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Voltage control is often time provided at the plant-level control of inverter-based resources (IBR). Addition of energy storage systems in an IBR power plant makes it feasible to have frequency control at the power plant level. While frequency control appears as a simple frequency-power droop control to adjust real power commands to inverter-level controls with measured frequency as an input, care must be taken to avoid interactions among the plant frequency control with communication delays, inverter-level control effects, and the frequency sensor, usually a phase-locked-loop (PLL). This paper present two types of interaction scenarios that makes frequency control design challenging. The first interaction scenario may occur if the frequency control's gain is large, while the second interaction scenario may occur at a small control gain if the plant-level PLL lacks sufficient damping. We contribute to the fundamental understanding of the causation of stability issues due to plant frequency control through the derivation of a simplified feedback system focusing on the frequency and power relationship, and the follow-up frequency-domain analysis for gaining insights. For validation, we also design a data-driven approach to obtain models from data generated from an electromagnetic transient (EMT) simulation testbed. The findings from analysis have all been validated by EMT simulation. Finally, we contribute to mitigating strategies and also the understanding of the role of additional proportional integration power feedback control. This addition has been demonstrated as an efficient stability enhancement strategy to mitigate the effect of communication delay. 

Grid-forming inverters (GFMIs) have been identified as critical assets in ensuring modern power system reliability. Their ability to synthesize an internal voltage reference while emulating synthetic inertia has sparked extensive research. These characteristics have recently piqued interest in their capacity to provide blackstart ancillary services. The blackstart of a bulk power system poses significant challenges, namely the large transients from the energization of unloaded transformers, rotational motor loads, and long transmission cables, which have been effectively studied using conventional synchronous generators (SGs). The concept of an inverter-based resource (IBR)-based blackstart continues to be an open research area necessitating further investigations due to the known limitations of IBRs such as low short-circuit current capabilities. This paper presents a blackstart case study of a bulk power system investigating the performances of a conventional SG to a GFMI when utilizing hard switching methods. The paper qualitatively investigates the transient inrush currents from the transformer and rotational load energization sequences. Additional examinations into the significance of the GFMI’s current-limiting schemes and voltage control loop compensator gains are presented. Furthermore, the harmonic distortions from the transformer energization sequence are also evaluated. Finally, a full network energization case is presented to demonstrate how both sources can provide blackstart provisioning services. The models are developed in EMTDC/PSCAD using real-world transmission planning data. 

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. 

Forced oscillation events have become a challenging problem with the increasing penetration of renewable and other inverter-based resources (IBRs), especially when the forced oscillation frequency coincides with the dominant natural oscillation frequency. A severe forced oscillation event can deteriorate power system dynamic stability, damage equipment, and limit power transfer capability. This paper proposes a two-dimension scanning forced oscillation grid vulnerability analysis method to identify areas/zones in the system that are critical to forced oscillation. These critical areas/zones can be further considered as effective actuator locations for the deployment of forced oscillation damping controllers. Additionally, active power modulation control through IBRs is also proposed to reduce the forced oscillation impact on the entire grid. The proposed methods are demonstrated through a case study on a synthetic Texas power system model. The simulation results demonstrate that the critical areas/zones of forced oscillation are related to the areas that highly participate in the natural oscillations and the proposed oscillation damping controller through IBRs can effectively reduce the forced oscillation impact in the entire system.