Assessment of Overvoltage and Insulation Coordination in Mixed HVDC Transmission Lines Exposed to Lightning Strikes

Many geographical constraints and aesthetic concerns necessitate the partial use of cable sections in the High Voltage DC (HVDC) transmission line, resulting in a mixed transmission line. The overhead sections of mixed lines are exposed to lightning strikes. The lightning strikes can not only result in flashover of overhead line (OHL) insulators but can enter the cable and permanently damage its insulation if adequate insulation coordination measures are not taken. In this work, we have analyzed the factors that affect the level of overvoltage inside the cable by simulating a fast front model in PSCAD. It has been determined that surge arresters must be provided at cable terminals when the length of cable sections is less than 16 km to limit the core-ground overvoltage within the lightning impulse protective level (LIPL). The level of sheath-ground overvoltage is independent of the length of cable; however, it can be limited within LIPL by lowering the sheath grounding impedance to 1.2 Ω. Insulation coordination measures do not impact the likelihood of OHL insulators’ flashover. The flashover performance of OHL can be improved by lowering the footing impedance of the second tower closest to the cable terminals, which is otherwise most likely to flashover.

Tree crowns grow into self-similar shapes controlled by gravity and light sensing

Plants have developed different tropisms: in particular, they reorient the growth of their branches towards the light (phototropism) or upwards (gravitropism). How these tropisms affect the shape of a tree crown remains unanswered. We address this question by developing a propagating front model of tree growth. Being length-free, this model leads to self-similar solutions after a long period of time, which are independent of the initial conditions. Varying the intensities of each tropism, different self-similar shapes emerge, including singular ones. Interestingly, these shapes bear similarities to existing tree species. It is concluded that the core of specific crown shapes in trees relies on the balance between tropisms.

Boundary conditions in problems of studying the stability of a plane stationary detonation wave

The research’s problems of a plane stationary detonation wave’s stability are considered. It is shown that the boundary conditions for the two-front model allow estimating the main parameters of the internal structure of gas detonation. Such a model can serve as the basis for development of mathematical support and software for an intellectual decision support system for the problems of explosion-proof and explosion protection. An attempt has been made to systematize the problem of setting boundary conditions in studies of the stability of a detonation wave in order to further create a decision support system (DSS) on problems of explosion safety and explosion protection. The following models of a plane stationary detonation wave were considered, which the stability problem is stated for: 1) the Chapman-Jouget detonation model is the simplest model where the shock-detonation front is modeled by a direct shock wave, and all chemical transformations are assumed to occur instantaneously, directly at the front; 2) a two-front (single-stage, square-wave) model based on the assumption that chemical transformations also occur instantaneously, not on the leading shock front, but in a plane (called the instantaneous combustion front), which is separated from the leading shock front by the induction zone; 3) a multistage model that approximates the continuous distribution of parameters behind the leading shock front piecewise constant function; 4) a model with a continuous distribution of parameters behind the leading shock front, which most accurately reflects the real physical processes in a stationary detonation wave. These models are fundamentally different in boundary conditions, which small pertur-bations in the region separating the regions of the initial combustible medium and detona-tion products satisfy. The advantages and disadvantages of the models described above are both assessed from the standpoint of the correctness of the physical analysis of the detonation process and from the point of view of applicability for the mathematical support of DSS on problems of explosion safety and explosion protection. It is shown that the boundary conditions for the two-front model allow to estimate the main parameters of the internal structure of the gas detonation. Such model can be as the basis for the development of mathematical support and software of DSS for problems of explosion safety and explosion protection

Capillary water absorption by a porous cylinder

Capillary absorption (imbibition) of water by a porous cylinder is described by means of a Sharp-Front model. The cumulative absorption increases as (time)1/2 at early times, but more slowly as the wet front approaches the cylinder axis. Results are given in terms of dimensionless variables. Experimental data on plaster cylinders are in good agreement with theory. Estimates of the sorptivity and effective porosity of the material can be obtained. The model may be useful in testing drilled cores and may also be applied to radial flow through the wall of a porous tube (hence to conduits and arches).