Inverter-based Distributed Energy Resources Research Articles

A new voltage sensitivity-based distributed feedback online optimization for voltage control in active distribution networks

Rising sustainability, environmental, and decarbonization considerations have prompted a recent increase in the integration of inverter-based distributed energy resources (DER), such as photovoltaics (PV), into the power grid. This trend raises concerns about potential voltage violations within distribution systems, emphasizing the growing relevance of effective voltage regulation. Utilizing smart inverters for active and reactive power control proves highly efficient in addressing voltage rise issues within active distribution networks (ADN). This work proposes a distributed feedback online optimization (FOO) strategy for voltage regulation in ADNs. The FOO strategy utilizes online recursive least squares (RLS) sensitivity-based feedback, relying on local measurements to iteratively compute optimal reactive power set points for DERs, ensuring bus voltage regulation to within acceptable limits. The distributed control algorithm employs a primal-dual update approach, enabling coordinated information exchange between neighboring controllers. This online feedback-based control offers a promising solution for real-time coordinated voltage control in distribution grids, particularly with the increasing integration of inverter-based DERs at the grid's edge. The approach demonstrates robustness under varying conditions, communication dynamics, and topology changes, validated using real distribution feeders with actual solar PV production and consumption data. Advancements in sensing, communication systems, and smart grid tools further enhance the feasibility and scalability of this strategy for evolving power systems.

A Review of Control Techniques for Inverter-Based Distributed Energy Resources Applications

A Review of Control Techniques for Inverter-Based Distributed Energy Resources Applications

POWER SYSTEM STABILITY CONSIDERING THE INFLUENCE OF DISTRIBUTED ENERGY RESOURCES ON DISTRIBUTION NETWORKS

The increasing penetration of distributed energy resources (DERs) into power systems has transformed the landscape of energy distribution, bringing both opportunities and significant challenges. This study examines the impact of DER integration on power system stability, focusing on voltage stability, frequency stability, and transient stability. A comprehensive review of 50 high-quality studies from peer-reviewed journals and reputable conference proceedings was conducted, utilizing advanced modeling, simulation, and empirical analysis methods. The findings reveal that the intermittent nature of renewable DERs, such as solar and wind, leads to voltage fluctuations, necessitating advanced control strategies to maintain stability. Additionally, the displacement of traditional synchronous generators by inverter-based DERs reduces system inertia, posing severe frequency stability challenges that require innovative solutions like synthetic inertia and fast frequency response mechanisms. Transient stability issues are also exacerbated by DER integration, highlighting the need for advanced inverter controls and enhanced fault ride-through capabilities. Energy storage systems (ESS) are identified as crucial for buffering the variability of renewable DERs, providing essential services such as frequency regulation and voltage support. However, high costs and scalability issues remain barriers to widespread ESS adoption. The study underscores the importance of supportive regulatory and policy frameworks in facilitating the seamless integration of DERs while maintaining grid stability. Effective policies that promote smart grid technologies and DER-friendly regulations are essential for ensuring a stable, resilient, and sustainable power grid. This research contributes to a deeper understanding of the complex dynamics introduced by DERs and offers insights into developing robust strategies to address stability challenges in modern power systems.

A Risk Assessment Framework for Cyber-Physical Security in Distribution Grids with Grid-Edge DERs

Integration of inverter-based distributed energy resources (DERs) is reshaping the landscape of distribution grids to fulfill the socioeconomic, environmental, and sustainability goals. Addressing the technological challenges of DER grid integration requires an adaptive communication layer for efficient DER management and control. This transition has given rise to a cyberphysical system (CPS) architecture within the distribution system, causing new vulnerabilities for cyberphysical attacks. To better address potential threats, this paper presents a comprehensive risk assessment framework for cyberphysical security in distribution grids with grid-edge DERs. The framework incorporates a detailed CPS model accounting for dynamic DER characteristics within the distribution grid. It identifies vulnerabilities in DER communication systems, models attack scenarios, and addresses communication latency crucial for inverter control timescales. Subsequently, the quantification of attack impacts employs an attack probability model including both the vulnerability and criticality of cyber components. The proposed risk assessment framework was validated through testing on the modified IEEE 13-node and 123-node test feeders.

