Inverter-interfaced Distributed Energy Resources Research Articles

A privacy-preserving anomaly diagnosis scheme for AC microgrids

The integration of cyber infrastructure in renewable energy source based AC microgrids (MGs) raises concerns about potential cyber attacks on such systems. An adversary can fabricate the information being exchanged between system components, which may cause false tripping of the installed protection devices. This creates the need for an anomaly diagnosis scheme that can distinguish between faults and cyber attacks to attain a reliable and secure system under large signal disturbances. This work presents a machine learning-based anomaly diagnosis scheme which characterizes these anomalies. The scheme uses the well-known XGBoost approach, and requires local measurements (such as, active power, reactive power, and frequency) of inverter-interfaced distributed energy resources (IIDERs). Since such measurements may be difficult for participants in a system to share across organizational boundaries due to commercial or regulatory concerns, the proposed approach obfuscates the measurements by additive noise while preserving detection accuracy. This scheme was tested on an AC MG network with four IIDERs, modeled on a real-time simulation platform in an OPAL-RT environment. The results validate the effectiveness and accuracy of the proposed scheme under different conditions.

Optimal Design of Volt/VAR Control Rules for Inverter-Interfaced Distributed Energy Resources

The IEEE 1547 Standard for the interconnection of distributed energy resources (DERs) to distribution grids provisions that smart inverters could be implementing Volt/VAR control rules among other options. Such rules enable DERs to respond autonomously in response to time-varying grid loading conditions. The rules comprise affine droop control augmented with a deadband and saturation regions. Nonetheless, selecting the shape of these rules is not an obvious task, and the default options may not be optimal or dynamically stable. To this end, this work develops a novel methodology for customizing Volt/VAR rules on a per-bus basis for a single-phase feeder. The rules are adjusted by the utility every few hours depending on anticipated demand and solar scenarios. Using a projected gradient descent-based algorithm, rules are designed to improve the feeder’s voltage profile, comply with IEEE 1547 constraints, and guarantee stability of the underlying nonlinear grid dynamics. The stability region is inner approximated by a polytope and the rules are judiciously parameterized so their feasible set is convex. Numerical tests using real-world data on the IEEE 141-bus feeder corroborate the scalability of the methodology and explore the trade-offs of Volt/VAR control with alternatives.

Frequency and Voltage Control Techniques through Inverter-Interfaced Distributed Energy Resources in Microgrids: A Review

Microgrids (MG) are small-scale electric grids with local voltage control and power management systems to facilitate the high penetration and grid integration of renewable energy resources (RES). The distributed generation units (DGs), including RESs, are connected to (micro) grids through power electronics-based inverters. Therefore, new paradigms are required for voltage and frequency regulation by inverter-interfaced DGs (IIDGs). Notably, employing effective voltage and frequency regulation methods for establishing power-sharing among parallel inverters in MGs is the most critical issue. This paper provides a comprehensive study, comparison, and classification of control methods including communication-based, decentralized, and construction and compensation control techniques. The development of inverter-dominated MGs has caused limitations in employing classical control techniques due to their defective performance in handling non-linear models of IIDGs. To this end, this article reviews and illustrates advanced controllers that can deal with the challenges that are created due to the uncertain and arbitrary impedance characteristics of IIDGs in dynamics/transients.

A proportional power sharing method through a local control for a low-voltage islanded microgrid

In this paper, the proportional power sharing is focused through proposing a modified reverse droop control (RDC) scheme in a low-voltage (LV) islanded microgrid. The power is shared proportionally among parallel inverter-interfaced distributed energy resources (DERs) in an LV microgrid to distribute the change in the load based on their power capacity. The proposed control method shares real power inversely proportional in accordance with the real droop gain when the DERs have the same initial voltage values, while reactive power is also shared proportionally depending on reactive droop gains when different DERs’ rate-of-change of voltage angles (i.e. system frequency) are equal. The stability margins of the droop gains are analysed by dint of extended dynamic phasor model (DPM). An LV microgrid with inverter-interfaced DERs is demonstrated to run simulations during load impedance changing scenario. The simulation results demonstrate that the proposed controller provides better response with lesser oscillations than the conventional reverse droop controller. Also, the performance of sharing power in an LV islanded microgrid is validated.

