New 400 kV Line & GIS Substation Signal Major Investment in Czech Power Grid
UTILITY PRACTICE & EXPERIENCE
New 400 kV Line & GIS Substation Signal Major Investment in Czech Power Grid
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2011 marks the start of what is expected to be a rapidly accelerating growth in the asset base of CEPS, the stateowned operator of the transmission system in the Czech Republic. While investment spending last year amounted to about 2.5 billion CZK (circa US$ 165 million), this year the figure will nearly double and is expected to reach an annual level of as much as 7 billion CZK (US$ 465 million) by 2018. INMR visits two major grid projects now underway as part of this program – one a new double circuit 400 kV line and the second a new 400 kV GIS substation. Both are nearing completion and together will account for more than two-thirds of the utility’s total investment spending this year.
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The Czech Republic has for years been an exporter of electrical energy. Geographically situated in the middle of Europe, the country has therefore developed important interconnections with neighbors such as Poland, Austria, Germany and the Slovak Republic.
capacity means that we will have to spend on our grid so that it will be in the best position to handle this additional power. Apart from meeting all the technical requirements, this will also represent a challenge for us in terms of financing as well as finding the human resources needed.”
These interconnections now need to be strengthened as a result of the incorporation of new domestic power generation sources, specifically the planned 2 x 1700 MVA nuclear facilities at Temelin, the new 880 MVA gas powered Pocerady plant and a 660 MVA coal-fired generation plant soon to come on stream in Ledvice, in the north of the country.
Velek goes on to explain that among the top priorities in this regard will be adding and renewing transmission lines, many of which date back to the 1960s and 70s while at the same time reinforcing assets to meet the growing needs of both interconnections and local demand. Especially important, he emphasizes, will be increasing short circuit withstand capabilities at substations, some of which may need to be partially or even entirely reconstructed.
Photo: INMR ©
Jiri Velek is Head of Engineering & Design at CEPS and in this regard finds himself at the center of major plans to refurbish and expand the Czech power transmission system. Says Velek “the expanding generation
Work at site of new 400 kV line section near Ledvice.
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Changing conductors has been one of the options to increase power flow along existing lines. But Velek points out that re-conductoring older lines may mean having to increase existing tower heights by up to 4 or more meters so as to meet local requirements for permitted electric and magnetic field resulting in higher currents induced in human body underneath the line. This could therefore mean higher towers in some sections of a line and there is often public opposition to this.” As a result, he feels that application of high capacity conductors is restricted mostly to spans where there are special technical requirements versus for any whole line. Price, he notes, is also an issue. Says Velek, “I am not aware of any electrical utility in Europe that presently uses such conductors for a whole line.” The CEPS power grid consists of nearly 3000 km of 400 kV and about 1370 km of 220 kV lines,
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Photo: INMR ©
“All this new power generation means that we will have to spend on our grid so that it will be in the best position to handle the added load.”
New 400 kV line employs strings of 3 porcelain long rods, whose diameters vary whether in suspension or tension application.
incorporating some 14,000 towers and 39 substations. Tower types generally depend on whether they carry a single or double circuit line and include such well-known designs as portal, cathead and delta as well as the classical Donau (Danube) shape. The large majority of insulators used in the Czech Republic have historically been porcelain long rods and these still dominate most of the country’s 400 kV and 220 kV networks. According to Velek, long rod porcelain has proven very reliable, with the exception of one particular design dating back to the 1970s, where problems of separation of the head from the body were found to be the result of bad design and improper cementing of end fittings. Apparently, such insulators, (made with C-110
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Photo courtesy of EGU HV Laboratory
Defective design of porcelain long rod cementing caused mechanical separation of fitting.
quartz porcelain bodies instead of the C-120 alumina mass used since 1985) had a sharp edge and the cement did not fill the gap between the porcelain body and end fitting. This caused radial stress cracks to develop over time, which in some cases evolved into full mechanical fracture. These insulators have progressively been fully replaced by better designs with rounded edges and where cementing is done such that there is no direct contact between porcelain and metal.
Long rod porcelain has proven very reliable, with the exception of one particular design dating back to the 1970s, where problems of separation of the head from the body were found to be the result of improper cementing of end fittings.
Since the mid 1990s there has also been growing application of silicone insulators on overhead lines and particularly on the 110 kV network which, in the case of the Czech Republic, is divided regionally among several private distributors. The motivation for selecting these insulators in most cases has involved the need to shorten string length so as to take advantage of any opportunities to increase ground clearance and thereby reduce electric fields, which are not allowed to exceed 5 kV/meter outside the right-of-way.
comparative ease they offer for inspection. He reports that where there have been problems with glass in the past has mainly been due to poor quality of production, where up to 20 percent of discs self-shattered even before being installed. “But today,” he says, “quality has improved and the situation is much different.”
Photos courtesy of Elektrotrans
Velek notes that glass insulators are also being used and indeed personally favors these due to what he feels is their excellent dynamic behavior under loading as well as the
(Left) Assembly of silicone insulators and hardware on site is typically performed on special ground sheets to protect them from dirt and damage from rocks. (Right) Installation of 220 kV composite tension string.
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Installation of porcelain long rod tension string on new 400 kV line. Insulators covered by protective material until all work is completed.
