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Power electronic transformer (PET) is the key equipment to realize the power conversion and routing of AC/DC hybrid distribution network, which can greatly enhance the flexibility and controllability of power grid. At present, PET with cascaded H-bridge (CHB) and series resonant dual active bridge (SRDAB) is widely used because of its modular structure and high efficiency. In this paper, a 1.5 MW 10 kV AC-750 V DC PET uesd in practical engineering is analyzed. Firstly, the different switching transient characteristics under the forward and reverse power flow direction are described, and the reasons for the difference are analyzed. Secondly, the current on the rectifier side of SRDAB only flows through the diode. Therefore, the switching loss of SRDAB can be reduced by locking the power semiconductor device on the rectifier side, so as to improve the efficiency of PET. Finally, the experimental test on the PET project site proves the correctness of the loss analysis and the effectiveness of the efficiency optimization method. Using the proposed method, the PET operation efficiency can be improved by about 0.2%.

With the rapid development and wide range application of AC/DC distribution network,its stability has also become the focus. The low-frequency oscillation at the DC side in the commissioning stage of an AC/DC distribution network project is analyzed. Based on the small signal modeling,the eigenvalue analysis method is used to analyze the influence laws of short circuit ratio (SCR),power level and control parameters on low-frequency oscillation. Next,the measures of changing controller parameters and adding additional controllers to suppress oscillation are proposed. The theoretical analysis and simulation test show that weak grid affects the stability of voltage source converter (VSC). Low frequency oscillation occurs when the 375 V DC side power increases to exceed the rated power. The integral coefficient of the Buck converter current inner loop proportional integral (PI) controller does not affect the stability of the system,but it affects the oscillation frequency after the oscillation occurs. If the proportional coefficient of the voltage outer loop PI controller is too small or the integral coefficient is too large,the system is unstable. Changing voltage outer loop PI controller parameters of Buck converter and adding additional controller can effectively suppress low-frequency oscillations. The theory and simulation analysis of the actual AC/DC distribution network oscillation influence factors are of reference significance for the commissioning and operation analysis of other AC/DC distribution network projects.

The vocal cord polyp is easy to relapse after surgery, but if the patient has recurrence in a short term, it is necessary to consider it as postoperative vocal cord contact granuloma. If the patients with contact granuloma after surgical treatment had severe impact on the pronunciation, it is necessary to be operated and confirmed by pathology and given the treatment of acid suppression, in order to avoid postoperative recurrence.

Chronic rhinonasal diseases and chronic obstructive pulmonary disease (COPD) cause impairment to the quality of life, increase the hospitalization and mortality. But we know little about the relationship between chronic rhinonasal disease (such as rhinitis, sinusitis, nasal polyps and so on) and COPD. This review aimed to reveal the correlation of rhinonasal disease and COPD, as in epidemiology, mechanism, clinical presents, treatment and so on.

Objective: To explore the application of 3D-reconstruction of lacrimal sac under computer., Method: To the 3D model of lacrimal passage was reconstructed with CT scanning., Result: The anatomy of lacrimal passage could be displayed with 3D-reconstruction under computer and the relation between lacrimal sac and the axilla of the middle turbinate could be observed and measured freely., Conclusion: The technology of 3D-reconstruction under computer is greatly valuable in the anatomic research of lacrimal sac.