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1. Transient Algebraic Impedance Derivations and Applications for PLL-Synchronized IBRs

   https://doi.org/10.1109/TPWRD.2023.3338968  

   Fan, Lingling; Miao, Zhixin; Ramasubramanian, Deepak (February 2024, IEEE Transactions on Power Delivery)

   In system-level dynamic studies, grid-following inverter-based resources (IBRs) have been treated as current sources synchronized to the main grid via phase-locked-loops (PLL), while the interconnected transmission line is usually treated as a constant complex impedance. In this letter, we present the derivation of transient algebraic impedance of the transmission line and demonstrate its superiority over constant impedance. We show significant accuracy improvement in predicting transient stability and oscillations when the constant impedance is replaced by a transient algebraic impedance. Furthermore, we derive a small-signal model by use of the transient algebraic impedance and this model is successful in explaining the interaction between the PLL and the grid. On the other hand, if constant impedance is assumed, such stability issues cannot be predicted. 

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2. Model Predictive Current Control with Inherent Damping for Inverters under Weak Grid Conditions

   Lee, S. and (January 2019, Proceedings of the 52nd Frontiers of Power Conference)

   null (Ed.)

   Increased capacity associated with renewable energy sources has created a need for improved methods for controlling power flows from inverter-based generation. This research provides a comparative study of finite-control-set model predictive current control (FCS-MPC-based) with respect to conventional proportional-integral-based (PI-based) synchronous current control for a three-phase voltage source inverter (VSI). The inverter is accompanied by an inductive-capacitive-inductive (LCL) filter to attenuate pulse width modulation (PWM) switching harmonics. However, an LCL filter introduces a resonance near to the control stability boundary, giving rise to substantial complexity from a control perspective. In order to avoid potential instability caused by the resonance, active damping can be included in the PI-based current control. Though properly designed active damping can improve inverter stability, in practice the robustness of standard PI control is not attainable due to variability in the grid inductance at the point of common coupling (PCC). This is due to impedance variations causing large shifts in the LCL resonance frequency. Weak grid conditions (i.e., a low short-circuit ratio) and a correspondingly high line impedance are particularly susceptible to LCL induced resonance instabilities. As an approach to operate with grid impedance variations and weak grid conditions, FCS-MPC has the potential to produce superior performance compared to PI-based current control methods. This comparative study indicates that FCS-MPC has improved resonance damping and fast dynamic capability in a system with renewable energy sources under weak grid conditions. Detailed results from MATLAB/SimPower are presented to validate the suggested FCS-MPC method where it is robust to uncertainty in the grid impedance variations. Overall results indicate an improvement over conventional PI-based current control methods. 

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3. Impact of the Exciter and Governor Parameters on Forced Oscillations

   https://doi.org/10.3390/electronics13163177  

   Thotakura, Naga Lakshmi; Burge, Christopher Ray; Liu, Yilu (August 2024, Electronics)

   In recent years, the frequency of forced oscillation events due to control system malfunctions or improper parameter settings has increased. Tuning the parameters of exciters and governor models is crucial for maintaining power system stability. Traditional simulation studies typically involve small transient disturbances or step changes to find optimal parameter sets, but existing optimization algorithms often fall short in fine-tuning for forced oscillations. Identifying the sensitive parameters within these control models is essential for ensuring stability during large, sustained disturbances. This study focuses on identifying these critical exciter and governor model parameters by analyzing their influence on sustained forced oscillations. Using Kundur’s two-area system, we analyze common exciter models such as SCRX, ESST1A, and AC7B, along with governor models like GAST, HYGOV, and GGOV1, utilizing PSS®E software version 34. Sustained forced oscillations are injected at generator-1 of area-1, with individual parameter changes dynamically simulated. By considering a local oscillation frequency of 1.4 Hz and an inter-area oscillation mode of 0.25 Hz, we analyze the impact of each parameter change on the magnitude and frequency of forced oscillations as well as on active and reactive power outputs. This novel approach highlights the most influential parameters of each tested model—such as exciter, governor, and turbine gains, as well as time constant parameters—on the impact of forced oscillations. Based on our findings, the sensitive parameters of each tested model are ranked. These would provide valuable insights for industry operators to fine-tune control settings during oscillation events, ultimately enhancing system stability. 

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4. Transient Stability and Active Protection of Power Systems with Grid-Forming PV Power Plants

   https://doi.org/10.1109/TPWRS.2022.3165704  

   Roy, Soummya; Villegas Pico, Hugo N. (April 2022, IEEE Transactions on Power Systems)

   Photovoltaic (PV) power plants with grid-forming technology must withstand severe disturbances and remain operational. To address this challenge, this paper sets forth a grid-forming strategy for PV solar power plants so that they can ride through power system faults. This capability is accomplished by leveraging two-axis proportional-integral regulators with anti-windup functionality. This paper also demonstrates that fluctuations of solar irradiance can cause significant dc-link voltage variations and loss of synchronism of grid-forming PV plants. Hence, we develop an active dc-link protection method which depends on estimation in solar irradiance. The contributions of this paper are demonstrated via positive-sequence simulations of modified versions of the WSCC 9- and IEEE 39-bus grids. 

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5. Exploring the Leakage Inductance of Transformers Used in Dual Active Bridge

   https://doi.org/10.1109/NAPS46351.2019.9000267  

   Soria, Denisse A.; Ranade, Satish; Lavrova, Olga (October 2019, 2019 North American Power Symposium (NAPS))

   Battery Energy Storage Systems (BESS) are critical to achieving reliability and efficiency in the modern electric grid. Dual Active Bridges (DAB) are often proposed for such integration since they can allow multiple sources and electrical isolation via a high-frequency transformer. In defined applications, DABs rely on the magnetizing and leakage inductance of their high-frequency transformers to achieve their performance requirement. In this paper, the inductance parameters of two types of two-winding transformers are investigated using the Finite Element Analysis (FEA) software ANSYS Maxwell. The variation of the inductance values due to the windings' distribution and core geometry was studied by establishing a parametric analysis. Prototypes were built and tested to compare the inductances actual measurements versus the simulation results. Exploring the possibility of characterizing the inductances of a transformer from a simulation standpoint is of interest in order to construct customized transformers with the optimal characteristic required in a specific DAB design. 

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