The steady growth in the demand for electricity, as well as increasing the grid operation reliability and maintaining an ever higher level of the energy supply security generate the need to expand and upgrade the distribution grid. The magnitude of required changes is also large due to the age of a significant part of the 110 kV grid. Investments of this type include the construction of new lines as well as increasing current capacity and upgrades of existing 110 kV power lines.
In response to market demand, as well as the occasion of amendments to normative requirements, i.e. the introduction in 2016 of a National Annex to PN-EN 50341-1:2013-03 Overhead electrical lines exceeding AC 1 kV, the process of designing supporting structures has taken off, which with their new solutions can contribute to standardization in the design of high voltage lines in Poland.
2. Climate impacts in the context of PN-EN 50341
Since 2010, the current guideline for the design of power lines in Poland has been the system of European standards PN-EN 50341, which is based on a basic standard – common for European countries, and national annexes, which consider, among other things, the local climatic conditions. Due to the interdisciplinary specificity of the issues defined, PN-EN 50341 often refers to Eurocodes, despite being excluded from their composition. In 2013, the Polish translation of the updated common part of the standard was published as: PN-EN 50341-1:2013-03 AC overhead power lines above 1 kV – Part 1: General requirements – Common specifications. The National Annex was adopted in 2016 as: PN-EN 50341-2-22:2016.
The National Annex update from the previous 2010 version introduced the following amendments:
- S2 zone range expanded at the expense of S1 zone
- increased icing load in zone S1
- changed wind action determination method
- modified conductor and support load combinations
- changed use of partial factors
- changed method of compressed element dimensioning
- abolished requirement to test towers.
The PN-EN 50341-1:2013-03 standard defines the type of loads and how they should be considered. The basic loads are:
- Wind action on any line subassembly given by the formula:
where: qp(h) – peak wind speed pressure; h – reference height above the ground (used for a specific line subassembly); Gx – structural factor (for a specific line subassembly); Cx – aerodynamic drag coefficient; Ax – area of the projection of the subassembly onto a plane perpendicular to the wind direction.
- Icing load included on conductors, components fitted on conductors, and insulators. The standard relates the conductor grade to the conductor diameter and the region (icing load zone).
The standard specifies the wind and icing load cases and their combinations, at the same time indicating for each case the allowable tension specified as a percentage of the conductor's rated tensile strength (RTS) (Tab. 2).
Fig. 1. Application area of Pylon supports in wind and icing load zones as per PN-EN 50341-2-22:2016
Tab. 1. Specific icing loading as per PN-EN 50341-2-22:2016
Tab. 2. Load cases for conductors as per PN-EN 50341-2-22:2016
Tab. 3. Insulation clearance as per PN-EN 50341-2-22: 2016
3. Line design guidelines as per PN-EN 50341-2-22:2016
For specific load variants, the standard also specifies the requirements for each condition. The above applies at minimum to:
- external insulation clearances – to terrain and intersecting objects
- internal insulation clearance – between individual conductors, as well as between conductors and the line supporting structures.
The main task of a high voltage grid designer is to ensure compliance with the standard in terms of external and internal insulation clearances. To this end, the designer can use the following methods:
- support foundation elevation selection (location by terrain profile)
- individual span length adjustment
- support height adjustment
- support type selection (with consideration of allowable support operating conditions).
4. Lattice support solution optimisation vs support size
Sometimes meeting the requirements of site terrain profile and land development, associated with the fulfilment of normative requirements, causes many problems when designing an overhead line. Often, these problems result from the limited performance of the available support structures. Each individual modification of a support height and/or size is time consuming and expensive. It seems that the ideal solution could be to create a maximally universal design, providing significant reserves in relation to the values required by the standard. However, this approach would increase the costs of implementing typical projects, where site conditions are not that demanding. There is no doubt that a desirable solution would be improved towers, i.e. towers with a wide scope of applicability, but designed that their widespread use is economically justified. The ideal moment to introduce support structure improvements is an amendment to the normative requirements, when – by creating new support designs – actual needs can be considered.