Current-Based Coordination of Distributed Energy Resources in a Grid-Connected Low-Voltage Microgrid: An Experimental Validation of Adverse Operational Scenarios

Low-voltage grid-connected microgrids rely on the exploitation of inverter-interfaced distributed energy resources (DERs) in order to feed loads and to achieve bidirectional power flow controllability at their point of common coupling (PCC) with the upstream grid. However, adverse operational conditions, such as the existence of DERs of different operation natures, DERs of non-equal power ratings, as well as the occurrence of non-steady and non-sinusoidal grid voltage scenarios, bring complications to microgrid energy management. Consequently, control strategies employed to coordinate DERs in dispatchable microgrids need to be resilient to such non-ideal conditions. Hence, this paper demonstrates that a multi-purpose strategy, so-called the Generalized Current-Based Control (GCBC) approach, is capable of steering DERs under such adverse operational scenarios, ensuring proportional current sharing among them while also regulating the microgrid power dispatchability at the PCC. The discussions are supported by an extensive experimental validation on a laboratory-scale single-phase microgrid prototype, demonstrating that the GCBC approach allows DERs of different operational natures to be coordinated, respecting their power ratings, and allowing the single-controllable microgrid to endure operation under distorted voltages and support voltage ride-through conditions.

Integrated Local and Coordinated Overvoltage Control to Increase Energy Feed-In and Expand DER Participation in Low-Voltage Networks

Large amounts of active power injection by inverter-interfaced distributed energy resources (DER) is a common cause of overvoltage in low-voltage networks. Hence, local active and reactive power control (i.e., Volt/Watt and Volt/VAR, respectively) are adopted to limit voltage rise, leading to active power curtailment. This paper proposes an automatic control strategy to steer non-dispatchable (nd-DER) and dispatchable (d-DER) inverters in low-voltage networks, mitigating overvoltage through integrated local and coordinated Volt/Watt and Volt/VAR functionalities. Moreover, active power curtailment is avoided whenever possible. The method does not require <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) the implementation of optimization algorithms or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i> ) knowledge about line impedance parameters or the location of DERs. The control approach exploits the power flow dispatchability of low-voltage networks comprising one point-of-common coupling with the distribution grid, allowing DERs close to the distribution transformer to also contribute to voltage regulation by only using a low-bandwidth communication link. Simulation results show the flexibilities of the proposed approach and demonstrate that active power injection can be increased by up to 25% for the considered scenario in comparison to conventional local Volt/Watt or Volt/VAR schemes. Experimental results based on a laboratory prototype with three inverter-interfaced DERs certify the applicability of the approach to real-life implementations.

OC/OL Protection of Droop-Controlled and Directly Voltage-Controlled Microgrids Using TMF/ANN-Based Fault Detection and Discrimination

In this article, we propose a generalized overcurrent (OC) and overload (OL) protection scheme for the droop-controlled and directly voltage-controlled (DVC) inverter-interfaced distributed energy resources (IIDERs) to be used for the protection studies of the alternating-current (ac)-islanded (autonomous) microgrids, and provide a management strategy during and after short circuit (SC) and consequent OLs. The fault is detected, and its type is discriminated by a new method that takes the advantages of both artificial neural networks (ANNs) and transient monitoring function (TMF). By using the proposed strategy, the output current magnitudes of the distributed energy resource (DER) units are limited, and after fault clearance, the microgrid is restored to its normal operating conditions. The proposed OC/OL protection scheme limits the output power of the DERs. In this regard, a generalized fault model is presented, which considers the effect of the control system of the IIDERs. This model exploits a current-limiting strategy (CLS) that can be implemented in different reference frames to enhance the microgrid fault ride-through (FRT) capability. Finally, to demonstrate effectiveness, accuracy, authenticity, and feasibility of the proposed OC/OL protection scheme along with the TMF/ANN-based fault detection method and their desirable performance, offline digital time-domain simulations are done in MATLAB/Simulink environment, and the results are experimentally verified by using the OPAL-RT real-time digital simulator (RTDS).

Adaptive robust energy management for isolated microgrids considering reactive power capabilities of distributed energy resources and reactive power costs

This paper presents an adaptive robust Energy Management (EM) model for an isolated Microgrid (MG) to decide for the day-ahead optimal dispatch to effectively manage the MG power sources considering uncertainties in Renewable Energy Sources (RESs). The aim of this model is to minimize the operation costs, reactive power costs, spinning reserve and load shedding. Usually, fuel consumption costs of diesel generators are considered to be dependent on active power generation only. However, neglecting the related reactive power costs might result in increased operation costs and deviations from optimal dispatches. Hence, this paper optimizes the costs related to both active and reactive powers of diesel generators. Furthermore, simultaneous active/reactive power dispatch can lead to accurate operation decisions compared to separate dispatch for active or reactive power alone. In addition, this study considers the reactive power capability of inverter-interfaced Distributed Energy Resources (DERs) so that reactive powers can be supplied from inverter-interfaced DERs. Moreover, detailed models for the different resources are presented, especially for diesel generators and inverter interfaced DERs where actual capability curves are used instead of the widely used box constraints. The problem is formulated as nonlinear programming problem using GAMS program and solved by the CONOPT solver.

