New Elements of the Dielectric System of MV Overhead Lines

Dr. Sava Handjiev & Ivan Handjiev, MEng

Typically, 110 – 400 kV transmission lines are built very conservatively, with technology that is almost from the Stone Age. In normal overhead line construction, the insulation distances from phase to phase in the pole are determined by the need to maintain an acceptable level of insulation under certain weather conditions. It means that the distance between the conductors in the pole of a conventional overhead line is significantly over-engineered from an electrical point of view.

Particular attention is paid to transmission lines, while distribution lines are often ignored. However, this should not mean that medium voltage power lines are of less importance. On the contrary, they are the backbone of the electricity industry, distributing loads from distribution stations to consumers.

The construction of distribution power lines 11 – 36 kV is done using technology from the 1960s – concrete or wooden intermediate poles and lattice steel towers for anchoring poles. Techniques are developing fast and a lot of new technologies are being introduced, but the technology used for the construction of medium voltage lines is still from the middle of the last century.

With the development of composite materials, the gradual replacement of glass insulators with composite ones began. But this is not enough. New technologies, together with the construction of generating stations from renewable sources, require power lines to be able to transmit the generated power over long distances when the construction of high-voltage substations is not feasible.

Composite Poles as a New Paradigm

Composite poles are the new player in the power line system. Until now, concrete/lattice steel towers have not been considered as an element of the insulation system of the overhead line, as reinforced concrete and steel structures are able to conduct electric current. Live conductors must be insulated from grounded conductive brackets and poles.

Taking into account the experience gained in the operation of polymer insulators and supporting structures made of fiberglass in construction in different countries of the world, experiments began on the creation of composite, primarily poles. The pioneers of the practical application of such poles were the electricity grids of the USA and Canada. This is due to challenging weather conditions in these countries: frequent hurricane winds, severe winter conditions, etc. Under such conditions, the significantly greater elasticity of fiberglass compared to reinforced concrete or steel allows the poles to withstand temporary overloads without damage and irreversible deformations.

Equally, the development of composite materials opens up broad opportunities in the construction, repair and operation of overhead power lines of all voltage classes. The use of strong and lightweight poles allows us to reconsider the principles of ensuring reliability under mechanical loads. Despite the fact that the materials for their production, fiberglass and epoxy resin, are insulating, it is difficult to say whether the poles as a final product are insulators without additional tests.

Three Module Composite Poles 11 – 33 kV

Since 2012, under the Power-line brand, fiberglass composite poles have been developed and manufactured by Handjiev Ltd. for the European MV network.

Replacing 24 kV concrete/lattice steel towers in inaccessible mountainous areas without roads or in harsh conditions is a challenging, complex and labor-intensive process due to their length and weight. Special transport is required, and reaching the site is almost impossible in rainy and snowy winter weather.

The installation of long and heavy poles that require a concrete base is a great challenge for the engineers from the distribution companies. In severe conditions, such as sub-zero temperatures, strong winds and blizzards, it takes a week just to restore the foundations and for only one concrete/lattice steel tower to be installed per day and that is valid only for flat terrain. For partially accessible mountainous areas, restoration works can take even longer.

Imagine an emergency team that can quickly restore a damaged pole within 2 – 3 hours without a crane, even in a severe blizzard and sub-zero temperatures! This is not science fiction, as fantastic as it sounds.

Handjiev Ltd. has developed and patented special constructions that solve the problems associated with the regular or emergency replacement of existing concrete/lattice steel towers. They are designed and manufactured in accordance with Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds, three-modular composite poles (3CUP), to be "bird safe" – fully dielectric. Relevant dielectric tests have been carried out for confirmation, showing high dielectric strength: 40 MΩ at height; and 217 MΩ at the periphery.

The modular design, small dimensions and light weight allow the modules for two composite poles to be transported by off-road vehicles to the accident site and carried by hand to the installation site by two linemen. No crane or concrete foundation is required. All that is needed is a pre-dug trench. Installation is carried out only with "backfilling". For the flanged anchor pole, only an existing concrete base from a damaged steel lattice tower is required. The restoration/replacement will take hours instead of weeks!

The presence of V0 as a fire protection level and mechanical strength of the composite material as steel will give utility companies the advantage of being able to restore 3 – 4 damaged poles within one business day in severe weather conditions, in hard-to-reach areas without roads, dramatically reducing the duration of power outages for consumers.

