Study of vacuum arc plasma transport characteristics during the DC interrupting process

The mechanical DC vacuum circuit breaker based on forced-over-zero technology will inevitably generate vacuum arc during the actual interrupting process. Since the current drop frequency is usually very high, the vacuum arc usually exhibits obvious transient characteristics, and the excessive transient characteristics may even become a key factor limiting the interruption capacity. In order to improve the mechanical DC vacuum circuit breaker arc interrupting capability, this paper establishes a vacuum arc transient magneto-hydrodynamic simulation model in the DC interrupting process and studies the plasma transport characteristics of the vacuum arc under different DC interrupting conditions. The results show that the ion pressure, ion density and ion temperature decrease with decreasing arc current, while the ion velocity gradually increases during the DC interrupting process. The increase in breaking current and current drop frequency will increase the ion density in the arc column at the moment of current crossing zero, resulting in more difficult vacuum arc interrupting. The results of the study can provide an important theoretical basis for a deeper understanding of the vacuum arc transient process in the DC interrupting process and improve the DC vacuum circuit breaker arc interruption capability.

Investigation on the Transient Characteristics of Self-Priming Pumps with Different Hub Radii

Self-priming pumps, important fluid equipment, are widely used in the disaster relief and emergency fields. Meanwhile, the impeller is the only rotational unit of the self-priming pump, which plays an essential part in the power capability of the pump. In this paper, impellers with different hub radii are proposed; by comparing the internal flow characteristics, blade surface load, pressure pulsation characteristics, and radial force distribution of each scheme, the relationship between transient characteristics and hub radius is obtained. The results present that the impeller with a large hub radius can not only weaken the pressure pulsation, blade surface load, and radial force distribution, but also improve the ability of the blade to work on the internal flow field. Finally, the relevant hydraulic experiment is conducted, with the difference between the experiment and calculation below 3%, which ensures the accuracy of the calculation results.

Atypical start-up experiments on pump as turbine

At present, research on the characteristics of pumps as turbine (PATs) during the start-up process is still insufficient. To reveal the transient characteristics of a centrifugal PAT during the atypical start-up process, a test rig for the transient performance of the PAT was built; in addition, experiments on the transient hydraulic performance of three kinds of steady speed and three kinds of steady flow were conducted. Through these experiments, the evolution characteristics of the transient performance parameters of the PAT during the atypical start-up process were analyzed over time. Moreover, three dimensionless coefficients were employed to deeply reveal the transient characteristics of the PAT during atypical start-up. Results showed that the rise curves of flow rate and outlet static pressure exhibited shock phenomena. With the increase in the stable running speed after start-up, the impact phenomenon of the outlet static pressure presented a delayed trend. The dimensionless head and flow coefficients reached the maximum value at the initial stage of the atypical start-up process and then rapidly dropped to the minimum value before slowly rising to the final stable value. The dimensionless power coefficient had a maximum value at the initial stage of atypical start-up and then rapidly dropped to the final stable value.

Research on Consistency of Transient Response Characteristics of Current Transformers for DC application in DC Engineering

In ultra high voltage(UHV) DC system, the transient characteristics for fault current monitoring of current transformers for DC application (DCCTs) and the consistency of response characteristics when different types of DCCTs are used for differential protection have become important factors affecting the safe and stable operation of DC system. In this paper, transient characteristics of all-fiber DCCTs and shunt DCCTs mainly used in DC system were studied, the main parameters of the transient characteristics including delay time and maximum peak instantaneous error were tested, and the key parameters influencing the response consistency of DCCTs were analyzed. The results show that the maximum peak instantaneous errors of all-fiber DCCTs and shunt DCCTs can meet ±5% limit requirement, meeting the demand for protection application in DC system. The delay time is the main factors affecting the maximum peak instantaneous error of DCCT. The longer the delay time is, the greater the maximum peak instantaneous error is, which may even exceed the limit value of ±5%. If necessary, the delay time of DCCTs participating in differential protection can be compensated. The research results provide a theoretical basis for preventing the malfunction of protection device caused by the inconsistent response characteristics of DCCTs in DC system.