Inverter-based Distributed Generation System Research Articles

An Islanding Detection Method Based on the Reactive Power Disturbance for Multiple Inverter-Based DG Systems

In this paper, an islanding detection method for multiple inverter-based distributed generation systems is proposed, which is based on perturbing reactive power output. An asymmetric form of positive and negative reactive power disturbance is presented so that there always exists a certain total amount of disturbance in the system. To decrease the disturbance amplitude without expanding the non-detection zone, a positive feedback of frequency difference is added to the disturbance. A reactive power disturbance synchronization method without communication among inverters is proposed so that the averaging effect caused by unsynchronized reactive disturbances can be eliminated and the island can be detected in a multiple-DG system. Additionally, the islanding detection criterion is improved by establishing the relationship between the direction of disturbance and the numerical symbol of frequency difference, so there is no misjudgment of islanding detection in a weak grid. Finally, an experimental platform consisting of three 6kW three-phase grid-connected inverters and a 99.9kVA islanding detection test device is established. The experimental and simulation results indicate that the proposed method performs fast and precise islanding detection for the multiple-DG system with small NDZ and little impact on power quality. Additionally, no misjudgment happens in a weak grid.

Sampled-Data Cooperative Output Regulation: An Adaptive Distributed Continuous-Discrete Observer Approach

This paper studies the sampled-data cooperative output regulation problem for linear multi-agent systems. Firstly, a novel adaptive distributed continuous-discrete observer is established to recover the leader's state, which, on one hand, only relies on the sampling output of the leader, and on the other hand, does not need to store the sampled message from neighbors. Secondly, by solely making use of the digital states of both the agent and the distributed observer, a certainty equivalence control law is synthesized featuring a time-varying feedforward gain. It is rigorously proven that, with this time-varying feedforward gain, the proposed control approach can achieve exponential convergence of the tracking errors given arbitrary time-varying leader's signal, and the upper bound for the sampling intervals is explicitly given. Thirdly, for the class of chain-integrator multi-agent systems, the proposed control approach does not need any restriction on the upper bound of the sampling intervals, and thus would be more practical in certain application scenarios from the perspective of energy saving. The performance of the proposed control approach is validated by the simulation results of inverter-based distributed generation systems.

Decentralized Robust Control of a Network of Inverter-Based Distributed Generation Systems

This paper presents the design of decentralized robust controllers for a network of inverter-based distributed generation systems with LC filters in the scale of a nanogrid. Using overlapping decomposition, the network of inverters is clustered into several subnetworks such that all inverters within a subnetwork are strongly coupled and there is no or a weak coupling effect between any two inverters from different subnetworks. For the inverters within the same subnetwork, decentralized robust controllers are designed sequentially in the μ-synthesis framework. In addition, all controllers are designed to be robust against ±10% variations in the LC filter parameters. To assess the performance of the proposed sequentially-designed controllers and compare it to that of the benchmark independently-designed ones, the distances between two neighboring inverters from the same and different subnetworks are considered to be 200 (m) and 800 (m), respectively. In this case, time-response and robustness analysis results illustrate the superiority of the proposed sequentially-designed controllers in the overlapping decomposition framework over the benchmark independently-designed ones. Moreover, transient overload and nonlinear load analyses demonstrate that the proposed sequentially-designed decentralized controllers are able to keep the load voltage within ±10% of the nominal value and the harmonic voltage distortions to less than 4%.

Sequential Design of Decentralized Robust Controllers for Strongly Interconnected Inverter-Based Distributed Generation Systems: A Comparative Study versus Independent Design

Internal oscillations among multiple generation systems in low-voltage stand-alone nanogrids and small-scale microgrids can cause instability in the entire generation system. This issue becomes worse when the coupling strength between the generation systems increases, which is a result of a shorter distance between them and a smaller reactance to resistance ratio. Previous approaches, which were based on the independent control design and considered the coupling effect as disturbances, may fail to tackle this issue when the two generation systems become strongly coupled. Therefore, in this paper a novel method is proposed to handle this coupling effect by designing robust decentralized controllers in a sequential manner to address the problem of voltage and frequency control in a nanogrid. This proposed sequential design is a general technique that is applicable to multiple inverter-based generation systems in a nanogrid or small-scale microgrid. For the ease of demonstration, in this paper the case of two interconnected inverters with LC output filters is studied. Two robust decentralized controllers are designed for the two inverter systems by using the μ-synthesis technique. The sequential design takes into account the interconnection line between the two inverters. Moreover, the controllers are designed to be robust against all the parameter variations in the system including the LC filter and interconnection line parameters. The simulation results demonstrate the superior performance of the proposed controller over the independently-designed controllers for the case of two generation systems that are highly coupled due to the short distance between them. Moreover, the proposed controller is shown to be robust against the LC filter and interconnection line parameter uncertainties as compared to the sequentially-designed linear quadratic Gaussian controllers.

