Overhead lines constructed with covered conductors in lieu of bare conductors began in Finland in the 1970s. Covered conductors for medium-voltage (MV) overhead lines offered increased reliability in adverse weather conditions, satisfied ecological considerations and also proved to be cost-effective for the complete life cycle. Utilities in Sweden and Norway soon adopted this system, which has since spread to utilities throughout Europe, the United States, Australia and Japan.

Elektro-Ljubljana (EL, Ljubljana, Slovenia) built its first MV overhead line using covered conductors in accordance with the Finnish technology (PAS System) in 1993. This decision, based on operational performance, led the utility to adopt the PAS System as a standard form of construction.

A survey of Slovene experience in 2000 showed overhead lines with covered conductors withstood three successive severe winters without damage. Although the sample size was too small for statistical-based conclusions, sufficient evidence confirmed a reduction in both outages caused by falling trees and touching branches loaded with heavy snow and ice.

EL's System Development

Slovene utility Elektro-Gorenjska (EG), located in the northwestern section of Slovenia, introduced covered conductors in the reconstruction of the HE Savica-Komna 20-kV line in 1992. However, the first application on a new overhead line was the 20-kV Rob-Purkarce circuit, which EL built in December 1993. The utility selected the Finnish PAS System that had increased the reliability of MV networks in Finland by a factor of five. Furthermore, the superior performance of this overhead-line technology resulted in the erection of 10,000 km (6215 miles) throughout Europe.

EG built the first line according to the Finnish technical standards for covered conductors using materials imported from the Finnish manufacturer Ensto. EL constructed the second line using instructions derived from a study by the Electrical Institute in Milan, Italy, which reviewed installations of covered conductor circuits from around the world. The use of covered conductors continues to increase. In the Slovene power-supply network, the five utilities have constructed a total of 864 km (538 miles), of which 35% comprises Ensto equipment. Table 1 shows the 10-kV and 20-kV system statistics for the Slovenian utilities.

The Slovene utility continued its experience with the Finnish PAS technology through extreme winter weather conditions of ice, heavy snow, falling trees and branch contact in 1995/96, 1996/97 and 1997/98. Reduced outages during natural phenomena resulted in an increase in construction of MV networks with covered conductors.

Table 1. Slovene power utilities 10-kV and 20-kV system statistics for 2000.

Slovene utility	Underground cables km (miles)	Overhead lines bare conductors km (miles)	Overhead lines covered conductors km (miles)	Covered/bare conductors %
Elektro-Celje	410 (255)	2631 (1635)	172 (107)	6.54
Elektro-Gorenjska	405 (252)	705 (438)	157 (98)	22.27
Elektro-Ljubljana	1126 (700)	2669 (1659)	389 (242)	14.57
Elektro-Maribor	545 (339)	2913 (1811)	8 (5)	0.27
Elektro-Primorska	466 (290)	2399 (1491)	138 (86)	5.75
Total	2952 (1836)	11,317 (7034)	864 (538)	7.63

Covered-Conductor System

From 1993 to 1996, in the absence of acceptable technical solutions from competitors, the Slovene power industry used the PAS System exclusively. Since then, the power industry has shared this market between the Finnish product and those manufactured in Slovenia, which offer alternative methods of anchoring the conductor to the insulator and enhanced methods to protect from overvoltage. The Finnish have developed a new system that uses Slovene crossarms and arc protection horns to replace the arcing protection device to protect the conductor from atmospheric overvoltages.

• Poles

  Wood poles are used to support overhead lines with covered conductors. Concrete poles are mainly applied when suspension and terminal poles are replaced.

• Crossarms

  Since the use of Ensto crossarms, Slovene designers are now able to offer a range of alternatives for specific applications.

