The increasing penetration of inverter-based resources (IBRs) is changing grid dynamics and challenging safe and reliable grid operation. Particularly, the increasing integration of IBRs may cause the small-signal stability issues resulting from the dynamic interaction between the IBR inverter controls and the power network in a power system with high penetration of IBRs. It is challenging for assessing the small-signal stability in such a power system due to the complex interaction between IBRs interconnected through the power network. The assessment complexity is further increased when considering variable IBR generation. To address the challenges, this paper proposes a method for small-signal stability analysis of a multi-IBR power system under uncertain renewable generation. First, we derive that the small-signal stability of a multi-IBR power system can be estimated based on the smallest eigenvalue of a weighted Laplacian matrix of the power network. Then, a robust optimization problem is formulated to analyze the small-signal stability of a multi-IBR power system under variable renewable generation. The efficacy of the proposed method is demonstrated on a power system with three IBRs by eigenvalue analysis and electromagnetic transient simulations.

The growing penetration of renewable resources such as wind and solar into the electric power grid through power electronic inverters is changing the grid dynamics and challenging grid protection. Due to the advanced inverter control algorithms, the inverter-based resources present fault responses different from conventional generators, which can fundamentally affect the way that the power grid is protected and thus is challenging grid protection engineers. This paper studies inverter dynamics during the system restoration period and their impact on protection schemes in a grid-connected solar photovoltaic (PV) system following grid disturbances. To this end, the solar PV system with a detailed inverter model that consider inverter switching dynamics along with inverter blocking and deblocking functions are constructed for the hardware-in-loop simulation based on a real-time digital simulator (RTDS). It is found that undesired transient events such as three-phase overvoltage and overcurrent as well as negative sequence current may occur after the inverter is deblocked to reinject energy into the system during the restoration period following a grid disturbance, especially when the system is under weak grid conditions. The undesired transient events may lead to misoperation of instantaneous overcurrent and negative sequence overcurrent protection schemes.

The growing penetration of renewable resources such as wind and solar into the electric power grid through power electronic inverters is challenging grid protection. Due to the advanced inverter control algorithms, the inverter-based resources present fault responses different from conventional generators, which can fundamentally affect the way that the power grid is protected. This paper studied solar inverter dynamics focused on negative-sequence quantities during the restoration period following a grid disturbance by using a real-time digital simulator. It was found that solar inverters can act as negative-sequence sources to inject negative-sequence currents into the grid during the restoration period. The negative-sequence current can be affected by different operating conditions such as the number of inverters in service, grid strength, and grid fault types. Such negative-sequence responses can adversely impact the performance of protection schemes based on negative-sequence components and potentially cause relay maloperations during the grid restoration period, thus making system protection less secure and reliable.

The increasing integration of renewable energy resources (RERs) such as wind and solar onto the electric power grid through power electronic interface is challenging safe and reliable grid operation. Particularly, the high penetration of the inverter-based RERs (IB-RERs) may drive the grid towards weak grid conditions, which may cause grid stability issues. Grid strength assessment is helpful to identify these weak grid issues. However, it is challenging to assess grid strength while considering the impact of uncertain renewable generation. This paper presents an approach for quantifying the probabilistic characteristics of grid strength under uncertain renewable generation based on the probabilistic collocation method, which is a computationally efficient technique to reduce the computational burden without compromising the result accuracy compared with traditional Monte Carlo simulation. The efficacy of the proposed approach is demonstrated on the modified IEEE 9-bus system.

The increasing use of natural gas power generation has strengthened the interdependence between the power and natural gas subsystems in the integrated power and gas system (IPGS). Due to the interactions between the two subsystems, the disturbances in one system may spread to the other one, triggering a disruptive avalanche of subsequent failures in the IPGS. This paper presents a survey of cascading failure analysis for the IPGS. First, we identify the important features characterizing cascading dynamics in individual power and gas subsystems. Then, we will discuss the features for the cascading failure analysis in the IPGS and future research.

The increasing penetration of inverter-based resources (IBRs) is changing grid dynamics and challenging grid planning, operation, and protection. Particularly, the increasing integration of IBRs may drive the power grid towards weak grid conditions, where potential dynamic stability issues may become significant. Recently, it was reported that the unintended loss of solar generation occurred in Southern California over a large geographic area. One of the major reasons for this generation loss is the tripping of solar generators due to the overvoltage in a less than one cycle time frame (i.e., sub-cycle overvoltage) experienced by solar photovoltaic (PV) inverters, especially when solar PV inverters enter the momentary cessation operation mode in response to abnormal grid disturbances. In this paper, the impact of grid strength on sub-cycle dynamics resulting from momentary cessation is investigated in a power system with distributed solar PV integration. In this investigation, distributed solar PVs are modeled with detailed grid-following inverter models considering inverter switching dynamics and momentary cessation function. It is found that undesired sub-cycle overvoltage has a positive relation to grid strength at points of integration (POIs) of solar PVs. At the weak POIs, server sub-cycle overvoltage occurs not only when momentary cessation starts to cease energy injection but also when momentary cessation restarts to inject energy into the system during the restoration process. Furhtermore, the interaction between solar PVs making POIs weak exacerbates the severity of the sub-cycle overvoltage at the POIs. Thus, in the power system with high penetration of solar PVs, it is important to improve grid strength in power system planning and quickly recover grid strength in power system operation following disturbances.