Analysis of Grid-Forming Inverter Controls for Grid-Connected and Islanded Microgrid Integration

Autonomous grid-forming (GFM) inverter testbeds with scalable platforms have attracted interest recently. In this study, a self-synchronized universal droop controller (SUDC) was adopted, tested, and scaled in a small network and a test feeder using a real-time simulation tool to operate microgrids without synchronous generators. We presented a novel GFM inverter control adoption to better understand the dynamic behavior of the inverters and their scalability, which can impact the distribution system (DS). This paper provides a steady-state and transient analysis of the GFM power inverter controller via simulation to better understand voltage and frequency stabilization and ensure that the critical electric loads are not affected during a prolonged power outage. The controllers of the GFM inverter are simulated in HYPERSIM to examine voltage and frequency fluctuations. This analysis includes assessing the black start capability for photovoltaic microgrids, both grid-connected and islanded, during transient fault conditions. The high photovoltaic PV penetration levels open exciting opportunities and challenges for the DS. The GFM inverter control demonstrated appropriate response times for synchronization, connection, and disconnection to the grid. The DS has become more resilient and independent of fossil fuels by increasing the penetration of inverter-based distributed energy resources (DERs).

Systematic Decoupling Grid-Forming Control for Utility-Scale Inverter-Based Distributed Energy Resources in Weak Distribution Grids

Systematic Decoupling Grid-Forming Control for Utility-Scale Inverter-Based Distributed Energy Resources in Weak Distribution Grids

Anti-Islanding Techniques for Integration of Inverter-Based Distributed Energy Resources to the Electric Power System

Anti-Islanding Techniques for Integration of Inverter-Based Distributed Energy Resources to the Electric Power System

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.

Estimating and Calibrating DER Model Parameters Using Levenberg–Marquardt Algorithm in Renewable Rich Power Grid

The proliferation of inverter-based distributed energy resources (IBDERs) has increased the number of control variables and dynamic interactions, leading to new grid control challenges. For stability analysis and designing appropriate protection controls, it is important that IBDER models are accurate. This paper focuses on the accurate estimation and parameter calibration of DER_A, a recently proposed aggregated IBDER model. In particular, we focus on the parameters of the reactive power–voltage regulation module. We formulate the problem of parameter tuning as a non-linear least square minimization problem and solve it using the Levenberg–Marquardt (LM) method. The LM method is primarily chosen due to its flexibility in adaptively selecting between the steepest descent and Gauss–Newton methods through a damping parameter. The LM approach is used to minimize the error between the actual measurements and the estimated response of the model. Further, the computational challenges posed by the numerical calculation of the Jacobian are tackled using a quasi-Newton root-finding approach. The proposed method is validated on a real feeder model in the northeastern part of the United States. The feeder is modeled in OpenDSS and the measurements thus obtained are fed to the DER_A model for calibration. The simulation results indicate that our approach is able to successfully calibrate the relevant model parameters quickly and with high accuracy, with a total sum of square error of 3.57×10−7.

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.

Voltage-sag mitigation by stability-constrained partitioning scheme

Microgrids have the ability to function in islanded or grid-connected modes of operation. The increased penetration of inverter-based Distributed Energy Resources (DERs) in the system promotes the concept of partitioning the system into self-governing and self-adequate microgrids. Most of the partitioning techniques determine virtual boundaries and did not consider the survivability of the constructed microgrids. In this paper, a stability-constrained partitioning scheme is proposed based on small-signal stability to ensure microgrids' survivability when physically partitioned. Moreover, a sensitivity analysis of active power droop gain is utilized to define a novel index for the microgrid’s marginal stability. The application targeted in this paper is the mitigation of voltage-sag events caused by low impedance faults. The system will be partitioned into clusters of survivable microgrids during faults to isolate the faulted zone that caused the voltage-sag event. By isolating the voltage-sag origin from the rest of the system, voltage-sag mitigation is accomplished. Also, a microgrid re-connection method is proposed. This method allows multiple droop-controlled DERs to adjust the frequency, phase, and magnitude of their output voltages to facilitate seamless re-connection of microgrids. Simulation results are given that show a seamless re-connection of two different microgrids when the proposed method is utilized. The effectiveness of the proposed mitigation algorithm is validated using a modified IEEE 33-bus distribution system simulated on MATLAB/SIMULINK platform.

