Inverter-based Resources Research Articles

Implementation and Applications of Grid-Forming Inverter with SiC for Power Grid Conditioning

Sustainable energy technologies have become promising solutions to global warming and climate change. Operation of the electric power grid has been dominated in the past by synchronous generation, wherein conventional sustainable energy inverters are designed simply to feed maximum real power into the grid. With the increasing penetration of renewable energy, it is undergoing a rapid shift toward the power generation of inverter-based resources (IBRs). As a result, transitioning to a power grid with more IBRs requires introducing advanced inverter technology that can respond to various disturbances in frequency and voltage occurring on the grid. The grid-forming (GFM) inverter equipped with an energy storage system featuring frequency and voltage support functionalities is vital for the stability of the micro power grid system. The GFM inverter also offers PV output smoothing, low voltage ride through, and low frequency ride through. In addition, the GFM inverter functions as a voltage source inverter to supply energy under off-grid state when the main grid is in fault conditions. A SiC-based 30 kVA GFM inverter is presented with a 3-phase 3-level neutral-point-clamped (NPC) topology for high-frequency operation to achieve high efficiency and power density. Designchallenges in gate driver design, PCB layout, and thermal consideration are addressed. The performances of the designated GFMinverter are measured and tested under on-grid and off-grid operations to verify the functionalities of the advanced inverter, as well.

Three-Terminal Lines Fault Location Using Two Main Terminals Data in the Presence of Renewable Energy Sources

Penetration of renewable energy sources (RESs) in power systems is increased considerably and fault location algorithms face considerable problems with the presence of RESs. Also, the third terminal of three-terminal lines is expanded as a cost-effective solution to energize the remote areas of medium voltage (MV) systems, while its data may not be measured and easily available. In addition, in cases where this data is obtainable, due to the excessive harmonics of the inverter-based resources (IBRs) in that terminal, the measured data includes high error. Therefore, providing a method for fault location in three-terminal lines in the presence of RESs is challenging. This paper proposes an accurate method to identify the fault location on these lines incorporating RESs without requiring the third terminal data. Due to current transformers saturation and transient response of capacitive voltage transformers, the fault location based on the faulted phase data contains significant errors. The proposed method combines both the faulty and healthy phases data to improve the accuracy of fault location. Its performance is evaluated on a MV system for different scenarios including various fault resistances, locations and types, and third terminal integration of RESs. The obtained results confirm superiority of the proposed method.

Reducing blackouts: Supporting grid transformation [In my view

Electrification and decarbonization are two seismic shifts that pose significant reliability challenges to power grid planning and operation. The emerging power supply portfolio is dominated by renewable inverter-based resources, which are sensitive to widespread and long-duration weather conditions and are electrically decoupled from the power system. This projected resource mix will behave quite differently from synchronous generation, which dominates the past and was closely coupled to the power system. As older generation technology is being replaced, a fundamental shift is needed in the understanding of generating capacity, energy supply, and load-serving needs. In addition to this, now more than ever, industry, federal, state, and provincial energy regulators and policy makers need to work together to provide a reliable, resilient, and secure grid.

Review of RoCoF Estimation Techniques for Low-Inertia Power Systems

As the traditional generation is gradually replaced by inverter-based resources, a lack of rotational inertia is now a common issue of modern power systems, which leads to an increasingly larger rate of change of frequency (RoCoF) following contingencies and may result in frequency collapse. As a crucial index of the frequency security and stability of power systems, the accurate estimation of the RoCoF can be a foundation for the development of advanced operations and control techniques of the future power system. This paper firstly analyzes the role of the RoCoF in typical blackouts occurring in recent years and discusses the physical and numerical nature of the RoCoF; then, by introducing the frequency spatial distribution of the power system, the paper discusses the concept of the “center” RoCoF that can present the frequency security and stability of the entire system. The estimation and prediction techniques of the maximal power system RoCoF following a contingency and the existing real-time tracking techniques of the power system RoCoF are comprehensively reviewed. Finally, the open questions and related research topics of the RoCoF estimation are discussed.

