Grid-connected Operation Modes Research Articles

Power management system for hybrid AC/DC microgrid based on high frequency isolated interlink converter

The hybrid AC/DC microgrid systems have been popular and being developed as the next generation power systems because of the comprehensive combination of both AC and DC microgrid systems. Power management scheme is one of the most critical operation aspects for hybrid microgrids because the system is operating with various generation sources and loads such as renewable energy sources, energy storage systems and AC and DC loads. Therefore, in this research, control strategies and power management scheme is considered for all possible operation modes of standalone and grid-connected conditions. In the existing microgrid system in Electrical Power Engineering department in Yangon Technological University, rooftop PV plant and battery are cooperating to supply the electricity. The inverter in this system allows the unidirectional power flow from DC to AC and there is no specific power management system for grid-connected and standalone operation mode. To provide bidirectional power flow in the existing system, the configuration of converter and control strategies for power management system are developed in this research. To provide the bidirectional power flow between AC bus and DC bus, the bidirectional interlink power converter with high frequency isolation is applied. The control system including the power balancing between generation and demand, DC link voltage control and AC link voltage and frequency control is considered. By applying droop control method in the developed system, power flow balancing between AC bus and DC bus is maintained. And also, using high frequency isolation transformer in interlink converter provides fast response of the system performance and maintaining continuous power supply within each AC system and DC system during disturbance condition in one subsystem. The performance of the proposed power management system is demonstrated by using MATLAB/Simulink.

Review and Comparison of Grid-Tied Inverter Controllers in Microgrids

Grid-tied inverters are widely used for interfacing renewable energy sources or storage devices to low-voltage electrical power distribution systems. Lately, a number of different control techniques have been proposed to address the emerging requirements of the smart power system scenario, in terms of both functionalities and performance. This article reviews the techniques proposed for the implementation of current-controlled or voltage-controlled inverters in microgrids. By referring to a voltage source inverter with an LCL output filter, different control architectures are classified as single, double, and triple loop. Then, the functionalities that are needed or recommended in the grid-connected, islanded, and autonomous operating modes of the grid-tied inverter are identified and their implementation in the different control structures is discussed. To validate the analysis and to better illustrate the merits and limitations of the most effective solutions, six control strategies are finally implemented and experimentally compared on a single-phase, grid-connected inverter setup.

Economic Dispatch of Micro-grid Based on SQP—Validation and Results

An economic dispatch strategy of micro grid based on sequential quadratic programming is introduced and formulated in the authors’ former paper in the context of mathematical model and formulations. In this paper, a small micro grid system in Yantai City, established by Shandong Electric Power Company, is used to investigate the performance of the proposed model in more detail. The results show that: in the independent micro grid operating mode, when the load is small, the operating costs of micro grid have a large difference before and after the control. In the grid-connected operation mode, the micro grid generates as recommended is more economical than full-power generation. Therefore, this paper can achieve the optimal output of each micro source and the economic operation with satisfying the load requirements.

Optimal Siting and Sizing of Grid-Connected Microgrid System for an Educational Institution Using HOMER Software

This paper analyses the sizing of PV system installation in a selected location for optimal operation using Hybrid Optimization Model for Electric Renewable, HOMER software. The main objective is to reduce the total net present cost (NPC) and the size of the system. A grid-connected mode of operation is considered in which the main grid is used as a backup power system. The hourly real-time electric load demands, monthly temperature, and solar irradiance data are collected and used as input for the analysis. A centralized and distributed type of load demand is compared and found the optimal solution which gives the optimal size and costs of the system.

