Inverter-based Renewable Generators Research Articles

T&D equipment: Evolving to meet current demands

Transmission and distribution (T&D) systems are evolving to meet the demands posed by an increasing number of utility-scale and distributed inverter-based renewable generation and energy storage installations. At the same time, T&D asset owners are faced with aging physical T&D infrastructure that is vulnerable to failures. While the combination of aging assets and increasing installation of inverter-based renewables presents challenges, it also presents opportunities to take advantage of new technologies and concepts to upgrade existing equipment, put in new equipment, and improve the operation and maintenance of T&D equipment.

Improvement of Power System Frequency Stability With Universal Grid-Forming Battery Energy Storages

Large-scale integration of inverter-based renewable generation leads to the reduction of power systems’ natural inertia. Therefore, the dynamics of the future power systems will be more sensitive than in the traditional systems with high-inertia rotating synchronous generators. This development is a potential risk for frequency stability and requires utilization of rapidly controllable resources for dynamic frequency stability support. Simultaneously, development of new synchronization and control methods for inverter-based resources is needed in order to ensure the frequency and synchronization stability of future power systems. In this paper, a grid-forming and supporting universal frequency-locked-loop -based control and grid synchronization for inverter-based resources is utilized to improve the frequency stability of a small high-voltage network. The simulations are done with PSCAD software and the main focus is on the battery energy storages to evaluate the effect of their location, enhanced control schemes as well as operation mode on frequency stability. In the studies, for example, the effect of battery storages location, active power response related control parameters, communication time delay and input frequency determination on frequency support are studied during charging and discharging of the batteries. Based on the simulations, also new solutions to improve the frequency stability of future variable inertia power systems with universal grid-forming battery storages are proposed.

Frequency Dynamics of the Northern European AC/DC Power System: A Look-Ahead Study

In many power systems, the increased penetration of inverter-based renewable generation will cause a decrease in kinetic energy storage, leading to higher frequency excursions after a power disturbance. This is the case of the future Nordic Power System (NPS). The look-ahead study reported in this paper shows that the chosen units participating in Frequency Containment Reserves (FCR) cannot keep the frequency above the prescribed threshold following the outage of the largest plant. This analysis relies on a detailed model of the Northern European grid. The latter is compared to the classical single-mass equivalent, and the impact of voltage-dependent loads is assessed in some detail. Next, the paper focuses on emergency power control of the HVDC links that connect the NPS to the rest of the European grid, which can supplement or even replace part of the FCR. The proper tuning of that control is discussed. Finally, the analysis is extended to the HVDC links connecting the future North Sea Wind Power Hub under two configurations, namely low and zero inertia. The impact of outages in the latter sub-system is also assessed. The material to simulate the system with industrial software is made publicly available.

Synchronization in electric power networks with inherent heterogeneity up to 100% inverter-based renewable generation.

The synchronized operation of power generators is the foundation of electric power network stability and a key to the prevention of undesired power outages and blackouts. Here, we derive the conditions that guarantee synchronization in power networks with inherent generator heterogeneity when subjected to small perturbations, and perform a parametric sensitivity analysis to understand synchronization with varied types of generators. As inverter-based resources, which are the primary interfacing technology for many renewable sources of energy, have supplanted synchronous generators in ever growing numbers, the center of attention on associated integration challenges have resided primarily on the role of declining system inertia. Our results instead highlight the critical role of generator damping in achieving a stable state of synchronization. Additionally, we report the feasibility of operating interconnected electric grids with up to 100% power contribution from inverter-based renewable generation technologies. Our study has important implications as it sets the basis for the development of advanced control architectures and grid optimization methods that ensure synchronization and further pave the path towards the decarbonization of the electric power sector.

A Scalable Method of Adaptive LVRT Settings Adjustment for Voltage Security Enhancement in Power Systems With High Renewable Penetration

Recent expansion of inverter-based renewable generation (RG) has introduced unprecedented stability issues. One major challenge stems from tripping RGs offline during disturbances based on predefined operation thresholds (ride through curves) on voltage magnitudes and frequency at the point of common coupling (PCC). The current practice uses a single set of thresholds for the whole year across all RGs in the system. This could result in loss of these resources during critical times, particularly endangering system voltage security, i.e a significant reduction in the load margin (LM). On the other hand, relaxing these limits could result in them staying online even during islanding scenarios, which is a safety concern to the restoration crew. Thus, an approach to find minimal relaxations to these limits to enhance voltage security is proposed here. This process first constructs an optimization model aiming to minimize adjustments on RG voltage limits to reach anticipated LM increase and then solves it with a multi-stage screening method including base power-voltage (P-V) curve acquisition, critical RGs identification, successive screenings of candidate solutions based on sensitivities, and optimal solutions validation. This largely increases the scalability of the algorithm, which is demonstrated on the IEEE 14-bus, 118-bus systems, and a 500-bus system.

