E-REDES adopts EN 50522: Touch voltages on secondary substations – observability & mitigation solutions

The Portuguese DSO (E-REDES) developed a new procedure for supervising earthing systems in Secondary Substations (SS). A Working Group with experts from E-REDES and EDP LABELEC (EDP’s Centre of Technical Excellence) was created to study and define the rules and methodologies for the new earthing measurement approach and to conceive and evaluate mitigation solutions. In the first phase, standards and regulations on earthing measurement and touch voltage evaluation were analysed. A comparative analysis was conducted, including field tests, on equipment with potential use in this new approach. In the second phase, the focus of this paper, mitigation solutions were evaluated by simulation. Two existing SS were used to build and validate the models, considering their dimensional and electrical characteristics, as well as soil resistivity. Validation showed that measured and simulated values were very close. Various soil resistivity models were considered to obtain a range of earthing resistances for assessing different mitigation solutions, including crushed rock, asphalt, electro-welded steel mesh, and concrete pavement with an electro-welded steel mesh. The latter proved to be the most effective, regardless of SS earthing resistance value, soil resistivity, and electrode characteristics. Surface mitigation techniques were the best for minimizing touch voltages and easier to implement.