Deriving DERs VAR-Capability Curve at TSO-DSO Interface to Provide Grid Services

The multitudes of inverter-based distributed energy resources (DERs) can be envisioned as distributed reactive power (var) devices (\textit{mini-SVCs}) that can offer var flexibility at TSO-DSO interface. To facilitate this vision, a systematic methodology is proposed to derive an aggregated var capability curve of a distribution system with DERs at the substation level, analogous to a conventional bulk generator. Since such capability curve will be contingent to the operating conditions and network constraints, an optimal power flow (OPF) based approach is proposed that takes inverter headroom flexibility, unbalanced nature of system and coupling with grid side voltage into account along with changing operating conditions. Further, the influence of several factors such as compliance to IEEE 1547 on the capability curve is thoroughly investigated on an IEEE 37 bus and 123 bus distribution test system along with unbalanced DER proliferation. Validation with nonlinear analysis is presented along with demonstration of a scenario with T-D co-simulation.

A Hybrid Controller With Hierarchical Architecture for Microgrid to Share Power in an Islanded Mode

One challenge of utilizing inverter-based distributed energy resources in islanded microgrids is the proper power distribution between them. Improper power allocation causes resource overload and disturbs the balance of supply and demand, which can be eliminated using a droop controller. However, different operating points and loading conditions decrease the accuracy of this method. This paper aims to present a plan to improve the power distribution in islanded microgrids. To enhance the performance of droop control, the virtual impedance is exploited, giving the operator the ability to control the impedance between the inverter and the load effectively. The proposed method is evaluated and validated in the case of linear, nonlinear, and unbalanced loads.

Power Hardware-in-the-Loop (PHIL): A Review to Advance Smart Inverter-Based Grid-Edge Solutions

Over the past decade, the world’s electrical grid infrastructure has experienced rapid growth in the integration of grid-edge inverter-based distributed energy resources (DERs). This has led to operating concerns associated with reduced system inertia, stability and intermittent renewable power generation. However, advanced or “smart” inverters can provide grid services such as volt-VAR, frequency-Watt, and constant power factor capabilities to help sustain reliable grid and microgrid operations. To address the challenges and accelerate the benefits of smart inverter integration, new approaches are needed to test both the impacts of inverter-based resources (IBRs) on the grid as well as the impacts of changing grid conditions on the operation of IBRs. Power hardware-in-the-loop (PHIL) stands out as a strong testing solution, enabling a real-time simulated power system to be interfaced to hardware devices such as inverters which can be implemented to determine interactions between multiple inverters at multiple points of common coupling on the grid and microgrids. This paper presents a review of PHIL for grid and microgrid applications including recent advancements and requirements such as real-time simulators, hardware interfaces and communication and stability considerations. An illuminating case study is summarized followed by exemplary PHIL testbed developments around the world, concluding with a proposed research paradigm to advance the integration of smart grid-following and grid-forming inverters.

Synchronization Stability of Interconnected Microgrids with Fully Inverter-based Distributed Energy Resources

Synchronization Stability of Interconnected Microgrids with Fully Inverter-based Distributed Energy Resources

Time-Domain Protection Scheme for Microgrids With Aggregated Inverter-Based Distributed Energy Resources

This paper proposes a time-domain-based protection scheme for radial and loop microgrid systems with inverter-based resources (IBRs), such as solar photovoltaic (PV) and type-4 wind turbines. The protection scheme is designed to function during both grid-interconnected and grid-isolated modes. The proposed scheme provides an ultra-high-speed sub-cycle directional element aided with low bandwidth communication between relays. Like directional comparison schemes, relays identify whether faults are in-zone or out-zone. The directional element is based on time-domain superimposed quantities and Park’s transformation algorithms. More specifically, the element calculates the superimposed positive-sequence direct component of transient energy during faults. Superimposed voltage and current quantities are calculated using delta filters and decoupled double synchronous reference frame (DDSRF) filters. The proposed filtering method improves the reliability of the superimposed directional element when IBRs are present. The protection scheme is evaluated on a modified IEEE 34-bus distribution system simulated using an electromagnetic transients program.

