Power transformers are the most important assets of electric power substations. The reliability in the operation of electric power transmission and distribution is due to the correct operation and maintenance of power transformers. The parameters that are most used to assess the health status of power transformers are dissolved gas analysis (DGA), oil quality analysis (OQA) and content of furfuraldehydes (FFA) in oil. The parameter that currently allows for simple online monitoring in an energized transformer is the DGA. Although most of the DGA continues to be done in the laboratory, the trend is online DGA monitoring, since it allows for detection or diagnosis of the faults throughout the life of the power transformers. This study presents a review of the main DGA monitors, single- or multi-gas, their most important specifications, accuracy, repeatability and measurement range, the types of installation, valve or closed loop, and number of analogue inputs and outputs. This review shows the differences between the main existing DGA monitors and aims to help in the selection of the most suitable DGA monitoring approach according to the needs of each case.

This paper presents the steady and dynamic thermal balances of an overhead power line proposed by CIGRE (Technical Brochure 601, 2014) and IEEE (Std.738, 2012) standards. The estimated temperatures calculated by the standards are compared with the averaged conductor temperature obtained every 8 min during a year. The conductor is a LA 280 Hawk type, used in a 132-kV overhead line. The steady and dynamic state comparison shows that the number of cases with deviations to conductor temperatures higher than 5 • C decreases from around 20% to 15% when the dynamic analysis is used. As some of the most critical variables are magnitude and direction of the wind speed, ambient temperature and solar radiation, their influence on the conductor temperature is studied. Both standards give similar results with slight differences due to the different way to calculate the solar radiation and convection. Considering the wind, both standards provide better results for the estimated conductor temperature as the wind speed increases and the angle with the line is closer to 90 • . In addition, if the theoretical radiation is replaced by that measured with the pyranometer, the number of samples with deviations higher than 5 • C is reduced from around 15% to 5%.

The transformers lifespan depends importantly on its refrigeration. Mineral oils perform this work in the majority of the power transformers. However, this type of coolant has two main drawbacks: low biodegradability and low ignition point. Several alternative liquids are being developed in order to overcome these drawbacks. This paper compares their thermal-fluid behavior with a mineral oil by means of several parameters, such as temperature, flow rate, fluids velocity, convective heat transfer coefficient (h) and the cooling criterion (P). These are calculated using the numerical results of the simulation of a 3D-model of a Low Voltage Winding that belongs to a power transformer with ONAN cooling. The software COMSOL Multiphysics has allowed the simulation of the geometry using a physical model in which buoyancies and viscous forces are the only considered establishing the natural convection. As a result of the comparison, it is clear that the mineral oil is the best coolant liquid. Among the alternative liquids, silicone oil would be the second best coolant fluid, followed by the synthetic and natural esters, respectively. On the other hand, it seems to be clear that the 3D simulations can be used to compare properly the cooling capacities of the liquids.

High-temperature fixed points (HTFPs) based on eutectic and peritectic reactions of metals and carbon are likely to become, in the near term, reference standards at high temperatures. Typically for radiation thermometry applications, these HTFPs are generally formed of a graphite crucible, with a reentrant well, an included 120 • cone, and a nominal aperture of 3 mm. It is important to quantify the temperature drop at the back wall of the cavity, and to understand the influence of the crucible configuration and furnace conditions on this drop. In order to study these influences, three different situations have been modeled by means of the finite volume method for numerical analysis. The first investigates the influence of the furnace temperature profile on the temperature drop by simulating four different furnace conditions. The other two study variations in the crucible configuration, namely, the thickness of the graphite back wall and the length of the blackbody tube.

This study forms part of the European Metrology Research Programme project implementing the New Kelvin to assign thermodynamic temperatures to a selected set of high-temperature fixed points (HTFPs), Cu, Co-C, Pt-C, and Re-C. A realistic thermal model of these HTFPs, developed in finite volume software ANSYS FLUENT, was constructed to quantify the uncertainty associated with the temperature drop across the back wall of the cell. In addition, the widely applied software package, STEEP3 was used to investigate the influence of cell emissivity. The temperature drop, T , relates to the temperature difference due to the net loss of heat from the aperture of the cavity between the back wall of the cavity, viewed by the thermometer, defining the radiance temperature, and the solid-liquid interface of the alloy, defining the transition temperature of the HTFP. The actual value of T can be used either as a correction (with associated uncertainty) to thermodynamic temperature evaluations of HTFPs, or as an uncertainty contribution to the overall estimated uncertainty. In addition, the effect of a range of furnace temperature profiles on the temperature drop was calculated and found to be negligible for Cu, Co-C, and Pt-C and small only for Re-C. The effective isothermal emissivity (ε eff ) is calculated over the wavelength range from 450 nm to 850 nm for different assumed values of surface emissivity. Even when furnace temperature profiles are taken into account, the estimated emissivities change only slightly from the effective isothermal emissivity of the bare cell. These emissivity