000	2001	2002	2003
Fault rates	0.74	0.64	0.46	0.37	0.17	0.11	0.09	0.08	0.07	0.04	0.07
No. of faults	102	145	113	105	53	53	55	59	58	38	72
No. of nests	138	228	246	283	316	502	589	712	814	895	1005