Entire Microgrid Research Articles

Verifikacija modela distribuiranih izvora energije za proračune kratkih spojeva mikromreža

The emerging microgrids are mainly powered by renewable distributed energy resources (DERs), such as solar and wind, with batteries as a backup power. These DERs are decoupled from the grid by inverters and thus, their fault currents are dictated by the control strategies programmed in the inverter itself. The inverters’ control strategies are in most cases dictated by the Grid Code requirements, in order to help the microgrid ride through the fault as painless as possible. Thus, in order to have accurate results for microgrid fault calculations, crucially important for setting the relay protection and protecting the entire microgrid, these DERs must be accurately modelled. Unfortunately, these models have not yet been fully developed nor standardized. In this paper, a recently developed model for inverter-based DERs are integrated into the fault current calculation based on the IEC 60909 standard for fault calculations and tested in the state-of-the-art hardware-in-the-loop environment. The test results are very promising, which opens the possibility to standardize these novel models, filling the seriously dangerous gap of not having the standardized fault models for inverter-based DERs.

Control design and small-Signal stability analysis of inverter-Based microgrids with inherent current limitation under extreme load conditions

A novel droop controller that ensures a desired inverter current limitation and guarantees the stable operation of inverter-based microgrids under extreme load conditions, is proposed in this paper. Opposed to existing dq framework-based droop controllers that align the output voltage on d axis, here the inverter current is aligned on d axis in order to achieve two main goals: i) limitation of the RMS value of each inverter current during transients, without a need for saturation units that guarantee only steady-state limitation and require adaptation techniques for adjusting their limits and ii) a rigorous proof of closed-loop system stability for the entire microgrid. In particular, the proposed approach significantly simplifies the stability analysis of the microgrid, since it can be investigated through a Jacobian matrix of reduced size. Simulation results are given to highlight the superiority of the proposed controller when compared to a conventional droop controller under extreme load conditions, while experimental validation in a lab-scale microgrid is also provided.

Novel Improved Adaptive Neuro-Fuzzy Control of Inverter and Supervisory Energy Management System of a Microgrid

In this paper, energy management and control of a microgrid is developed through supervisor and adaptive neuro-fuzzy wavelet-based control controllers considering real weather patterns and load variations. The supervisory control is applied to the entire microgrid using lower–top level arrangements. The top-level generates the control signals considering the weather data patterns and load conditions, while the lower level controls the energy sources and power converters. The adaptive neuro-fuzzy wavelet-based controller is applied to the inverter. The new proposed wavelet-based controller improves the operation of the proposed microgrid as a result of the excellent localized characteristics of the wavelets. Simulations and comparison with other existing intelligent controllers, such as neuro-fuzzy controllers and fuzzy logic controllers, and classical PID controllers are used to present the improvements of the microgrid in terms of the power transfer, inverter output efficiency, load voltage frequency, and dynamic response.

Sliding Mode Observer-Based Fault-Tolerant Secondary Control of Microgrids

This paper proposes two sliding mode observer (SMO)-based fault-tolerant secondary control schemes for microgrids. The first scheme consists of a central SMO-based fault tolerant controller that uses outputs from the microgrid and estimates all states in the microgrid as well as the fault. The estimated fault is then used to reject the effect of faults on the microgrid. The second scheme is decentralised, where each distributed generator (DG) has its own SMO-based fault tolerant controller which would estimate faults affecting that DG alone, and compensate for faults in only that DG. By rejecting the effect of faults in each DG, the effect of faults on the entire microgrid can be negated. Finally, we simulate an example using both schemes, and its results verify the efficacy of the schemes for fault-tolerant secondary control of microgrids.

Research on Attack Detection Method of Photovoltaic Grid-connected Interface Device Based on Gradient Boosting Decision Tree

Photovoltaic grid-connected interface devices are an important class of smart devices in microgrids. The authenticity and reliability of the data they acquire, as well as the safety and stability of operation, are related to the safe and reliable operation of the entire microgrid system. However, in the context of microgrid intelligence and informatization, information / network attacks will become the norm, making network-dependent information interaction methods subject to various security risks. The photovoltaic grid-connected interface device involves an open operating environment and is extremely vulnerable to network attacks. The attack information will occupy the space or resources of the photovoltaic grid-connected interface device, making the photovoltaic grid-connected interface device unable to respond to other important requests or instructions in a timely manner, and in severe cases, will cause the device to be paralyzed and affect the normal system operation. Aiming at the above problems, this paper presents an attack detection method based on the gradient-upgraded decision tree model, and gives a detailed design process of attack detection model of the photovoltaic grid-connected interface device. That is, the important data flow in the photovoltaic grid-connected interface device is used as the input of the gradient-upgraded decision tree model, and then the gradient-upgraded decision tree model detect or classify flows, finally, intercept the data flow with attack behavior and give a warning prompt, and forward data without attack behaviors normally.