An Intelligent Adaptive Overcurrent Protection System for an Automated Microgrid in Islanded and Grid-Connected Operation Modes

As the penetration of distributed generation sources increases at the distribution level, the necessity of designing microgrids is highlighted. Microgrids can operate in either islanded (separated from the utility grid) or grid-connected mode. Apart from the technical and economic benefits of microgrids, they may cause some protection issues in distribution systems. The most important of these issues is the difference between the short circuit levels in islanded and grid-connected operation modes. In this paper, an intelligent overcurrent relay that can adapt to the microgrid operation mode is proposed; that is, the relay detects the microgrid operation mode by means of an adaptive algorithm. The proposed adaptive method optimizes the accuracy and speed of the algorithm for detecting the microgrid operation mode. Then, the optimal settings that have been already stored in the relay are activated according to the microgrid operation mode. All internal processes of the relay use the voltage and current sampled at the relay location. As a result, the relay can be used in all networks without communication infrastructures. Finally, the proposed method is implemented in a sample microgrid. The superiority of the intelligent relay performance over conventional overcurrent relays is shown by numerical results.

Towards Grid of Microgrids: Seamless Transition between Grid-Connected and Islanded Modes of Operation

With the ever-increasing number of blackouts in distribution systems arising from a variety of natural and manmade disasters, the frequent and necessary isolation/reconnection of loads without power deviations/fluctuations has become an important issue. Grid of microgrids (MG)s is a promising solution towards a highly resilient and efficient power grid operation. To facilitate this implementation, seamless transition with the utility grid is a key feature the today's MG control needs to possess. This transition ability is of great prominence, especially after the accelerated adaptation of distributed renewable energy sources (RES) in MGs. This ability of the MGs should ensure uninterrupted energy services to critical loads and infrastructures. Thus, the implementation of MG control strategies to enable smooth transition between grid-connected (GC) and islanded (IS) operation modes is mandatory. The control scheme implemented should therefore be capable of mitigating the stirring voltage/current deviations due to frequency/phase misalignment during the transition process. This paper provides an overview of the various MG control schemes that enable seamless transition between GC and IS modes of operation. The main purpose of this paper is to provide a generic overview of the challenges and existing techniques available in literature to mitigate the voltage and frequency ( V-f ) fluctuations at the MG's point of common coupling (PCC) and that of the utility grid; during the transition process. It aims at motivating the development of advanced control schemes in this area of MG related research.

Average Model-Based Feedforward and Feedback Control for PUC5 Inverter

This paper proposes new average model based control strategies for a 5-level Packed U-cell (PUC5) inverter in both standalone and grid-connected modes of operation. First, a simple feedforward controller (FFC) is designed, using only two pulse width modulation (PWM) carrier signals, for the PUC5 inverter operating in standalone mode. This proposed control technique ensures self-balanced operation with high steady-state performance. Moreover, the employment of the proposed FFC leads to a decrease in the capacitor’s value as well as the minimization of the Total Harmonic Distortion (THD). Then, a feedback linearizing control technique is designed to improve the transient and steady-state performances. In grid-connected mode, a reduced-sensor technique based on the FFC and the state feedback (FC) techniques was applied. Simulations and experimental results are presented to prove the high performance of the proposed solutions for standalone and grid-connected operating modes.

An Implementation of Solar PV Array Based Multifunctional EV Charger

In this article, an implementation of solar photovoltaic (PV) array powered grid-connected residential electric vehicle (EV) charger is presented, which caters the need of an EV, household loads, and the grid. The charger is enabled to operate autonomously using a PV array for providing an uninterruptible charging and power to household loads. However, in the absence of the PV array or insufficient PV array generation, the grid-connected mode of operation is presented. Moreover, the charger is supported with the synchronization and seamless mode switching control, so that the charger automatically connects/disconnects from the grid without disturbing the EV charging and household supply. The charger is also enabled with the vehicle-to-grid active/reactive power support to the grid and vehicle-to-home power transfer for supporting the local loads in an islanded condition. The charger is also controlled to operate as an active power filter for achieving the unity power factor operation and total harmonic distortion of the grid current within 5%. Moreover, for achieving energy management, a dc-link voltage regulation based energy management strategy is used and a sliding mode control is used for regulating the dc-link voltage. For satisfactory operation under distorted voltage condition, a second-order generalized integrator frequency-locked loop with dc offset rejection is used to generate the sinusoidal reference grid current. The charger is designed for a single-phase 230-V, 50-Hz grid and it is experimentally validated in the laboratory.

