Zeroth-Order Negative Curvature Finding: Escaping Saddle Points without Gradients

We consider escaping saddle points of nonconvex problems where only the function evaluations can be accessed. Although a variety of works have been proposed, the majority of them require either second or first-order information, and only a few of them have exploited zeroth-order methods, particularly the technique of negative curvature finding with zeroth-order methods which has been proven to be the most efficient method for escaping saddle points. To fill this gap, in this paper, we propose two zeroth-order negative curvature finding frameworks that can replace Hessian-vector product computations without increasing the iteration complexity. We apply the proposed frameworks to ZO-GD, ZO-SGD, ZO-SCSG, ZO-SPIDER and prove that these ZO algorithms can converge to $(\epsilon,\delta)$-approximate second-order stationary points with less query complexity compared with prior zeroth-order works for finding local minima

Exploration on the upgrading of personnel training mode of safety engineering specialty from the perspective of Internet +

in the context of the Internet + era, in order to eff ectively respond to the new situation of upgrading the personnel training mode of safety engineering major in Colleges and universities, and fundamentally solve the problems of single personnel training mode, low enthusiasm of students and unsatisfactory teaching quality of the major, Based on the in-depth analysis of the current situation of personnel training of traditional safety engineering specialty, this paper explores a new path of professional personnel training under the condition of Internet popularization, puts forward the positioning of innovative talents, and comprehensively reforms the teaching content of personnel training of safety engineering specialty, It is believed that this has important guiding signifi cance for the cultivation of modern talents and the improvement of the adaptability of talents to future jobs

Multiscale power fluctuation evaluation of a hydro-wind-photovoltaic system

The hybrid energy systems are required to operate stably in different time scales. Previous studies on the stability are carried out under the unrealistic assumption of discontinuous time scales. Therefore, a second time scale model for the hybrid energy systems is presented in this study. To overcome the possible uncertainty caused by the discontinuous time scale assumption, a new method is introduced to analyze power fluctuations for the hybrid power system considering the hydroelectric power station (HPS) and PV-wind complementarity. Compared with traditional statistics, the proposed three indices, discussed in terms of variation frequency, have the ability to show the stability and complementarity characteristics of the hybrid system with the time scale varying from second to hour, The results show that the volatility of wind power and photoelectric increase with the increase of time scale. In (10 0, 10 2) seconds, the HPS could not compensate for as they do not meet flexibility demand in that particular frequency domain, and hydro-electric power is able to compensate wind and PV power sources well when the time scale is over 10 2 s. The obtained stability evolution law has important reference significance for the subsequent studies on the stability of hybrid energy systems

Nitropyrazole based tricyclic nitrogen-rich cation salts: A new class of promising insensitive energetic materials

Nitrogen-rich compound 5,5′-(4-nitro-1H-pyrazole-3,5-diyl)-bis-(4H-1,2,4-triazole-3,4-diamine) (4) and its energetic salts (5–8) were synthesized. Compounds 4–7 posesse satisfactory mechanical sensitivity (IS > 40 J, FS > 360 N). 4 and 5 have the higher decomposition temperatures (4, 318 °C and 5, 304 °C) than TNT (TNT, 295 °C). Compared with TATB (8114 m•s−1), the calculated detonation velocity of 8 (8716 m•s−1) has a certain advantage. Multi-factor analysis shows that these compounds are promising nitrogen-rich energetic materials

Sensitivity analysis and low frequency oscillations for bifurcation scenarios in a hydraulic generating system

A hydraulic generating system (HGS) may undergo Hopf bifurcations or limit point bifurcations before the occurrence of low-frequency oscillations (LFO). These oscillations severely limit the power trans-mission capacity and even threaten the stability of the power grid in some situations. Hence it is increasingly crucial to have a clear understanding of LFO in bifurcation scenarios. This problem becomes even more complicated with parametric uncertainties in power systems as it is always challenging to efficiently handle LFO under such conditions. To address this challenge, a mathematical model of a HGS is established which is then utilized to evaluate the sensitivity of state variables against parametric uncertainties. Subsequently, two bifurcation scenarios with variations in two sensitivity parameters (the proportional and integral adjustment coefficients) are analyzed to demonstrate the interactions between oscillation modes and bifurcations. Finally, the results are presented in terms of oscillation indicators of bifurcation scenarios. These indicators include the eigenvalue, frequency, damping and participation factor. One of the most important results shows that the governor provides positive damping to HGS and introduces ultra-low frequency oscillations to the system under certain conditions. The proposed approach and analytical results provide a useful tool for analyzing LFO in a HGS for bifurcation scenarios. (c) 2020 Elsevier Ltd. All rights reserved

Application of Evaporative Cooling Technology in Transformer for Mine Tunnels

The traditional dry-type mine transformer has some disadvantages, such as incomplete fire prevention, large volume, high loss and not enough environmental protection. Based on evaporative cooling technology, this paper proposes a scheme of mine transformer using fluorocarbon as coolant which can truly realize the incombustible transformer scheme and meet the strict environmental requirements. The transformer adopts three-dimensional wound core structure to reduce the loss and improve the anti-short-circuit ability. The volume of transformer is greatly reduced due to the efficient heat dissipation ability of evaporative cooling technology. The scheme of plate fin heat exchanger is more suitable for the ventilation condition of the mine. The filling technology is used to reduce the amount of expensive coolant. Through the life cycle cost analysis, evaporative cooling transformer has better economic benefits than traditional mine transformer, but also has better safety benefits. A prototype is manufactured and tested, and the results show that the prototype meets the requirements of IEC standard temperature rise limit and achieves the expected requirements

Vibration Characteristics of a Hydroelectric Generating System with Different Hydraulic-Mechanical-Electric Parameters in a Sudden Load Increasing Process

In a sudden load increasing process (SLIP), the hydroelectric generating system (HGS) experiences a severe vibration response due to the sudden change of the hydraulic-mechanical-electric parameters (HMEPs). The instability of HGS limits the ability of sudden load increase, and its flexibility and reliability are reduced. Thus, in this study, a new transient nonlinear coupling model of HGS is proposed, which couples the hydro-turbine governing system (HTGS) and the hydro-turbine generator shafting system (HGSS) with the hydraulic-mechanical-electric coupling force, rotating speed, flow rate, hydro-turbine torque, electromagnetic torque, and guide vane opening. By using numerical simulation, the influences of different HMEPs on the vibration characteristics of HGS in SLIP are analyzed. The result shows that, compared with stable operating conditions, the vibration amplitude of HGS increases sharply in SLIP. The increase of the sudden load increasing amount, blade exit flow angle, mass eccentricity and excitation current, and the decrease in guide bearing stiffness and average air gap between the stator and rotor cause abnormal vibration of different degrees in the HGS. Hydraulic factors have the greatest influence on the nonlinear dynamic behavior of HGS. The maximum vibration amplitude of HGS in SLIP is increased by 70.46%, compared with that under stable operating conditions. This study provides reasonable reference for the analysis of the nonlinear dynamic behavior of HGS in SLIP under the multiple vibration sources