Soobae Kim

This paper presents an adaptive virtual oscillator control in coordination with an adaptive filter to mitigate subsynchronous oscillations in grid-forming converters caused by series compensation. Although series compensation enhances power transfer capability and transient stability margins, it can introduce subsynchronous resonance, leading to subsynchronous oscillations. Virtual oscillator control fed with set points is made dispatchable for grid-forming control to ensure the power-sharing, fast-synchronization, and subsynchronous oscillation damping capability of inverters. In this paper, taking advantage of power reserves in grid-forming operation, virtual oscillator control law is modified to dynamically change the set power point during low-resonance conditions to mitigate subsynchronous oscillations. Moreover, to overcome the limited damping capability of adaptive VOC during severe-resonance conditions, a coordinated adaptive adjustment of the grid-side filter inductance based on the modified power set point is designed. The IEEE’s first benchmark model is altered by integration with a 1000 MW grid-forming inverter in a MATLAB R2024b/Simulink environment. The previously proposed dispatchable virtual oscillator control and electronic-based FACT device, i.e., thyristor-controlled series capacitor, are implemented and analyzed under the same test system for the sake of comparison with the designed coordinated strategy. The simulation results are investigated in the time domain and frequency domain, and by calculating performance indices to verify the effectiveness of the proposed scheme. The overall analysis justifies the mitigated, low transient overshoot and high power quality of subsynchronous oscillations by using the designed strategy with varying compensation levels.

With the increasing share of renewable energy in power systems, the instability of the power systems is becoming increasingly significant. Consequently, power system stability has become a critical issue, and non-transmission alternatives have been examined as potential solutions. Among non-transmission alternatives, the synchronous condenser can enhance power system stability by providing inertia support and reactive power compensation, especially in systems with a high share of renewable energy. The placement and voltage settings of synchronous condensers significantly impact system stability. This paper proposes a methodology for determining the optimal placement and optimal voltage setting of synchronous condensers for enhancing their voltage stability and transient stability; the improved voltage stability index and synchronizing torque coefficient are used for enhancing the voltage stability and transient stability, respectively. A case study with a focus on specific stability aspects and involving scenarios where the size and number of synchronous capacitors are varied while maintaining a constant inertia energy is presented. The results of the case study show that strategically optimizing the placement and voltage setting of synchronous condensers can enhance the stability of a power system significantly.

The integration of power electronic equipment with complex internal structures, which are represented by switching elements or black-box models, is increasing because of the growing penetration of renewable energy into the power grid. The increase in model complexity causes greater computational workload and presents challenges for grid stability analysis. To address this issue, this paper proposes a method for estimating the parameters of a simple generic model capable of emulating the dynamic behavior of complex power-electronic models. For the estimation, the frequency scanning method is utilized, involving the injection of various frequency inputs into the complex model. The responses obtained are then utilized in the optimization process as the objective function. The use of frequency scanning is reasonable because it can cover a wide frequency range, thus comprehensively capturing the dynamic properties of the model. The optimization process aims to minimize the difference in responses to frequency scanning between the complicated and simple generic models. The accuracy of the generic model with estimated parameters is verified by Bode plot comparison and time-domain simulations. Simulation results demonstrated that the generic model, optimized via parameter estimation using the frequency scanning method, accurately replicated the response of the reference model, particularly in the low-frequency range. The proposed method allows for the integration of power electronic equipment, which may represent switching-based components or lack internal information, into stability analysis using existing power-system analysis tools.

The integration of renewable energy sources ( RES ) into the grid predominantly utilizes voltage source converters ( VSC ) with a grid-following ( GFL ) connection strategy. This approach, however, often faces instability issues in low-inertia grid regions. To enhance grid resilience in renewable-dominated power systems, grid-forming ( GFM ) connection strategies have been introduced, with the virtual synchronous machine ( VSM ) technique based on GFM control being particularly prevalent. Consequently, transient stability investigations of power systems employing both GFL and GFM connection techniques have gained increasing relevance. This paper proposes an innovative fast direct transient stability analysis ( DTSA ) approach that leverages Lyapunov’s direct method to assess the transient stability of a GFM with VSM control technique, incorporating the influence of parallel-connected VSC with GFL control strategy. The proposed DTSA approach models the GFL as a current source with parallel internal reactance and the GFM as a voltage source with series internal reactance. Detailed analytical derivations of the DTSA approach using transient energy functions, along with the DTSA criterion for the interconnected system, are presented to determine the critical clearing time ( CCT ) of the GFM -employed VSC . Furthermore, Matlab/Simulink-based electromagnetic transient ( EMT ) time-domain ( t - d ) simulation responses are utilized to validate the accuracy and computational effectiveness of the proposed fast DTSA approach. In addition, the proposed DTSA approach offers considerable benefits for efficiently assessing and managing power systems that heavily rely on RES in terms of operational planning and contingency management.