Implementation of a Stable Solar-Powered Microgrid Testbed for Remote Applications

An intrinsically stable microgrid, operated by inverter-interfaced distributed energy resources (I-DERs) is introduced in this paper. The microgrid is built upon a systematic design method, which is adapted from the operation of the synchronous machine (SM). The proposed method analogizes the dynamics of the dc-link in I-DERs to the rotor dynamics in synchronous generators (SGs) and utilizes the capacitor as energy storage. Thus, the proposed mechanism relaxes battery usage for frequency control, and by using the capacitive stored energy, provides a high fault ride-through capability, which is suitable for both on-grid and off-grid applications. Based on stability analysis of the SG and the dynamic state matrix eigenvalues for multimachine power system, the dc-link capacitor of I-DERs is characterized in the context of microgrid. The dc-link capacitor stores kinetic energy similar to the rotor of the SG and provides inertia in transients without the need of battery storage. The inverter angle responds to the change of the dc link voltage (energy). The dc-link voltage is then controlled similar to the field control pertaining to the SG. Finally, a governor-like mechanism is applied to maintain dc-link voltage stability. Simulation and experimental results are provided to show the effectiveness of the proposed design mechanisms.

Data Driven Stochastic Energy Management for Isolated Microgrids Based on Generative Adversarial Networks Considering Reactive Power Capabilities of Distributed Energy Resources and Reactive Power Costs

This paper proposes a two-stage stochastic energy management (EM) in an isolated microgrid (MG) to decide for the day-ahead optimal dispatch. The dispatch aims to effectively manage the MG power sources, including intermittent renewable energy sources (RESs), battery energy storage systems (BESSs), and diesel generators such that the expected operation costs, reactive power costs, spinning reserve, and load shedding are minimized. The Generative Adversarial Networks (GANs) is utilized in this paper as a data driven scenario generation method to model the uncertainties in the output power of the RESs to be used in the stochastic programming formulation. Then, the fast forward scenario reduction algorithm is used to reduce the number of scenarios with the help of SCENRED/GAMS software. Usually, fuel consumption costs of diesel generators are considered to be dependent on active power generation only. However, neglecting the related reactive power costs might result in increased operation costs and deviations in the dispatches from the optimal solutions. Hence, this paper co-optimizes the costs related to both active and reactive powers of diesel generators. In addition, this study considers the reactive power capability of inverter-interfaced distributed energy resources (DERs). Moreover, the detailed models for the different resources are presented, especially for diesel generators where the actual capability curves are used instead of the widely used box constraints. The problem is formulated as a nonlinear programming problem in the General Algebraic Modeling System (GAMS) software and is solved by the CONOPT solver.

Influence of load dynamic response on the stability of microgrids during islanding transition

Microgrids (MGs) are characterized by faster dynamics than conventional grid-connected distributions systems. This is in part due to the lack of inertia of inverter-interfaced distributed energy resources such as photovoltaic and energy storage systems. In this context, load dynamics considerably affects the transient stability performance of MGs, especially if disconnected from the main grid. The aim of the paper is to analyze the influence of the load modeling and the load composition in the transient stability assessment of a medium voltage MG during islanding transition. The MG we analyze consists of a photovoltaic power plant, two battery energy storage systems, a synchronous generator and different classes and types of load, which we consider representative for the problem of interest. The system is implemented in the EMTP-RV simulation environment. The importance of appropriate load modeling for the adequate analysis of the transient response of an islanding microgrid is also addressed and discussed.

Day-Ahead Energy Management for Isolated Microgrids Considering Reactive Power Capabilities of Distributed Energy Resources and Reactive Power Costs

In this paper, a day-ahead energy management (EM) for isolated microgrids (MGs) is proposed to obtain the optimal dispatch such that the costs related to the fuel consumption of diesel generators, load shedding, and renewable energy curtailment are minimized. Usually, fuel consumption costs of diesel generators are considered to be dependent on active power generation only. However, neglecting the related reactive power costs might result in increased operation costs and deviations in the dispatches from the optimal solutions. Hence, this paper co-optimizes the costs related to both active and reactive powers of diesel generators. In addition, this study considers the reactive power capability of inverter-interfaced distributed energy resources (DERs). The energy management problem is formulated as a nonlinear programming problem in GAMS environment and is solved using the CONOPT solver. Four different cases are presented to show the effect of the reactive power costs of diesel generators and the reactive power capability from the inverter-interfaced DERs on the operating costs of the isolated MG.

