Inverter-interfaced Distributed Generators Research Articles

Fault detection and classification scheme for power islands with inverter interfaced distributed generators

The low fault current contribution from inverter-interfaced distributed generators (IIDGs) and bidirectional flow of current in the power networks introduces protection challenges when islanded. The existing fault detection scheme based on the transient monitoring function (TMF) fails under such scenarios. A new decentralised machine learning (ML) based fault detection and classification scheme is proposed in this paper using terminal voltage information of IIDG (vo−abc). TMF of vo−abc, along with the zero sequence component of vo−abc, is utilised for training and testing of ensemble bagged trees ML algorithm. The proposed scheme is implemented inside each IIDG, and hence, it does not require communication. It is validated on a modified CIGRE test benchmark microgrid for all shunt faults at various locations with different loading, fault resistances, and inception angles. The accuracy of detection and classification of fault (DCF) during fault cases with unbalanced loads, noise, harmonic loads, and no-fault scenarios such as load and DG switching proves the effectiveness of the proposed method with varying scenarios. The performance of the proposed scheme is found to be superior when compared with the existing threshold based TMF and Hilbert–Huang transform (HHT) based techniques considering accuracy, dependability, security, and response time.

Relay protection sensitivity integrated optimal placement and capacity of inverter interfaced distributed generators in distribution networks

The relay protection sensitivity is one of the determined factors in the power system, however, it is often overlooked in current distribution network (DN) planning. The relay protection sensitivity can be decreased to below the minimum values, failing to meet the requirements for electrical installations. To address this challenge, a new optimization model integrated with the relay protection sensitivity to maximize the inverter interfaced distributed generator (IIDG) penetration level while minimizing IIDG investment was proposed in this paper. The IIDG effect on the relay protection sensitivity was analysed and the relay protection sensitivity re-evaluation method was developed. The relay protection sensitivity evaluation was integrated into the proposed model and the particle swarm optimization (PSO) algorithm was developed to solve the nonlinear issue. The proposed optimization method was tested on different cases, and results confirmed the effectiveness of the method. Furthermore, the relay sensitivity profiles obtained through the proposed method and the optimization without considering the relay sensitivity limits were compared. The proposed method improves the average and minimum sensitivity factors by 28.77 % and 51.76 %, respectively, when the DTO protection functions as the backup for the protected line in the thirty-three-node system. When DTO acts as the backup for the adjacent line, the average and minimum values increase by 29.91 % and 50.95 %, respectively. Comparative analysis confirms the efficacy of the proposed method. The new method extends the power system panning approaches and can be integrated into the DN planning tools to support the low-carbon initiatives.

Fault nature identification and location scheme for distribution network based on active injection from IIDG

In view of the difficulty in passively identifying the nature of faults in distribution networks and the high equipment cost of existing active detection schemes, a permanent fault identification and location scheme using an inverter-interfaced distributed generator (IIDG) connected to feeders for signal injection is proposed. After the faulty feeder is isolated, the power electronics in the IIDG are switched on for a short time and a voltage signal is injected into the feeder through the modified LCL filter and grid-connected transformer. The analysis shows that the aerial mode voltage of the feeder will not remain 0 under permanent faults. A highly sensitive criterion with fixed thresholds is constructed using approximate entropy (ApEn) to identify fault nature. Then the fault distance is further calculated based on the detected mutation point of the aerial mode voltage under permanent faults. The correctness of the theory and the effectiveness of the scheme are verified in PSCAD/EMTDC simulations.© 2017 Elsevier Inc. All rights reserved.

A Current Differential Protection Scheme for Distribution Networks with Inverter-Interfaced Distributed Generators Considering Delay Behaviors of Sequence Component Extractors

The high-level proliferation of inverter-interfaced distributed generators (IIDGs) in modern distribution networks (DNs) has changed system topologies and fault current signatures, which compromises the protective relays in DNs. Investigating IIDG fault behaviors-based protection scheme will benefit the grid’s safety and stability. This paper proposes a novel current differential protection (CDP) scheme that considers the delay behaviors of positive- and negative-sequence component extractors for IIDGs in DNs. A frequency-domain analytical model of the fault current for a grid-connected IIDG with the PQ control strategy and a low-voltage ride-through (LVRT) capability is investigated. The dynamic behavior of the IIDGs considering the sequence-component extractor based on the Pade approximation is presented, where the T/4 delay extractor of the IIDGs causes a two-stage behavior in the fault transient process. It is found that a 5 ms error between the measured and actual values after the fault will affect the transient characteristics of the IIDGs. The transient current generated by the IIDGs during grid faults contains a large number of low-order harmonic components within the range of 0–200 Hz, which is significantly different to the current provided by the power grid. Therefore, the proposed CDP scheme uses protective relays at both terminals to obtain the required transient electric quantity using the Prony method. By constructing the frequency-characteristics ratio (FCR) and the exchanging FCR between two terminal relays, the developed protection criteria are implemented. The accuracy of the fault analysis method, whose maximum computational error is below 0.1%, and the feasibility of the proposed protection scheme are demonstrated by using a 10 kV DN in a PSCAD/EMTDC simulation, which can be applied to various fault conditions and traditional DNs without IIDGs.

