With the proposal of carbon peak and carbon neutrality target, the tank multi-break vacuum circuit breakers with environment-friendly gas insulation is an effective methods to replace SF6 gas in the field of high voltage and ultra-high voltage. Firstly, the electric field simulation of tank multi-break vacuum circuit breakers is carried out. The scale test electrode is designed for the key position according to the electric field distribution, and the lightning breakdown test with different pressures, electrode distances and electrode structures are carried out. The results show that 0.9 MPa dry air and 0.9 MPa CO2 can satisfy the insulation configuration requirements of the tank multi-break vacuum circuit breaker. In addition, the surface flashover characteristics of the ceramic shell interface of vacuum interrupter are studied, the effects of gas pressure, metal particle attachment location on the surface flashover voltage of the ceramic shell are investigated, which provides guidance for the development of vacuum interrupters in ultra-high voltage field.
In this paper, polypropylene (PP) film samples were obtained at different stretching ratios. The dielectric properties of PP samples were studied. The crystal morphology and orientation degree of the materials were characterized. The findings indicate that the PP film sample with the stretching ratio 5 x 5 has the lowest conductivity and the highest breakdown strength. Moreover, the effect mechanism of the stretching ratio on the dielectric properties of PP films was explored. A reasonable stretching ratio can make the molecular chains of PP aligned, form an orderly crystalline morphology, and improve the crystallinity. When the stretching ratio is too high, the orientation degree of PP will decrease and the spherulite structure will be destroyed, leading to a decrease in dielectric properties. The research results can serve as a reference for improving the dielectric properties of PP films.
To address the technical challenges of DC fault clearing in DC grid, a multiport DC circuit breaker (DCCB) integrating current-limiting and fast fault clearing capabilities is proposed. Due to the topology design of the H-bridge LCSs and the additional bridge arm provided in the H-bridge component for connecting to the DC bus, the proposed DCCB possesses the capability to clear DC bus faults. Three control strategies were designed, including energization, fault interruption following current limiting and reset following current limiting. The proposed topology realizes a large reduction of peak current value by employing a current limiting component. The time required for fault clearing is minimized by implementing a bypass component. The proposed control strategy enables the pre-charge of circuit breaker capacitor by multiplexing a portion of the topology loop. The operating sequence of circuit breaker is optimized through a two-step process: preliminary fault judgment and secondary confirmation. This process further reduces the current value at the break time. A five-terminal DC grid simulation model is built in PSCAD/EMTDC, and various operating conditions of circuit breakers are simulated and verified. The results show that the proposed multiport DCCB can quickly and reliably clear faults in multiple DC lines and DC bus, reduce the peak current value, and minimize the time required for fault interruption under all working conditions.
The fractional level modular multilevel converter (FLMMC) can effectively improve the voltage quality of MMC with small quantities of submodules. However, the switching loss of FLMMC under existing fractional level modulation (FLM) strategy is high. In order to reduce the switching loss of FLMMC, a modified FLM strategy including submodule energy balancing segment and submodule classification inserting segment was proposed in this paper. During the energy balancing segment, both the conventional submodule and the fractional level submodule participated in sorting and inserting at the time of each level change, so as to achieve energy balance between the two types of submodules. During the classification inserting segment, the conventional submodule and the fractional level submodule were sorted and inserted separately according to the integral level or the fractional level, so as to decrease the switching times of power devices. Simulation results based on Matlab/Simulink reveal that the proposed modified FLM strategy can significantly reduce the switching loss of FLMMC, and obtain excellent effect on level multiplying.
With the increasing penetration of renewable energy and power electronic equipment, the characteristics of frequency in a power system have been significantly changed. This paper analyzes the space-time distribution of frequency in a renewable power system. Firstly, the frequency-response model is established for different types of renewable integrated multi-generator power systems. Next, quantitative indices for the evaluation of frequency distribution are presented. Then, with simulation studies, the characteristics of frequency responses in different types of grid are analyzed. Finally, the factors affecting the space-time distribution of frequency, including location of renewable generator, line reactance and location of disturbances, are investigated.