An MPC-Based Dual-Solver Optimization Method for DC Microgrids With Simultaneous Consideration of Operation Cost and Power Loss

In this paper, a dual-solver framework based on model predictive control (MPC) is proposed, $E-$ solver and $L-$ solver. The economic scheduling problem is formulated using mixed-integer linear programming (MILP), which can be solved in an efficient way by using commercial solver ( $E-$ solver). While the transmission loss problem is formulated using non-linear programming (NLP), which can be solved in the interior point method, namely $L-$ solver. The $E-$ solver provides an economic priority power scheduling plan for the $L-$ solver, and the $L-$ solver solves the entire microgrid accurate power flow scheduling plan. The proposed planning model decomposition technique aims to solve the planning model in a time-sharing manner and combines the characteristics of the two optimizers with a reasonable matching algorithm to achieve economic, efficient, and fast real-time control. A case study of a DC microgrid is employed to assess the performance of the online optimization-based control strategy. Simulations based on eight-node DC microgrid show that the method reduces the operating cost by $12.10\%$ and increases the calculation speed by $80.18\%$ compared with the traditional interior point method. With a shorter delay, the proposed optimization method will facilitate the implementation of real-time control and optimization.

Protection of multi‐terminal remote DC microgrids with overhead lines against temporary and permanent faults

Multi-terminal low-voltage DC (MT-LVDC) remote microgrids are subjected to faults with low current magnitudes. Although such faults do not disturb microgrid steady-state operation, their continuous existence can lead to permanent (prm) power losses and personnel hazard. Yet, given that faults are often temporary (tmp) on overhead lines, instantaneous de-energisation of the entire microgrid upon fault detection can result in prolonged loss of infeed, i.e. loss of load. This study proposes a centralised protection scheme to achieve tmp-fault resilient MT-LVDC microgrids. The proposed approach is comprised of local intelligent electronic devices (IEDs) at each terminal and a central IED at one of the MT-LVDC microgrid terminals. Low-bandwidth communication is utilised between local and central IEDs. Local IEDs are equipped with overcurrent function to take fast protective action under faults with high current magnitudes. On the other hand, the central IED consists of two passive oscillators and DC choppers. The passive oscillators facilitate the detection and identification of tmp and prm faults with low current magnitudes. DC choppers are used for online pole voltage rebalancing upon tmp fault clearance. Testing the centralised protection scheme on a ± 750 V , TN-S grounded, MT-LVDC microgrid reveals that the approach is fast, sensitive, selective and resilient under various conditions.

Simultaneous Active and Reactive Power Sharing with Frequency Restoration and Voltage Regulation for Single Phase Microgrids

Abstract The control of microgrids at secondary hierarchical level offers multiple challenges. When the entire microgrid is seen as a Multiple-Input-Multiple-Output (MIMO) system and the Voltage Source Inverters (VSI) connected as control actuators, distributed controllers have been proposed to independently achieve active power sharing, reactive power sharing, frequency restoration, or voltage regulation around nominal values. Here we address these issues simultaneously in generic single phase mesh-micro-grids with arbitrary non-linear loads, in order to derive formal conditions that can be used to check if a given controller allows to reach all four mentioned control objectives. The paper is complemented with a simulation example where the main characteristics of the proposed methodology can be seen.