Fast and accurate method for short-circuit current calculation in distribution network with IIDGs

The fault behavior of Inverter Interfaced Distributed Generators (IIDGs) diverges from that of synchronous generators. When a substantial number of IIDGs are integrated into the distribution network, the conventional short-circuit current calculation technique, grounded in a mechanical framework, struggles to simultaneously satisfy the demands for both rapid computation and precision. This study introduces an alternative approach reliant on data analysis, enabling swift and precise computation of short-circuit currents across the IIDG-infused distribution grid. The investigation delves into the efficacy of multi-output regression techniques and the selection of the foundational learning algorithm. Specifically, two strategies are advanced: Multi-Target Regressor Stacking and Regressor Chains, both utilizing the Light Gradient Boosting Machine as the base learner. The influence of varying sample sizes on the multi-output computational model's performance is assessed. To ascertain the real-world viability of the multi-output regression methodology, two distinct fault characteristics pertaining to data-driven short-circuit current calculation are scrutinized using existing distribution network measurement conditions. The study reveals that, given a known distribution network structure, the proposed method can compute network-wide short-circuit currents across diverse IIDG penetration levels and fault scenarios through a singular model, maintaining a balance between computation precision and speed. If the distribution network's structure undergoes minor modifications, the model demonstrates reasonable adaptability, yet a retraining of the short-circuit current calculation model is recommended to ensure sustained accuracy in the face of significant network changes.

Distributed Adaptive Control Framework for Enhanced Voltage and Frequency Regulation in Inverter Interfaced Autonomous Distribution Network

This article presents a unique design method for an adaptive neural network based backstepping-like control (ANNBC) scheme. The technique is employed for synthesizing the primary controller for inverter interfaced distributed generators (IIDGs) integrated to an autonomous distribution network. Further, an optimal distributed secondary control framework is developed for a multiple IIDGs-based autonomous distribution network. The secondary controller facilitates optimal gain selection to regulate the frequency of the system and voltage of the critical bus to their desired set points. The framework also achieves accurate real and reactive power sharing among the IIDGs according to their power ratings. The novel design procedure of the proposed control framework takes into account the entire system dynamics of the IIDG including the uncertain terms (viz., load current and network dynamics) and is completely independent of the system parameters information. Suitable update laws are designed for estimating the unknown weights of the neural network and the uncertain system parameters. Lyapunov analysis is used to show that the tracking errors and parameter estimation errors are uniformly ultimately bounded. Finally, case studies are conducted on a typical autonomous distribution network having a single and multiple IIDGs modeled in MATLAB/Simulink platform.

Adaptive Coordination of Relays in AC Microgrid Considering Operational and Topological Changes

Conventional protection schemes are vulnerable to varying changes in a microgrid with different types of distributed generators (DGs), which alter the fault current levels, leading to miscoordination of directional over-current relays (DOCRs). Due to limited storage in numeric relays, having one setting for each network scenario in the relays is infeasible. Also, schemes with one setting considering grid-connected mode (GCM) and islanded mode (IM) of operation with multiple scenarios may be infeasible for a microgrid with inverter-interfaced DGs due to low short-circuit level in IM. Thus, this article proposes an adaptive trip characteristics-based relay coordination scheme for a microgrid with different DGs, where two trip characteristics are assigned to each DOCR, one each for GCM and IM. Also, a novel framework for selecting the suitable trip characteristic is proposed. Hardware-in-loop simulations using Numerical Relay Development Environment for a meshed CIGRE microgrid, modified IEEE 34-bus distribution system, and comparison with previous works validate the proposed method's performance for various scenarios, such as the outage of a DG or line, variations in solar irradiation and wind speed, and network reconfiguration for different fault types during GCM and IM.