Protection of microgrids using voltage-based power differential and sensitivity analysis

Microgrids are emerging as an alternative mode of operation for distribution systems integrated with Distributed Energy Resources (DERs). With appropriate management and control of the DERs, a section of a distribution system can operate isolated from the main grid thereby enhancing the reliability and security of supply to consumers. However, the integration of DERs has raised many technical challenges including protection. The traditional distribution system protection cannot provide reliable protection to the microgrid in the isolated mode due to the limited short-circuit capacities of the converter-interfaced DERs. This paper proposes the application of a new voltage-based relay type for the protection of microgrids. The relay algorithm achieves its protection function through active power differential and sensitivity calculations based on voltage measurements within a specified protection zone. In the paper, the new relay type is modelled in Digsilent PowerFactory software and installed at the nodes of a microgrid test system. The performance of the relay type is investigated under variety of faults. The relay is shown to operate correctly and effectively to detect and identify faults in both radial and meshed microgrids integrated with inverter-interfaced DER technologies.

Real-Time Model-Free Coordination of Active and Reactive Powers of Distributed Energy Resources to Improve Voltage Regulation in Distribution Systems

This paper proposes and implements a model-free optimal strategy to regulate the voltage in distribution systems by effective control of Inverter-Interfaced Distributed Energy Resources (I-DERs) output power. In distribution systems, where the ratio of R/X is considerable, reactive power control is not always an optimum solution for voltage regulation. With the aim of improving voltage regulation, this paper proposes an alternative method to optimally coordinate active and reactive power of I-DERs in the grid. The proposed method does not need to have a system model and phase measurement, which are barriers in control and operation of behind-the-meter assets. The paper formulates the problem of voltage regulation in form of equilibrium-map-with-optimum, without any restriction on system configuration, amount of reverse power flow and losses. Using the extremum seeking control technique and without an explicit model of distribution circuit, the ratio of active and reactive power is coordinated in real time. The amount of exchanged apparent power is also adjusted simultaneously. Simulation results for a typical distribution circuit verify the effectiveness of the proposed approach in improving the voltage regulation performance. The experimental results also show a 20 percent reduction in use of system capacity to provide the desired voltage regulation performance.

Seamless Transition of Synchronous Inverters Using Synchronizing Virtual Torque and Flux Linkage

The introduction of virtual-synchronous machine and generator as well as synchronous inverter (synchronverter), in recent years, has facilitated the integration of inverter-interfaced distributed energy resources (IIDERs) into the synchronous generator (SG) dominant microgrids and power grid. In this concept, the voltage source inverter (VSI) is controlled in such a way that it mimics the behavior of SG. To provide a reliable utilization of the IIDERs, a seamless transition of the VSIs between islanded and grid-connected operation modes is required. This paper integrates a novel virtual torque and virtual flux linkage-based synchronizing technique into the synchronverter controller to provide a plug ‘n’ play capability of the VSIs and their smooth transition between these two operating modes. Comprehensive mathematical modeling, design, and dynamic stability analysis as well as dSPACE-based real-time simulation are provided to validate the effectiveness of the proposed technique.

Frequency-Based Decentralized Conservation Voltage Reduction Incorporated Into Voltage-Current Droop Control for an Inverter-Based Islanded Microgrid

Conservation voltage reduction (CVR) aims to decrease load demands by regulating bus voltages at a low level. This paper proposes a new strategy for decentralized CVR (DCVR), incorporated into the current-based droop control of inverter-interfaced distributed energy resources (IDERs), to improve the operational reliability of an islanded microgrid. An I dq controller is developed as an outer feedback controller for each IDER, consisting of Id-V controllers for the DCVR and I d -ω and I q -V controllers for power sharing. In particular, the I d -V controllers adjust the output voltages of the IDERs in proportion to the frequency variation determined by the I d -ω controllers. This enables the output voltages to be reduced by the same amount, without communication between the IDERs. The I q -V controllers are responsible for reactive power sharing by adjusting the voltages while taking into account the Id-V controllers. Small-signal analysis is used to verify the performance of the proposed DCVR with variation in the I d -ω and Iq-V droop gains. Case studies are also carried out to demonstrate that the DCVR effectively mitigates an increase in the load demand, improving the operational reliability, under various load conditions determined by power factors and load compositions.