The increasing penetration of renewable energy resources such as solar and wind via power electronic inverters is challenging grid dynamics, as well as grid planning, operation, and protection. Recently, the North American Electric Reliability Corporation (NERC) has reported a series of similar events of the unintended loss of solar generation in Southern California over a large geographic area following the transmission-level disturbances. These events highlight the importance of understanding the characteristics of the transmission-side disturbances propagating into the distribution systems and their impacts on the operation of inverter-based resources. In this paper, a real-time electromagnetic simulation testbed is constructed for real-time electromagnetic simulations to generate realistic transmission-level disturbances and investigate their impacts on the solar PV operation under different fault types and locations, solar penetration levels, and loading levels. Through the simulation analysis and grid strength assessment, it is found that the grid strength at points of integration (POIs) of solar PVs significantly affects the transient stability of solar generators. Particularly, undesirable transient stability events are more likely to occur at the weak POIs following the transmission-level disturbances. Moreover, undesirable transient stability events become severer when the transmission-level disturbance is closer to the weak POIs or the disturbances become more serious. Additionally, the impact of the transmission-level disturbances on the solar PVs at the weak POIs exacerbate with the increasing solar penetration levels and loading levels. Thus, it is important to study and develop new technologies for grid planning, operation, and protection in weak grid conditions to address the emerging issues of integrating the high penetration of solar PVs and other IBRs.

This paper investigates the impact of inverter modeling on the dynamics in a less than one cycle time frame (i.e., sub-cycle dynamics). Two types of inverter models (i.e., the detailed and average inverter models) are used to create the solar PV test systems for real-time electromagnetic simulation analysis. It is shown that the detailed inverter model that includes the inverter switching dynamics is more appropriate than the average inverter model for accurate sub-cycle dynamic analysis, especially important for power systems with high penetration of solar PVs. Also, increasing the system heterogeneity may reduce the severity of undesired sub-cycle transient events.

Recently, the North American Electric Reliability Corporation (NERC) has reported that the unintended loss of solar generation occurred in Southern California over a large geographic area. One of the major reasons for this generation loss is the tripping of solar generators due to the overvoltage in a less than one cycle time frame (i.e., sub-cycle overvoltage) experienced by solar PV inverters when the grid suffers voltage drop following the transmission-level disturbances. Moreover, the sub-cycle dynamics become more complicated when solar PV inverters enter the momentary cessation operation mode in response to abnormal grid conditions. In this paper, solar PV systems with detailed inverter models that consider inverter switching dynamics and momentary cessation function are constructed for RTDS-based real-time electromagnetic simulations to explore the impact of momentary cessation on sub-cycle dynamics. It is found that undesired sub-cycle transient events not only occur when momentary cessation starts to cease energy injection but also happen when momentary cessation restart to inject energy into the system during the restoration process. Furthermore, the undesired sub-cycle transient events become more significant when increasing the number of solar PV inverters or under weak grid operating conditions. On the other hand, the severity of sub-cycle transient events can be reduced by increasing the diversity in the recovery time of solar PV inverters during the restoration process after momentary cessation.

While the increasing penetration of renewable energy resources into the electric power grid improves energy efficiency and reduces greenhouse gas emissions, it may drive the grid towards weak power grid conditions, under which grid stability issues may affect the operation of inverter-based renewable generators. The short-circuit ratio (SCR) with some modifications has been used to analyze power grid strength. However, the existing SCR-based methods for grid strength assessment neglect the impact of the interactions between shunt capacitors interconnected through the power network, which may cause over-optimistic results of grid strength assessment. To account for the impact, a novel method for grid strength assessment is proposed in this paper by theoretical analysis of how the interconnected capacitors affect static voltage stability. The efficacy of the proposed method is demonstrated through numerical simulation case studies on the IEEE 39-bus system.

The electric power grid is undergoing significant changes in the mix of generation source types. The increasing penetration of renewable energy resources (RERs), such as wind and solar, is improving energy efficiency, but meanwhile it is also challenging grid planners and operators to maintain reliable electricity services. The high penetration of RERs may drive the power grid toward weak grid conditions, which may cause grid stability and reliability issues. One of ways to address these issues is to select appropriate points of interconnection for integrating RERs into power grids. In previous works, grid stability analysis and grid reliability assessment are evaluated separately for selecting the points of interconnection. This paper presents an integrated method for selecting such points by coordinating grid strength assessment and grid reliability evaluation as well as investment cost. The efficacy of the proposed method is demonstrated on the IEEE reliability test system.

While the increasing penetration of renewable energy resources (RERs) into the electric power grid reduces greenhouse gas emissions, it may drive the grid’s parts towards the “weak grid” conditions under which grid instability may occur due to small disturbances leading to voltage oscillations affecting the operation of renewable generation plants. To avoid such grid weak issues, one effective way is to reinforce the power grid by updating its components. A cost-effective grid reinforcement needs the information of critical components whose updates can significantly strengthen the power grid for renewable energy operation. In this paper, we study the identification of such critical components by the impact analysis of power network structure on grid strength. It is shown that the critical network components are shared by critical transmission paths between renewable energy resources and synchronous generation resources in a power grid. Moreover, critical transmission paths can be identified with the network structure information represented in node admittance matrix and node impedance matrix. Numerical results are demonstrated in the IEEE-9 bus and IEEE-39 bus systems.