This paper presents a comparative study of various grid-forming control strategies under different series compensation levels to suggest the optimal control scheme for compensated lines. Series compensation is a technique used to insert a fixed capacitor in transmission line to maximize its power-transfer capacity and enhance the stability of the power system, however it may result in sub-synchronous oscillations issues. Virtual oscillator control and matching control schemes are recently proposed grid forming control strategies to ensure the power-sharing and synchronization capability of inverters under parallel operation. The mentioned schemes are designed for grid-forming control of inverter-based resources, with keeping traditionally used droop control as a benchmark for comparison. The IEEE first-benchmarked test model is modified by inserting series capacitance and energizing it with 1000MW grid forming inverter-based source in MATLAB/ Simulink. Distinct grid forming schemes, namely droop, matching, and dispatchable virtual oscillator control are implemented and critically analyzed for sub-synchronous oscillations under the same specifications as the test model. The simulation response of subsynchronous oscillations are investigated in time domain, frequency domain, and by calculating performance indices to verify the results in every aspect. The overall analysis justifies the damped, low transient overshoot and fast settling of sub-synchronous oscillations with the virtual oscillator control scheme at various compensation levels.

This article presents an investigation of sub-synchronous resonance (SSR) issues in doubly-fed induction generator (DFIG) based wind plant and proposes an optimal control scheme for its mitigation. The article firstly reviews the causes of SSR, namely compensation level and wind speed in DFIG-based wind plant. The proportional resonance controller with harmonic compensators (PR+HC) is designed to mitigate SSR without compromising total harmonic distortions (THD) in the power signal. The series compensated IEEE first benchmarked model energized with a 100MW DFIG-based wind plant is used as a platform in MATLAB/Simulink for the implementation of the proposed scheme. The comparative analysis of the designed PR+HC controller is carried out with the conventional Proportional Integral (PI) controller to validate the optimal response of PR+HC controller. In addition, high performance of PR+HC in coordination with a flexible AC transmission (FACT) device in terms of damped oscillations and low THD is achieved. The overall simulation results are analyzed in both time domain and frequency domain to authenticate the damped, low harmonic, and smooth response of proposed scheme under varying compensation level and wind speeds.

As the integration of inverter-based resources (IBRs) is rapidly increasing in regard to the existing power system, switching from grid-following (GFL) to grid-forming (GFM) inverter control is the solution to maintain grid resilience. However, additional overcurrent protection, especially during fault transition, is required due to limited overcurrent capability and the high magnitude of spikes during fault recovery in IBRs, specifically in the GFM control mode. Furthermore, the power system stability should not be compromised by the employment of additional fault ride through (FRT) schemes. This article presents the design and implementation of an adoptive fault ride through (FRT) scheme for grid-forming inverters under symmetrical fault conditions. The proposed adoptive FRT scheme is comprised of two cascaded power electronic-based circuits, i.e., fault current ride through and a spikes reactor. This adoptive FRT scheme optimizes the fault variables during the fault time and suppresses the fault clearing spikes, without affecting system stability. A three-bus inverter-based grid-forming model is used in MATLAB/Simulink for the implementation of the proposed scheme. Further, a conventionally used FRT scheme, which includes fault current reactors, is simulated in the same test environment for justification of the proposed adoptive scheme. The adoptive FRT scheme is simulated for both time domain and frequency domain to analyze the response of harmonic distortion with the suppression of the fault current. Moreover, the proposed scheme is also simulated under the GFL mode of IBRs to justify the reliability of the scheme. The overall simulation results and performance evaluation indices authenticate the optimal, fault tolerant, harmonic, and spike-free behavior of the proposed scheme at both the AC and DC side of the grid-forming inverters.

New techniques and approaches are constantly being introduced to analyze and enhance the transient stability of renewable energy-source-dominated power systems. This review article extensively discusses recent papers that have proposed novel and innovative techniques for analyzing and enhancing the renewable source-dominated power system’s transient stability. The inherent low-inertia characteristics of renewable energy sources combined with fast-acting power electronic devices pose new challenges in power systems. Different stability concerns exist for grid-following and subsequent grid-forming converter/inverter connections to power grids; hence, distinct solutions for enhancing the transient stability have been devised for each. Moreover, the fundamental concepts and characteristics of converter/inverter topologies are briefly discussed in this study. Recent discussions and reviews of analysis and enhancement techniques in transient stability could lead to new ways to solve problems in power systems that rely primarily on renewable energy sources.

This article demonstrates a new topology for optimization of the electrical variables and enhancement of low-voltage-ride-through (LVRT) capacity of a grid-tied photovoltaic system (PS) during asymmetrical faults. The proposed topology comprises a fuzzy-logic controller (FLC) based on gradient descent (GD) optimization in association with parallel-resonance-type fault current limiter (PRFCL) as an LVRT circuitry. Gradient descent is an iterative process to minimize the objective function by updating the variable in the opposite direction of the gradient of the objective function. The PRFCL optimizes the fault variables, specifically preventing voltage sag without any transitional spikes. A 100-kW detailed model of grid-tied PS is used in MATLAB/Simulink to analyze the response of the proposed topology at the point of common coupling (PCC) and PV side variables. A keen comparative study of the conventionally adopted proportional-integral (PI) controller in association with crowbar circuitry is carried out for the justification of the proposed topology. The simulation findings of the proposed topology authenticate the optimal response of variables in terms of stability, robustness, smoothness, and fault tolerance at both the grid side and the PV side.