The dc circuit breaker is an indispensable building block for dc network systems. Hybrid circuit breakers that combine mechanical and solid-state switches have more potential to be utilized in dc distribution networks because of their lower conduction losses and fast interrupt speed. The efficient energy dc interrupter with surge protection (EDISON) removes any solid-state circuit in the main current path, thereby substantially reducing the conduction losses during normal operations. However, it requires a dedicated control with fast dynamics because any false triggering of the mechanical switch or solid-state switches of the breaker would lead to a commutation failure and a potential unquenchable arc. This article proposes an FSM-based control scheme that guarantees the accurate and fast control response during a fault event. Due to this fast response requirement, a dual-core-CPU-based control architecture is applied featuring parallel taskings and negligible communication delays between the two cores. Furthermore, a control-law accelerator is employed to further reduce the latency of the program execution by 33%. The controller hardware-in-the-loop (CHIL) model of the EDISON breaker is implemented in OPAL-RT to verify the control scheme and derisk the prototype test. A fifth-order RLC network is developed to characterize the mechanical switch behavior such that the hybrid circuit model can be simulated in the CHIL solely by circuit components. Finally, the proposed control scheme is implemented and validated in a prototype test with a 3 kA interrupting current and a 60 A/ ${\mu }$ s current commutation rate.

Surface creeping discharge at solid-liquid interface is regarded as one of the reasons for oil-immersed transformers failure. In order to restrain the surface creeping discharge, we investigated the effects of TiO ₂ nanoparticles on creeping discharge and flashover characteristics of the oil/pressboard interface under AC and impulse voltages. Partial discharge and creeping flashover tests of oil/pressboard (OPs) and nanofluid/- pressboard (NPs) were carried out. The results show that TiO ₂ nanoparticles can improve both partial discharge inception voltage (PDIV) and creeping flashover strength of oil-impregnated pressboards. Thermally Stimulated Depolarization Current (TSDC) and Electrostatic probe techniques were used to measure space charge behaviors of oil/pressboard before and after modification of TiO ₂ nanoparticles. It reveals that the space charge dissipation rate at the surface of NPs is much faster than that of OPs. Moreover, TiO ₂ nanoparticles can greatly increase the shallow trap density and lower the shallow trap energy level of oil-impregnated pressboard. It is believed that electrons may be repeatedly trapped and de-trapped by shallow traps in NPs, which can reduce the accumulation of space charges and result in improving the creeping flashover strength of oil/pressboard interface.

Piezoelectrically actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for protecting medium-voltage direct-current systems. As the desired actuation speed approaches the driving limit of a piezoelectric actuator, excessive vibration starts to dominate its underdamped travel curves, resulting in insulation failures and delayed switching in the fast mechanical switch. To improve the underdamped travel curves into critically damped travel curves, several switching motion control schemes with active damping filters are proposed in this paper. The switching motion controllers are based on an electromechanical model of a fast mechanical switch with experimentally identified parameters. According to hardware implementation tests, the proposed closed-loop control schemes optimized the switch travel curves with a 56% reduction in the undershoot and a 75% reduction in the settling time. Equipped with the critically damped travel curves, the piezoelectrically actuated fast mechanical switch is better suited for sequential submodule operations to reduce the interruption time, the peak current and the fault energy to be absorbed in hybrid circuit breakers.

We propose a thermally efficient and power-dense high-temperature superconducting (HTS) cable (CLEAN Cable) as well as a thermally efficient, mechanically rugged, and easily attachable/detachable cryogenic-to-ambient power link (CLEAN Connector). The bundled design enables high gravimetric power density and node-less tapping of power. The CLEAN Connector minimizes ambient heat leaking into the cryogenic environment by magnetically coupling the cryogenic section to the ambient counterpart via a high-frequency isolated DC/DC converter. The proposed high-frequency operation reduces the core size of the cryogenic/ambient transformer that provides electrical and thermal isolation while enabling quick connection. The proposed CLEAN technologies provide the following advantages - (1) Reduced heat load; (2) High gravimetric power density; (3) N –1 system redundancy; (4) Minimum ambient heat leak into the cryogenic system; (5) High vibration/mechanical stress tolerance; (6) Partial discharge (PD) free at high-altitudes; (7) Easy cryogenic and ambient system connection; (8) Inherent fault detection and isolation.

In this paper, seven groups of sheet insulation for use in liquid nitrogen immersed power transformers have been tested under three different standard electrodes from ASTM D149-97a. The seven groups consist of three different types of paper/film, non-adhesive Kapton 100HN (Kapton), THERNOMID 14.20 paper (NMN) and Nomex 410 paper (Nomex). The porous paper, Nomex 410, is found to have ac dielectric strength of 62-68 kV/mm, which is likely to be independent from its thickness. The non-porous film, Kapton, shows the dielectric strength of 288 kV/mm, whereas NMN paper has a dielectric strength of 74-78 kV/mm, larger than Nomex with similar thickness. Liquid nitrogen gaps near the triple junction of electrodes as well as liquid nitrogen pockets between the fibers are the main sources of partial discharge (PD) for NMN and Nomex. For Kapton, PDs are more likely to occur in the cavities filled with liquid nitrogen, either on the surface of the electrode or around the impurities on the surface of the film. Electrodes with a small stressed volume are more appropriate for dielectric tests of thin paper/film insulation such as Kapton, due to the lower influence of cavities on the electrode surface. In contrast, electrodes with a large stressed volume and uniform electric field distribution are suitable for the papers such as Nomex and NMN.

Numerous studies have been carried out on the ageing of Kraft paper used in transformer insulation, especially on the production rate of the ageing by-products such as 2-furfural and CO ₂ , which were then used as the index to infer the status of ageing of the transformer insulation. However, as this study found out, the ageing of pressboard produces the same byproducts at a much higher rate under exactly the same environment, i.e., at the same temperature, with the same moisture and oxygen concentration, etc. Thus the ageing diagnosis of transformer insulation based on the concentration of above byproducts could be misled by the interference of pressboard ageing. A comparative study was carried out to identify the difference between the above two processes, to figure out the scenario when the pressboard ageing can confuse the ageing diagnosis bases on the ageing curve of Kraft paper, and to point out a potential new method to monitor the hot spots occurrence in transformer winding caused by sudden changes such as winding deformation by lightning invasion, or the blocking of oil flow channel by flowing chunk objects. Several other observations in the ageing experiments are discussed.