Porcelain long rods were the insulators selected for CEPS’ new 28.5 km long 400 kV line now being built in North Bohemia. This line, which Velek says represents an investment of some 820 million CZK (circa US$ 55 million), started in the summer of 2010 and is expected to be commissioned by the end of November this year. The line replaces an older 220 V line that was completely dismantled and, in order to most effectively utilize the existing right of way, most suspension towers are of a design referred to as ‘barrel’ type. Velek points out that, as in most other European countries, the process of applying for and obtaining new rightof-ways for power lines in the Czech Republic is fraught with difficulty and can even take more than a decade. This also represents a potential risk for all new projects, especially since existing contracts with landowners can be challenged should the property subsequently be sold. For example, pointing to the situation in Germany where 3000 km of new transmission lines are apparently required to meet Towers are given two coats of paint (yellow followed by green) while special bolts near base are designed to reduce problems of theft.
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Photo courtesy of EGU HV Laboratory
Problems have developed at bolted connection points where any humidity seeping into the joint has caused corrosion to form on overlapping portions of the tower. Problems at tower joints are the result of humidity penetration. the needs of integrating new renewable sources, Velek says that less than a 100 km have so far been built.
Work underway in preparation for delivery of transformer..
According to Steinbauer and Velek, one of the past problems for transmission lines in the Czech Republic has been corrosion of towers made with a special anti-corrosive brand of steel that does not require additional galvanizing or paint protection. This material sees rapid formation of a thin first layer of rust that in principle should effectively shield the tower from further corrosion. However, problems have developed at bolted connection points where any humidity seeping into the joint has caused corrosion to form on overlapping portions of the tower. CEPS has developed a special process to repair such damaged connections which requires dismantling the affected section, followed by cleaning and painting.
Photo: INMR ©
Photo courtesy CEPS
Aerial view of existing 220/110 kV Chotejovice Substation.
The company responsible for erecting CEP’s new 400 kV line in North Bohemia is Elektrotrans, a private turnkey contractor also based in Prague. Managing Director, Jiri Steinbauer, explains that insulators specified for this construction project were porcelain long rods that in suspension have a 60 mm diameter while in dead-end applications are 85 mm in diameter. In both cases, strings consist of 22 sheds per insulator with a length of 1270 mm in the first case and 1310 mm for the dead-ends.
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Josef Hlavac is Supervisor for one of the crews building the new 400 kV line. He notes that to reduce corrosion, each tower on this line is specified to have a hot-dip galvanizing thickness of 84 microns over which a 60-micron thick layer of yellow paint is then applied. Finally, each tower receives an 80-micron thick layer of green paint. Says Hlavac, “we estimate that each year some 8 to 10 microns are lost due to the aggressive atmospheric conditions and this means that each tower should encounter no corrosion problems for at least 20 years.” He also notes that using two different colors of paint allows maintenance staff to see when the outer paint layer has been depleted since the yellow undercoat starts to show through.
All bushings at Chotejovice Substation were specified as silicone-housed SF6 type.
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Typically, it takes a crew of 7 workers about three 10-hour days to finish each suspension tower prior to installation of insulators and the start of conductor stringing. This includes
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all concrete work and tower erection and this estimate is doubled to 6 days in the case of tension towers. Once the line has been completed, Velek explains that future maintenance will involve annual ‘walking inspection’ supported by helicopter flybys every three years to check for signs of corona and excessive heating. Preventive ‘climbing inspections’ will then conducted once every 5 years with more detailed climbing inspections performed every 10 years.
In this case, the existing 220/110 kV substation needed to be expanded to handle the new incoming line but land constraints were such that an air-insulated design was not feasible. Says Velek, “generally, we prefer conventional substations due to their easier maintenance, but in this case we had to go with GIS.” The project began early in 2010 with delivery of the GIS portion in June of this year. According to Velek, the substation is expected to be in operation by December. While the 400 kV GIS at Chotejovice has two transformer bays, it will operate initially with only one power transformer. Still, construction of infrastructure at both bays will soon be fully ready whenever the decision is made to bring in the second. Testing of bushings and spacers at the GIS facility was recently completed and saw a mobile impulse generator brought in to verify both the lightning and switching impulse withstand of the new facility in accordance with EN 62271-203 requirements. Looking at some of the future changes that will be imposed on the Czech Power Grid due to the new power generation soon to come on stream, Vaclav Sklenicka, Director of the
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Apart from the new 400 kV line which will run to the Chotejovice substation near the Ledvice power plant, the construction of a new 400 kV GIS station is another major new investment by CEPS, with a budgeted cost of some 882 million CZK (circa US$ 60 million).
Views of new 400 kV GIS substation. EGU High Voltage Laboratory in Prague observes that there will be a clear need to increase the capacity of local transmission lines. This, in turn, will have an impact on the short circuit performance requirements at substations, which for 400 kV will have to increase from the present 50/125 kA (thermal/dynamic) to 63/160 kA. There will also be some impact on insulator strings on those towers located within 4 km of stations, where short circuit design requirements will grow from 35 kA to 50 kA. “These demands,” says Sklenicka, “will require new hardware on strings such as arcing horns with larger dimensions to ensure power arcs do not damage insulators.” Sklenicka also remarks that, in order to meet the new transmission capacities needed in the Czech Republic as well as similar tendencies now evident throughout Europe due to the incorporation of large sources of renewables, there will be a number of options to consider. One could be
employing conductors with larger cross-sections or new high capacity conductors. A second could be conversion to DC, although he sees this as a relatively expensive solution, except perhaps to separate some portions of the system. Utilizing shift transformers as part of a flexible AC transmission system is yet another option to control power flows. But such measures will only protect part of the system while moving the risk of overload to another part of the interconnected network. Eventually, he expects that the best solution will probably be the longdiscussed supergrid – a transmission ‘super highway’ – whether AC or DC – with one central control over power flows, and to which each of the individual European networks would then be connected. This would enable all countries to handle their interconnection needs while also allowing the growing number of renewable sources of energy to be safely brought in.
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