The extensive use of power electronic devices has led to supraharmonic issues.The concept of harmonic groups has been widely used to minimize energy leakage. Moreover, the distribution of supraharmonics is related to the switching frequency of harmonic sources. Given the diversity of switching frequencies and emissions distribution, harmonic groups need to be flexibly adjusted. A switching frequency identification method based on correlation analysis is proposed in this work. In addition, the switching frequencies of supraharmonic sources are identified through a correlation analysis of the supraharmonic emissions of a measurement point. Furthermore, a flexible supraharmonic group method in which bandwidth can be adjusted according to switching frequency is proposed. This method can avoid inconsistency between the emissions and group centers and minimizes energy leakage successfully. Finally, the method is validated via simulation and test data.

Modular multilevel converter (MMC) is widely employed in high-voltage direct current (HVDC) systems for the long-distance renewable energy transmission, where the larger submodule (SM) capacitors significantly increase its size, weight and cost. Conventional capacitor voltage fluctuation suppression methods, such as composite harmonic injection (CHI) strategies, can achieve lightweight MMC. However, these approaches often neglect the dynamic constraints between harmonic injection parameters and their coupled effect on modulation wave, which not only leads to suboptimal global solutions but also increases the risk of system overshoot. Therefore, this paper proposes a comprehensive CHI parameters optimization method to minimize capacitor voltage fluctuations, thereby allowing for a smaller SM capacitor. First, the analytical expression of SM average capacitor voltage is developed, incorporating the injected second-order harmonic circulating current and third-order harmonic voltage. On this basis, an objective function is defined to minimize the sum of the fundamental and second-order harmonic components of the average capacitor voltage, with the harmonic injection parameters and modulation index as optimization variables. Then, these parameters are optimized using a particle swarm optimization (PSO) algorithm, where their constraints are set to prevent modulation wave overshoot and additional power loss. Finally, the optimization method is validated through a ±500 kV, 1500 MW MMC-HVDC system under various power conditions in PSCAD/EMTDC (version 4.6.3). In addition, simulation results demonstrate that the proposed method can achieve a 13.33% greater reduction in SM capacitance value compared to conventional strategies.

A novel loop Constrained Vapor Bubble (CVB) heat exchanger system was used to study instabilities in an evaporating extended meniscus of ethanol. To simultaneously measure the curvature and apparent contact angle of the unstable liquid film, an image analyzing technique was used. The film's motion was first characterized by a small amplitude and high oscillation frequency. During this period the contact angle was relatively constant but curvature doubled. In the second phase, the film spread out to flood the surface and both the curvature and contact angle decreased. Finally the spreading film receded, leaving the first two fringes on the flat surface ahead of the main meniscus for thin film evaporation. We found that this kind of unstable film can increase heat transport capacity.