Grid-connected Operation Modes Research Articles

A New Impedance-Based Main and Backup Protection Scheme for Active Distribution Lines in AC Microgrids

Microgrids active characteristics such as grid-connected or islanded operation mode, the distributed generators with an intermittent nature, and bidirectional power flow in active distribution lines lead to malfunction of traditional protection schemes. In this article, an impedance-based fault detection scheme is proposed as the main protection of microgrids by applying the proposed equivalent circuits for doubly-fed lines. In this scheme, relay location data and positive sequence voltage absolute value of the other end of the line are used. It can detect even high impedance faults in grid-connected and islanded modes. It is robust against load and generation uncertainties and network reconfigurations. Low sampling rate and minimum data exchange are among the advantages of the proposed scheme. Moreover, a backup protection scheme based on the conductance variations is suggested. No requirement for the communication link is a distinguished advantage of the proposed backup protection scheme. The proposed schemes have been simulated using PSCAD and MATLAB software and the results confirmed their validity.

An Adaptive Parameter-Based Control Technique of Virtual Synchronous Generator for Smooth Transient Between Islanded and Grid-Connected Mode of Operation

This paper deals with the development of a control technique for the control of voltage source converters (VSCs) as an interface between renewable energy sources (RESs) and power grids. The lack of inertia in converter-based generators and their unconventional dynamic behavior increase the negative impacts on the power grid, unlike the operation of the conventional synchronous generators (SGs). In this sense, the proposed control technique emulates the behavior of SGs in the control loop of the interfaced converter acting as a grid-forming power generator. Thus, supportive functionalities for enhancing electric grid stability (e.g., frequency oscillation damping) will be provided by interfaced converter as a virtual synchronous generator (VSG). The main targets of the proposed control technique include virtual inertia emulation, frequency oscillation damping, primary and secondary frequency regulation, and voltage regulation. The main contribution of this work over the existing methods is that the optimal values for swing equation parameters are employed to ensure well-designed damping feature of the VSG-based converter which adapts grid-connected and islanded operating modes, enabling a smooth resynchronizing to the grid as its plug-and-play capability requirement. The control structure is evaluated through simulation in MATLAB/Simulink.

Power Coordinated Control of Micro Sources of MMC Series Microgrid Based on Model Prediction

To improve the utilization rate of micro sources of MMC series structured microgrid in a gridconnected operation mode, a prediction-based coordination control strategy of micro-source power of bridge arm is proposed in this paper. The connection between the output power and the delta carrier is determined by analyzing each generation unit’s output power. Each micro source’s output power is controlled independently by using the carrier-wave variable amplitude phase-shift modulation method. A finite control set-model predictive control algorithm is used to predict the next cycle’s power using the carrier amplitude of the current cycle as the controlled input. The optimal switching state for the next cycle is determined according to the coordination principle of “more work for more energy.” Through simulation, the coordination strategy can effectively control the maximum power output of the bridge arm and improve the micro source’s utilization rate.

Fault Protection in Microgrid Using Wavelet Multiresolution Analysis and Data Mining

The protection problems in microgrid effect the reliability of the power system caused due to high distributed generator penetrations. Therefore, fault protection in microgrid is extremely important and needs to be resolved to enhance the robustness of the power system. This manuscript proposes a combined signal processing and data mining-based approach for microgrid fault protection. In this study, first the multiresolution decomposition of wavelet transform is employed to preprocess the voltage and current signals to compute the total harmonic distortion of the voltage and current. Then, the statistical indices of standard deviation, mean, and median of the total harmonic distortion and the negative sequence components of active and reactive power are used to collect the input data. After that, all the available data is provided to the random forest-based classifier to evaluate the efficiency of the proposed scheme in terms of the detection, identification, and classification of faults. This study used different aspects for data collection by simulating various fault and no-fault cases for both looped and radial configurations under grid-connected and islanded modes of operation. The simulations were performed on a standard medium voltage microgrid using MATLAB/SIMULINK, whereas the analysis for testing and training of the random forest were conducted in Python. It is recognized that the proposed method performed better than support vector machines and decision tree that are reported in the literature. The results further demonstrate that the proposed method can also detect simultaneous faults, and it is also effective against measurement noise.

