Three‐Phase Water‐Rheostat Dump Loads in Hydropower Plants: Failure Analysis of a Flawed Retrofit

When hydraulic regulation and spillways are unavailable or insufficient, grid-connected hydropower plants may require electrical dissipation systems to absorb power during load rejection and abnormal operating conditions. This paper analyses a refurbishment of a three-phase water-rheostat dump load installed in a large hydropower plant on the Po River (Italy), where immersion-depth regulation was replaced by electrode-spacing regulation. Field tests at incremental voltage levels (1--5--10 kV) and three-dimensional stationary current simulations were used to reconstruct the causal chain from observed phenomena to root causes. The retrofit exhibits intrinsic criticalities: (i) strong nonlinearity between the control variable (electrode spacing) and dissipated power, which amplifies positioning errors; (ii) local dielectric overstress driven by electrode geometry, with simulated peak electric field exceeding the design limit by more than three times at rated voltage; and (iii) phase-to-phase discharge triggered by the overflow water paths, which place line-to-line voltage across adjacent tanks. The results show that, for large-scale applications, design validation must jointly consider thermal-resistivity coupling, field enhancement and hydraulic layout of overflow paths. Practical design implications are provided for refurbishment and specification of water-rheostat dump loads in hydropower contexts.