Strength assessment of electric power systems containing inverter-based distributed generation

Large-scale penetration of inverter-based distributed generation (IBDG) systems such as wind and solar PV plants into conventional power systems is raising concerns for traditional methods of power system operation and control. System strength at the point of interconnection (PoI) of the IBDG is one such concern, measured by the short circuit ratio (SCR). However, SCR and other SCR-based methods disregard interactions and electrical connections that exist amongst IBDGs. This study proposes an alternative method using the theory of power system network structural characteristics. The network response short circuit ratio (NRSCR) method captures the interaction among IBDGs and with nearby synchronous generators without having to solve the power flow problem. In addition, it does not require inversion of the systems’ admittance matrix. Comparative analysis of the proposed method with other SCR-based methods is demonstrated by showing the relationship between system strength, voltage stability, and active power transfer.

Value iteration and adaptive optimal output regulation with assured convergence rate

In this paper, we investigate the learning-based adaptive optimal output regulation problem with convergence rate requirement for disturbed linear continuous-time systems. An adaptive optimal control approach is proposed based on reinforcement learning and adaptive dynamic programming to learn the optimal regulator with assured convergence rate. The above-mentioned problem is successfully solved by tackling a static optimization problem to find the optimal solution to the regulator equations, and a dynamic and constrained optimization problem to obtain the optimal feedback control gain. Without requiring on the accurate system dynamics or a stabilizing feedback control gain, a novel online value iteration algorithm is proposed, which can learn both the optimal feedback control gain and the corresponding feedforward control gain using measurable data. Moreover, the output of the closed-loop system is guaranteed to converge faster or equal to a predefined convergence rate set by user. Finally, the numerical analysis on a LCL coupled inverter-based distributed generation system shows that the proposed approach can achieve desired disturbance rejection and tracking performance.

An Optimal Islanding Detection Scheme for an Inverter-based Distribution Generation System

In this paper, an optimal controller-based islanding detection scheme is proposed for an inverter-based distributed generation system (DGS). Islanding is both harmful and unsafe but unavoidable state in a DGS. Therefore, islanding state is required to be detected quickly and accurately. A reactive power control strategy for islanding is presented in this work. It is reported that the active and reactive power mismatches in a DGS forces its voltage or frequency to deviate. That deviation is recorded continuously in this scheme and islanding state is reported when the deviation touches the thresholds. To help in this tracking process, a modified H8-based optimal controller whose parameters are tuned with LMI is proposed. It can offer both feedback and feed-forward control actions. The proposed DGS is modelled using MATLAB/SIMULINK and tested taking several case studies. It is found that, islanding condition is rapidly detected compared to that of the uncontrolled scheme yielding an optimal solution. Further, it is an inexpensive, robust, compact in structure and very easy to implement too.

Resiliency-Oriented Optimization of Critical Parameters in Multi Inverter-Fed Distributed Generation Systems

In the modern power grid, with the growing penetration of renewable and distributed energy systems, the use of parallel inverters has significantly increased. It is essential to achieve stable parallel operation and reasonable power sharing between these parallel inverters. Droop controllers are commonly used to control the power sharing between parallel inverters in an inverter-based microgrid. In this paper, a small signal model of droop controllers with secondary loop control and an internal model-based voltage and current controller is proposed to improve the stability, resiliency, and power sharing of inverter-based distributed generation systems. The distributed generation system’s nonlinear dynamic equations are derived by incorporating the appropriate and accurate models of the network, load, phase locked loop and filters. The obtained model is then trimmed and linearized around its operating point to find the distributed generation system’s state space representation. Moreover, we optimize the critical control parameters of the model, which are found using eigenvalue analysis, and Grey Wolf optimization technique. Through time-domain simulations, we show that the proposed method improves the system’s resiliency, stability, and power sharing characteristics.

Comparative Analysis of Some Remarkable Islanding Detection Techniques in Inverter-Based Distributed Generation Systems

Recently, there is large scale of infiltration of renewable energy resources based distributed generation Systems (DGSs) into the power grid as cheap power can be available at local-end with less maintenance. However, islanding state of DGSs may enforces severe stability and security issues. Therefore, islanding condition in power system network need to be detected and addressed as soon as possible. For that an appropriate islanding detection technique is required. Vast research going on this topic and numerous techniques are found in literature till 2020 and some latest papers of 2021. New researchers need idea about available detection techniques. This paper may server for that purpose. Although some other significant review works on islanding detection technique are also available in literature in previous years but the necessary extensive analysis is also feature of this paper. Also, it includes some of the latest developed but distinct islanding detection techniques. In this paper, a thorough description and their classification of islanding detection techniques has been discussed. The classification structure has been built on control strategy, control variable types of circuitry, non-detection zone and weakness. This paper is intended to serve as a convenient reference for future researchers working on islanding issue for the DG power system.