Quantifying the impact of inverter-based distributed energy resource modeling on calculated fault current flow in microgrids

Microgrid fault calculations are crucially important for setting the relay protection in both modes of operation – grid-connected and islanded. To obtain accurate results for this class of analysis, distributed energy resources within the microgrid must be accurately modeled. As most of distributed energy resources within the emerging microgrids are inverter-based, their precise models are of a significant importance. Thus, the aim of this research is to quantify the impact of the model accuracy on short circuit calculation results as well as quantifying the differences in the results obtained by models that are in a complete accordance with various Grid Code requirements, and simplified models proposed in the most commonly used International Standard for fault calculations – IEC 60909. To accomplish this goal, in this paper, various shares of inverter-based distributed energy resources’ penetration in the total generation power of a microgrid – from 50% to 90%, are thoroughly analyzed, and the differences between the results obtained by using standardized models relative to more accurate modeling that considers Grid Code requirements are presented. The obtained results suggest when the share of inverter-based resources in the total generation power of a microgrid is considerable, especially in the islanded operational mode, simplified models from the IEC 60909 standard become unreliable. According to these observations, we lay out cases in which the simplified and standardized models for inverter-based distributed energy resources are reliable to use. Additionally, we provide recommendations on improvement of the standardized models in critical cases of microgrids with high penetration of these resources, that operate in the islanded mode.

Design of an Optimal Adoptive Fault Ride through Scheme for Overcurrent Protection of Grid-Forming Inverter-Based Resources under Symmetrical Faults

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.

Grid Interconnection Modeling of Inverter Based Resources (IBR) Plant for Transient Analysis

The increase in penetration levels of inverter-based resources (IBRs) is changing the dynamic performance of power grids of different parts of the world. IBRs are now being more and more integrated into the grid at a single connection point as an IBR plant. Due to the complex nature and dynamicity of each inverter model, it is not realistic to build and analyze full complex models of each inverter in the IBR plant. Moreover, simulating a large plant including detailed models of all the IBRs would require high computing resources as well as a long simulation time. This has been the main issue addressed in the new IEEE Std 2800-2022. This paper proposes a novel approach to model an IBR plant, which can capture the transient nature at the plant level, detailed IBR control at the inverter level, interactions of multiple IBR groups in a plant structure, and a collector system connecting the IBRs to the grid. The IBRs in the plant use a voltage source inverter topology combined with a grid-connected filter. The control structure of the IBR includes a cascaded loop control where an inner current control and outer power control are designed in the dq-reference frame, and a closed-loop phase-locked loop is used for the grid synchronization. The mathematical study is conducted first to develop aggregated plant models considering different operating scenarios of active IBRs in an IBR plant. Then, an electromagnetic transient simulation (EMT) model of the plant is developed to investigate the plant’s dynamic performance under different operating scenarios. The performance of the aggregated plant model is compared with that of a detailed plant model to prove the effectiveness of the proposed strategy. The results show that the aggregated EMT simulation model provides almost the same result as the detailed model from the plant perspective while the running time/computation burden is much lower.

Non-synchronous Inertia Estimation in a Renewable Energy Integrated Power System With Reduced Number of Monitoring Nodes

Accurate estimation of non-synchronous inertia in renewable energy (RE)-integrated power systems is challenging through conventional approaches, as emulated inertia from inverter-based resources (IBRs) is fundamentally different from the inherent inertial response of synchronous generators. In this context, this paper presents a novel realistic approach for estimating the non-synchronous inertial response in a large-scale RE-integrated power system. The method incorporates an optimal number of frequency and voltage monitoring nodes while ensuring an accurate estimation of non-synchronous inertia. In the proposed method, the synchronous inertial response for a frequency disturbance is estimated first using the conventional approach, followed by an estimate of the loads’ inertial contribution. The latter is calculated using center-of-inertia frequency and voltage measurement across the power system with a voltage-controlled zone approach. Finally, the non-synchronous inertial response is estimated by segregating the aggregate inertial response of synchronous generators and the loads from the overall estimated inertia of the system. The proposed approach can help the system operator determine inertial contributions from the loads and IBRs depending on loading conditions and the availability of virtual inertia from RE sources, estimate the minimum required synchronous inertia, and select the appropriate proportional gains and time constants of virtual inertial controllers. The proposed method is validated by implementing it on a modified IEEE 39-bus system and a real-life Gujarat State grid model (in India).