Sliding Mode Control of Hybrid Renewable Energy System Operating in Grid Connected and Stand-Alone Mode

Abstract This paper studies innovative application of sliding mode control (SMC) for a Hybrid Renewable Energy System (HRES) in grid-connected and autonomous modes of operation. The considered HRES includes a photovoltaic (PV), wind turbine (WT) based on a Permanent Magnet Synchronous Generator (PMSG). The PV generator is coupled to the common DC bus via a DC/DC converter. The latter is controlled by an MPPT algorithm based on the Adaptive Perturbation and Observation Algorithm Method (APOAM) to search the optimum working of this source. A SMC is utilized to manage the PV voltage to achieve the Maximum Power Point (MPP) by altering the obligation duty cycle. The battery interfaced by a bidirectional buck-boost DC/DC converter can be charged or discharged depending on the production situation. On the one hand, the wind turbine conversion chain is equipped with a PMSG and a rectifier controlled to regulate the operating point of the wind turbine to its optimum value. During a Stand-Alone Mode (SAM) operation, the Voltage Source Converter (VSC) was used for controlling the output voltage in terms of amplitude and frequency delivered to the AC load. However, in Grid-Connected Mode (GCM) operation, the VSC was adapted to control the electrical parameters of the grid. To better appreciate the advantages of the proposed SMC approach, we have proposed a series of comparative tests with the conventional PI control in the operating modes GC and SA and under different scenarios. The proposed control strategy has undeniable advantages in terms of control performance and very low total harmonic distortion THD value compared with the conventional PI control. Finally, It is concluded that the proposed approach improves the quality and provides a stable operation of the HRES.

A pilot-based unit protection scheme for meshed microgrids using apparent resistance estimation

Microgrids (MGs) are very attractive for electrical power generation/consumption owing to increased reliability and resiliency, however, protection against different short-circuit faults is a major challenge in implementation. This paper proposes a simple algorithm for on-line estimation of apparent resistance per-phase of a three-phase ac power line. The idea is to compare the estimated apparent resistance with the real resistance per-phase of the power line. Then, any reasonable difference between the two resistances will indicate a short-circuit fault occurrence along the line; hence, the line has to be tripped immediately. This pilot-based unit protection scheme presents salient advantages when applied to protect the power lines in the meshed MGs. Effectiveness of the proposed protection scheme is verified by simulation in the MATLAB/SIMULINK environment using a test MG with inverter-based distributed generations. The results confirm reasonable performance of the protection scheme for the meshed MGs with various topologies under both the grid-connected and islanded modes of operation and with different faults including the symmetrical/asymmetrical short-circuit faults, high impedance short-circuit faults, and close-in-internal short-circuit faults.

Highly applicable small hydropower microgrid operation strategy and control technology

In this paper, a small hydropower microgrid solution with high applicability is proposed to solve the problem of high line failure rate and long maintenance time, which can improve the reliability of distribution network in mountainous areas. In this scheme, small hydropower and local load constitute a low-cost but efficient microgrid operation to improve the reliability of rural electricity. In this scheme, small-sized hydropower and local load constitute microgrid operation. In case of failure, the scheme makes full use of the independent operation of microgrid to provide qualified power quality to local load. This scheme is easy to realize and does not need additional energy storage, so it has good economy. Firstly, the frequency disturbance characteristics of small hydropower microgrid is analyzed, which lays a theoretical foundation for small hydropower microgrid operation. Then it introduces the island operation mode to grid-connected operation mode transition of small hydropower, including the key technologies such as frequency adjustment strategy in island operation stage. Finally, the simulation studies show the effectiveness of the method.

Multi-year load growth-based optimal planning of grid-connected microgrid considering long-term load demand forecasting: A case study of Tehran, Iran

Although much efforts have been devoted to the optimal design of the energy systems, there is a research gap about the multi-year load growth-based optimal planning of microgrids. This paper tries to fill such a research gap by developing a novel method for the optimal design of the grid-connected microgrids based on the long-term load demand forecasting. The multilayer perceptron artificial neural network is used for time-series load prediction. The impacts of the annual load growth are analyzed under various cases based on the consideration and determination methods of yearly load growth. The proposed method is applied to an actual microgrid in Tehran, Iran, using HOMER (Hybrid Optimization of Multiple Energy Resources) software. The load modeling’s capabilities of HOMER software, as a well-known software for the optimal design of energy systems, are used, which have received less attention. Since most existing research works in Iran focused on the off-grid operating mode, the study of an actual microgrid under grid-connected operating mode is one of the most contributions of this paper. The comparison of the obtained results and other available methods illustrate the impacts of the adequately precise estimation of annual load growth in the design of energy systems.