• Conductors

  Aluminium alloy conductors having a cross-sectional area of 70 and 35 mm² (0.109 and 0.054 inches²) have been widely used; Elektro-Primorska has used covered conductors in these sizes on circuits. Elektro-Gorenjska has used 50-mm² (0.078-inches²) conductors and is currently considering the use of 150-mm² (0.234-inches²) conductors. In 2000, Elektro-Celje introduced the use of 95-mm² (0.148-inches²) conductors on the Slovene networks.

• Insulators

  The use of PAS technology (Fig. 2) introduced the special pin insulator SDI 37 equipped with a pulling sleeve and composite insulators, the latter also being used on bare conductor lines. However, the SDI 37 is not universal; this has lead to the development of the swinging clamp on VHD insulators. The swinging clamp (Fig. 3) can be used with various supporting insulators made of different materials but some weaknesses have been identified when used with glass insulators.

  Though several designs are available on the market, in-service operational experience has provided the opportunity to test, confirm and approve some of the innovative solutions available. A classic example was the practice of not removing the XLPE insulation prior to binding the conductor to an insulator. This practice, contrary to that recommended by the PAS System, resulted in several circuit failures; therefore, insulation is now removed before binding the conductor to an insulator.

• Overvoltage Protection

  Initially, the covered conductor lines used APD protection with arc protection horns where the interphase conductor spacing was 700 mm (28 inches). Following the replacement of conductors on existing circuits built with an interphase conductor spacing of 1300 mm (51 inches), the PAS System with two arc protection horns have been used. The PAS System is now used throughout Europe for new lines with interphase conductor spacing of 400 to 600 mm (16 to 24 inches) without any reported overvoltage defects offering price and simplified construction advantages over APD protection.

Table 2. Faults on Slovene overhead lines with covered conductors.

Fault Cause		No. of Faults	Circuit Outages
Trees touching conductors		5	2
Trees falling on conductors		25	7
Trees falling on arc protection horns		11	10
Broken conductors — Ice loading		7	7
Partial discharge
Swinging clamp to glass insulator		3	3
Equipment failure — Jumpers		1	1
	Insulator	13	12
	Pole	4	4
	Conductor	1	1
	Splice joint	6	6
	Surge arresters	3	3
Lightning — direct strike conductor damage		22	22
Totals		101	78
Note: Faults recorded by Elektro-Ljubljana, Elektro-Celje, Elektro-Primorska and Elektro-Gorenjska.

Fewer Faults

Scandinavian countries have more than 30 years experience in the use of overhead lines with covered conductors. Statistics confirm that the number of faults dropped to 0.9 per year per 100 km (1.45 per 100 miles) compared to 4.5 per year per 100 km (7.2 per 100 miles) for lines with bare conductors. Table 2 shows a summary of the recorded outages and their fault cause.

The Slovene power-supply industry has about 400 overhead lines with covered conductors, and this new technology in the construction of MV overhead lines has already effectively confirmed high operational reliability, particularly in adverse weather conditions. Furthermore, this ecological, reliable and cost-effective solution has encouraged the Slovene power utilities to adopt the construction of MV overhead lines with covered conductors as standard.

Janez Hostnik, director of quality in Elektro-Ljubljana, has more than 35 years experience in the operation, maintenance, development and planning of power-supply systems and networks. He is a member of the CIGRÉ's Technical Committee and Study Committee 21 (cables). Hostnik also is a member in Office for Standardization and Measurement in the Republic of Slovenia, chairman of Technical Committee 42 (High-Voltage Testing Techniques) and chairman of Revision Committee for Studies in the field of operation and maintenance at the Electrical Institute in Milan.

Viktor Lovrencic is technical manager at C&G Ljubljana working as a consultant in the design and construction of covered conductored circuits for the Slovene utilities. He graduated from the University of Ljubljana, Faculty of Electrical Science, and earned the MS degree in power-supply engineering. Being fluent in Slovene, Croatian and English, Lovrencic is a CIGRÉ member participating in the Slovene and Croatian CIGRÉ activities.