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Ph.D. Thesis Colloquium
July 23 @ 10:00 AM - 12:00 PM IST
Colloquium Announcement
Title of the Thesis : Power Swing Blocking Protection in Presence of Large Scale Grid Following PV Generation
Name of the Student : Meenu Jayamohan
Name of the Advisor : Dr. Sarasij Das
Degree Registered : PhD (Eng.)
Date and Time : 23rd July, 2024, 10 AM
Location : C 241, MMCR, Electrical Engg Dept
Abstract:
The penetration of Inverter-Based Resources (IBRs) is increasing in power grids due to environmental concerns. The fault behaviour of IBR is quite different than that of Synchronous Generators (SGs). In addition, IBRs usually do not have inherent inertia. As a result, the existing protection schemes, which are traditionally developed for SG-dominated systems, can become ineffective. Stable power swings (SPS) and Unstable Power Swings (UPS) caused by oscillations generated during system disturbances may trigger undesired relay operations. Power swing Blocking (PSB) and Out-of-Step Tripping (OST) techniques have been employed to stop distance relays from malfunctioning during SPS and UPS, respectively. PSB schemes commonly use the magnitude of the rate of change of positive sequence impedance (|dZ/dt|) for SPS detection. This research work focuses on the PSB protection issues in the
presence of large-scale Grid-Following (GFOL) PV generation. A modified IEEE-39 bus system is used for all the studies presented in this thesis.
As the converter controls determine how PV generators behave during transients, the behaviour of SGs used in conventional power systems differs significantly from that of PVs. As a result, existing protection methods, including PSB methods, must be modified to protect the IBR-integrated power systems. This work examines how integrating GFOL PV generation affects power swing impedance (Z) trajectories and |dZ/dt|. The research reveals that the
GFOL PV systems can significantly alter the Z trajectories observed during power swings compared to that of an SG-dominated system. The results presented demonstrate that the penetration of GFOL PV may increase the speed of Z trajectories and, hence, |dZ/dt|, which may, in turn, cause maloperations of the PSB and OST functions. The findings emphasize the critical need to revisit and potentially adapt existing PSB and OST schemes to account for the growing presence of IBRs in power grids.
In the GFOL control strategy, the injected power is controlled with respect to the grid voltages measured at the terminal by the Phase-Locked Loop (PLL). Considering a PLL bandwidth in the range of 2−15 Hz for a weak grid, the PLL dynamics play a significant role in the power swing dynamics. In this work, the impact of various types and control parameters of PLLs on |dZ/dt| and Z trajectories are analyzed using mathematical analysis. Synchronous Reference Frame PLL with additional Low pass filter (LSRF PLL), Multiple Reference Frame (MRF) PLL and Dual Second-Order Generalized Integrator (DSOGI) PLL are used for the study. The impacts of varying penetration of PV and relay locations are also investigated. This study shows that the PLL parameters and bandwidth influence the operation/maloperation of the PSB during SPS.
During Fault Ride-Through (FRT), the PV system can provide additional reactive power to the grid to maintain the voltage at its terminals. This is achieved through the dynamic voltage or reactive power support and is provided in proportion to the drop in terminal voltage using the K-factor. The study also highlights the importance of considering the active power recovery rate to mitigate the oscillatory behaviour of IBR during the fault recovery process. The findings reveal that, following fault removal, the dynamic behaviour of inverters would be significantly influenced by both the K-factor and the active power recovery rate, which may affect the power swing characteristics. This work emphasizes the need for a comprehensive understanding of how dynamic voltage support features and active power recovery interact with the power swing dynamics and influence PSB operation.
Auto-Reclosing (AR) of a circuit breaker is a technique that attempts to re-energize the faulted line after a predetermined time delay. While IEEE Std C37.104-2012 provides guidelines for minimum AR dead time based on arc de-ionization, these may not be sufficient for grids with a high penetration of IBRs. This work explores how varying the three-phase AR dead time can influence the severity of power swings that may occur after consecutive Low-Voltage Ride-Through (LVRT) events in a GFOL PV plant. This finding highlights the potential need to revise
existing minimum AR dead time standards for grids with high IBR penetration levels to ensure reliable system operation.
The studies presented in the previous sections show that existing impedance-based PSB methods might fail in the presence of GFOL PV generation. The lack of inherent inertia of the GFOL PV is one of the reasons behind the increased |dZ/dt| which may cause maloperation of the existing impedance-based PSB schemes. Hence, a novel PSB method is proposed, which uses nodal inertia to re-evaluate the |dZ/dt| values. The effectiveness of the proposed method is verified for both the SG-dominated system and the GFOL PV-integrated system using PSCAD simulations.