Refractory Performance of the Axial Tensile Welded Hollow Spherical Joint

Xiaobin Qiu,Bingsheng Huang,Haoyu Song,Zhen Zhang 대한토목학회 2023 KSCE Journal of Civil Engineering Vol.27 No.6

To study the refractory performance of the axial tensile welded hollow spherical joint, the steady-state test of heating under constant load was carried out on the joint specimens with different load levels. The failure modes, temperature field, displacement-time curves, and refractory performance of the spherical joints exposed to fire are investigated and analyzed. The experimental results indicate that the failure modes of the joints exposed to fire are pull-out failures. The failure location is at the intersection of the steel tube-hollow sphere. The lowest temperature is located at the intersection of the steel tube-hollow sphere. The closer you get to the equator of the hollow sphere, the higher the spherical temperature. The smaller the load level, the higher the refractory temperature and the longer the refractory limit of the joints. The correctness of the numerical model was verified by comparison with the experimental result. Numerical simulations show that with the increase of the load levels, the refractory performance of the joints gradually decreases, and the downward trend gradually slows down. Compared to the refractory temperature code-recommended, the refractory temperature of the tensile joints is very conservative.

Study of Fire Resistance Performance of Stiffened Welded Hollow Spherical Joint Under Axial Tension

Xiaobin Qiu,Tao Chen,Bingsheng Huang,Jingrui Zhu,Zhen Zhang,Haoyu Song 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.2

To study the fire resistance performance of stiffened welded hollow spherical joints under axial tension, two specimens with load ratios of 0.4 and 0.6 were subjected. The temperature distribution, failure mode, and fire resistance performance of stiffened welded hollow spherical joints under axial tension were obtained. The test results show that the failure mode of the joint was a pull-out failure. During the heating process, the closer to the spherical equator, the higher the spherical temperature, and the temperature of the sphere at the stiffener is lower than that of the sphere at the non-stiffener. The load ratio has a great influence on the refractory performance of the stiffened welded hollow sphere joint. When the load ratio of the specimen is reduced from 0.60 to 0.40, the fire resistance time of the specimen increases by 4.47 min, and the critical temperature increases by 21.3 °C. According to the European Code, the finite element model for the stiffened welded hollow spherical joints is established. The effect law of various influencing factors on the fire resistance performance of stiffened welded hollow spherical joints is studied. The research results show that reducing the load ratio and increasing the thickness of the sphere can significantly improve the fire resistance performance of the joint. The calculation formula of the critical temperature of steel members in the current standard not suitable for the calculation of the critical temperature of the welded hollow spherical joint. The critical temperature values with different load ratios recommended by the current standard were relatively more conservative. Taking the ambient temperature as the fire resistance temperature, the finite element analysis results were fitted, and the formula for calculating the fire resistance temperature of the stiffened welded hollow spherical joint was proposed.

Influence of Different Frequency Harmonic Generated by Rectifier on High-speed Permanent Magnet Generator

Qiu, Hongbo,Wei, Yanqi,Yang, Cunxiang,Fan, Xiaobin The Korean Institute of Electrical Engineers 2018 Journal of Electrical Engineering & Technology Vol.13 No.5

Since the stator winding of High-Speed Permanent Magnet Generator (HSPMG) has few winding turns and low inductance value, it is more prone to be influenced by harmonic current. Moreover, the operation efficiency and the torque stability of HSPMG will be greatly influenced by harmonic current. Taking a 117 kW, 60 000 rpm HSPMG as an example, in order to analyze the effects of harmonic current on HSPMG in this paper, the 2-D finite element electromagnetic field model of the generator was established and the correctness of the model was verified by testing the generator prototype. Based on the model, the losses and torque of the generator under different frequency harmonic current were studied. The change rules of the losses and torque were found out. Based on the analysis of the influence of the harmonic phase angle on torque ripple, it is found that the torque ripple could be weakened through changing the harmonic phase angle. Through the analysis of eddy current density in rotor, the change mechanism of the rotor eddy current loss was revealed. These conclusions can contribute to reduce harmonic loss, prevent demagnetization fault and optimize torque ripple of HSPMG used in distributed power supply system.

Influence Analysis of Power Grid Harmonics on Synchronous Hydro Generators

Qiu, Hongbo,Fan, Xiaobin,Feng, Jianqin,Yang, Cunxiang The Korean Institute of Power Electronics 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.5

The content of harmonic current increases with an increase in the number of power electronic devices in power grid. When a generator is directly connected to the power grid through a step-up transformer, the influence of the harmonic currents on the generator is inevitable. To study the influences of harmonics on generators, a 24-MW bulb tubular turbine generator is taken as an example in this paper. A 2-D transient electromagnetic field model is established. Through a comparative analysis of the data of experiments and simulations, the correctness of the model is verified. The values of the air gap magnetic density, torque and losses of the generator under various conditions are calculated using the finite element method. Taking the rated condition as a reference, the influence of the harmonic currents on the magnetic flux density is analyzed. It is confirmed that the time harmonic is a key factor affecting the generator performance. At the same time, the effects of harmonic currents on the torque ripple, average torque and eddy current loss of the generator are studied, and the mechanism of the variation of the eddy current loss is also discussed.