Stability and Control Aspects of Microgrid Architectures–A Comprehensive Review

Self-governing small regions of power systems, known as “microgrids”, are enabling the integration of small-scale renewable energy sources (RESs) while improving the reliability and energy efficiency of the electricity network. Microgrids can be primarily classified into three types based on their voltage characteristics and system architecture; 1) AC microgrids, 2) DC microgrids, and 3) Hybrid AC/DC microgrids. This paper presents a comprehensive review of stability, control, power management and fault ride-through (FRT) strategies for the AC, DC, and hybrid AC/DC microgrids. This paper also classifies microgrids in terms of their intended application and summarises the operation requirements stipulated in standards (e.g., IEEE Std. 1547-2018). The control strategies for each microgrid architecture are reviewed in terms of their operating principle and performance. In terms of the hybrid AC/DC microgrids, specific control aspects, such as mode transition and coordinated control between multiple interlinking converters (ILCs) and energy storage system (ESS) are analysed. A case study is also presented on the dynamic performance of a hybrid AC/DC microgrid under different control strategies and dynamic loads. Hybrid AC/DC microgrids shown to have more advantages in terms of economy and efficiency compared with the other microgrid architectures. This review shows that hierarchical control schemes, such as primary, secondary, and tertiary control are very popular among all three microgrid types. It is shown that the hybrid AC/DC microgrids require more complex control strategies for power management and control compared to AC or DC microgrids due to their dependency on the ILC controls and the operation mode of the hybrid AC/DC microgrid. Case study illustrated the significant effects of microgrid feeder characteristics on the dynamic performance of the hybrid AC/DC microgrid. It is also revealed that any transient conditions either in the AC or DC microgrids could propagate through the ILC affecting the entire microgrid dynamic performance. Additionally, the critical control issues and the future research challenges of microgrids are also discussed in this paper.

Stability analysis and decentralized control of inverter-based ac microgrid

This work considers the problem of decentralized control of inverter-based ac micro-grid in different operation modes. The main objectives are to (i) design decentralized frequency and voltage controllers, to gather with power sharing, without information exchange between microsources (ii) design passive dynamic controllers which ensure stability of the entire microgrid system (iii) capture nonlinear, interconnected and large-scale dynamic of the micro-grid system with meshed topology as a port-Hamiltonian formulation (iv) expand the property of shifted-energy function in the context of decentralized control of ac micro-grid (v) analysis of system stability in large signal point of view. More precisely, to deal with nonlinear, interconnected and large-scale structure of micro-grid systems, the port-Hamiltonian formulation is used to capture the dynamic of micro-grid components including microsource, distribution line and load dynamics as well as interconnection controllers. Furthermore, to deal with large signal stability problem of the microgrid system in the grid-connected and islanded conditions, the shifted-Hamiltonian energy function is served as a storage function to ensure incremental passivity and stability of the microgrid system. Moreover, it is shown that the aggregating of the microgrid dynamic and the decentralized controller dynamics satisfies the incremental passivity. Finally, the effectiveness of the proposed controllers is evaluated through simulation studies. The different scenarios including grid-connected and islanded modes as well as transition between both modes are simulated. The simulation conforms that the decentralized control dynamics are suited to achieve the desired objective of frequency synchronization, voltage control and power sharing in the grid-connected and islanded modes. The simulation results demonstrate the effectiveness of the proposed control strategy.

Multi-Objective Configuration Optimization for Isolated Microgrid With Shiftable Loads and Mobile Energy Storage

For remote structures in military applications located in high mountains and outlying islands, the configuration of isolated microgrids constructed in these locations is highly important. To denote oxygen generation/seawater desalination devices and military battery packs in practical applications, shiftable load models and mobile energy storage models are used. A multiobjective optimization model, including the annual system cost, demand shortage rate, and the ratio of diesel energy supply, is constructed for configuration design. This model is solved using the improved preference-incentive coevolution algorithm (PICEA-ng) presented in this paper. Based on practical data and numerical simulation, the utilization strategy in terms of lost time and output power for mobile energy storage, stationary energy storage, diesel generators, and shiftable loads are generated and validated. Finally, the operational performance of the shiftable load and mobile energy storage over the entire microgrid system is analyzed and discussed.