An adaptive protection scheme for multiple single-phase grounding faults in radial distribution networks with inverter-interfaced distributed generators

Conventional zero-sequence current (ZSC) protection relays for low-resistance grounded systems (LGSs) are confronting challenges due to the risk of multiple single-phase grounding faults (MSGFs) and the ongoing increase in the penetration of inverter-interfaced distributed generators (IIDGs). Such issues concerning false or failure tripping require a reconsideration of traditional solutions. This paper presents a novel adaptive ZSC protection scheme using the current-compensation method for LGSs with IIDGs. The mechanism for in-phase MSGFs and out-of-phase MSGFs in LGSs with IIDGs is formulated. The ZSC calculation method for LGSs with IIDGs is introduced via the zero-sequence compensation factor, where the feeder ZSC generated by MSGFs is converted to its counterpart during a single fault. With the manufacturing message specification (MMS) protocol in IEC 61850 that is selected to help share IIDG information, the adaptive ZSC protection philosophy is implemented, which automatically amends the ZSC and sets the protection settings of all relays in real-time. The validity of theoretical analyses and the performance of the protection scheme have been demonstrated by using the PSCAD/EMTDC simulation under various scenarios.

Optimal Protection Coordination of Islanded Microgrids Utilizing an Adaptive Virtual Impedance Fault Current Limiter

Outages may impact power grids, e.g., lines or generation forced out of service. Thus, solving the optimal protection coordination (OPC) problem for the main network topology results in protection miscoordination under contingencies. Further, the limited fault currents of inverter-interfaced distributed generators (IIDGs) require a highly sensitive and reliable protection scheme. This paper proposes an overcurrent protection scheme for islanded microgrids powered by droop-based IIDGs. The inverter controller is modified to include a virtual impedance-fault current limiter (VI-FCL) to protect inverter switches from overcurrent and limit IIDG fault currents. The VI-FCL is designed such that it adapts to fault severity. OPC of directional overcurrent relays (DOCRs) is achieved using a two-stage optimization algorithm. The first Stage calculates the short-circuit currents involving various fault resistances. Next, constraints on the DCORs operation times are formulated for each topology resulting from an N-1 contingency and the main topology. The OPC problem is formulated as a constrained nonlinear programming problem. Finally, the second Stage is dedicated to obtaining the DOCRs optimal settings. The OPC method is tested on a radial microgrid that is part of a Canadian urban distribution system.

Distributed Secondary Control for Island Microgrids With Expected Dynamic Performance Under Communication Delays

In island microgrids, the increasing integration of inverter-interfaced distributed generators (DGs) degrades the dynamic performance of the control system. The operation voltage and frequency may appear unacceptable overshoots and fluctuations under disturbances. To achieve rapid convergence while efficiently suppressing overshoot, this paper develops a distributed control method for island microgrids with expected dynamic performance. In addition to the typical distributed consensus design, an auxiliary dynamic reshaping function is introduced into the algorithm to increase the control degree of freedom of the operation dynamics. Thus, the proposed method can realize independent and flexible configurations of the damping ratio and natural frequency of the control system, and obtain the expected state convergence and overshoot suppression performance according to the actual requirements of microgrids. Besides, different delays between DGs are also considered. Instead of simply adopting the conventional global fixed delay time, we preserve the difference of delays to the greatest extent. Such processing can effectively reduce the conservatism of control and improve the response rate. A nonlinear programming problem is also established for DGs to calculate the allowable delays under the specific performance. Finally, several case studies and an RT-Lab experiment evaluate the effectiveness of the method.

High Frequency Resonance Analysis and Resonance Suppression of a Grid-Connected Inverter Coupled With a Long Feeder

With the development of distributed generation (DG) technologies, various inverter-interfaced DG systems have been connected to the grid, which can cause serious harmonic issues. For an inverter-interfaced DG source connected to the grid via a long feeder, multiple resonances may occur due to the non-negligible distributed capacitance of the long line. Multiple resonances can cause current distortions when the DG delivers power to the grid and the harmonic voltage may be amplified because of the resonances. Many inverter control strategies have been studied, but few can suppress harmonics. To solve this issue, this paper derives the voltage transmission equations to analyze the transmission characteristics of harmonic voltages along a long feeder and reveals the reasons for the multiple resonances in the current. Based on the transmission line theory, a new control strategy of inverter is proposed to suppress the multiple resonances in current and the amplification of harmonic voltages. Finally, the effectiveness of the proposed control strategy is verified by the simulation and experimental studies.

An adaptive Distance Protection for Distribution Network With Inverter-Interfaced Distributed Generation

Inverter-Interfaced Distributed Generation (IIDG) accessed to the high-voltage distribution network may cause protective device tripping missing when using regular distance protection. In case of non-metallic faults, the generation of additional impedance may lead to faulty action of the distance protection, which threatens the safety and stability of the distribution network. Therefore, an adaptive distance protection plan is suggested, which corrects the measured impedance according to the diagram, thus the corrected measured impedance can track the fault impedance. The suggested plan is insensitive to the fault resistance, fault locations and IIDG’s output. The simulation results indicate that the suggested plan can reflect the fault impedance correctly, and accurately identify internal and external faults.