Multi-time Scale Optimal Power Flow Strategy for Medium-voltage DC Power Grid Considering Different Operation Modes

Direct current (DC) power grids based on flexible high-voltage DC technology have become a common solution of facilitating the large-scale integration of distributed energy resources (DERs) and the construction of advanced urban power grids. In this study, a typical topology analysis is performed for an advanced urban medium-voltage DC (MVDC) distribution network with DERs, including wind, photovoltaic, and electrical energy storage elements. Then, a multi-time scale optimal power flow (OPF) strategy is proposed for the MVDC network in different operation modes, including utility grid-connected and off-grid operation modes. In the utility grid-connected operation mode, the day-ahead optimization objective minimizes both the DER power curtailment and the network power loss. In addition, in the off-grid operation mode, the day-ahead optimization objective prioritizes the satisfaction of loads, and the DER power curtailment and the network power loss are minimized. A dynamic weighting method is employed to transform the multi-objective optimization problem into a quadratically constrained quadratic programming (QCQP) problem, which is solvable via standard methods. During intraday scheduling, the optimization objective gives priority to ensure minimum deviation between the actual and predicted values of the state of charge of the battery, and then seeks to minimize the DER power curtailment and the network power loss. Model predictive control (MPC) is used to correct deviations according to the results of ultra short-term load forecasting. Furthermore, an improved particle swarm optimization (PSO) algorithm is applied for global intraday optimization, which effectively increases the convergence rate to obtain solutions. MATLAB simulation results indicate that the proposed optimization strategy is effective and efficient.

Parameters analysis and operation area calculation of VSG applied to distribution network

Virtual synchronous generator (VSG) technology is an effective way to realize the coordinated energy supply of active distribution network and comprehensive energy at present. This paper starts from the two perspectives of grid-connected operation mode and island operation modes. Based on the mathematical model of VSG, referring to the existing grid standards, comprehensively considering its active and reactive-power loop stability and dynamic performance, the influence factors of the related control parameters K, D q , J, D p are analyzed. And the selection range of K, D q , J, D p are proposed, which ensures the stability and robustness in grid-connected and island operation modes. In this paper, the method can be used as a reference for the control strategy research and practical engineering application of VSG. Finally, the effects of parameter selection on the stability and dynamic performance of active and reactive-power loops are analyzed by using the zero-pole map and Bode diagram, and the feasibility of the scheme is verified by PSCAD / EMTDC.

23-level Single DC Source Hybrid PUC (H-PUC) Converter Topology With Reduced Number of Components: Real-Time Implementation With Model Predictive Control

In this paper, a new configuration of single DC source hybrid packed U-cell (H-PUC) converter with reduced number of components is proposed. The proposed H-PUC only requires one dc source, twelve power switches, and three capacitors to provide 23-level output voltage. It is comprised of two high voltage low frequency (LF) and low voltage high frequency (HF) sub-modules which leads to less power losses and higher efficiency of the proposed H-PUC converter. Moreover, a finite control set model predictive control (FCS-MPC) method is proposed to generate 23-level staircase output voltage with low THD and to regulate voltages of three capacitors to their desired values simultaneously. A real-time model of the proposed 23-level H-PUC converter and its suggested FCS-MPC are developed and implemented in OPAL-RT OP4510 platform to evaluate and validate the feasibility of the proposed H-PUC in grid-connected operation mode. The provided real-time implementation results verify and demonstrate the performance and viability of the proposed 23-level H-PUC and its associated FCS-MPC to provide low THD 23-level output voltage and all three capacitors voltages balancing.

Operation Mechanism and Strategies for Transactive Electricity Market With Multi-Microgrid in Grid-Connected Mode

Under the policy support of electricity market reform and the promotion of microgrids in grid-connected operation mode construction in China, the sales side of electricity market is gradually liberalized. Microgrids are developing rapidly through participating in the electricity market competition. In order to find the multi-player game relationship of real-time electricity market with multi-microgrid in grid-connected operation mode and provide an incentive for electricity trading among microgrids, we propose a transactive real-time market trading mechanism based on the principle of proportional distribution. Through the analysis of multi-player game in real-time market, we build cost-benefit models of microgrids with power surplus and those with power shortage. Based on the game theory, non-cooperative game models with multi-buyer and multi-seller of microgrids with power surplus and those with power shortage are established. The existence of Nash equilibrium for the game models of microgrids with power surplus and those with power shortage has been proved and the Nash equilibrium is solved to obtain their optimal bid and selling strategies. The feasibility and validity of the proposed mechanism and models are proved through case study. The result demonstrates that all microgrids can obtain the best profits with their optimal strategies.

