Abstract:With the widespread deployment of smart meters, power grids have accumulated vast amounts of raw electricity consumption data. However, data loss remains a challenge due to the complex operational environments of data acquisition equipment. This study addresses the problem of incomplete electricity consumption data by accounting for the influence of Gaussian noise and proposing a robust completion method. First, a electricity consumption data matrix is constructed by reorganizing the sequences of individual users, and the ideal electricity data matrix is approximated using nonnegative matrix factorization (NMF). Second, both the Frobenius norm and the nuclear norm are employed to regularize the Gaussian noise and promote low-rank characteristics of the ideal matrix, thereby formulating an optimization model. Finally, within a block coordinate descent framework, the EM algorithm and a direct updating method are applied alternately to update the matrix factors derived from NMF, enabling accurate and complete data reconstruction. Simulation and experimental results validate the proposed algorithm’s effectiveness and accuracy.

Abstract:Accurate characterization of tie-line power transfer capability is essential for ensuring the feasibility and optimality of cross-regional power exchanges. However, existing methods typically consider only the base operating scenario and neglect N-1 contingencies, leading to inaccurate assessments of regional tie-line transfer capabilities. This paper addresses this gap by focusing on the tie-line exchange capability of power systems under N-1 contingency constraints. A fast determination strategy is proposed based on the coupling relationship between the base case and N-1 scenarios, using a decomposition approach. Furthermore, the impacts of various generator control modes on tie-line exchange capability under N-1 contingencies are analyzed to support the optimal allocation of cross-regional power transfers. The proposed methods is validated using a provincial-level power system, demonstrating its accuracy and practical effectiveness.

Abstract:To address the issue of subsynchronous oscillation (SSO) arising when photovoltaic virtual synchronous generators (VSG_PV) transmit power through modular multi-level converter-based high-voltage direct current (MMC-HVDC) systems, a linearized mathematical model of the system is developed. An improved virtual synchronous control strategy is proposed. Using the eigenvalue analysis method, the study reveals that, under power disturbances, several factors-including the active and reactive control links, virtual inductance, MMC-HVDC bridge arm inductance, and current vector control loop-significantly affect the damping and frequency characteristics of SSO in the VSG-PV system. These findings are validated through simulation on the PSCAD/EMTDC platform. Results show that the integration of the VSG function introduces a sub-synchronous oscillation mode in which both the photovoltaic VSG and MMC-HVDC participate during power disturbances in the outgoing system. On the VSG side, an excessively large active frequency modulation coefficient K_f or a too-small virtual inductance L_v can lead to system instability; on the MMC-HVDC side, increasing the integral coefficient of the voltage loop K_i4 may induce system instability, whereas increasing the bridge arm inductance L_g enhances system stability.

Abstract:Under turbulent wind excitation, the drive chain of the shaft system in doubly-fed wind power generation systems experiences wideband forced torsional vibrations in the low-frequency range, compromising the stable operation of the turbine. To address this issue, a model reference adaptive control (MRAC) method for torsional vibration suppression is proposed. A wideband forced torsional vibration model is established, incorporating optimal torque control and bandpass filter parameters. To overcome the challenge of unmeasurable transmission chain state variables, a feedforward controller and an adaptive control law are designed based on system inputs and outputs. The bandpass filter parameters are adaptively adjusted to provide active damping, enabling the transmission chain’s dynamic response to track a stable reference model. Using a 1.5 MW doubly-fed wind power generation system as a case study, the effectiveness of MRAC is validated through simulations under sustained turbulent wind conditions. The torsional vibration suppression performance of the MRAC method is compared with that of the electrical damping reshaping method. Results show that the proposed MRAC method achieves stable tracking of the controlled object, with an average tracking error not exceeding 4.94%. Compared to the electrical damping reshaping method, MRAC improves wideband torsional vibration suppression by 21.77%, and significantly reduces fluctuations in both shaft torque and generator active power output.

Abstract:Threshold voltage drift in silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs) poses a significant challenge to device reliability in practical applications. This paper reviews the characteristics and existing theoretical models of threshold voltage drift in silicon carbide MOSFETs and proposes a novel gate driving method and circuit to mitigate this issue. The proposed circuit differentiates the device’s turn-off dynamic process from the post turn-off gate voltage by introducing an intermediate voltage level, thereby effectively suppressing threshold voltage drift while retaining the benefits of a negative gate turn-off voltage. An experimental platform was constructed to evaluate the proposed driving circuit. Experimental results indicate that, under the specified conditions, the new circuit reduces threshold voltage drift by 37% compared to conventional driving methods.

Abstract:The earth-to-air heat exchanger (EAHE) is a shallow geothermal energy technology capable of significantly reducing building energy consumption. When combined with a solar chimney (SC), a novel passive ventilation and cooling system is established in which fresh air is cooled and naturally drawn into the indoor space. This study investigates the effect of SC-EAHE system on the indoor thermal environment in regions characterized by hot summers and cold winters, and compares the building envelope performance with and without the system. Experimental results show that, compared to a reference chamber, the EAHE reduces air temperature by 3.2 ℃ in summer and increases it by 1.6 ℃ in winter, with corresponding air change rates ranging from 2.2ACH (air changes per hour) to 10.1 ACH in summer and 3.4ACH to 13.5ACH in winter. Relative to the ambient air, the indoor air temperature shows a maximum decrease of 5.9 ℃ in summer and a maximum increase of 15.5 ℃ in winter. The south-facing exterior wall and the roof receive the highest solar irradiation among all envelop surfaces. In summer, the average temperature of the interior surface of the southern wall is reduced by about 1.1 ℃, while the roof temperature remains unchanged. In winter, the average interior temperatures of the southern wall and roof increase by 1.5 ℃ and 1.7 ℃, respectively.

