Supervisor:
Professor Claus Leth Bak

Co-Supervisor:
Associate Professor Filipe Miguel Faria da Silva)

Assessment Committee:
Professor Francesco Iannuzzo, AAU Energy (Chair)
Professor Pantelis N. Mikropoulos, Aristotle University of Thessaloniki
Professor Juan A. Martinez-Velasco, Technical University of Catalonia

Moderator:
Associate Professor Peter Omand Rasmussen

Abstract:

The recent development of modern renewable energy and the growing electrical power system bring a huge demand for environmentally friendly transmission lines. Aiming to this goal, the 'Power Pylons of the Future (PoPyFu)' project designed an innovative composite material pylon with a 'Y' shape profile for 400 kV AC transmission lines. This pylon consists of lightweight composite materials and has a compact configuration. Compared with the conventional pylons made of steel, the composite pylon requires less right-of-way, lower manufacturing cost, fewer maintenance efforts, and presents an attractive appearance. Meanwhile, this kind of pylon has been confirmed to possess a perfect lightning shielding performance due to its special structure. With these attractive advantages, this pylon is believed to be a promising candidate as a next-generation of transmission tower.

However, the down-lead configuration, which directly concerns the transmission pylon safe operation when lightning strikes the tower, has not been determined. In this project, a conductor going through the hollow cross-arm and pylon body is proposed as the down-lead system. In order to inhibit the partial discharge inside the pylon, the gap between the down-lead and cross-arm is filled with a suitable insulating material. Based on this down-lead scheme, the Ph.D. research work mainly focuses on the optimization of the down-lead system, insulation verification, and proposing empirical formulas to estimate the backflash critical current for this novel pylon. The primary research contents are summarized as follows:

The lightning performance of the pylon with the conductor passing through the filled cross-arm and pylon body as a down-lead system has been studied. The electromagnetic transient (EMT) model is established in PSCAD/EMTDC to obtain the transient response to lightning surges. In the modeling of the equivalent circuit, the surge impedance of the inclined down-lead circumscribed by composite materials is expressed in integral form and validated by the Finite Element Method (FEM). In addition, the mutual coupling effect between separated down-leads and the parasitic capacitance is considered. The simulation results show that the lightning performance of the Y-shaped pylon is worse than that of conventional steel Eagle and Donau towers at the same voltage level, and the down-lead system is needed to be optimized to improve the composite pylon's lightning performance. Then, potential multi-factors that may affect the lightning performance have been studied including the downlead configuration, pylon span, the permittivity of the filling material, and parasitic capacitances. The results show that the configuration of the down-lead has a large influence on the lightning performance, but the effect of the filling material can be neglected. Finally, two down-leads through cross-arms and connecting with a steel mast are proposed as the optimal grounding form. With this optimized down-lead system, the backflashover rate of the Y-shaped pylon is much lower than that of these traditional steel ones.

To make the cross-arm lightweight and easier to assemble, the cable as down-lead through a hollow cross-arm is proposed as the next step of the optimization strategy. The feasibility of this scheme is investigated from the perspectives of electrical insulation and lightning performance. Based on experimental tests on a scaled cross-arm model combined with the electrostatic simulation, the corona inception characteristic as a function of the cable radius is obtained. In addition, the impact of the corona under switching overvoltage on the cable down-lead has also been evaluated. Then, the appropriate cable dimensions to meet the insulation requirements are determined. Considering the corona sheath around the cable, the lightning critical current is obtained through the EMT simulation. The cable down-lead insulation strength under lightning impulse overvoltage has been verified by impulse test on the scale cross-arm combined with the FEM simulation. The research shows that the pylon with cable as a down-lead has satisfying insulation properties and lightning performance, and the design of cable used as a down-lead is feasible for a Y-shaped composite pylon.

Since the down-lead configuration of the composite pylon is different from the traditional lattice tower, it is necessary to improve empirical formulas to realize a fast lightning performance estimation for this novel pylon. First, the equivalent circuit of the down-lead system is simplified to facilitate expression by formulas. Then, the bifurcation structure of the down-lead is considered in the expression. Based on the lattice diagram method originally proposed in CIGRE Technical Brochure 63, the new formulas are derived and modified to be applied for both Y composite and traditional steel towers. Compared with the original method, the results obtained by this improved method demonstrate a fairly good agreement with the simulation results for both traditional and composite towers. The critical current at the full range of front time is obtained, and the errors for the Y-shaped pylon by the new method are restricted within 13%, while the error of the CIGRE original method exceeds 23% when the lightning front time is in the 97.5% confidence interval. Finally, the Monte Carlo procedure proves the superiority of the new empirical formulas.

In summary, in this Ph.D. project, efforts have been made to design and validate the down-lead system for the Y-shaped composite pylon. Meanwhile, new empirical formulas are proposed to realize the rapid lightning performance evaluation. The Ph.D. work is of great significance for the design and application of Y-shaped composite pylons in electrical engineering.