An Adaptive Auto-Reclosing Scheme to Preserve Transient Stability of Microgrids

Conventional auto-reclosing algorithm (CRA) jeopardizes the transient stability of synchronous generator based distributed energy resource (SGBDER) and, accordingly, the transient stability of entire microgrid (μG). The fixed dead time associated with CRA is the origin of this deficiency. In this paper, an adaptive auto-reclosing algorithm is devised to overcome the transient stability problem in SGBDER-integrated μGs. The proposed method aims at reclosing the interrupted feeder in the least possible time while preserving the transient stability of μG even during permanent faults. To do so, the most appropriate reclosing instant (MARI) is determined based on the SGBDER potential energy assessment. MARI is an instant where the potential energy associated with SGBDER is minimum. Two indices are introduced to specify MARI. The indices are forecasted to determine the time to MARI for readiness purposes. To realize this, the least square error approach is deployed to elicit the temporal characteristics of the indices and their future behavior. The evaluation of proposed indices necessitates the availability of rotor angle of SGBDER as well as the associated rotor angular velocity deviation and its first order derivative. These signals are provided using the dynamic equations of the SGBDER which are fed to the algorithm.

A new multifunctional converter based on a series compensator applied to AC microgrids

This paper proposes a new multifunctional converter based on a series voltage compensator applied to centralized AC microgrids. Besides the basic functionalities required for a centralized converter in microgrids, such as grid-supporting and grid-forming voltage support and smooth transition from operating modes, the proposed converter can change its connection topology from series to parallel and vice-versa, depending on the system’s condition. Thus, it assumes all the basic defining features of series and shunt power converters. The devised control algorithm allows on-line change of converter operating modes either as voltage- or current-controlled sources. These flexible functionalities allow the converter to operate as: (i) a grid-forming converter providing voltage and frequency references for the entire microgrid, keeping the voltages at the point of common coupling (PCC) synchronized with the utility voltage; (ii) grid-feeding active power into the microgrid; and (iii) grid-supporting, performing ancillary services such as voltage regulation, voltage sag and swell compensation, along with active power filtering, reactive power, unbalance and harmonic compensation. The converter can be applied to both single-phase and three-phase three- or four-wire networks. The multifunctional control is implemented using a Texas Instruments TMS320F28335 digital signal processor, and then validated through a hardware-in-the-loop simulation developed in the Typhoon HIL 600 platform.

Low‐voltage ride‐through of a droop‐based three‐phase four‐wire grid‐connected microgrid

The ability of riding through the grid disturbances can increase the integration of microgrids into the distribution system. Consequently, a grid-connected microgrid should provide ancillary services such as low voltage ride-through (LVRT) capability and reactive power support to sustain the power system operations during abnormal grid conditions. The objective of this paper is to propose an LVRT scheme that improves the power quality of the entire microgrid. The developed method is implemented as the controller of the interface voltage-sourced converter (VSC) of a distributed energy resource and consists of primary and secondary control levels. The former includes the cascaded voltage and current control loops and the droop controller, while the latter controls the reactive power injection during the balanced/unbalanced voltage sags/swells. The proposed scheme is developed based on the independent control of each phase and does not require calculation of symmetrical components. Moreover, it can be employed in the VSC control systems with various reference frames and is effective for droop-based grid-connected microgrids with both single-phase and three-phase four-wire configurations. The proposed strategy is implemented using the hierarchical control system and preserves the plug and play capability. Several case studies are presented to verify the effectiveness of the proposed strategy.

Direct and Efficient Simulation of Battery Models for Standalone PV-Battery Microgrids

With renewable energy based electrical systems becoming more prevalent in homes across the globe, microgrids are becoming widespread and could pave the way for future energy distribution. Accurate and economical sizing of a standalone power system components has been an active area of research, but current control methods do not make them economically feasible. Typically, batteries are treated as a black box that does not account for their internal states in current microgrid simulation and control algorithms.1 This might lead to under-utilization and over-stacking of batteries. In contrast, detailed physics-based battery models, accounting for internal states, can save a significant amount of energy and cost, utilizing batteries with maximized life and usability.2 However, typical microgrid controls including feedback algorithms cause a significant time-delay as detailed physics-based battery models are coupled to the entire microgrid system.3 It is important to identify how efficient physics-based models of batteries can be included and addressed in current grid control strategies. In this talk, we will show that perhaps the best way to integrate detailed battery models is to write the microgrid equations in mathematical form and then identify an efficient way to solve those models simultaneously with battery models, which gives better results. Implementation of the maximum power point tracking controller algorithm and physics-based battery model along with microgrid components as differential algebraic equations will be discussed. The results of the proposed approach will be compared with the conventional control strategies and improvements in performance and speed will be reported. Acknowledgements This work was supported by the Clean Energy Institute located in University of Washington, Seattle and Washington Research Foundation.

