Energies

The long-term operational reliability of a valve-side bushing in High-Voltage Direct Current (HVDC) converter transformers depends heavily on the integrity of the spring contact. The existing literature has primarily focused on static defects under steady-state conditions. This study analyzes time-dependent temperature and electric field characteristics under both normal and spring contact defect conditions using a time-varying model accounting for harmonic-induced heating and temperature-dependent conductivity. Harmonic components were derived from operational waveforms of the Bukdangjin LCC-HVDC converter transformer valve-side bushing. Over an 8640 h simulation reaching a maximum contact hot-spot temperature of 107.789 °C, results revealed that under normal conditions, the radial field temporarily flattened at 937 h, with the gap between the first and final layers narrowing by 86.7% (from 2.593 to 0.345 kV/mm) before recovering at a steady state. Conversely, contact defects caused an electric field inversion: the final-layer field surged by 122.3% (from 1.39 to 3.09 kV/mm) at 937 h, exceeding the first layer (1.05 kV/mm) by 2.04 kV/mm, and remained inverted at 8640 h (2.97 vs. 2.14 kV/mm). Consequently, these findings demonstrate that bushing insulation experiences the most severe electro-thermal stress during the transient state, where contact-induced temperature gradients govern dielectric stress intensification at the outermost layers.

Multi-effect distillation with thermal vapor compression (MED-TVC) is widely used for seawater desalination, but the mixing process in the steam ejector is associated with exergy destruction. This study investigates a six-effect MED system integrating turbine-driven mechanical vapor compression (MED-TDC), together with four selected feedwater preheating configurations. A steady-state model is developed to compare the energy, exergy, and exergoeconomic performance of the proposed systems with conventional MED-TVC. At a first-effect evaporation temperature of 60 °C, the baseline MED-TDC achieves a net gained output ratio (GOR) of 10.41, compared with 7.61 for MED-TVC, an increase of 36.9%. Among the four selected preheating configurations, TDC3 gives the highest net GOR of 10.87. Under the adopted nominal cost assumptions, MED-TVC has the lowest specific freshwater production cost of 1.175 $/m³, while TDC3 has the lowest value among the MED-TDC cases, at 1.194 $/m³. Within the explicitly resolved component boundary, the exergy-destruction sum is 398.26 MW for MED-TVC and 235.25–241.92 MW for the MED-TDC cases; the baseline MED-TDC has the lowest sum among the MED-TDC cases, whereas TDC3 has the highest. The results indicate that turbine-driven vapor compression increases net GOR under the reference conditions, while the selected feedwater arrangements produce configuration-dependent changes in component-level exergy destruction and nominal freshwater production cost.

High penetration of distributed photovoltaic (PV) generation complicates coordinated PV–energy storage system (ESS) planning and network operation, especially where discrete and continuous reactive-power devices coexist. This study develops a screening-assisted planning–operation framework incorporating capacitor banks (CBs) and static var compensators (SVCs). A time-averaged voltage-stress index (TVSI) and network-loss sensitivity (NLS) identify candidate installation buses. The screened layout A1 is compared with the reference layout A0 at matched PV/ESS ratings under prescribed and coordinated reactive-power control. Under coordinated control, A0 achieves 1.14% lower daily operating cost and 13.17% lower physical voltage deviation than A1; thus, the screened layout is not adopted for subsequent capacity planning. At the retained A0 sites, PV/ESS capacities and seasonal dispatch are jointly optimized using single-level mixed-integer second-order cone programming (MISOCP) with relaxed branch-flow constraints. The formulation includes PV availability and curtailment, ESS state-of-charge dynamics, voltage and branch-current limits, CB configuration-change limits and stage-operation costs, and a unified upstream electricity-purchase boundary. Four-season profiles and probability-weighted PV–load–price scenarios support deterministic and scenario-based planning, with common-input evaluation and independent alternating-current (AC) power-flow verification. In the IEEE 33-bus system, coordinated control reduces A0’s physical voltage-deviation index by 42.96% relative to prescribed control, at a higher daily cost. Scenario-based planning yields a 0.43% lower observed annualized test cost than deterministic planning on the common 30-day sample, while both satisfy the checked AC limits.