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1. A Heterogeneous Multiscale Method for Efficient Simulation of Power Systems with Inverter-Based Resources

   https://doi.org/10.1109/TPWRS.2025.3539567  

   Huang, Kaiyang; Xiong, Min; Liu, Yang; Sun, Kai (February 2025, IEEE Transactions on Power Systems)

   As inverter-based resources (IBRs) penetrate power systems, the dynamics become more complex, exhibiting multiple timescales, including electromagnetic transient (EMT) dynamics of power electronic controllers and electromechanical dynamics of synchronous generators. Consequently, the power system model becomes highly stiff, posing a challenge for efficient simulation using existing methods that focus on dynamics within a single timescale. This paper proposes a Heterogeneous Multiscale Method for highly efficient multi-timescale simulation of a power system represented by its EMT model. The new method alternates between the microscopic EMT model of the system and an automatically reduced macroscopic model, varying the step size accordingly to achieve significant acceleration while maintaining accuracy in both fast and slow dynamics of interests. It also incorporates a semi-analytical solution method to enable a more adaptive variable-step mechanism. The new simulation method is illustrated using a two-area system and is then tested on a detailed EMT model of the IEEE 39-bus system. 

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2. A perturbation solution to the full Poisson–Nernst–Planck equations yields an asymmetric rectified electric field

   https://doi.org/10.1039/D0SM00417K  

   Hashemi Amrei, S. M.; Miller, Gregory H.; Bishop, Kyle J.; Ristenpart, William D. (August 2020, Soft Matter)

   null (Ed.)

   We derive a perturbation solution to the one-dimensional Poisson–Nernst–Planck (PNP) equations between parallel electrodes under oscillatory polarization for arbitrary ionic mobilities and valences. Treating the applied potential as the perturbation parameter, we show that the second-order solution yields a nonzero time-average electric field at large distances from the electrodes, corroborating the recent discovery of Asymmetric Rectified Electric Fields (AREFs) via numerical solution to the full nonlinear PNP equations [Hashemi Amrei et al. , Phys. Rev. Lett. , 2018, 121 , 185504]. Importantly, the first-order solution is analytic, while the second-order AREF is semi-analytic and obtained by numerically solving a single linear ordinary differential equation, obviating the need for full numerical solutions to the PNP equations. We demonstrate that at sufficiently high frequencies and electrode spacings the semi-analytical AREF accurately captures both the complicated shape and the magnitude of the AREF, even at large applied potentials. 

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3. An Efficient Network Solver for Electromagnetic Transient Simulation of Power Systems Based on Hierarchical Inverse Computation and Modification

   https://doi.org/10.1109/NAPS52732.2021.9654532  

   Zhang, Lu; Wang, Bin; Sarin, Vivek; Shi, Weiping; Kumar, P. R.; Xie, Le (November 2021, 2021 North American Power Symposium (NAPS))

   In both power system transient stability and electromagnetic transient (EMT) simulations, up to 90% of the computational time is devoted to solve the network equations, i.e., a set of linear equations. Traditional approaches are based on sparse LU factorization, which is inherently sequential. In this paper, EMT simulation is considered and an inverse-based network solution is proposed by a hierarchical method for computing and store the approximate inverse of the conductance matrix. The proposed method can also efficiently update the inverse by modifying only local sub-matrices to reflect changes in the network, e.g., loss of a line. Experiments on a series of simplified 179-bus Western Interconnection demonstrate the advantages of the proposed methods. 

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4. Task-Level Control and Poincaré Map-Based Sim-to-Real Transfer for Effective Command Following of Quadrupedal Trot Gait

   https://doi.org/10.1109/Humanoids57100.2023.10375183  

   Bhounsule, Pranav A; Torres, Daniel; Hinojosa, Ernesto Hernandez; Alaeddini, Adel (December 2023, IEEE)

   The ability of quadrupedal robots to follow commanded velocities is important for navigating in constrained environments such as homes and warehouses. This paper presents a simple, scalable approach to realize high fidelity speed regulation and demonstrates its efficacy on a quadrupedal robot. Using analytical inverse kinematics and gravity compensation, a task-level controller calculates joint torques based on the prescribed motion of the torso. Due to filtering and feedback gains in this controller, there is an error in tracking the velocity. To ensure scalability, these errors are corrected at the time scale of a step using a Poincar´e map (a mapping of states and control between consecutive steps). A data-driven approach is used to identify a decoupled Poincar´e map, and to correct for the tracking error in simulation. However, due to model imperfections, the simulation-derived Poincar´e map-based controller leads to tracking errors on hardware. Three modeling approaches – a polynomial, a Gaussian process, and a neural network – are used to identify a correction to the simulation-based Poincar´e map and to reduce the tracking error on hardware. The advantages of our approach are the computational simplicity of the task-level controller (uses analytical computations and avoids numerical searches) and scalability of the sim-to-real transfer (use of low-dimensional Poincar´e map for sim-to-real transfer). A video is in this shortened link: http://tiny.cc/humanoids23 

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5. Protocol for inferring epithelial-to-mesenchymal transition trajectories from single-cell RNA sequencing data using R

   https://doi.org/10.1016/j.xpro.2023.102819  

   Najafi, Annice; Jolly, Mohit Kumar; George, Jason T (March 2024, STAR Protocols)

   The epithelial-to-mesenchymal transition (EMT) provides crucial insights into the metastatic process and possesses prognostic value within the cancer context. Here, we present COMET, an R package for inferring EMT trajectories and inter-state transition rates from single-cell RNA sequencing data. We describe steps for finding the optimal number of EMT genes for a specific context, estimating EMT-related trajectories, optimal fitting of continuous-timeMarkov chain to inferred trajectories, and estimating inter-state transition rates. 

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