Identification of avian polyomavirus and its pathogenicity to SPF chickens

The research aimed to study an Avian polyomavirus strain that was isolated in Shandong, China. To study the pathogenicity of APV in SPF chickens, and provide references for epidemiological research and disease prevention and control of APV. The genetic characterization of APV strain (termed APV-20) was analyzed and the pathogenicity of APV was investigated from two aspects: different age SPF chickens, and different infection doses. The results revealed that the APV-20 exhibits a nucleotide homology of 99% with the other three APV strains, and the evolution of APV In China was slow. In addition, the APV-20 infection in chickens caused depression, drowsiness, clustering, and fluffy feathers, but no deaths occurred in the infected chickens. The main manifestations of necropsy, and Hematoxylin and Eosin staining (HE) showed that one-day-old SPF chickens were the most susceptible, and there was a positive correlation between viral load and infection dose in the same tissue. This study showed that SPF chickens were susceptible to APV, and an experimental animal model was established. This study can provide a reference for the pathogenic mechanism of immune prevention and control of APV

An economical hybrid DC circuit breaker with pre-current-limiting capability

Multi-terminal flexible high-voltage-direct-current (HVDC) transmission and DC grid technology are developing rapidly, and the hybrid DC circuit breaker (DCCB) is usually used to protect the DC system, which can quickly remove the fault current and make sure the safe and stable work of system. However, the existing hybrid DCCBs have restrictions regarding one or more of the following characteristics: economy, fast adaptive reclosing, low on-state losses, and no additional pre-charge power source. This paper presents an economical hybrid DCCB with pre-current-limiting capability (EP-HCB), in which the main branch only is made up of mechanical switch (MS) and the fault handling branch consists of fault current transfer circuit, current limiting circuit and breaking circuit. And the proposed EP-HCB itself has inherent pre-charging and adaptive reclosing capabilities and low on-state losses, which efficiently limit the magnitude and rise rate of fault current and thus weaken the opening requirements on the DCCB. Detailed topology structure of the EP-HCB is presented, and the work principle of the fault current commutation and limiting are analyzed. Then, a design method for parameters of the proposed EP-HCB is provided. Finally, based on the 4-terminal simulation system in PSCAD/EMTDC, the feasibility and economy of the EP-HCB are proved by comparison with ABB hybrid DCCB

A novel current-limiting hybrid DC breaker based on thyristors

Since the DC current has no zero-crossing in high-voltage and large-capacity DC scenarios, and the low line impedance results in a large growth rate of fault current, the breaking requirements of DC circuit breakers (DCCB) are further increased. Hybrid circuit breaker (HCB) represent the techniques orientation of high-voltage DC interruptions and the HCB can quickly interrupt fault currents to ensure reliable and safe operation of the grid, which is an indispensable piece of equipment in flexible DC systems. Utilizing the characteristics of thyristor with strong current capacity, low on-state loss and zero-crossing natural turn-off of unidirectional current, this article proposes a hybrid circuit breaker scheme based on forced zero-crossing of thyristors (T-HCB). The proposed T-HCB could achieve arcless breaking of mechanical switch (MS), which improves the service life of the MS. In addition, it has the limiting capability of fault current, which reduces the growth rate and amplitude of the fault current and improves the security level and economics of the system. Firstly, the working principle of the proposed topology is introduced and deduced. Then, the key parameters affecting the commutation capacity and current limiting capacity are analyzed extraordinary. Finally, a comparison of the breaking performance in a 4-terminal DC system with existing fault current limiter (FCL) and ABB hybrid DCCB demonstrates the feasibility and superiority of the proposed topology

A variable frequency injection method for modular multilevel converters in variable speed drives

Modular multilevel converter (MMC) is a novel circuit that has appeared in the last few decades, and has been extensively applied to medium/high voltage scenarios, especially in motor drive owing to its scalability and easy to supply a higher number of output voltage levels. As theoretical analysis shows, the sub-module (SM) capacitor voltage magnitude is positively related to the converter output current amplitude and conversely connected to the fundamental frequency. Therefore, the prime challenge of applying MMC to variable speed drives is that there is a huge voltage oscillation in SM capacitor voltage during low-speed working. An innovative high-frequency voltage injection modality to diminish the fluctuation of the capacitor voltage in variable speed drives is proposed in this paper, in which the frequency of the injected component can be changed with the operating frequency of the converter and the low frequency component of power can be converted into high frequency component. The discussed measure not only significantly reduces the voltage float in capacitor, but also avoids the overmodulation of arm voltage modulation waveform caused by high frequency of injection component when converter works in low frequency. With the improved method, the performance of motor at low frequency is enhanced. Finally, the feasibility and effectiveness of the proposed improved high-frequency voltage injection method are verified by simulation and experiment