Autonomous Restoration of Networked Microgrids Using Communication-Free Smart Sensing and Protection Units

This paper presents autonomous restoration of networked microgrids using distributed energy resources (DERs), inverter-based resources (IBRs) in general, and controllable assets of intelligence without communication to achieve a extremely resilient power system restoration. The main innovation of this work lies in use of real-time smart sensing and decision making that enable autonomous recombination of islanded microgrids, which are individually black-started by local grid-forming (GFM) inverter units. The smart circuit breakers (SCBs) designed and demonstrated here play critical roles in the restoration process to achieve autonomy; the autonomous restoration scheme is not pre-engineered and may be subject to encounter overloading, live or downed segments as the boundary of the microgrids dynamically expands without prior knowledge and only with local measurement. We demonstrate autonomous microgrid restoration both in simulation of a modified IEEE 13-bus system, and in a hardware testbed comprising 24 GFM-IBRs and 6-SCBs, depicting several scenarios to evaluate their functionalities and interoperability. This work validates high potential of extremely resilient microgrid using novel approaches for inverter-based bottom-up system restoration in a communication-free paradigm.

Distinguishing Between Cyber Attacks and Faults in Power Electronic Systems—A Noninvasive Approach

With the increased cyberinfrastructure in large power systems with inverter-based resources (IBRs), it remains highly susceptible to cyber-attacks. Reliable and secure operations of such a system under a large signal disturbance necessitate an anomaly diagnosis scheme, which is substantial for either selective operation of relays (during grid faults) or cybersecurity (during cyber-attacks). This becomes a challenge for power electronic systems, as their characteristic response to such large-signal disturbances is very fast. Hence, we accumulate our efforts in this article to characterize them accurately within a short time frame. A novel noninvasive anomaly diagnosis mechanism for IBRs is presented, which only requires locally measured voltage and frequency as inputs. Mapping these inputs in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$XY$ </tex-math></inline-formula> -plane, the characterization process is able to classify between the anomalies within 5 ms. To the best of our knowledge, this mechanism provides the fastest decision in comparison to the existing techniques, which also assists the equipped protection/cybersecurity technology to take corresponding decisions without enforcing any customization. The proposed scheme is validated on many systems using real-time (RT) simulations in OPAL-RT environment with HYPERSIM software and also on a hardware prototype. The results verify the effectiveness, scalability, and accuracy of the proposed mechanism under different scenarios.

그리드포밍 인버터 자원 기반 전력계통의 블랙스타트 및 계통 동기화 영향 연구

The increasing use of inverter-based resources, while reducing synchronous generators, decreases the system"s inertia, leading to an increased risk of power outages. The shift from a centralized power generation structure to a decentralized structure can be effective for stable operation of inverter-based power generation sources and to mitigate the risks of power outages. The grid forming inverter can perform the same role as a synchronous generator and can contribute to energizing primary restorative transmission systems. Grid-forming inverters must meet several requirements to replace synchronous generators in power systems. One of the most important requirements is the ability to operate as AC voltage sources. This means that the grid-forming inverter must be able to provide a stable AC voltage to the grid, just like a synchronous generator. To address this, a black start study using inverter resources was conducted, and a pumping generator-IBR cooperative black start was simulated through cooperation with an existing black start resource. The paper proposed the grid-synchronization method of grid-forming inverter and emphasizes the need for independent and quick recovery of inverter resources, which must operate as AC voltage sources to replace synchronous generators and contribute the primary power system restoration. The effectiveness of the proposed method to increase the input capacity and speed by stably inputting through synchronization has been studied and were verified through PSCAD/EMTDC simulation.

Estimation Method of Short-Circuit Current Contribution of Inverter-Based Resources for Symmetrical Faults

This paper proposes a practical approach to estimate the symmetrical short-circuit current (SCC) levels in overcurrent protection devices (OCPDs) installed on radial feeders for any penetration level of inverter-based distributed energy resources (DERs). The proposed method restores the lost phase protection coordination by estimating SCC values and changing the TMS of OCPDs accordingly. The method is validated by comparing the results with simulations on the IEEE 34-Node Test Feeder using MATLAB/Simulink, which shows an average error of 1.5% and a maximum error of 3.0%. For a 100% penetration level, the SCC variation through OCPDs installed on the main fault trunk (MFT) exceeds ± 10%, leading to compromised phase protection coordination. The SCC flowing reversely through OCPDs on lateral branches and the fault on the MFT could cause improper tripping. Higher SCC levels are estimated and measured for fault impedances equal to zero. The phase protection is restored by changing the TMS of OCPDs using the estimated values. The study proposes two phase protection schemes to accommodate inverter-based DERs injecting 1.2 pu and 2.0 pu of SCC for a 100% penetration level. This study contributes to improving the protection coordination of distribution networks with high penetration levels of DERs. The findings have practical implications for distribution system operators and planners to maintain safe and reliable operation of distribution feeders.