Non-Pilot Protection of the Inverter- Dominated Microgrid

Without utilizing costly communication systems, the existing protection strategies fail to reliably detect the occurrence and direction of faults in the inverter-dominated microgrid. To address this issue, this paper introduces a selective and reliable non-pilot protection strategy for the inverter-dominated microgrid. The proposed protection strategy (i) does not require communication signals, (ii) incorporates phase- and sequence-domain protective elements for reliable detection of symmetrical and asymmetrical faults, (iii) improves the existing sequence-domain directional elements and effectively utilizes them for accurate determination of the fault direction in the presence of inverter-interfaced distributed energy resources, (iv) selectively protects the inverter-dominated microgrid against internal and external faults, (v) is robust against the grid-connection mode of the microgrid, and (vi) enables fuse protection of laterals and non-critical circuits. The acceptable performance of the proposed protection strategy is verified through comprehensive fault studies conducted on a realistic study system simulated in the PSCAD/EMTDC software environment. This paper also demonstrates that the proposed protection strategy can be implemented using an off-the-shelf digital relay.

Adaptive Impedance-Based Fault Location Algorithm for Active Distribution Networks

Modern fault location methods are robust; however, they depend strongly on the availability of the measurements given by Distributed Energy Resources (DER). If the communication or synchronism of this information is lost, the fault location is not possible. This paper proposes an adaptive impedance-based fault location algorithm for active distribution systems. The proposal combines information provided by Intelligent Electronic Devices (IEDs) located at the substation, the knowledge of the network topology and parameters, as well as the distributed power sources, to estimate the fault location. Its adaptive feature is given by the use of a Distributed Energy Resources (DER) electrical model. This model is used to estimate the DER current contribution to the fault, in case the information provided by a local IED is not available. The method takes two types of DER technologies into account: Inverter non-interfaced DER (INIDER) and Inverter-interfaced DER (IIDER). The proposed method is validated on a modified IEEE 34-node test feeder, which was simulated with ATP/EMTP. The results obtained using the IEDs information, presented a maximum error of 0.8%. When this information is not available, the method’s performance decreases slightly, obtaining a maximum error of 1.1%. The proposed method showed better performance when compared with two state of the art methods, indicating potential use for real-life applications.

Coordinated control of three- and single-phase inverters coexisting in low-voltage microgrids

This paper proposes the third generation of the Power-Based Control, that is, an approach to effectively coordinate the operation of single- and three-phase inverter-interfaced distributed energy resources that can be arbitrarily connected among the phases of three-phase four-wire low-voltage microgrids. The aim of the approach is to precisely regulate the power exchanged with the main grid by fairly exploiting the available distributed resources while respecting their own local constraints. This allows to achieve demand-response, unbalance compensation, and improved voltage profiles, which is valuable for limiting stress conditions to the distribution infrastructure. The technique is based on a master/slave microgrid architecture where the distributed inverters act as slave units driven by a centralized master controller. This latter employs the Power-Based Control in order to steer the contribution of the inverters on the basis of the microgrid power status. In particular, active, reactive, and unbalance power terms are processed by the master controller and the corresponding microgrid's power needs shared among the distributed energy resources, to achieve the compensation target at the point-of-common-coupling with the main grid. The strategy proposed herein drives the three-phase inverters to operate balanced, which avoids unwanted voltage fluctuations at the DC-side of the inverters; instead, single-phase inverters, which can be connected arbitrarily among the phases of the distribution grid, are exploited to compensate any residual unbalances (i.e., negative- and zero-components). The control technique is analyzed and demonstrated by computer simulations considering a real urban meshed distribution network. Steady-state and dynamic results and performances are reported and discussed considering typical demand and generation profiles.

Optimal Power Flow Pursuit

This paper considers distribution networks featuring inverter-interfaced distributed energy resources, and develops distributed feedback controllers that continuously drive the inverter output powers to solutions of ac optimal power flow (OPF) problems. Particularly, the controllers update the power setpoints based on voltage measurements as well as given (time-varying) OPF targets, and entail elementary operations implementable onto low-cost microcontrollers that accompany power-electronics interfaces of gateways and inverters. The design of the control framework is based on suitable linear approximations of the ac power-flow equations as well as Lagrangian regularization methods. Convergence and OPF-target tracking capabilities of the controllers are analytically established. Overall, the proposed method allows to bypass traditional hierarchical setups where feedback control and optimization operate at distinct time scales, and to enable real-time optimization of distribution systems.