Influence Analysis of Power Grid Harmonics on Synchronous Hydro Generators

Hongbo Qiu,Xiaobin Fan,Jianqin Feng,Cunxiang Yang 전력전자학회 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.5

The content of harmonic current increases with an increase in the number of power electronic devices in power grid. When a generator is directly connected to the power grid through a step-up transformer, the influence of the harmonic currents on the generator is inevitable. To study the influences of harmonics on generators, a 24-MW bulb tubular turbine generator is taken as an example in this paper. A 2-D transient electromagnetic field model is established. Through a comparative analysis of the data of experiments and simulations, the correctness of the model is verified. The values of the air gap magnetic density, torque and losses of the generator under various conditions are calculated using the finite element method. Taking the rated condition as a reference, the influence of the harmonic currents on the magnetic flux density is analyzed. It is confirmed that the time harmonic is a key factor affecting the generator performance. At the same time, the effects of harmonic currents on the torque ripple, average torque and eddy current loss of the generator are studied, and the mechanism of the variation of the eddy current loss is also discussed.

Influence of Different Frequency Harmonic Generated by Rectifier on High-speed Permanent Magnet Generator

Hongbo Qiu,Yanqi Wei,Cunxiang Yang,Xiaobin Fan 대한전기학회 2018 Journal of Electrical Engineering & Technology Vol.13 No.5

Since the stator winding of High-Speed Permanent Magnet Generator (HSPMG) has few winding turns and low inductance value, it is more prone to be influenced by harmonic current. Moreover, the operation efficiency and the torque stability of HSPMG will be greatly influenced by harmonic current. Taking a 117 ㎾, 60 000 rpm HSPMG as an example, in order to analyze the effects of harmonic current on HSPMG in this paper, the 2-D finite element electromagnetic field model of the generator was established and the correctness of the model was verified by testing the generator prototype. Based on the model, the losses and torque of the generator under different frequency harmonic current were studied. The change rules of the losses and torque were found out. Based on the analysis of the influence of the harmonic phase angle on torque ripple, it is found that the torque ripple could be weakened through changing the harmonic phase angle. Through the analysis of eddy current density in rotor, the change mechanism of the rotor eddy current loss was revealed. These conclusions can contribute to reduce harmonic loss, prevent demagnetization fault and optimize torque ripple of HSPMG used in distributed power supply system.

Comparison of Two Kinds of Momentum Control Variables in 3DVAR During Assimilating Low‑resolution Observations in a Convective‑scale Model: a Case Study of Torrential Rainfall in North China

Qiru Dong,Xuelian Wang,Shuiyong Fan,Yinghua Li,Xiaobin Qiu,Lili Liu 한국기상학회 2022 Asia-Pacific Journal of Atmospheric Sciences Vol.58 No.5

The x and y components of wind (U and V, respectively) are widely used as control variables in radar assimilation; therefore,it is common to choose (U, V) as the control variables for multi-scale data assimilation (DA) in convective-scale. When themodel resolution reaches the convective scale, whether (U, V), as the momentum control variables, are still more suitablethan the stream function (ψ) and unbalanced velocity potential (χu), it needs to be studied further examination. This studyuses 3-km resolution forecast samples to calculate the background error covariance (B) with two different pairs of momentumcontrol variables ((ψ, χu) and (U, V)) by the National Meteorology Center (NMC) method. In single-observation experiments,the analysis wind field is most sensitive to the two pairs of B, and the temperature is insensitive. When using (U, V) as thecontrol variables, the local characteristic is more evident according to vertical and horizontal wind increments. The studyassimilates low- resolution conventional observations to compare different momentum control variables, (ψ, χu) and (U, V),in numerical simulation experiments of the torrential rainfall in North China. In addition, the impacts of the two controlvariables options are also compared in terms of the 15 continuous days of cases in flood season. The main results are as follows:(1) the wind field is the critical difference between the two assimilation experiments at the analysis time. Using (U, V)as the control variables, the analysis field of wind from both the surface and different vertical levels is superior. The analysisfield closer fits the wind observation; (2) the use of (U, V) control variables improves the short term (0~3-h) in surface windprediction; and (3) the use of (U, V) control variables enhances the 24-h TS (threat score) in moderate rain and heavy rain.