Search

The loading of Automatic Identification System equipment on low-orbiting satellites can adapt to the demand of exchanging data and information with greater “capacity” brought by the AIS data information of ships in deep waters that cannot be covered by land-based stations. The information in the satellite AIS data contains a large number of potential features of ship activities, and by selecting the ship satellite AIS data of typical months in the South China Sea in 2020. Data mining, geographic information system, and traffic flow theory are used to visualize and analyze the ship activities in the South China Sea. The study shows that the distribution of ship routes in the South China Sea is highly compatible with the recommended routes of merchant ships, and the width of the track belt is obviously characterized. The number of ships passing through the southern waters of the Taiwan Strait has increased significantly, and the focus of traffic safety in the South China Sea should also focus on major route belt and important straits.

This article proposes a four-degree-of-freedom overmodulation strategy for five-phase space vector pulsewidth modulation (SVPWM) in order to reduce the low-order harmonics of the output voltage during overmodulation. Based on the nearest-four-vector SVPWM algorithm, this overmodulation strategy calculates the corresponding third harmonic injection rate according to the modulation ratio in the overmodulation region. What is more, this strategy optimizes and controls the third harmonic so that basic voltage vectors of the fundamental wave subspace and the third harmonic subspace can be controlled simultaneously, thus effectively decreasing the low-order harmonic contents without reducing control degrees of freedom. The whole overmodulation region is divided into three subregions with linearized approximation to reduce the complexity of computation. Simulation and experimental results demonstrate that under the same modulation ratio during overmodulation, the proposed overmodulation strategy can improve the dc bus voltage utilization of a five-phase inverter, make the third harmonic controllable with four control degrees of freedom, and also reduce the low-order harmonic contents of the output voltage.