Real Time-Based under Frequency Control and Energy Management of Microgrids

In this paper, an efficient under frequency control and the energy management of a distributed energy resources (DERs)-based microgrid is presented. The microgrid is composed of a photovoltaic (PV), double-fed induction generator (DFIG)-based wind and diesel generator with critical and non-critical loads. The system model and the control strategy are developed in a real time digital simulator (RTDS). The coordination and power management of the DERs in both grid-connected and islanded operation modes are implemented. During power imbalances and frequency fluctuations caused by fault or islanding, an advanced automatic load shedding control is implemented to regulate and maintain the microgrid frequency at its rated value. One distinct feature implemented for the load shedding operation is that highly unbalanced critical loads are connected to the microgrid. The diesel generator provides the required inertia in the islanded mode to maintain the microgrid rated frequency by operating in the isochronous mode. The International Council on Large Electric Systems (CIGRE) medium voltage (MV) test bench system is used to implement the DERs and their controller. The proposed control approach has potential applications for the complete operation of microgrids by properly controlling the power, voltage and frequency in both grid-connected and island modes. The real time digital simulator results verify the effectiveness and superiority of the proposed control scheme in grid connected, island and fault conditions.

Virtual Power-Based Control for Operation and Grid Synchronization of Induction Generator

A novel approach on soft transition from stand-alone (SA) mode to grid-connected (GC) mode for uncontrolled prime mover-driven squirrel cage induction generator (SCIG) is presented in this article. The distributed generation unit (DGU), consisting of SCIG along with back-to-back converter, is initially controlled to supply SA load with fixed voltage and frequency. A virtual power-based control (VPC) is developed for the DGU to ensure zero power exchange with the grid, for smooth grid synchronization (GS). The proposed controller is generating the required magnitude and angle of the inverter voltage without use of the phase-locked loop. The load-side voltage source converter is controlled by the VPC during GS and GC operation. Variation in power reference, in GC mode of operation, is also established with necessary change in the angle of the inverter voltage vector. The machine-side voltage source converter is controlled by three-stage control technique, to maintain the dc-link voltage, throughout the operation. Implementation of the proposed control is performed in simulation environment successfully. The effectiveness of the control is demonstrated experimentally on a scaled laboratory prototype.

Photovoltaic power plants in electrical distribution networks: a review on their impact and solutions

Photovoltaic (PV) technology is rapidly developing for grid-tied applications around the globe. However, the high-level PV integration in the distribution networks is tailed with technical challenges. Some technical challenges concern the stability issues associated with intensive PV penetration into the power system are reviewed in this study. To mitigate the voltage disturbances in a system with massive PVs integration, some techniques are devoted such as frequency regulation techniques, active power curtailment, reactive power injection (RPI), and storage energy. Also, with a high penetration level of distributed generators, the potential of dynamic grid support is discussed. Islanding operation and microgrid, operating using different control techniques, which ensure a smooth transition between grid-connected and islanded operation modes as well as synchronisation between the two modes, are discussed.

A Composite Magnitude-Phase Plane of Impedance Difference for Microgrid Protection Using Synchrophasor Measurements

This article presents a protection scheme based on the magnitude-phase plane of impedance difference (ID) using wide-area positive sequence components of current and voltage signals for the microgrid. The computation of the ID of the line is performed by extracting signals from the phasor measurement units complied with IEEE C37.118 standards. The composite magnitude-phase plane obtained from the magnitude of the ID and the angle of ID is considered as the key index for the detection of faults. The feasibility to include the proposed microgrid protection technique is examined by the extensive fault cases simulation with substantial variation in fault conditions, including types of fault and fault at different line length and for the grid-connected and islanding modes of microgrid operation. The fault study is carried out on a medium-voltage 15 bus test microgrid and a standard IEEE 34 bus microgrid testbed. The impact of several critical no-fault conditions is also investigated to analyze the possibility of false operation. The proposed ID-based scheme is extensively tested on MATLAB/Simulink platform and validated on the real-time digital simulator. The test results indicate that the magnitude-phase plane of the ID-based scheme can provide a dependable microgrid protection measure.