Dynamic grid stability in low carbon power systems with minimum inertia

The power system transition to large penetrations of renewable generation has become a core target of decarbonisation roadmaps in many countries. However, switching from synchronous to variable inverter-based resources has prompted technical concerns linked to the reduction in system inertia and short circuit levels. This may result in significant deviations in frequency and bus voltages during fault events. Hence, this paper provides a comprehensive review of power system stability challenges when integrating up to 100% inverter-based resources. The potential solutions from several enabling technologies (i.e., energy storage, controllable loads, wind, solar PV, etc.) are also assessed to mitigate anticipated issues. The review further details the role of grid codes and international standards in maintaining dynamic stability in power systems with extremely high up to 100% variable renewable generation. Finally, discussion and recommendation pathways are provided to inform industries and accelerate a secure power system transition to meet Net Zero targets.

Voltage and frequency stabilization control strategy of virtual synchronous generator based on small signal model

Power system with large scale Inverter-based Resources (IBR) penetration is challenged with insufficient voltage and frequency support capacity. This paper restructures the control blocks of virtual synchronous generator (VSG) from the perspective of traditional synchronous generators’ control, the control path of the system over its active and reactive power output is clarified and simplified, which has facilitated the tuning of virtual inertia and damping parameters. Firstly, the small signal model of VSG control IBR is established, including instantaneous power calculation module, LC filter and power loop, voltage and current loops. In the meantime, the improved VSG voltage and frequency regulation control strategy is proposed based on the stability analysis of the small signal model. The stable operation range of the VSG control grid-connected inverter system is studied with the objective to improve the stability and robustness of the VSG-controlled grid-connected inverter system. Finally, MATLAB/Simulink simulation verifies the correctness of the proposed VSG’s dynamic model of power grid voltage and frequency regulation. The effectiveness of the restructures of VSG is verified as well. The results show that the control strategy not only improves the inertia of the system, but also enables the IBR to support the frequency and voltage of the system.

Data-Driven Time-Varying Inertia Estimation of Inverter-Based Resources

This letter proposes a data-driven inertia estimator for inverter-based resources (IBRs) with grid-forming control. It is able to track both constant and time-varying inertia. By utilizing the Thevenin equivalent, the virtual frequency inside IBRs is first estimated with only its terminal voltage and current phasor measurements. The virtual frequency and the measurements are then used together to derive the state-space swing equation model. Then, an enhanced adaptive Unscented Kalman filter (EAUKF) is developed to estimate IBR inertia. Numerical results on the modified IEEE 39-bus power system demonstrate that the proposed inertia estimator remarkably outperforms the existing state-of-art methods both in tracking speed and accuracy.

Impedance-Based Root-Cause Analysis: Comparative Study of Impedance Models and Calculation of Eigenvalue Sensitivity

Impedance models of power systems are useful when state-space models of apparatus such as inverter-based resources (IBRs) have not been made available and instead only black-box impedance models are available. For tracing the root causes of poor damping and tuning modes of the system, the sensitivity of the modes to components and parameters are needed. The so-called critical admittance-eigenvalue sensitivity based on nodal admittance model has provided a partial solution but omits meaningful directional information. The alternative whole-system impedance model yields participation factors of shunt-connected apparatus with directional information that allows separate tuning for damping and frequency, yet do not cover series-connected components. This paper formalises the relationships between the two forms of impedance models and between the two forms of root-cause analysis. The calculation of system eigenvalue sensitivity in impedance models is further developed, which fills the gaps of previous research and establishes a complete theory of impedance-based root-cause analysis. The theoretical relationships and the tuning of parameters have been illustrated with a three-node passive network, a modified IEEE 14-bus network and a modified NETS-NYPS 68-bus network, showing that tools can be developed for tuning of IBR-rich power systems where only black-box impedance models are available.