Abstract:The air supply mode of air conditioners creates a dynamic indoor thermal environment, which can significantly affect the thermal comfort needs of elderly individuals. However, the traditional predicted mean vote (PMV) index is designed for steady-state conditions and fails to consider the impact of spatial and temporal fluctuations in thermal environments. This study investigates the thermal needs of the elderly in dynamic air-conditioned environments and proposes evaluation metrics to quantify the effects of these fluctuations. Seven elderly participants were recruited for a climate chamber experiment. Thermal environment parameters were monitored at 24 points (across three heights and eight directions) surrounding the subjects under four typical air supply modes of room air-conditioners in cooling conditions. The temporal and spatial characteristics of the thermal environment were quantified using two proposed indicators: uniformity (defined as the difference in environmental parameters between the chest and the back of the subjects) and fluctuation (defined as the average variability of environmental parameters across the eight directions). Results showed significant uniformity and fluctuation in the thermal environment surrounding the subjects. Thermal perception evaluation showed that reduced environmental fluctuation and improved uniformity were associated with more neutral thermal and draught sensations, thereby enhancing thermal comfort. Correlation analysis further confirmed a significant relationship between thermal perception and the proposed uniformity and fluctuation indicators. These findings suggest that the uniformity and fluctuation indexes can be used as effective evaluation metrics for assessing the heat demand and thermal comfort of elderly individuals in dynamic indoor air-conditioned environments.

Abstract:The incorporation of skylights significantly improves indoor natural lighting uniformity and illumination levels. Additionally, the use of an air layer helps mitigate hear transfer between indoor and outdoor environments, thereby improving the overall quality of the indoor light environment. Using a comprehensive market building in Handan City as a case study, this research designs a double-layer skylight transmission system comprising 27 combination configurations by integrating skylights and air layers through the Grasshopper simulation platform. With the help of Octopus, a multi-objective optimization solution is developed to balance energy consumption and light comfort. The independent variables and evaluation index datasets are further analyzed using the Hiplot platform to evaluate correlations between design parameters and light environment indicators. Results show that the optimal configuration is achieved when the air layer thickness is 0.8 m, the semi-transparent material thickness is 0.005 m, and the skylight area ratio is 0.3. Under these conditions, the proportion of indoor effective daylight illumination increases by 10.97%, the probability of daytime glare decreases by 39.40%, the brightness ratio between north-south entrances and exits and their background surfaces decreases by 61.45% and 45.10% respectively, and energy consumption per unit area is decreased by 18.42%. Correlation analysis reveals that interlayer height is negatively correlated with daylight coefficient, effective daylight illumination, and daytime glare probability, although these correlations are weak. The skylight area ratio is negatively correlated with indoor effective daylight illumination, while the thickness of the interior light-transmitting panel is positively and strongly correlated with daytime glare probability. Finally, the system’s economic viability, applicability, and feasibility are analyzed, offering a novel design approach and component model for enhancing the indoor light environment of large-scale public buildings in urban settings.

Abstract:Castellated beams are a new type of structural member fabricated by cutting the web of an I-section or H-sectionmember along with the polygonal lines and then welding the upper and lower halves together. Compared to the traditional solid-web beams, castellated beams exhibit the advantages of a high strength-to-weight ratio, high in-plane stiffness, and economic efficiency. Due to the increased depth and the presence of openings, castellated beams are more susceptible to buckling.?Studies have shown that? the distribution of residual stresses across the section significantly influences their overall stability behavior. In this study, the residual stress distribution of two solid beams, four beams with low-height web plates, and four castellated beam specimens was measured using the sectioning method to investigate the influence of the cutting and welding processes on the residual stresses in the castellated beam sections. Test results indicate that the shape of the flange residual stress distribution remains largely unchanged after cutting and welding, but the tensile residual stress at the flange-web junction increases significantly, and it can achieve the yield strength after welding.? The web cut region in the shallow and deep web specimens exhibit high levels of tensile residual stress. The residual stress distribution in the T-section of castellated beams is similar to that of the short and deep web specimens, while the web post section in castellated beams exhibits changes due to welding, with tensile residual stresses reaching yield strength at the welds. Based on the analysis of test results and existing models, a simplified model for longitudinal residual stress distribution was proposed for castellated beams, which could provide a reference for the stability analysis and design of castellated beams.

Abstract:Due to slippage between dissimilar materials during tensile loading, clamping aluminum cable steel reinforced (ACSR) conductors presents challenges, often resulting in significant errors in conventional tensile tests. To address this, the tensile mechanical properties of ACSR-300/25 at 20 ℃ were studied by separate tensile tests on the steel and aluminum strands. Finite element analysis was conducted to validate the experimental findings. The results show that aluminum strands exhibit neither a distinct strengthening stage nor a clear yield point, while the steel strands show a well-defined elastic region, yield stage, strengthening phase, and localized necking. Damage in the aluminum strands predominantly occurs between 1/2 to 1/3 of the specimen length, with the failure pattern following the twisting direction. In contrast, damage in the steel strands is generally concentrated on the same cross-sectional plane near the spcimen’s center. The results of the separated tests for both the steel and aluminum strands are in good agreement with the overall ACSR simulation result, confirming the feasibility and effectiveness of the proposed testing method.