Relay Coordination and Optimization techniques using DWT-Differentiation Algorithms for Fault Detection in Microgrid

The microgrid is assuming an indispensable role in the power area and greatly affects inexhaustible reconciliation. There are numerous specialized troubles to move so as to utilize high ability in the microgrid. The significant issue in the microgrid has its protection challenges as the bidirectional progression of current moves through transports. This paper proposes novel Discrete Wavelet Transform-Differential Algorithm Techniques, which help to secure Distributed Generations and other costly electronic parts from the defective condition. The occurrence of any shortcoming on the microgrid causes alters in amplitude and course, which prompts a hamper on the surviving protection framework. In the proposed scheme, short circuit current is pre-processed through discrete wavelet transform (DWT) transform to remove all noise and high pass and low pass content in the signal and undergoes state-of-art of adaptive protection for selection of relay. This algorithm implements on the Standard IEEE 9 bus system in MATLAB environment and with a single line to ground fault at 8 locations tested with pair of 21 relays for primary and backup protection in the grid-connected and separated mode of operation. The traditional methods compared with the proposed method in terms of Different parameters like PS (plug setting ), TDS (time dial span), CTD (coordination time duration), Top (operating time of relay). To prove the better implementation of results, one real-time system as a case study selected with the hybrid microgrid.

A Protection Method for Inverter-based Microgrid Using Current-only Polarity Comparison

The design of an effective protection system for inverter-based microgrids is a complicated engineering challenge. This is due to the fact that inverters have limited fault current capabilities, and that the conventional overcurrent protection is not suitable for inverter-based microgrids. This paper introduces a novel protection method for inverter-based microgrid using a current-only polarity comparison. The proposed method is based on the phase difference between the pre-fault and fault current components. The method responds to faults in both grid-connected and autonomous operation modes and provides a new way to identify faulted sections. Simulations of an inverter-based microgrid with a relay model are conducted using PSCAD/EMTDC software. The results show that the proposed method can detect faults in inverter-based microgrids.

Optimal power routing scheme between and within interlinking converters in unbalanced hybrid AC–DC microgrids

An optimal power routing (OPR) scheme between and within interlinking converters (ICs) in unbalanced hybrid AC–DC microgrids to minimise the power imbalance factor at the point of common coupling, active power losses, and voltage deviation indices for microgrids in grid-connected operating mode is proposed in this study. These goals are achieved through a multi-objective optimisation model by optimal distributing of mobile loads between available charging stations and at the same time, OPR within three phases of three-phase four-lag AC/DC converters. Numerical results obtained from implementing the proposed method on the modified IEEE 13-bus system, as an unbalanced hybrid microgrid, and IEEE 34-bus test system, as an unbalanced distribution system, demonstrate that proposed OPR algorithm is successful to satisfy the optimisation goals. For this purpose, four case studies are defined and studied to demonstrate the unique features of the proposed OPR comparing with other power routing schemes. In addition to simulation results, the OPR scheme between ICs is realistically implemented at Florida International University smart grid testbed to show the effect of the power routing on energy losses reduction.

Research on Peer-to-Peer Control Strategy for Microgrid Distributed Generation

In between a plurality of microgrid in complex coordination control problems, this paper proposed a droop control strategy for microgrid distributed generation, which can realize micro grid by cutting load, micro grid operation mode switch to smooth the distributed power supply voltage and frequency control. This method bases on droop control principle, research on the effects of P-f and Q-U droop characteristics, design of the droop controller output filter, power controller, voltage current double loop controller based on the established simulation model of the droop controller and the equivalent control strategy of micro grid simulation model. The results of simulation by Matlab/Simulink, analysis of the variation of the micro grid distributed power switch in islanded and grid connected operation mode and the frequency voltage and power, to verify the correctness and feasibility of droop control strategy.