Fig. 2. Wind load impact on the line operating conditions in the example of suspension tower
Fig. 3. 110 kV line designing in real circumstances
The support improvements introduced in the Pylon project included:
- admission of an up to 380 m long wind span as a solution for difficult site terrain conditions (soil or ownership problems) as well as economically justified (reduction of the total amount of steel used compared to other available supports assuming the typical 2.5 km long section)
- support operating angles (180˚ – 165˚ – 145˚ – 125˚ – 90˚) – ensuring maximum utilization of sizes (determined on the basis of bending angles of existing lines)
- consideration of the mechanical characteristics of the three most common phase conductor types (AFL-6 240 mm2, AFLs-10 310 mm2, AFLse-10 310 mm2) – no need to calculate supports in projects that include one of the above conductors
- consideration of the mechanical characteristics of the two most commonly used types of earth wires (AFL-1.7 70 mm2, AFL-1.7 95 mm2) – which allows the use of a large group of conductors connected to an optical fibre (OPGW), and reduction of cases whereby span would have to be shortened or support silhouette needs to be modified.
Fig. 4 shows the correlation of various types of conductors suspended according to PN‑EN 50341-2-22:2016-04 with reduced tension. Different mechanical characteristics of the conductors affect their mutual correlation, and ultimately the final support sizes.
Fig. 4. A comparative profile of sag curves of different conductor types (conductor saging conditions as per PN-EN 50341-2-22:2016-04, reduced tension)
Consequently, the HV line support sizes have been designed which comply with the allowable span length and match the technical feasibility and economic rationale of their use. In addition, the cases whereby the actual support working angles significantly deviate from those allowed for the specific support type have been reduced by narrowing their ranges. Moreover, the support structures enable the use of the most commonly used types of phase conductors and earth wires while ensuring their mutual cooperation.
5. Dimensioning and optimization
A spatial lattice structure model was used for static calculations. The need for power line supports dimensioning according to PN-EN 50341-1:2013 standard and Annexes G and H to PN-EN 1993-3-1:2008 standards instead of the general principles defined in PN-EN 1993-1-1 standard, necessitated the need to develop a calculation procedure specifically for designing lattice support structures. Based on the internal forces obtained from the calculation model, compression and tension bars were dimensioned using an original procedure combining requirements of the latest standards and increasing the calculation efficiency and correctness.
The starting point for power line support design is an analysis of the selection of the optimal tower body convergence taking into account the most favourable reaction force on the foundation. Next, the lattice pattern is selected for the maximum utilisation of the profile's cross-section while complying with the normative slenderness limits. The slenderness limits (as per PN-EN 50341-2-22:2016-04) cannot exceed:
- 120 for curbs and compressed chords and turrets
- 200 for the primary bracing
- 250 for secondary bracing.
It is worth noting that despite the dimensioning of compression elements based on the PN-EN 1993-3-1:2008 standard, the National Annex PN-EN 50341-2-22:2016-04 allows higher slenderness of the main lattice bars (200 instead of 180 for towers and masts). The selection of most lattice profiles is determined by bar slenderness limitation. In the case of cross-brace bars, the use of a steel with increased properties is not justified. The selection of the bracing usually results from the economically justified necessity to ensure the bars' design resistance and stability with the optimal profile cross-section utilisation. Once the above recommendations have been complied with, the bars are dimensioned for the ultimate limit state condition and the bolted connections are dimensioned.
A fundamental change in the power line support structure design introduced in PN-EN 50341-2-22:2016-04 is the need to assess the resistance of compressed bars taking into account Annexes G and H to PN-EN 1993-3-1:2008. The effective slenderness factor introduced to account for the method of bars in nodes fastening increases the bars' theoretical resistance, which results in economic benefits, particularly significant in the case of a repetitive design of line objects.
All supports were designed according to the same procedure. Energa Invest's database contains a set of data, with which designers can very quickly adapt, develop, and update and adjust the catalogue solutions to the actual standard and legal requirements, as well as the specific technical and site conditions, while maintaining their full functionality and consistency. This is very favourable in the case of subsequent operation and maintenance, since at any failure, repair and/or upgrade there is no problem with archival documentation.