3CUP poles are environmentally friendly with zero pollution during production and throughout their service life. The composite material is not harmful to the environment and can be easily recycled.

Several types of composite poles have been developed depending on the method of installation and their purpose.

• Intermediate composite pole type 3CUP with 3 modules with a maximum length of 5 m and a maximum weight of 60 kg, for direct burial with backfill;
• Anchor composite pole type A3CUP with 3 modules with a maximum length of 5 m and a maximum weight of 60 kg, for direct burial with backfill;
• Anchor composite pole type A3CUP-F with 3 modules with a maximum length of 5 m and a maximum weight of 60 kg, with a flange for installation on an existing latticed steel tower’s base, concrete floor or rocks with anchor bolts.

3CUP has been operating in Bulgaria since 2019 in the networks of ER South (formerly EVN Bulgaria) and ERP North. More than 50 poles have been installed.

Bird Protection Coatings

Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds requires distribution companies to use special materials to protect birds from being exposed to voltage when they land on or fly away from the conductor-insulator interface.

Silicone coating for conductors for voltages up to 36 kV is much more effective than XLPE insulated conductors for the following reasons:

• XLPE insulation is applied to the conductor only at the factory, which means that it can only be used when constructing new power lines. Laying such conductors on an existing overhead line means completely replacing the bare conductor with an insulated one, which reduces the efficiency of its use;
• XLPE insulated conductors are not safe to touch and must be treated as bare conductors. The silicone coating is supplied in rolls of a certain length and can be installed on an existing conductor on an 11 – 36 kV overhead line;
• The silicone coating has a much lower weight per linear meter than XLPE insulation, which means that there is no need to replace the poles;
• Silicone coatings are rated for voltages up to 36 kV

Bird Protection by Envertec

Dielectric tests conducted in the high-voltage laboratory of ERP North on silicone coatings show that an Envertec coating can withstand voltages up to 40 kV. Breakdown occurs through the air gap of the connecting units.

Envertec silicone coatings have been developed as a protective measure against bird electrocution and are certified for use in overhead distribution lines of class 0, with a phase-to-phase voltage of 36/√3 kV, and in distribution networks of class I, with a phase-to-phase voltage of 72,5/√3 kV. They are manufactured and tested in accordance with AENOR EA0058:2016, ENDESAGE-BNA001:2017 and the international standard Enel GSCC030. They are intended for installation on existing lines.

Their main purpose is to protect birds and other wildlife from electric shock, both from simultaneous contact with a conductor and a metal bracket, and from contact with two conductors or other live components.

Due to their composition and nature, these coatings repel moisture build-up and are permeable to oxygen.

• Compared to other materials, silicones stand out for their excellent and long-lasting hydrophobicity. Insulating components made of silicone rubber retain their hydrophobicity for many years. Leakage currents are minimized and discharges caused by dirt build-up are avoided even in the case of heavy surface contamination, as the hydrophobicity is transferred to the contaminated layer;
• Based on accelerated climatic aging tests and their excellent resistance to chemical agents and UV radiation, these components are estimated to have a service life of over 20 years;
• These coatings have a minimum silicone thickness of 3 to 6 mm (depending on the model), ensuring exceptional dielectric characteristics and UV resistance. They do not show color degradation or crystallization and remain effective even in areas with very high salt contamination;
• The mechanical tests already carried out on the covers, in accordance with section 8.3.2 of EA0058:2016 by AENOR and section 7.3.2 of the ENDESA GE-BNA001 standard, guarantee their use at minimum temperatures down to -30°C;
• On the other hand, the vulcanization process of the silicone used in all the coatings is carried out at a temperature between 180°C and 200°C, which allows their use at temperatures up to 200°C;
• This upper limit is further supported by the reaction of the silicone used in the manufacture of these coatings, according to ANSI C29.13–2000 and C29.11–1989 (1), carried out by the independent laboratory LAPEM, according to report no. K304-150/2010;
• Silicone coatings pose great conductivity features to the heat released from live equipment.

Ice Formation

A major problem for power lines during the winter time is icing. Poles are designed to withstand ice load up to 10 mm in diameter around the conductor.

