Overview on Resonance Characteristics and Resonance Suppression Strategy of Multi-Parallel Photovoltaic Inverters

Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.

Gender and magnetic resonance imaging classification-related differences in clinical and biochemical characteristics of Cushing＇s disease： a single-centre study

Background Cushing＇s disease （CD） presents a remarkable preponderance in female gender,and a significant minority of patients with CD presented with negative magnetic resonance imaging （MRI） findings.The aim of this study was to evaluate gender-related and MRI classification-related differences in clinical and biochemical characteristics of CD.Methods We retrospectively studied 169 patients with CD,comprising 132 females and 37 males,and 33 patients had no visible adenoma on MRI.Results We observed that male and MRI-positive patients presented with high adrenocorticotropic hormone （ACTH） values （P ＜0.05）.Female patients presented with higher prevalence of hirsutism and hyperpigmentation and lower prevalence of purple striae （P ＜0.05）.The prevalence of buffalo-hump and hypertension was greater in MRI-negative patients （P ＜0.05）.In addition,male patients with CD presented at a younger age compared with females （P ＜0.05）.Patients with fatigue and hypokalaemia presented significantly higher urinary-free cortisol,ACTH and cortisol levels compared with patients without these symptoms （P ＜0.05）.The prevalence of LH reduction,hyper total cholesterol （TC） and hyper low-density lipoprotein was more frequent in MRI-positive patients （P ＜0.05）.Hyper-TC levels and PRL reduction were more frequent in males （P ＜0.05）.T3,T4 and FT3 levels negatively correlated with age at diagnosis （r=-0.310,P ＜0.01; r=-0.191,P ＜0.05; r＝-0.216,P ＜0.05）.T3,T4,FT3 and FT4 levels significantly negatively correlated with 8-am plasma cortisol levels （r=-0.328,P ＜0.01; r=-0.195,P ＜0.05; r=-0.333,P ＜0.01; r＝-0.180,P ＜0.05）.Females presented higher total protein level （P ＜0.01） and lower blood urea nitrogen and serum creatinine levels （P ＜0.01）,compared with male patients.Conclusions Carefulness and caution are required in all patients with CD,because of the complexity of clinical and biochemical characteristics in CD patients of different gender and MRI classification,particularly male patients and MRI-negative patients.

Laser beam characteristic for laser resonators with diffraction optical elements

The matrix eigenvalue method is used to analyze a laser resonator composed of diffraction optical elements. The results show that this type of resonator can separate fundamental mode and high order modes effectively. The output beams can be designed for different requests.