Spatial-Temporal Recurrent Graph Neural Networks for Fault Diagnostics in Power Distribution Systems

Fault diagnostics are extremely important to decide proper actions toward fault isolation and system restoration. The growing integration of inverter-based distributed energy resources imposes strong influences on fault detection using traditional overcurrent relays. This paper utilizes emerging graph learning techniques to build new temporal recurrent graph neural network models for fault diagnostics. The temporal recurrent graph neural network structures can extract the spatial-temporal features from data of voltage measurement units installed at the critical buses. From these features, fault event detection, fault type/phase classification, and fault location are performed. Compared with previous works, the proposed temporal recurrent graph neural networks provide a better generalization for fault diagnostics. Moreover, the proposed scheme retrieves the voltage signals instead of current signals so that there is no need to install relays at all lines of the distribution system. Therefore, the proposed scheme is generalizable and not limited by the number of relays installed. The effectiveness of the proposed method is comprehensively evaluated on the Potsdam microgrid and IEEE 123-node system in comparison with other neural network structures.

Decentralized Distributed Convex Optimal Power Flow Model for Power Distribution System Based on Alternating Direction Method of Multipliers

This paper proposes a fully decentralized distributed convex optimal power flow model for inverter-based distributed energy resources (DERs) integrated electric distribution networks based on Semi-Definite Programming (SDP) and alternating direction method of multipliers (ADMM) namely (SDP D-ADMM). The proposed approach is based on the SDP relaxed branch flow model of distribution networks within an auto-tuned accelerated decentralized ADMM architecture. The approach is based on dividing the power grid network into subproblems representing individual areas by interchanging minimum network information. In the proposed model the requirement of a central processor is also waived thus making the proposed approach more robust toward cyber-attacks. The effectiveness and scalability of the proposed method are validated by implementing modified IEEE 123 and IEEE 8500 bus systems with different levels of DER penetration. It has been observed that the proposed architecture outperforms other distributed optimization variants in terms of accuracy, global optimality, scalability, and computational time.

On the Cyber-Physical Needs of DER-Based Voltage Control/Optimization Algorithms in Active Distribution Network

With the increasing penetration of distributed energy resources (DERs) and extensive usage of information and communications technology (ICT) in decision-making, the mechanisms to control/optimize transmission and distribution grid voltage would experience a paradigm shift. Given the introduction of inverter-based DERs with vastly different dynamics, real-world performance characterization of the cyber-physical system (CPS) in terms of dynamical performance, scalability, robustness, and resiliency with the new control algorithms require precise classification and identification of suitable metrics. It has been identified that classical controller definitions, along with three inter-disciplinary domains, such as (i) power system, (ii) optimization, control, and decision-making, and (iii) networking and cyber-security, would provide a systematic basis for the development of an extended metric for performance evaluation of DER controllers; while providing the taxonomy. Majority of the control algorithms operate in multiple time scales, and therefore, algorithmic time-decomposition facilitates a new way of performance analysis. Extended discussion on communication requirements with the algorithmic architectural subtleties is expected to identify the real-world deployment challenges of voltage control/optimization algorithms in terms of presence of cyber vulnerabilities and associated mitigation mechanisms affecting the controller performance with DERs. Detailed discussion provided identifies the modeling requirements of the CPS for real-world deployment, specific to voltage control, facilitating the development of a unified test bed.

A Coordinated Control Architecture With Inverter-Based Resources and Legacy Controllers of Power Distribution System for Voltage Profile Balance

In this article, a coordinated control architecture is proposed that utilizes inverter-based distributed energy resources (DERs) and legacy controllers such as voltage regulators in a power distribution system and coordinates the reactive power support such that the voltage balance at a particular node of interest can be achieved. The approach is developed based on the alternating direction method of multipliers and the optimal control framework, where a primary control loop is used to provide the necessary reactive power set point to mitigate the voltage deviation and a secondary control loop is used to balance the reactive power from multiple devices such as DERs. The methodology is tested on the IEEE 123-bus distribution feeder with multiple three-phase DERs in coordination with a voltage regulator. It has been observed that the proposed architecture can reduce regulator tap operation, improve the voltage deviation (at least by 20%), and, at the same time, mitigate the negative effect of the reverse operation of the regulators.