Ice-covering characteristics are often researched in transportation, communication, and electric power industry. China Hunan Power Grid can easily suffer from ice disaster because of its geographical and climate environment. In particular, in the 2008 ice disaster, icing on overhead transmission lines and facilities caused outage in some cities and counties with a total population of about 30 million people. The ice-covering characteristics of the China Hunan Power Grid were analyzed in this paper. The recorded icing and frost climate data were investigated and analyzed. The model of overhead power line icing was proposed.The ice accretion mechanism was thus revealed. The icing impact factors such as geographical situation, climate characteristics, freezing weather characteristics, microterrain, and elevation were also presented in detail. The necessary conditions for ice-covering formation and the glaze frost distribution of Hunan Province were obtained.
This study proposes a novel non-inverting buck-boost converter topology with extra-wide operational zones under both continuous current mode (CCM) and discontinuous current mode (DCM), by introducing a four-mode modulation (4MM) strategy. The proposed 4MM method is able to eliminate the operation dead zone, thus extending the operation zones of the buck-boost converter to contain boost mode, extended boost mode, extended buck mode, and buck mode. Rigorous analyses and design principles are presented in detail to demonstrate the four-mode function through CCM and DCM operational investigations and illustrations. Theoretical analyses and parameter design are verified by software simulation and hardware experimentation in this study, which validates the proposed 4MM method and the functionality of the proposed 4MM-based buck-boost converter. They clearly indicate the superiority of the proposed 4MM-based four-switch non-inverting buck-boost converter over its conventional counterparts, posing wide applicability in industrial practices.
The traditional stator single-phase-to-ground fault protective schemes are hard to completely satisfy the requirement of selectivity for bus-connected Powerformers. In order to overcome the aforementioned problem, a new stator single-phase-toground fault protective scheme based on the hierarchical clustering algorithm is proposed in this paper. The direction and magnitude of zero-sequence current and leakage current are discussed and selected as feature vectors. Then, the data set are divided into two groups by hierarchical clustering algorithm, and cluster center of each group is calculated. Finally, the space relative distance among detected pattern and two cluster centers is calculated to discriminate the faulty Powerformer. The fundamental component and the third harmonic component of the data set are combined to realize 100% coverage of fault detection for the stator windings of Powerformer. Simulation results have shown that, under different fault conditions, the new scheme can distinguish internal faults from external faults reliably, and can detect in which machine a stator single-phase-to-ground fault is occurring even if the fault resistance is at 5 kΩ.
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