A novel phaselet-based approach for islanding detection in inverter-based distributed generation systems

In this paper, a novel signal processing approach based on phaselet algorithm is proposed to detect the islanding phenomenon. Phaselets can effectively compute the phasors over data windows that are not limited to an integer multiplication of a half-cycle. Thereby, they can obtain features of the islanded and non-islanded situations without injecting any disturbance or high frequency signal into the power system, which makes the proposed method more accurate and reliable because there are no power quality reduction problems. In the proposed approach, phaselet algorithm uses the variable filtering window capability in order to phasor estimation of the negative sequence components of the voltage and current waveforms. To evaluate the performance of the proposed technique, various islanding and non-islanding events have been implemented using a grid-connected PV system modeled in a MATLAB/Simulink environment, which are based on the UL-1741 and IEEE-1547 standards. The simulation results show that our approach is able to accurately distinct islanding incident from other power quality disturbances. The ability to detect islanding occurrence in less than two cycles even in the case of perfect power match, the zero non-detection zone (NDZ) and no power quality degradation are the main advantages of the proposed method.

A Feedback-Based Passive Islanding Detection Technique for One-Cycle-Controlled Single-Phase Inverter Used in Photovoltaic Systems

Single-stage, single-phase photovoltaic (PV) distributed generation (DG) systems rated below 5 kW are becoming popular because of the reduction in the cost of PV panels and also due to various policy initiatives. One cycle control (OCC) based inverters are ideally suitable for such DG systems because of its cost-effectiveness, simple structure, and phase-locked loop free implementation. These inverter-based DG systems should have a robust islanding detection method (IDM) incorporated into it and should fulfill the recent standard requirements. This article proposes a novel feedback-based passive IDM, which is suitable for OCC-based grid-connected PV inverter system. The proposed method relies on the inbuilt feedback nature of OCC-based system, which drives the voltage at the point of common coupling outside the normal range of operation during an islanding condition. The proposed IDM works effectively under the new standard requirements with negligible nondetection zone. The IDM does not rely on additional computational effort or injection of external disturbance into the grid. Simulation studies are performed to verify the effectiveness of the proposed IDM. A laboratory prototype has been developed, and experimental studies are conducted to ascertain the viability of the proposed method.

Islanding Detection Scheme for Inverter-Based Distributed Generation Systems Using Cumulative Reactive Power Harmonics

In this paper, a new islanding detection scheme using cumulative reactive power harmonics (CRPH) for inverter-based distributed generation (DG) systems is presented. In the proposed method, first, any changes in the system are detected using a change detection index. Desired harmonic components of the voltage and current signals at DG terminals are extracted by a discrete Kalman filter (DKF). Then, a criterion based on CRPH is defined using the extracted harmonic signatures. The change detection index is based on the difference of measured current and its estimated fundamental component. At the initial moments of any events, reactive power controller of the inverter is disabled just for a short time interval. Since the main grid as the main source of reactive power loses in islanding condition, the reactive harmonic power of the system experiences significant variations. Then, islanding can be discriminated from other similar conditions by processing the captured signals of the mentioned time interval. Performance of the proposed method is assessed using plenty of simulations. Moreover, the superiority of the proposed method is confirmed by performing a comparison with other similar techniques.

Islanding Detection based on Variational Mode Decomposition for Inverter based Distributed Generation Systems

Abstract The ability to detect islanding condition is a mandatory requirement for inverter-based distributed generation system. In this study, an improved islanding detection scheme is developed using the variational mode decomposition (VMD) algorithm. The estimated modal voltage component of PCC voltage is processed through VMD and statistical parameters, namely standard deviation and kurtosis are obtained. The decision making is done based on the deviation of the monitoring parameters from its threshold value. To enhance the robustness of the scheme, the entire analysis is done in a noisy environment. The performance of the scheme is investigated under islanding as well as all possible non-islanding network disturbance conditions. The scheme is found working effectively and can detect the islanding condition within 2 cycles of its inception.

Control Strategy for Distribution Generation Inverters to Maximize the Voltage Support in the Lowest Phase During Voltage Sags

Voltage sags are considered one of the worst perturbations in power systems. Distributed generation power facilities are allowed to disconnect from the grid during grid faults whenever the voltage is below a certain threshold. During these severe contingencies, a cascade disconnection could start, yielding to a blackout. To minimize the risk of a power outage, inverter-based distributed-generation systems can help to support the grid by appropriately selecting the control objective. Which control strategy performs better when supporting the grid voltage is a complex decision that depends on many variables. This paper presents a control scheme that implements a smart and simple strategy to support the fault: the maximum voltage support for the lowest phase voltage. Therefore, the faulted phase that is more affected by the sag can be better supported since this phase voltage increases as much as possible, reducing the risk of undervoltage disconnection. The proposed controller has the following features: 1) maximizes the voltage in the lowest phase, 2) injects the maximum rated current of the inverter, and 3) balances the active and reactive power references to deal with resistive and inductive grids. The control proposal is validated by means of experimental results in a laboratory prototype.