Unified Controller for Bimodal Operation of Cuk Converter Assisted SEIG-Based DC Nanogrid

This article proposes a unified controller for Cuk converter assisted variable speed wind turbine driven self-excited induction generator (SEIG) in dc nanogrid configuration for rural electrification. Cuk converter assisted SEIG based wind energy conversion system (WECS) is considered as a master source in the proposed nanogrid, which operates in both grid-connected and islanded mode of operation. In grid connected mode, the system operates in maximum power point tracking (MPPT) under varying wind velocity. A single sensor, modified perturb and observe (P&O) MPPT algorithm, which is independent of the generator parameter is developed and implemented through Cuk converter. During a fault, the WECS necessitates to operate in islanded mode with voltage regulation to supply the local loads continuously. The unified control strategy is developed for selecting the mode of operation, which are MPPT and voltage regulation in grid-connected and islanded mode, respectively. Harmonic spectrum analysis is carried to justify the selection of diode bridge rectifier over semicontrolled/controlled rectifier. The effectiveness and feasibility of the proposed system is verified by means of MATLAB/Simulink. To demonstrate the operational capability, a laboratory scaled prototype is fabricated and tested under various dynamic scenarios.

Comprehensive power management scheme for the intelligent operation of photovoltaic‐battery based hybrid microgrid system

This study proposes a comprehensive power management scheme (CPMS) for the economical and smart operation of a photovoltaic (PV) battery based Hybrid Microgrid (HMG) system. The solar-PV, DC-loads and battery storage are interfaced to DC-bus of the HMG. The battery is integrated into the DC-bus using a bidirectional energy storage converter (BESC). The voltage and frequency of AC-bus are regulated by utility grid and bidirectional interlinking converter (BIC) during grid-connected and islanded modes of operation, respectively. The proposed CPMS obviates high-speed communication link between converters for maintaining the system stability during power disturbances. Therefore, the vulnerability to collapse due to communication link failure is eliminated, thereby enhancing system reliability. The operational performance of HMG is improved with the proposed CPMS by providing additional features (minimisation of operating cost of the HMG, enhancement in voltage regulation of the DC-bus etc.) using low-speed communication network. The proposed CPMS can also be extended to a larger HMG system by utilising the full bandwidth of the communication network. Further, the proposed CPMS enables HMG to seamlessly switch from grid-connected to islanded mode and vice-versa. Experimental results validating the efficacy of the proposed CPMS are presented.

Improvement of the power quality of single‐phase grid‐connected inverter by filter‐based control scheme

In this study, a filter-based control scheme is developed for a single-phase grid-connected inverter system with the local load. Improving the quality of the local load voltage in the grid-connected mode and injecting clean current to the grid at the same time is the main objective of the proposed scheme. Moreover, unity power factor at the grid side for different types of the local load is ensured by the proposed controller. Furthermore, this control approach ensures the seamless transfer between grid-connected and stand-alone operation modes without adjusting the controller structure and without any resynchronisation scheme. The proposed scheme is validated by the Lyapunov stability analysis. Moreover, an experimental testbed has been implemented to further validate the controller in the real-time environment, as well as, the performance is compared to a conventional approach.

A protection scheme for multi-distributed smart microgrid based on auto-cosine similarity of feeders current patterns

There are many technical challenges associated with microgrid (MG) operation and reliability. A fast and reliable protection scheme is persistently needed either for a grid-connected or island mode of operation. This paper presents a novel communication-less directional protection scheme for a typical microgrid at the common bus (PCC) supplied by hybrid (AC/DC) renewable distributed generations (DGs). This protection scheme is based on the continuously measuring of each DG phase's current feeder and calculating the auto cosine similarity factors for the sampled current with movable window patterns concept. The selected window width is ¼ cycle and the proposed algorithm is applied between the measured window and its simultaneous stored previous cycle window. Using a movable pattern enables consideration of the intermittent generation of the different DG units. Applying the cosine similarity for the proposed relay action increases the MG reliability. The magnitudes indicate the fault incident while the sign is used for the fault feeder selection. The main advantages of the introduced technique are fast, sensitive and do not need any extra instruments or communication links for practical implementation. Besides, it eliminates the effects of fault impedance and loads changing on the digital relay efficiency.

