Grid Synchronization Control for Grid-connected Voltage Source Converters Based on Voltage Dynamics of DC-link Capacitor

This paper proposes a grid synchronization control strategy for the grid-connected voltage source converters(VSCs)based on the voltage dynamics of the DC-link capacitor in the VSC.The voltage dynamics of the DC-link capacitor are used to regulate the frequency and phase angle of the inner potential of the VSC,synchronizing the VSC with grid.Firstly,in the proposed strategy,the active power regulation and grid synchronization of the VSC are combined,which are separated in the traditional control strategy.This can avoid the instability of the VSC in a weak grid with a low short circuit ratio(SCR),aroused by the dynamic interaction between the separated control loops in traditional control strategies.Secondly,the energy stored in the DC-link capacitor is directly coupled with the grid via the inner potential of the VSC,and the inertia characteristic is naturally featured in the inner potential by the proposed strategy.With the increase of the capacitance,the natural inertial response of the VSC is helpful to improve the grid frequency dynamic.Finally,simulation results are presented to validate the correctness and effectiveness of the proposed strategy in the enhancement of the grid frequency and voltage dynamic support capability.

An in situ monitoring campaign of a hard rock pillar at great depth within a Canadian mine

A recent research campaign at a Canadian nickel-copper mine involved instrumenting a hard rock sill drift pillar with an array of multi-point rod extensometers,distributed optical fibre strain sensors,and borehole pressure cells(BHPCs).The instrumentation spanned across a 15.24 m lengthwise segment of the relatively massive granitic pillar situated at a depth of 2.44 km within the mine.Between May 2016 and March 2017,the pillar’s displacement and pressure response were measured and correlated with mining activities on the same level as the pillar,including:(1)mine-by of the pillar,(2)footwall drift development,and(3)ore body stoping operations.Regarding displacements of the pillar,the extensometers provided high temporal resolution(logged hourly)and the optical fibre strain sensors provide high spatial resolution(measured every 0.65 mm along the length of each sensor).The combination of sensing techniques allowed centimetre-scale rock mass bulking near the pillar sidewalls to be distinguished from microstrain-scale fracturing towards the core of the pillar.Additionally,the influence and extent of a mine-scale schistose shear zone transecting the pillar was identified.By converting measured rock mass displacement to velocity,a process was demonstrated which allowed mining activities inducing displacements to be categorised by time-duration and cumulative displacement.In over half of the analysed mining activities,displacements were determined to prolong for over an hour,predominately resulting in submillimetre cumulative displacements,but in some cases multi-centimetre cumulative displacements were observed.This time-dependent behaviour was more pronounced within the vicinity of the plumb shear zone.Displacement measurements were also used to assess selected support member load and elongation mobilisation per mining activity.It was found that a combined static load and elongation capacity of reinforcing members was essential to maintaining excavation stability,while permitting gradual shedding of stress through controlled pillar sidewall displacements.

A review of investigations on ground support requirements in coal burst‑prone mines

Ground support is widely implemented to mitigate dynamic rock failures in underground mines.This paper investigated the ground support requirements in burst-prone mines to mitigate the catastrophic dynamic rock failures of rock and/or coal bursts.First,the ground support principles and considerations in burst-prone conditions are identified.The objective of a ground support system is to increase the capacity to accommodate rock fracturing in a rockburst and,in turn,to minimize the kinetic energy of the ejected material.The support capacities of various yielding rockbolts and integrated support systems are then investigated using the test results in the laboratory.Apart from the energy absorption and yielding deformation capacity,the initial stiffness and energy absorption rate are also critical factors when applying yielding rockbolts in practice.Adding rope lacing and mesh strap to surface support elements can substantially enhance the support performance of the system.In practice,semi-analytical and empirical approaches are often used to determine the ground support elements in burst-prone areas.Semi-analytical methods first evaluate the support demand in burst risk zones and then select support elements according to their laboratory test results.Alternatively,empirical methods determine the ground support elements according to the locally established empirical rating scheme,which usually ranks the support capacities of various support systems based on ground support conditions and damage conditions.The outcomes of this study can provide insights into ground support strategies and assist the mining industry to develop effective coal burst control technologies.

Time series prediction of mining subsidence based on a SVM

In order to study dynamic laws of surface movements over coal mines due to mining activities,a dynamic prediction model of surface movements was established,based on the theory of support vector machines(SVM) and times-series analysis.An engineering application was used to verify the correctness of the model.Measurements from observation stations were analyzed and processed to obtain equal-time interval surface movement data and subjected to tests of stationary,zero means and normality.Then the data were used to train the SVM model.A time series model was established to predict mining subsidence by rational choices of embedding dimensions and SVM parameters.MAPE and WIA were used as indicators to evaluate the accuracy of the model and for generalization performance.In the end,the model was used to predict future surface movements.Data from observation stations in Huaibei coal mining area were used as an example.The results show that the maximum absolute error of subsidence is 9 mm,the maximum relative error 1.5%,the maximum absolute error of displacement 7 mm and the maximum relative error 1.8%.The accuracy and reliability of the model meet the requirements of on-site engineering.The results of the study provide a new approach to investigate the dynamics of surface movements.

Effects of elastic support on the dynamic behaviors of the wind turbine drive train

The reliability and service life of wind turbines are influenced by the complex loading applied on the hub, especially amidst a poor external wind environment. A three-point elastic support, which includes the main bearing and two torque arms, was considered in this study. Based on the flexibilities of the planet carrier and the housing, a coupled dynamic model was developed for a wind turbine drive train. Then, the dynamic behaviors of the drive train for different elastic support parameters were computed and analyzed. Frequency response functions were used to examine how different elastic support parameters influence the dynamic behaviors of the drive train. Results showed that the elastic support parameters considerably influenced the dynamic behaviors of the wind turbine drive train. A large support stiffness of the torque arms decreased the dynamic response of the planet carrier and the main bearing, whereas a large support stiffness of the main bearing decreased the dynamic response of planet carrier while increasing that of the main bearing. The findings of this study provide the foundation for optimizing the elastic support stiffness of the wind turbine drive train.