A GPU-based Transient Stability Simulation using Runge-Kutta Integration Algorithm

Graphics processing units (GPU) have been investigated to release the computational capability in various scientific applications. Recent research shows that prudential consideration needs to be given to take the advantages of GPUs while avoiding the deficiency. In this paper, the impact of GPU acceleration to implicit integrators and explicit integrators in transient stability is investigated. It is illustrated that implicit integrators, although more numerical stable than explicit ones, are not suitable for GPU acceleration. As a tradeoff between numerical stability and efficiency, an explicit 4th order Runge-Kutta integration algorithm is implemented for transient stability simulation based on hybrid CPU-GPU architecture. The differential equations of dynamic components are evaluated in GPU, while the linear network equations are solved in CPU using sparse direct solver. Simulation on IEEE 22-bus power system with 6 generators is reported to validate the feasibility of the proposed method.published_or_final_versio

Application of non-linear programming for large-scale AC-DC power flow analysis

This paper proposes a robust, non-divergent model for large-scale AC-DC power flow analysis. By introducing relaxation variables into power flow equations, the conventional power flow model is turned into a non-linear programming model to minimize the L2 norm of the relaxation variables subject to operation constraints and DC station control modes. The interior point method (IPM) is used to solve the model. Due to the special structure of the proposed model, the size of coefficient of correction equations in the IPM can be reduced to the same as Jacobian in the conventional Newton method. Further, the coefficient is symmetrical positive-definite with a diagonal perturbation, which leads to a better convergence than Newton direction. The model and algorithm proposed also have the following features: (1) no special initial value is needed; (2) strong robustness for power grids with plenty of small/negative impedance; (3) applicable for searching a desirable operation point by appropriate settings of inequality constraints; (4) striving for ultimate close to power flow solution avoiding to stop too early at a local minimizer. Numerical simulations on a 6527 buses power grid in China with 8 HVDC lines show that the proposed model has robust convergent property in the feasible region, around the boundary of the feasible region and even beyond the feasible region of power flow. © 2012 IEEE.published_or_final_versio

Grazing activity increases decomposition of yak dung and litter in an alpine meadow on the Qinghai-Tibet plateau

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FHL2 regulates hematopoietic stem cell functions under stress conditions.

FHL2, a member of the four and one half LIM domain protein family, is a critical transcriptional modulator. Here, we identify FHL2 as a critical regulator of hematopoietic stem cells (HSCs) that is essential for maintaining HSC self-renewal under regenerative stress. We find that Fhl2 loss has limited effects on hematopoiesis under homeostatic conditions. In contrast, Fhl2-null chimeric mice reconstituted with Fhl2-null bone marrow cells developed abnormal hematopoiesis with significantly reduced numbers of HSCs, hematopoietic progenitor cells (HPCs), red blood cells and platelets as well as hemoglobin levels. In addition, HSCs displayed a significantly reduced self-renewal capacity and were skewed toward myeloid lineage differentiation. We find that Fhl2 loss reduces both HSC quiescence and survival in response to regenerative stress, probably as a consequence of Fhl2-loss-mediated downregulation of cyclin-dependent kinase-inhibitors, including p21(Cip) and p27(Kip1). Interestingly, FHL2 is regulated under the control of a tissue-specific promoter in hematopoietic cells and it is downregulated by DNA hypermethylation in the leukemia cell line and primary leukemia cells. Furthermore, we find that downregulation of FHL2 frequently occurs in myelodysplastic syndrome and acute myeloid leukemia patients, raising a possibility that FHL2 downregulation has a role in the pathogenesis of myeloid malignancies

Virtual synchroscope: a novel application of PMU for system restoration

Power system restoration is well recognized as one of the key technologies to improve the reliability of power systems. Generally, the restoration process is divided into several stages as preparation, system restoration, load restoration, synchronization and inter-connection. Currently, re-closure of breakers is done by field crews with synchroscope. A novel methodology, entitled as 'virtual synchroscope', is proposed in this paper. By utilizing Phasor Measurement Units (PMUs), the proposed methodology compares phasors of voltages at connection points from the two islands with the accurately time-stamped measurements. By adjusting voltage magnitude, frequency, and phase angle in the control center, all the requirements for synchronization are satisfied and the synchronization can be implemented. A case study on a power system is presented to validate the feasibility of the proposed method. It is proved that the proposed method can meet the requirements of all standards and resulting in smooth synchronization. © 2011 IEEE.published_or_final_versionThe 2011 International Conference on Advanced Power System Automation and Protection (APAP 2011), Beijing, China, 16-20 October 2011. In Proceedings of APAP, 2011, v. 1, p. 193-19