Model Predictive Control of a Voltage-Source Inverter With Seamless Transition Between Islanded and Grid-Connected Operations

Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy.

Adaptive Dynamic Programming and Adaptive Optimal Output Regulation of Linear Systems

This note studies the adaptive optimal output regulation problem for continuous-time linear systems, which aims to achieve asymptotic tracking and disturbance rejection by minimizing some predefined costs. Reinforcement learning and adaptive dynamic programming techniques are employed to compute an approximated optimal controller using input/partial-state data despite unknown system dynamics and unmeasurable disturbance. Rigorous stability analysis shows that the proposed controller exponentially stabilizes the closed-loop system and the output of the plant asymptotically tracks the given reference signal. Simulation results on a LCL coupled inverter-based distributed generation system demonstrate the effectiveness of the proposed approach.

A New Islanding Detection Scheme for Multiple Inverter-Based DG Systems

A new islanding detection technique is proposed for the application in multiple inverter-based distributed generation (DG) systems. In the proposed method, a harmonic current disturbance is imposed to the grid at one of the harmonic frequencies associated with the grid voltage. This is done by changing the equivalent parallel impedance of the grid-connected inverters. The output current of the grid-connected inverters at this specific harmonic frequency is used as the islanding detection signal. The harmonic resonance is imposed over a short time interval, hence the overall power quality is not significantly affected. As opposed to most of the active islanding detection methods, in the proposed method, no interference among parallel DGs is observed, which makes it well suited for the application in systems involving a large number of parallel inverter-based DGs. Comprehensive analyses are performed to investigate various technical aspects of the proposed scheme. The performance of the proposed method is verified using time-domain simulations carried out in Matlab/Simulink environment.

A Novel Islanding Detection Method for Inverter-Based Distributed Generation Systems

In the area of renewable energy technologies, islanding of distributed generation system needs to be prevented due to safety reasons and to ensure quality of power supplied to the customers. Several islanding detection methods based on passive and active schemes have been proposed in the literature. Passive methods have a large non detection zone (NDZ), While active schemes degrade power quality. This paper proposed a new passive islanding detection method which combines the Wavelet-transform and neural network techniques. This method can reduce the NDZ to zero without any perturbation that deteriorates the power quality. The simulation results show that the proposed islanding detection method is effective and robust in all kind of conditions.

Impact of Multiple Inverter Based Distributed Generation Units on Harmonic Resonance

With the growing concerns over environmental impacts and rebate programs offered by governments, small renewable technology based distributed generation (DG) units are increasingly being used in commercial and domestic electric power production. However, a large number of DG units in a distribution system may sometimes contribute to high levels of harmonic distortion even though the emission level of individual DG units comply with the current harmonic standards. In addition, the incorporation of a passive filter within these DG units also has a negative effect due to the resonance phenomenon. Thus, this paper aims to analyse the impact of multiple inverter based DG units on resonance and harmonic distortion. Furthermore, the effect of simplifying DG model by bundling the realistic model of multiple and small DG units into a single unit were investigated. The analysis was performed for an inverter based DG system connected to the IEEE 13 bus distribution test system. To investigate resonance phenomenon, frequency scan was conducted while the fast Fourier transform analysis was carried out to measure the voltage and current distortion at the point of the common coupling and the DG connected buses. The test system was simulated using MATLAB/SimPowerSystems. The result shows that a simplified model of multiple DG units can be used as a DG model for harmonic distortion and resonance phenomenon study.

Ultra Capacitor Based Sensorless Current Control of Grid Connected Inverter for Power Quality Improvement

The economical and environmental impacts of fossil fuels have forced governments and society to investigate sustainable solutions. The interest has focused principally on the green and clean benefits provided by renewable energy sources. The growth rate of new renewable energy supplies being installed has reached the incredible yearly mark of approximately 30% over the last few years. Consequently, investments in research and development in the field of power electronics have increased proportionally, especially in high voltage and high power grid connected systems. This paper presents ultra capacitor based sensorless current control of a three- phase inverter-based distributed generation system. a robust control scheme for high power quality grid connection of inductor-capacitor-inductor (LCL)-filtered distributed generation (DG) inverters. This paper also presents a modified adaptive Kalman filter for sensorless current control of a three-phase inverter-based distributed generation system. The control variable states can be estimated one sample in advance with a reduced number of sensors. Two current control loops with the steepest descent adaptive control for grid voltage estimation have been employed to boost robustness. Stability and dynamic performance have been examined. Simulation and results validate the proposed control approach. Simulation is done using Matlab software.