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Power flow, on both AC and DC overhead transmission lines, is limited to keep the conductor temperature below a maximum (TCMAX) specified to limit both conductor sag and the aging of conductors and splices over time. This power flow limit (line thermal rating) varies with weather conditions along the line corridor but, for simplicity, static line ratings (SLR) are normally calculated for “suitably conservative” annual or seasonal weather conditions. Dynamic line ratings (DLR) change with the real-time weather conditions along the line, are usually higher than SLR, and are more complex to use in system operation. Forecasting of DLR requires forecasting of line corridor weather conditions but makes DLR more useful to power system operations. This paper discusses DLR methods including forecast techniques and presents various field applications.

This chapter describes two of the most adopted software tools for the evaluation of the lightning performance of transmission and distribution lines, namely FLASH and LIOV-EMTP (Lightning Induced OverVoltage - ElectroMagnetic Transient Program), together with some application examples. The FLASH program has been developed by IEEE Working Groups. LIOV-EMTP is the result of the integration between the EMTP and the LIOV code, developed by Alma Mater Studiorum - University of Bologna (Italy), Ecole Polytechnique Federale de Lausanne - EPFL (Switzerland), with the support of University of Rome `La Sapienza' and of Centro Elettrotecnico Sperimentale Italiano `G. Motta' CESI S.p.A. (Italy).

The awareness of the potential safety risk is paramount in the safe operation of overhead transmission lines. The consequence of safety hazards from touch, step and transferred voltages due to Ground Potential Rise (GPR) cannot be ignored or under-estimated. A CIGRE Technical Brochure [1] has been developed to provide a brief overview of the source of the GPR problems related to ground current from ac line faults to the line grounding systems. The GPR issues are sensitive to the soil resistivity near the tower and to other factors. Any safety risk mitigation may require stringent measures at locations where public access is frequent and to a lesser degree when the access is extremely rare. The GPR mitigation should be designed to ensure any potential hazards be kept to acceptable levels.

This paper discusses the wide range of real-time line monitoring devices which can be used to determine the dynamic thermal rating of an overhead transmission line with the power system operating normally or during a system contingency. The most common types of real-time monitors are described including those that measure the line clearance, conductor temperature, and weather data in the line right of way. The strengths and weaknesses of the various monitoring methods are evaluated, concluding that some are more effective during system normal and others during system contingency conditions.

Recent reviews have shown that many methods can be used for estimating the dynamic thermal capacity of overhead transmission lines. A number of approaches are described and key features of each system are organized. Data from field trials provide a unique basis for assessing the variation in ten different parameters. Characteristics of distributed measurement systems are underscored and contrasted against point measurements systems.

In February and December 2012, SaskPower experienced two flashovers of "clean" Fiberglass-Reinforced Plastic (FRP) hot stick during cold weather live-line work under steady-state 230 kV ac system conditions. As for SaskPower, Manitoba Hydro had two separate live-line tool flashover events, in 1997 and 2002. Clean-fog tests and non-uniform wetting tests at Hydro-Quebec Research Institute (IREQ), and cold-fog tests at Kinectrics and UQAC were carried out to explain these flashovers. The laboratory tests demonstrated that inadvertent contamination of clean tools at a very light level () of Equivalent Salt Deposit Density (ESDD) likely reduced voltage withstand capability of FRP hot sticks, particularly under freezing conditions. The flashover mechanism was identified when surfaces become fully wetted by the environment or surrounding fog. However, the source of moisture in field flashovers well below 0°C, i.e. as low as -19°C, was not identified. This study is based on laboratory tests and on using available pollution flashover models to explain the results. In this paper, the main objective is to determine the potential and electric field distribution along a FRP hot stick under dry condition and non-uniform wetting. A commercial software, COMSOL MultiphysicsTM, based on the finite element method (FEM), was used for the three-dimensional modeling and simulations. The results obtained will be useful to improve knowledge on the determination of discharge initiation conditions based on electric field distributions along a FRP hot stick.

Controlled laboratory tests validated a hypothesis that extremely light (~2-3 μg/cm2) levels of Equivalent Salt Deposit Density (ESDD) with no Non-Soluble Deposit (NSDD) can reduce voltage withstand capability of Fiberglass-Reinforced Plastic (FRP) hot sticks under cold-fog conditions near the freezing point, where the tool surfaces are fully wetted by the environment or alternately surrounded by fog. However, the source of moisture in flashovers at temperatures below -13 °C was not established. The mechanism of tool surface wetting was explored through coupled Computational Fluid Dynamics (CFD) and Heat Transfer mathematical equations for a FRP hot stick modeled in Commercial software, COMSOL Multiphysics. The results show that the flow of partial discharge current could be sufficient to raise the temperature of an iced pollution layer just below freezing, where the cold-fog flashover mechanism prevails.

Two clean live-line tool flashovers in Manitoba at 500-kV line voltage and two recent ones in Saskatchewan at 230 kV during freezing conditions led to extensive laboratory tests to reproduce some of these flashover conditions in a controlled laboratory environment to understand the underlying mechanisms and to assess mitigation measures. This paper improves on the three-dimensional finite element method model developed in our previous study to compute the potential and electric field along and around a fiberglass-reinforced plastic hot-stick in a series of “true” cold fog tests at the UQAC. In the previous study, the simulations were carried out with a dry surface and non-uniform wetting. In simulating the non-uniform wetting conditions, since a non-continuous water film was considered to be on the hot-stick surface, there was no leakage current. However, in the flashover and cold fog tests at the UQAC, leakage current activities were observed between the HV and ground electrodes on the surface of the hot-stick. In the present study, the mathematical equations of the model were modified to allow for the presence of leakage current flowing through a continuous thin layer of ice on the surface of the hot-stick in freezing and cold conditions, as was the case for the flashover incidents experienced in Saskatchewan. The potential and electric field distribution along and around the hot-stick were calculated and compared with those in dry surface conditions.

This December 2014 issue of the IEEE Transactions on Dielectrics and Electrical Insulation is devoted to promoting new links between outdoor insulation knowledge and other domains of technology.