The weight of icing on conductors varies dramatically, depending on the type of ice (frost, sleet, wet snow), environmental factors (wind, temperature, water content), conductor size and location, ranging from negligible (frost) to massive loads such as 0,68 kg/m² to over 1,7 kg/m² or more for heavy frost/sleet, which significantly increases the mechanical stress on the line and the risk of collapse and total deformation of concrete/steel lattice poles, with densities ranging from light (0,01 g/cm³ for frost) to dense (0,9 g/cm³ for frost).

The density of ice on conductors varies significantly depending on its type – from light, fluffy frost (150 – 700 kg/m³) and wet snow (100 – 850 kg/m³) with low adhesion, to dense glazed ice (900 – 920 kg/m³), which is formed from supercooled droplets and is much heavier and strongly bonded, which affects the safety of the line and the flow of power.

Factors affecting ice weight:

• Density:

• Frost: 0,6 – 0,9 g/cm³;
• Wet snow: 0,1 – 0,7 g/cm³;
• Crystalline frost (hoarfrost): 0,01 – 0,08 g/cm³ (very light).

• Wind and droplet collision: Wind speed and droplet size (MVD) affect how much water (liquid water content) collides and freezes on the conductor, increasing the weight;
• Conductor type: Larger diameter conductors or bundles may accumulate a different amount of ice than single conductors, although research has shown that the differences between single and bundled conductors may be minimal under some conditions.

Impact on Power Lines

A large amount of ice accumulation dramatically increases the mechanical stress on the conductor, straining poles and clamps.

Engineers use sophisticated models and online monitoring to predict ice weight, often converting real-world observations to estimate icing on actual lines.

But despite all calculations, the number of fallen or destroyed poles due to mechanical overloading with ice exists and there is a tendency for it to increase further.

Icing Protection

Due to the hydrophobicity of silicone insulation, these silicone coatings do not allow the formation of ice on the conductor. The formed water droplets freeze and fall under their own weight.

The weight of Envertec´s silicone covers for SWP16 conductor (0,3 kg/m) and the weight of silicone coatings for SPSC and STSC terminals (1,2 kg) are included in the calculated icing weights, but eliminate the possibility of its formation in all climatic conditions.

The use of spiral coatings is impractical due to the need for cable ties placed every 30 – 40 cm to protect the coating from self-unfolding. The additional mechanical load on the conductor from their own weight is not taken into account. More importantly, the material from which they are made (PVC) does not have the hydrophobic properties of silicone and allows ice to form on them. In addition, PVC ages faster than silicone, which can cause frost damage and hence self-unfolding of the spiral coating.

The SWP16 cover is attached to the conductor with a special zip fastener at its lower end. For security, a self-vulcanizing tape is placed at both ends of the 20 m cover. Cable ties are not required.

In the networks of ER South, more than 12 000 pcs of STSC & SPSC are in operation, and more than 6000 meters of silicone covers SWP16 are in the networks of ERP North. The entire substation "Biovet Peshtera" 110/6 kV is fully equipped with Envertec silicone covers.

Conclusions

Three Module Composite Poles

• To be used for emergency or permanent regular replacement of reinforced concrete and steel lattice poles in inaccessible areas without the use of specialized transport and/or crane equipment;
• Easy transportation close to the installation site with an off-road vehicle and on foot to 60 poles can be loaded in a semi-trailer truck;
• No crane or other lifting equipment is required – a winch is sufficient;
• No foundation required. The buried poles are secured only with "backfill", and the flanged ones use a concrete foundation of a damaged steel lattice pole or monolithic rock;
• The high dielectric features allow them to be considered as an insulator for voltages up to 20 kV or to use insulators with one voltage level lower for 36 kV lines;
• Meet the requirements of Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds;
• Resistant to weathering, UV rays and ozone for a minimum service life of 80 years;
• 3CUP poles are environmentally friendly with zero pollution during production and throughout their service life. The composite material is not harmful to the environment and can be easily recycled;
• UL 94:2018 certified, providing a V0 fireproof.

Envertec’s Bird Protection Silicone Coatings

• Main approach is the bare conductor to be covered on the length of the span;
• Hydrophobic properties, preventing the conductor from icing in severe weather conditions with wind;
• Full insulation for overhead lines up to 36 kV;
• Can be used to cover high-temperature bare conductors;
• Protect the power line from disconnection from contact or falling branches in the clearings in strong winds;
• High degree of heat dissipation, have the same or even lower temperature than non-insulated live parts;
• Reducing the insulation distance between live and grounded elements;
• Allow reduction of the right-of-way of overhead lines up to 36 kV.

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