Enhanced Protection Coordination for Inverter Interfaced Distributed Generation (IIDG) with a Robust Multi-Loop Controller

Increasing penetration of inverter interfaced distributed generation (IIDG) in distribution networks has severely impacted protection coordination. False tripping, sympathetic tripping, protection blinding, failure of reclosing, and increase in short circuit levels are the critical complications and consequently disturbed the protection setting of the distribution network. This paper presents a novel multiloop current control scheme based on single integrator backstepping with a proportional resonant (PR) controller assisted by the whale optimization algorithm (WOA) to integrate IIDG in the distribution network. This control approach is designed to mitigate the maloperation of the overcurrent protection, due to the IIDG's current contribution during various types of faults. Inner loop backstepping is utilized to track converter side current and in addition, the PR controller is designed to track grid current in an outer loop. Using the proposed methodology, the injected current to the grid is effectively controlled to the rated level during several types of faults or disturbances. The performance of the proposed control strategy is verified by time-domain results and through eigenvalues trajectory and results have been compared with a current state-of-the-art which corroborates the effective control capability of the present controller.

Harmonic Dual-Setting Directional Overcurrent Protection for Inverter-Based Islanded Microgrids

Limited fault currents in inverter-interfaced islanded microgrids impose immense challenges on conventional overcurrent protection schemes. This paper proposes a sensitive and selective protection scheme for islanded microgrids using a third harmonic voltage generated by inverter-interfaced distributed generators (IIDGs). The generated harmonic voltage results in a harmonic layer formed during short-circuit faults and is decoupled from the fundamental fault current, i.e., limited by IIDGs. Further, the generated harmonic voltage is adaptively adjusted based on fault severity to enhance protection sensitivity and obtain a universal set of relays’ settings. The proposed protection scheme utilizes harmonic directional overcurrent relays (HDOCRs) equipped with a dual-setting time-current-voltage setting that sense the generated harmonic voltages and currents at the relay location to ensure optimal protection coordination (OPC) of islanded microgrids. The OPC with the proposed dual-setting is formulated as a constrained nonlinear program to determine the optimal forward and reverse relays’ settings. The proposed scheme is tested on the Canadian benchmark urban distribution system and compared to the conventional protection scheme, which relies only on a single time-current-voltage trip characteristic. The results ensure the ability of the proposed scheme to protect islanded microgrids without communication and its capability to reduce relays’ operation times.

A Differential Protection Scheme Based on Improved DTW Algorithm for Distribution Networks With Highly-Penetrated Distributed Generation

Differential protection has been introduced into the distribution network to address the ineffectiveness of traditional protection due to the uncertainties of power flow caused by the access of multiple inverter-interfaced distributed generation (IIDG). Designed for lower data synchronization costs, this paper proposes a novel differential protection scheme based on improved dynamic time warping (DTW) distance. The proposed algorithm can effectively alleviate the singularity caused by DTW’s excessive X-axis distortion through relaxation search and derivative estimation and minimize the impact of data synchronization errors and data loss. Based on the proposed algorithm, the protection scheme combined with the feeder terminal unit (FTU) for fast fault isolation is designed, which can make full use of the existing equipment and reduce costs. Moreover, multiple influencing factors considered in this scheme, including transition resistance, penetration rate, and output intermittency of IIDG, variable network topology, underground cables, and hybrid lines. The test results demonstrate that the proposed protection scheme can effectively isolate short-circuit faults in various scenarios.

Machine Learning Based Protection Scheme for Low Voltage AC Microgrids

The microgrid (MG) is a popular concept to handle the high penetration of distributed energy resources, such as renewable and energy storage systems, into electric grids. However, the integration of inverter-interfaced distributed generation units (IIDGs) imposes control and protection challenges. Fault identification, classification and isolation are major concerns with IIDGs-based active MGs where IIDGs reveal arbitrary impedance and thus different fault characteristics. Moreover, bidirectional complex power flow creates extra difficulties for fault analysis. This makes the conventional methods inefficient, and a new paradigm in protection schemes is needed for IIDGs-dominated MGs. In this paper, a machine-learning (ML)-based protection technique is developed for IIDG-based AC MGs by extracting unique and novel features for detecting and classifying symmetrical and unsymmetrical faults. Different signals, namely, 400 samples, for wide variations in operating conditions of an MG are obtained through electromagnetic transient simulations in DIgSILENT PowerFactory. After retrieving and pre-processing the signals, 10 different feature extraction techniques, including new peaks metric and max factor, are applied to obtain 100 features. They are ranked using the Kruskal–Wallis H-Test to identify the best performing features, apart from estimating predictor importance for ensemble ML classification. The top 18 features are used as input to train 35 classification learners. Random Forest (RF) outperformed all other ML classifiers for fault detection and fault type classification with faulted phase identification. Compared to previous methods, the results show better performance of the proposed method.

