Restoring distorted 110 Kv overhead lines after catastrophic damage in Slovenia

Over the past thirty years, more than forty breakdowns occurred on overhead lines (OHLs) from 110 kV to 400 kV on the Slovenian high-voltage transmission and distribution networks. In February 2014, a large part of Slovenia suffered catastrophic damage to low and medium-voltage distribution networks, 110 kV distribution and transmission lines, and 220 kV and 400 kV transmission lines. The consequences for lines between transformer stations (TSs) in the vicinity of Ljubljana and Divaca were particularly severe, where 220 kV and 400 kV overhead lines were destroyed. The additional load from glaze ice also caused towers to collapse, and damaged conductors.

Standardisation of towers and the possibility of cooperation between the companies during rapid restoration of the towers were highly significant in this process. This is also relevant in the wider area from Slovenia to Croatia, Austria, Hungary, Romania and Serbia, largely because the 2015 floods.

Slovenian statistics of damage caused by freezing rain and glaze ice is unfortunately full of events with significant consequences on the distribution network, including failure of cross-arms, entire towers and entire sections of overhead line routes.

The ice storm of February 2014 paralysed Slovenia, with damage to overhead lines of all voltages, including voltage (LV), medium voltage (MV) and high voltage (HV). The consequences were catastrophic, and more than 250,000 people were left without electricity for several days. Whole cities were without electricity. After a few days of complete darkness, generators were restarted, and LV and MV lines as well as some 110 kV OHLs were restored with ERS Towers.

It took several months for the transmission and distribution HV overhead lines to be successfully and permanently restored.

With this new technology the transmission utility (Elektro Slovenija-ELES) and distribution utility (Elektro Ljubljana) were able to restore important 110 kV overhead lines over several days.

Standardisation of towers and cooperation between companies during the restoration of collapsed towers were very important during this process. This is also relevant to the wider area from Slovenia to Croatia, Austria, Hungary, Romania and Serbia, largely because of last the floods of 2015 and the transmission lines destroyed by glaze ice in these countries.

Meteorological conditions of the freezing rain
The 2014 freezing rain storm was exceptional because it affected a large part of Slovenia, part of south Austria and west Croatia (Gorski Kotar), and caused considerable damage to forests,roads, rail and electrical infrastructure.

Geographically, the Republic of Slovenia occupies a small, yet meteorologically diverse area. At this junction of Alpine, Carpathian, Continental and Mediterranean climate the weather can vary significantly over small distances. The mixing of different climates is often the consequence of simultaneous weather systems and terrain, which can cause extreme weather conditions. The consequences of these on both natural and urban environments are classified as disasters. For several years, climatologists have warned that natural disasters are consequences of global warming with impact on everyday life.

Freezing rain accumulates on plants, objects and buildings. Freezing rain is caused by a warm atmosphere at higher altitudes, and colder (freezing) atmosphere on lower altitudes. It occurs frequently at junctions between the Mediterranean and the continental Alpine–Carpathian climate. This is the area along the “Carpathian barrier”; in Brkini, the area around Senozece and Vremscica (1027 m), the foot and slopes of Sneznik, Cicarija, Javornik, Nanos, Trnovski gozd, parts of the Posocje region, and the mountain-ranges Idrijsko-Cerkljansko hribovje and Skofjelosko hribovje.

Freezing rain is a natural phenomenon which occurs most winters, with varying scope, intensity and rainfall quantity. The thickness of the ice that accumulates on objects, and consequently the quantity of damaged trees [m³] in forests, represents the scope of freezing rain.

Restoration of 110kV OHL with emergency restoration towers
A few years ago, ELES and Elektro Ljubljana decided to import emergency restoration towers. ELES has at its disposal emergency towers and equipment, which enables the assembly of ten 110 kV towers or three 400 kV towers, and Elektro Ljubljana has at its disposal three 110 kV emergency towers.

The towers are modular, which enables arbitrary assembly combinations, so they can be used in restorations to 110, 220 and 400 kV networks.

The length of one section is approximately 2m and it weighs approximately 100 kg. Therefore the sections can be manually transported to the field, and their assembly is relatively straightforward, as a team of 6 can assemble one in less than a day. The tower components are stored in containers, which can be loaded onto a truck and brought to the site.

Assembly or emergency restoration towers
ELES used the emergency towers during the restoration of the freezing rain damages on 2×110 kV OHLDravograd–Velenje, establishing the 110 kV OHLSlovenj Gradec–Velenje, and Elektro Ljubljana restored power from TS Logatec and TS Cerknica, which were unconnected following failure of the 110 kV OHLKlece–Logatec, the 110 kV OHL Vrhnika–Logatec and the 20 kV OHLVrhnika–Logatec.

A skilled team of linemen can assemble a 110 kV emergency tower in a single day. After trained by the Canadian manufacturer finished, ELES achieved this by setting up a testing ground at 400 kV TS Maribor to train linemen. On this testing ground, ELES’s experts passed on knowledge, experience and all necessary technical and software knowledge for using the emergency towers to practically all linemen in the transmission groups.

Conclusion
Freezing rain is one of the most dangerous natural weather phenomenons for OHL infrastructure. Calculations of the static reliability of transmission lines are always based on primary (economic) calculations of the additional winter loads.

February 2014 will be remembered by all electrical engineers and more than 250,000 inhabitants of Slovenia as a disaster that showed us how vulnerable our transmission and distribution systems are to natural disasters such as freezing rain.

During these catastrophic events, the temperature all over Slovenia (expect in the coastal regions) was below zero. Therefore, after several days without electricity, the residential houses cooled significantly, causing significant inconvenience.

By using the 110 kV emergency towers, ELES quickly re-established a direct OHL connection between the valleys Saleska dolina and Dravska dolinaon the damaged 2×110 kV OHLDravograd–Velenje.With the help of eight 110 kV emergency towers, Elektro Ljubljana quickly re-established near-normal operations, and therefore enabled a power supply from TS Logatec and TS Cerknica.

The utilities ELES and Elektro Ljubljana once again proved that with appropriately qualified professionals and the help of innovative technical solutions, all unexpected and complex tasks are not a hindrance but a challenge and contribution to the high-technology levels in companies, in which the goal is to enable an undisturbed supply.

Standardisation of towers and the possibility of cooperation between the companies during rapid restoration of the towers were very important for this process.

This is also relevant in the wider area from Slovenia to Croatia, Austria, Hungary, Romania and Serbia, largely because the 2015 floods and the destruction of transmission lines by glaze ice in these countries.

For more details, contact:
inquiry@madhavengineers.com
www.madhavengineers.com