6. Support strength tests on a real scale
The correctness of the adopted computational approach, previously not used, has been confirmed by strength tests of supports on a real scale. Until 2016, the power line design standards imposed the obligation to test one suspension tower and one tension support from a newly designed series. Now, the National Annex PN-EN 50341-2-22:2016 recommends a trial assembly only, while the need for strength tests is left to be determined by the contracting authority in the design specification. For the sake of the project quality and at the request of Energa Operator, four selected supports were subjected to experimental tests. They were tested at the Celpi test station in Bucharest in reference to the standard PN‑EN 60652:2006 Loading tests on overhead line structures.
The five load cases most relevant for structural dimensioning were selected for the strength tests of the PLN supports. Each support was subjected to five non-destructive tests (up to 100% of design loads) and one destructive load test, which was a continuation of the last load case. Rope and strain gauges were mounted to the support in three directions, through which the resultant loads for each combination were applied. The horizontal force was obtained using high gantries. The forces were gradually applied to 50%, 75%, 90%, 95%, 100% of the specified loads. Under load No. 5, over 100% of the force was gradually applied, every 5%, until the structure's damage.
Test results are considered positive if, during all tests, the structure withstands loads at each level for at least 1 minute with no damage to its components, and 100% of the calculation loads for 5 minutes. After completion of the strength tests, steel samples were taken for analysis of the material's basic mechanical properties. Elements for sampling were selected after the tests, depending on the structure damage development.
7. Verification of normative assumptions in tests
All four supports were damaged at the bottom of the tower body by buckling the compressed curbs.
The lattice support structures were dimensioned for the ultimate limit state. The serviceability limit state did not determine the choice of profiles. Deflections calculated in the computer program and measured during the tests (both at 100% load and just before damage) did not exceed those permitted by the normative values, even with a significant margin (approx. 70% for intermediate support and approx. 50% for tension supports). The supports were strength-tested under the assumption of design loads (and not the characteristic values, which are assumed for the serviceability limit state), so the comparison of the measured and normative values is only indicative. It is conclusive to compare the displacement determined in the program on the model for the design load and the displacement measured in the test after applying 100% of the load. The differences are small (within 10%), which demonstrates the correctness of the adopted model and calculation procedure.
Fig. 5. Application diagram of equivalent loads at the strength tests
Tab. 4. Comparison of the calculated and tested displacements
After the tests, calculations were carried out on the model under the assumption of the actual strengths of the steel from each tested support. Loads were calculated, at which the structure resistance would be exceeded, taking into account the steel test results. It has been shown that there was a margin of approx. 2–8% in the modelled loads compared to the destructive loads. Given the excess margin between the parameters required for steel by the standard and the actual ones measured at the test, the theoretically calculated structure material stress is very similar to that obtained experimentally. The values determined on the calculation model correspond to those obtained experimentally.
8. Supports used in the construction of an 110 kV overhead-cable line
Five different types of double-circuit supports were used in the project of connecting Daszyna substation. A comprehensive set of solutions, which is the towers catalogue, in addition to simplifying the design and construction process, will also reduce the time to fix any future failures.
Photo 1. A support prepared for the strength test, set with ropes and signal wires
Photo 2. Damaged supports: PLN122 P (left), PLN 122ON165 (centre), and PLN211 P (right)
Tab. 5. Comparison of the test results with the model calculations
Photo 3. PLN211 P tower of the 110 kV line to Daszyna substation
The amendment to the normative requirements and the need to reconstruct the national distribution system is a perfect opportunity to introduce improvements in support structures that consider the actual market and technical needs. It should be emphasized that, with a view to improving the supports in order to increase their utility values, Energa Invest has developed supports, which in the example of a typical 2.5 km section, allow for less consumption of materials for the construction and reduce the interference in private land and the natural environment. Their wide applicability range allow the country-wide unification of support structures, which will enable faster failure repair, and having full calculation models – efficient assessment of their adaptability to non-normative operating conditions.
Seeing the advantages of the Pylon project, for which the catalogue of safe, economical, and standardized support structures has been developed, the Energa Invest team decided to implement their solutions not only in the Energa Group, but also in almost the whole of Poland. With the catalogue, high voltage lines can be unified based on one solution, and the standardization idea can be transferred to other distribution grid operators. The project is currently being developed, and work is underway to expand the catalogue with new solutions.