Space Vector Rotation-Based Controlled Decaying Current Injection for Islanding Detection of Inverter-Interfaced DG

Continuous injection-based islanding detection of Inverter-Interfaced Distributed Generator (IIDG) has been a general trend. The major issues in previous islanding detection methods are large Non-Detection Zone (NDZ), poor Power Quality (PQ), high implementation cost, and lack of real-time validation. Further, the threshold in most methods is not generic and depends on the system’s rating. Hence, this paper proposes a local voltage and current measurement-based islanding detection method for IIDGs. It utilizes parameters estimated from the fundamental phasors of voltage and current at the Point of Common Coupling (PCC) of IIDG using Space Vector Rotation (SVR) and a controlled self-decaying current injection. The proposed disturbance is injected only after disturbance detection by SVR parameters to mitigate the impact on PQ. Disturbance identification is easy due to the combined PCC voltage and current effects on parameter estimation. Real-Time Digital Simulator (RTDS) and Controller Hardware In the Loop (CHIL) setup-based results from the proposed method have been obtained and compared with previous methods. These results indicate the proposed method’s real-time efficacy (low islanding detection time) with no NDZ and negligible impact on PQ for different islanding events. Also, the proposed method is independent of the microgrid’s rating and reliable for non-islanding events.

Fast short-circuit current calculation of unbalanced distribution networks with inverter-interfaced distributed generators

This study proposes an approach for the fast calculation of short-circuit currents in unbalanced distribution networks with inverter-interfaced distributed generators (IIDG). It is capable of estimating the short-circuit current in unbalanced distribution networks with several IIDGs based on the symmetric component without calculating the system bus impedance matrix. Furthermore, the effect of the load on the short-circuit current was considered. Because the IIDG fault characteristic is determined by its fault ride-through control models, the short-circuit current calculation process is formulated as an iterative linear system calculation and can be solved using the simple generalized minimal residual (SGMRES) method. Finally, the analytical results were verified by comparisons with the time-domain simulation results of several test systems in PSCAD.

Triple Current Control of Four-Wire Inverter-Interfaced DGs for Correct Fault Type Identification

Inverter-interfaced distributed generators (IIDGs) have fault current signatures that could jeopardize protective relaying. This paper unveils the failure of phase selection methods (PSMs) utilized by commercial relays in the presence of four-wire IIDGs, which adversely impacts the grid resiliency and reliability. A triple current controller (TCC) is proposed to regulate the inverter’s sequence currents during unbalanced faults to ensure accurate fault type identification. The negative- and positive-sequence components are extracted using a decoupled double synchronous reference frame (DDSRF)-based phase-locked loop (PLL). Further, a second-order generalized integrator (SOGI)-based PLL is employed for zero-sequence component synchronization. The negative- and zero-sequence reference currents are generated to force the angles of IIDG sequence currents to behave like those from synchronous generators (SGs) and abide by the inverter’s current limits. Consequently, commercial PSMs can correctly identify the fault type. The proposed TCC scheme pertains to four-wire as well as transformer-less IIDGs. A performance evaluation using time-domain simulations is conducted on a CIGRE benchmark system to confirm the success of the proposed control scheme under different fault conditions.

Nonlinear Load Harmonic Mitigation Strategies in Microgrids: State of the Art

A generational shift has led to the evolution of distributed generation (DG) and microgrids. The loads connected in a microgrid can be both linear and nonlinear. Nonlinear loads deteriorate the power quality by drawing harmonic currents. Drawing of nonlinear currents will make the voltage waveform nonsinusoidal, which may prove harmful for other loads connected to the system. Previously, passive and active filters were used to mitigate harmonics, but they are expensive and bulky. The state of the art is to exploit multifunctional capabilities of inverter-interfaced DGs to deal with the power quality issues. This article presents the typical sources of generation of the harmonics, their deleterious effects, available standards, and detection techniques. Harmonic mitigation strategies for both grid-connected and islanded microgrids have been discussed in detail. The aim is to review and categorize the various strategies adopted by authors in recent times to mitigate harmonics and bring out the research gaps so that further avenues can be explored in this area.