Fault Diagnosis Based Approach to Protecting DC Microgrid Using Machine Learning Technique

The electronic equipment used in DC microgrids is in essential need of more secure protection against short circuit faults. Due to the high current at the time of fault occurrence, the whole system might be de-energized which would have a severely negative impact on the entire system. An effective method to detect, isolate, and protect the DC microgrid system against the effects of short circuit faults is extremely important. This research proposes a highly effective new method to protect the DC microgrid system using an Artificial Neural Network (ANN) that will detect and isolate the fault before it affects other parts of the system. It would, therefore, be more dependable for DC microgrid protection. This protection network is distributed all along the DC microgrid system protecting the entire microgrid network and is connected to the other protective devices in the system.

Modelling and Simulation of Natural Gas Generator and EV Charging Station: A Step to Microgrid Technology

Besides the negative impact on both the environmental and geological aspect because of the fossil fuel combustion-based power generation, the fossil fuels have almost been depleted, and hence an alternative fuel source is required. To meet the demand of the next generation power system, renewable and clean energy resources, such as solar, wind, tide, geothermal, and natural gas, can be the fuel of choice. In this paper, the concentration is limited to the natural gas generators and electric vehicle charging station besides the delineation of the entire microgrid testbed. In particular, this letter is associated with the modeling of the natural gas generators and the simulations for the different aspects and cases of the regarding system. Besides that, the EV charging station is also delineated with the necessary modelling, simulation, and analysis. All the results are verified by the Matlab/Simulink simulations and with necessary explanation.

Control Of Utility Interfaces In Low-voltage Microgrids

This paper presents a general control technique for utility interactive inverters in low-voltage microgrids. The Utility Interface (UI) is a three-phase power conversion unit, equipped with energy storage, which governs the interaction between the utility grid and the microgrid. The UI is in charge of several functions: in grid-connected operation, it performs as a voltage-supporting unit and compensates the reactive power, unbalance, and distortion caused by loads, whereas in islanded operation, it performs as a voltage- forming unit and sets the voltage and frequency for the entire microgrid. Moreover, the UI ensures seamless transitions from grid-connected to islanded operation and actively decouples the microgrid and the mains. Finally, the UI can perform as a centralized microgrid controller for distributed energy resources. The UI is therefore a crucial component, which needs to be analyzed carefully to ensure safe and reliable operation for the microgrid. This paper discusses a control approach that provides all required functionalities and ensures proper microgrid operation even in case of non- intentional islanding or severe load transients. The experimental results show the behavior of the system in different scenarios.

Passivity-Based Control of DC Microgrid for Self-Disciplined Stabilization

DC microgrids may have time-varying system structures and operation patterns due to the flexibility and uncertainty of distributed resources. This feature poses a challenge to conventional stability analysis methods, which are based on fixed and complete system models. To solve this problem, the concept of self-disciplined stabilization is introduced in this paper. A common stability discipline is established using the passivity-based control theory, which ensures that a microgrid is always stable as long as this discipline is complied by each individual converter. In this way, the stabilization task is localized to avoid investigating the entire microgrid, thereby providing immunity against system variations. Moreover, a passivity margin criterion is proposed to further enhance the stability margin of the self-disciplined control. The modified criterion imposes a tighter phase restriction to provide explicit phase margins and prevent under-damped transient oscillations. In line with this criterion, a practical control algorithm is also derived, which increases the converter's passivity through voltage feed forward. The major theoretical conclusions are verified by a laboratory DC microgrid test bench.

System Modeling and Simulation of Intentionally Islanded Reconfigurable Microgrid

Microgrid reconfiguration plays important role in improving continuity of power supply especially in the event of fault within the power grid or microgrid connected to the medium voltage distribution network. Due to the fault in power grid, intentional islanding through static switch is one possible solution while, fault within the microgrid emphasizes microgrid reconfiguration to recover maximum affected loads in each isolated zone. This paper presents multiagent system based (MAS) approach in order to withstand stably and autonomously in the event of fault within microgrid or power grid. MAS approach includes negotiation agent (NA) for the entire microgrid and bus agent (BA) for each bus within each zone. NA is responsible for selecting a Dominant-BA from each zone and prioritizing the zones by using region weight coefficient (RWC) method. Zone having excessive power will recombine with the zone having lesser power for maintaining power quality of sensitive and less sensitive loads, while normal loads are shed.