Role of Distributed Inverter-Based-Resources in Bulk Grid Primary Frequency Response Through HELICS Based SMTD Co-Simulation

In the recent years, there has been an increase in distributed inverter-based resources (D-IBRs) in the system that is impacting the grid performance. Dynamic transmission and distribution (T&D) co-simulation is an effective method to evaluate the impact of D-IBRs on the system response, however, implementation is a challenge. This article innovatively achieves accurate T&D model coordination with inclusion of dynamic models of power electronic controls. It also demonstrates that D-IBRs operating with grid-forming (GFM) controls with frequency support capability can aid in primary frequency regulation in a low-inertia power system. The article discusses the fundamentals of dynamic T&D co-simulation to analyze the primary frequency response on the IEEE 39-Bus transmission system coupled to IEEE 123-Bus distribution system with D-IBRs with GFM controls. To demonstrate scalability, a large system (286383 T&D buses) is co-simulated: IEEE 39-bus transmission system with all load buses coupled to a total of 2328 parallel IEEE 123-Bus distribution feeders containing a mix of D-IBRs is considered. The article also discusses the use of parallel computation used for distribution system solutions enabled in the scalable multi-timescale T&D co-simulation framework along with computation time comparisons between the large and small system simulations with GFM inverter dynamics.

A Blind Spot in the LVRT Current Requirements of Modern Grid Codes for Inverter-Based Resources

Modern grid codes (GCs) require that inverter-based resources (IBRs) inject both positive- and negative-sequence currents during asymmetrical low-voltage ride through (LVRT) conditions. This GC provision prioritizes the reactive currents and also demands maximizing the active positive-sequence current if the IBR has unused current generation capacity when the required reactive current is generated. A variety of inverter control schemes are available to generate positive- and negative-sequence active/reactive currents, and satisfying these GCs seems to be straightforward. However, this paper reveals that the reference current generation methods of existing inverter control schemes fail to fulfil some important requirements of recent GCs. For example, they do not fully utilize the inverter capacity to generate the maximum active and/or reactive current. It is shown that these so-far hidden GC violations can result in a large untapped generation capacity during asymmetrical faults. This paper also develops an algorithm that satisfies recent GCs by deriving the positive- and negative-sequence currents that maximize the IBR’s reactive and active currents while the reactive current is prioritized. The simulation of a grid with high IBR penetration verifies that this new algorithm can unlock the full potential of recent GCs by significantly increasing the power generated during LVRT.

Restricted Earth Fault Protection in Low-Impedance Grounded Systems With Inverter-Based Resources

A restricted earth fault (REF) protection element provides sensitive detection of ground faults near the neutral of a transformer or a generator. The sensitivity gains from the REF element are especially more valuable when the equipment is low-impedance grounded when the transformer phase differential element (87T) is ineffective. However, the REF element faces several challenges in low-impedance grounded systems, which are becoming more prevalent in inverter-based resource (IBR) plants. Dependability of the REF element may be compromised if the zero-sequence current at the transformer terminal is capacitive for an internal ground fault. Security of the REF element may be jeopardized for external faults, such as a phase-to-phase-to-ground or an evolving fault with current transformer (CT) saturation. Using field events from low-impedance grounded IBR plants and simulations, this paper discusses the challenges and design improvements to an REF element for low-impedance grounded systems. CT selection criteria and setting guidance for reliable operation of the REF element are provided. Finally, the coverage provided by the REF element is compared with the 87T element to highlight its benefits in low-impedance grounded systems.

A reactive power market for the future grid

As pressures to decarbonize the electricity grid increase, the grid edge is witnessing a rapid adoption of distributed and renewable generation. As a result, traditional methods for reactive power management and compensation may become ineffective. Current state-of-art for reactive power compensation, which rely primarily on capacity payments, exclude distributed generation (DG). We propose an alternative: a reactive power market at the distribution level designed to meet the needs of decentralized and decarbonized grids. The proposed market uses variable payments to compensate DGs equipped with smart inverters, at an increased spatial and temporal granularity, through a distribution-level Locational Marginal Price (d-LMP). We validate our proposed market with a case study of the US New England grid on a modified IEEE-123 bus, while varying DG penetration from 5% to 160%. Results show that our market can accommodate such a large penetration, with stable reactive power revenue streams. The market can leverage the considerable flexibility afforded by inverter-based resources to meet over 40% of reactive power load when operating in a power factor range of 0.6 to 1.0. DGs participating in the market can earn up to 11% of their total revenue from reactive power payments. Finally, the corresponding daily d-LMPs determined from the proposed market were observed to exhibit limited volatility.