Evaluating power system network inertia using spectral clustering to define local area stability

The transitioning from synchronous machine generation to large-scale integration of inverter-based renewable generation introduces new challenges in power system frequency stability. This paper proposes a new approach for evaluating power system frequency stability based on network clusters. First, the most common metrics for power system frequency stability are presented, with their advantages and disadvantages. Then, we establish the link between frequency response and network topology. Based on the pros and cons of current frequency stability metrics and the influence of network topology, a proposal is made for a new approach to evaluate frequency stability using Spectral Graph Theory to partition a power system network into clusters for analysis. Consider the displacement of synchronous generator inertia, together with the spatial–temporal fluctuations/noise fed in from spatially distributed variable renewable energy sources. Then the proposed clustering approach has two main advantages over traditional methods for evaluating power system frequency robustness. The first is a spatial awareness of a power system network in terms of frequency-stable areas. The second advantage is the ease of interpretation of the metric result for robust network planning.

Emulation Strategies and Economic Dispatch for Inverter-Based Renewable Generation under VSG Control Participating in Multiple Temporal Frequency Control

As the increasing penetration of inverter-based generation (IBG) and the consequent displacement of traditional synchronous generators (SGs), the system stability and reliability deteriorate for two reasons: first, the overall inertia decreases since the power electronic interfaces (PEIs) are almost inertia-less; second, renewable generation profiles are largely influenced by stochastic meteorological conditions. To strengthen power systems, the concept of the virtual synchronous generator (VSG) has been proposed, which controls the external characteristics of PEIs to emulate those of SGs. Currently, PEIs could perform short-term inertial and primary frequency responses through the VSG algorithm. For renewable energy sources (RES), deloading strategies enable the generation units to possess active power reserves for system frequency responses. Additionally, the deloading strategies could provide the potential for renewable generation to possess long-term frequency regulation abilities. This paper focuses on emulation strategies and economic dispatch for IBG units to perform multiple temporal frequency control. By referring to the well-established knowledge systems of generation and operation in conventional power systems, the current VSG algorithm is extended and complemented by the emulation of secondary and tertiary regulations. The reliability criteria are proposed, considering the loss of load probability (LOLP) and renewable spillage probability (RSP). The reliability criteria are presented in two scenarios, including the renewable units operated in maximum power point tracking (MPPT) and VSG modes. A LOLP-based economic dispatch (ED) approach is solved to acquire the generation and reserve schemes. The emulation strategies and the proposed approach are verified by simulation.

Transient Stability Assessment of Prone-to-Trip Renewable Generation Rich Power Systems Using Lyapunov's Direct Method

Utilities across the world are seeing increased penetration of inverter-based renewable generation (RG) in their systems. These RGs have ride through curves programmed in them which define operating conditions that need to be satisfied during electrical disturbances. If they are violated, the RGs are tripped offline. In systems with large amounts of RGs tightly coupled electrically, there can be disturbances that cause sudden loss of a large number of RGs, which will then considerably exacerbate the stability of the system. Such a phenomenon is not captured by the existing direct methods for TSA. In this paper, by treating RG-rich systems as non-linear switched systems as opposed to the traditional approach of treating this tripping phenomenon as an instability, an approach utilizing multiple low voltage ride through constrained stability regions (CSRs) is proposed for capturing unstable fault clearing times. The CSRs are estimated through Lyapunov functions found using sum of squares programming. The effectiveness of the proposed technique is demonstrated using a three-machine system.

Using inverter-based renewable generators to improve the grid power quality—A review

With the increasing penetration of renewable power in the grid, renewable generators are expected to play more important roles rather than merely working as active power sources. Considering the high controllability and considerable idle capacity of the inverter-based renewable generators, various auxiliary functions for them have been studied to benefit the grid. Among these auxiliary functions, the grid power quality improvement is very promising. This paper presents a review of the emerging control strategies for inverter-based renewable generators to improve grid power quality. The related papers are classified with different power quality issues and technical routes, and a comprehensive comparison is presented. The future trends of this research area are also discussed.