A Unified Control Structure for Grid Connected and Islanded Mode of Operation of Voltage Source Converter Based Distributed Generation Units Under Unbalanced and Non-Linear Conditions

This manuscript develops a unified control structure for Distributed Generation (DG) units based on Voltage Source Converters considering unbalanced and non-linear operating conditions. This control structure works for both the Islanded and the Grid-connected modes of operation of the Micro-Grid (MG). The objective of this control scheme is to regulate the line currents of the DG unit in such a manner that the voltage at the Point of Common Coupling (PCC) remains balanced despite the line currents of the DG unit being unbalanced and distorted. Multiple adaptive P-R controllers have been proposed for the current control loop of the Voltage Source Converter (VSC). These controllers have been implemented with resettable integrators so as to limit the DC components in the post fault current of the VSC. The Battery Energy Storage System (BESS) is interfaced to the DC link of the VSC through bi-directional dc-dc converters. An improved control structure for the bi-directional dc-dc converter has been developed. The effectiveness of these control structures have been presented and tested in PSCAD/EMTDC in an IEEE 34 node distribution system model being fed by two identical DG units.

Rotor Electrical Fault Detection of Wind Turbine Induction Generators Using an Unscented Kalman Filter

This paper presents a fault detection method for induction generators using an unscented Kalman filter. In some applications such as wind turbine energy conversion systems, the induction generators experience oscillations caused by wind speed variation; therefore, the fault detection method should be able to detect fault in the presence of these oscillations. On the other hand, the induction generator may operate in islanded mode of operation, the situation that does not happen in motor application. In the present paper, the type of fault is the rotor electrical asymmetry and the proposed method is able to detect the fault in both grid-connected and islanded modes of operation. Several scenarios are simulated, and the efficacy of the proposed technique is evaluated by means of simulation results. Moreover, experiments are performed to validate the ability of the proposed method.

New hybrid‐microgrid topology using a bidirectional interleaved converter as a robust power interface operating in grid‐connected and islanded modes

This study presents a new microgrid topology that uses a bidirectional interleaved converter performing a power interface between DC buses in a hybrid microgrid allowing for both grid-connected and islanded modes. The authors propose a new control strategy and controllers' design method aiming at achieving a high-performance dynamic response regarding the converter load and generation disturbance rejection capability. In the grid-connected mode, the interleaved converter operates in the buck mode providing a high-power-quality DC microgrid voltage. In the islanded mode, the interleaved converter operates in the boost mode and it is responsible for regulating the DC link of the back-to-back converter that connects the main grid to the AC microgrid. A detailed mathematical model is presented to obtain a MIMO system that takes into account the system's disturbances to analyze both stability margins and disturbance-rejection response. Simulations of the proposed topology are carried out in PSCAD/EMTDC in a microgrid operating in grid-connected and islanded operation modes. Experimental results are provided in order to validate the proposed control tuning method.

Unidirectional isolated high‐frequency link DC/AC converter for grid integration of DC sources

Most of the converters used for the grid integration of DC resources are multi-stage DC/AC converters with a huge intermediate DC-link capacitor. This bulky DC-link capacitor increases the converter volume and cost, reduces its lifetime and reliability and, causes converters fault ride-through problems. For this purpose, DC/AC converters without any intermediate energy storage components are proposed in the literature named high-frequency AC (HFAC) link converters. However, due to a large number of active switches, modulation and commutation methods of these converters are overly complex compared with their conventional multi-stage versions. In this study, a new isolated unidirectional converter without any intermediate energy storage components, and with a reduced number of active switches are proposed for the grid integration of DC sources. The proposed HFAC converter structure is simple but more efficient and reliable than its conventional isolated multi-stage and HFAC versions. The converter modulation and commutation methods which are more simple than those of its conventional HFAC version, and its voltage and current gains are also presented. The performance of the proposed converter is verified through simulation in the islanded and grid-connected modes of operation. A simple experimental setup is also provided to validate the simulated converter.