Post arc current can reflect the residual plasma in the electrode gap and the zero zone characteristics, so it has a great influence on the interruption of the breakers. In this paper, a permanent magnet-repulsive force hybrid actuator is designed, and the post arc current measurement system is established. Based on the forced current zero method, a series of high frequency DC interruption experiments are carried out in the synthetic test circuit with vacuum interrupters. The influence of the commutation frequency and recovery voltage on the vacuum DC interruption is investigated. The results show that a short lag phase about 80 ns exists between the post arc current and recovery voltage. High commutation frequency leads to the rise of the post arc current, which is disadvantageous for the vacuum DC interruption. The post arc current also rises with the increase in the recovery voltage. The interruption is not just affected by the di/dt and du/dt. In this study, failing interruption at the first current zero happens when the transient recovery voltage is near 2800 V. The current chopping occurs easily at high commutation frequency. And the chopping current value is proportional to the commutation frequency, which is up to 120 A at 4.0 kHz.

The postarc current (PAC) is an important parameter, which can indicate the interrupting capacity of breakers. According to the amplitude of the PAC, the appearance of the arc before current zero and the quantity of residual metal vapor particles in the electrode distance after current zero can be studied qualitatively. In order to investigate the characteristics of PAC in direct current (dc) interruption, a high-speed hybrid actuator was designed, and the forced current zero method was used to carry out a series of dc interrupting experiments in the synthetic test circuit. Also, the equipment for measuring the PAC was established to investigate the factors influencing on the PAC, including the amplitude of rated breaking current, electrode distance, and commutation frequency. The experimental results indicated that PAC presented to be instable in a certain range, due to the different contact structures, arcing positions, and so on. To some extent, the instability of PAC reflects the sheath growth process after the current zero. In addition, the amplitude of PAC changes with the variation of electrode distances. For the same distances, PAC keeps stable in a certain range. When the rated breaking current is less than 800 A, PAC increases with a high growth rate. However, the one increases with a low growth rate when the rated breaking current varies within 800–2000A. For different commutation frequencies, PAC has a significant growth with the increase of the commutation frequency. It is for the reason that more residual metal vapor particles exist in the electrode distance after current zero when the commutation frequency is higher, which goes against to dc interruption.

This paper reports on investigations of the air-coupled ultrasonic (ACU) method to detect common defects in solid timber panels made of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.). The ACU technology is a non-contact method for nondestructive timber testing with quicker scanning rates compared to contact methods. A testbed was set up consisting of commercially available piezo-ceramic ACU transducers and in-house manufactured signal processing circuits. To demonstrate the suitability of the ACU technique, through-transmission measurement results are presented for samples with defects such as knots, wormholes, and cracks. Pulse compression methods (Barker-coded method) were used to improve the power of received signals based on cross-correction algorithms. Results showed defects of timber panels made of Chinese fir can be detected with a thickness of less than 40 mm. Defects larger than 3 mm in diameter could be detected with high precision. Applying the pulse compression method showed better results than using common sine signals as excitation signals since it increased the signal-to-noise ratio, which is especially important for air-coupled measurement of high-attenuation materials like timber materials. The measurement results on reference samples demonstrated that ACU technology is a promising method for timber defect detection, especially for the quality assessment of engineered wood products.

Surface dielectric barrier discharge (DBD) has recently received growing interest for enormous application potential in various fields. In this paper, the optical observation of surface DBD is conducted by an intensified charge-coupled device camera from top view in the atmospheric air. Spacial and transient characteristics of discharge phenomenon in atmospheric pressure air have been, respectively, visualized with the exposure time of 100 ms and 5 μs. The discharge area is obtained by calculating the number of pixels in MATLAB software at different electrical parameters and different high-voltage electrode configurations with microsecond time scale. Experimental measurements show that the diffuse discharge during the negative-half cycle has good uniformity and stability compared with filamentary discharge during the positive-half cycle. The results offer a new estimated method for the discharge area.