Microgrid Protection Strategy Based on the Autocorrelation of Current Envelopes Using the Squaring and Low-Pass Filtering Method

To resolve the protection issues caused by high penetration of distributed energy resources, this paper proposes an efficient protection scheme for microgrids based on the autocorrelation of three-phase current envelopes. The proposed strategy uses a squaring and low-pass filtering approach for evaluating the envelope of the current signal. Then, the variance of the autocorrelation function is used to extract the hidden information of the distorted envelope to detect the fault signatures in the microgrid. Furthermore, the reactive power is used for determining the fault direction. The performance of the proposed protection scheme was verified on a standard medium-voltage microgrid by performing simulations in the MATLAB/Simulink environment (Version: R2017b). The proposed scheme was shown to be easy to implement and have good performance under looped and radial configuration for both grid-connected and islanded operation modes. The simulation results showed that the scheme could not only detect, locate, classify, and isolate various types of short-circuit faults effectively but also provide backup protection in case of primary protection failure.

Unbalance mitigation strategies in microgrids

Unbalance or asymmetry in the distribution network is a well-known power quality issue. In the modern active distribution system, with the increasing penetration of renewables, this phenomenon becomes more pronounced. In the context of microgrids (MGs), several works have been proposed for the management and mitigation of the unbalance, for both the sharing of unbalanced load and maintaining the voltage quality in the islanded mode and for the control of distributed generators in the grid-connected mode during unbalanced conditions. This study comprehensively reviews, summarises, and classifies the various strategies of the unbalance mitigation techniques for the islanded and grid-connected modes of operation for three-phase MGs and presents the possible challenges and avenues for future investigations on the topic.

Smart overcurrent relay for operating in islanded and grid-connected modes of a micro-grid without needing communication systems

As the penetration of distributed generation resources in distribution networks increases, the need for designing micro-grids also increases. Micro-grids can operate in islanded and grid-connected modes. When a microgrid is connected to the main grid, it economically exchanges power with the upstream grid. However, in the islanded mode, the micro-grid can provide its own local demand. In addition to the technical and economic advantages due to the presence of micro-grids, there are some issues about the protection of distribution systems. The most important issue is the short circuit level in both islanded and grid-connected modes of operation. In this paper, a smart overcurrent relay is proposed that can adapt itself to the operation modes of a micro-grid. This relay detects the operation mode of the micro-grid using an adaptive algorithm. Two main features of the suggested adaptive method are the optimized accuracy and higher execution speed of the algorithm in determining the operation mode of the micro-grid. The optimal setting that is predefined in the relay is then activated based on the corresponding mode. The entire processing is performed in the relay itself by sampling the voltage and current of the relay line. Consequently, this relay can be installed and used in any location of a network without needing any communication infrastructure. Finally, the proposed method is implemented on a micro-grid. According to the numerical results, the coordination performance superiority of this smart relay over conventional overcurrent relays is proven.

Design of Networked Microgrid‟s Control towards Power Coordination and Voltage Frequency Regulation

One of the state-of-the-arts of smart grid system is the concept of networked microgrid that provides flexible integration and coordination of distributed renewable energy resources in multiple microgrids. However, due to the variable nature of renewable generations, load changes and grid contingencies, the system suffers from significant fluctuations of power, voltage and frequency. This paper seeks to present comprehensive converter control solutions for power sharing coordination and stabilization of voltages and frequency to ensure high power quality and reliability in a networked microgrid system regardless of its grid-connected and islanded modes of operation. The control solutions are provided in d-q axes for both active and reactive powers. The multilevel inverter topology that is designed to interface the distributed energy resources results in better suppression of output harmonics compared to that with a conventional 2-level inverter. The paper also features an interlinking algorithm framework of particle swarm optimization (PSO) to tune the control parameters for the optimized performance of converters. A prototype of grid-connected system of three microgrids is simulated in PSCAD platform to validate the efficacy of the proposed solution.