Microgrid Topology Research Articles

Synchronisation of two separate zones in a standalone microgrid

Standalone microgrids that comprise distributed generation sources (DGSs) or energy storage systems (ESSs) exist on off-shore islands or remote areas. When a fault occurs in a standalone microgrid, the microgrid is separated into several zones with pre-specified DGS or ESS supplying power to local loads. After the fault is cleared, the three-phase voltages and frequencies may differ among these zones. To restore the original microgrid topology, these zones must be reconnected to each other by synchronisation. A synchronisation controller of the DGS/ESS that accounts for slightly different three-phase voltages and frequencies in two separate zones is presented. The Clark transform and Mamdani fuzzy rules are used to implement the controller, providing fast and seamless synchronisation. Simulation results using a standalone microgrid show the practical applicability of the proposed method.

Optimal Planning of Loop-Based Microgrid Topology

In microgrid planning, topological design is a critical concern for ensuring certain features such as high reliability in islanded operation. This paper proposes a graph partitioning and integer programming integrated methodology for the optimal loop-based microgrid topology planning while considering the distributed energy resources in the microgrid. The proposed methodology is applied to a microgrid test system and the planning results are discussed. The results demonstrate that the proposed planning methodology is able to accurately and efficiently determine an optimal loop structure for microgrids, and exhibit the potentials for applying the proposed planning methodology in practical microgrid applications.

Optimal configuration analysis for a campus microgrid\u2014a case study

The foremost issues of 21st century are challenging demand of electrical energy and to control the emission of Green House Gases (GHG) emissions. Renewable energy resources based sustainable microgrid emerges as one of the best feasible solution for future energy demand while considering zero carbon emission, fossil fuel independency, and enhanced reliability. In this paper, optimization and implementation of institutional based sustainable microgrid are discussed based on cost analysis, carbon emission, and availability of energy resources. Various microgrid topologies are considered for addressing the most ideal solution. The metrological data such as irradiance is acquired from solar satellite data of NASA (National Aero Space Agency) while the data for wind speed is taken from synergy enviro engineer’s site. HOMER® simulation tool is used for modelling and optimization purpose.

Evaluation model for multi‐microgrid with autonomous DC energy exchange

This paper proposes an evaluation model to analyze the impact of microgrid topologies on self‐sufficiency for a given size of batteries and photovoltaic (PV) panels (resources). Three topologies are evaluated for a community of 19 houses: centralized resources (ideal case), stand‐alone resources, and a multi‐microgrid topology with autonomous exchange. Depending on the ratio of PV and battery size, the topology with stand‐alone resources has a clear disadvantage in terms of self‐sufficiency compared to the centralized, ideal topology. To counteract this, we propose a hybrid topology: households are interconnected so that they can exchange energy between each other based on an autonomous energy exchange algorithm we developed. We show that for a well‐chosen ratio of batteries and PV, the interconnected system can improve the stand‐alone design by up to 10% without requiring any additional resources. This topology can approach performance similar to that of a centralized microgrid but its design is more flexible and resilient to failures or accidents. The evaluation model computes the self‐sufficiency ratio (SSR) for the three topologies for 0–20 kWh batteries and 1–14 kWp PV sizes. Furthermore, seasonal differences in SSR per topology are analyzed for an actual community with real resources. We also calculate the savings in PV and battery due to the interconnected topology. Finally, the third topology's feasibility is demonstrated on a full‐scale platform in Okinawa on which the autonomous energy exchange software was tested for over a year in a community of 19 houses. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Decentralised control method for DC microgrids with improved current sharing accuracy

A decentralised control method that deals with current sharing issues in dc microgrids (MGs) is proposed in this study. The proposed method is formulated in terms of ‘modified global indicator’ concept, which was originally proposed to improve reactive power sharing in ac MGs. In this work, the ‘modified global indicator’ concept is extended to coordinate dc MGs, which aims to preserve the main features offered by decentralised control methods such as no need of communication links, central controller or knowledge of the microgrid topology and parameters. This global indicator is inserted between current and voltage variables by adopting a virtual capacitor, which directly produces an output current sharing performance that is less relied on mismatches of the multi-bus network. Meanwhile, a voltage stabiliser is complementary developed to maintain output voltage magnitude at steady state through a shunt virtual resistance. The operation under multiple dc-buses is also included in order to enhance the applicability of the proposed controller. A detailed mathematical model including the effect of network mismatches is derived for analysis of the stability of the proposed controller. The feasibility and effectiveness of the proposed control strategy are validated by simulation and experimental results.

Plug-and-Play Voltage Stabilization in Inverter-Interfaced Microgrids via a Robust Control Strategy

This paper proposes a decentralized control strategy for the voltage regulation of islanded inverter-interfaced microgrids. We show that an inverter-interfaced microgrid under plug-and-play (PnP) functionality of distributed generations (DGs) can be cast as a linear time-invariant system subject to polytopic-type uncertainty. Then, by virtue of this novel description and use of the results from theory of robust control, the microgrid control system guarantees stability and a desired performance even in the case of PnP operation of DGs. The robust controller is a solution of a convex optimization problem. The main properties of the proposed controller are that: 1) it is fully decentralized and local controllers of DGs that use only local measurements; 2) the controller guarantees the stability of the overall system; 3) the controller allows PnP functionality of DGs in microgrids; and 4) the controller is robust against microgrid topology change. Various case studies, based on time-domain simulations in MATLAB/SimPowerSystems Toolbox, are carried out to evaluate the performance of the proposed control strategy in terms of voltage tracking, microgrid topology change, PnP capability features, and load changes.

Improved efficiency, enhanced reliability and reduced cost: The transition from static microgrids to reconfigurable microgrids

Thanks to smart switches, the current generation of static microgrids is about to transition to a new generation of reconfigrable microgrids (RMG). RMGs use remotely controlled switches to control and change the microgrid topology to ensure that desired objectives can be achieved. RMGs can provide customers with a greater degree of cost-effectiveness, efficiency, reliability, and power quality, though challenges remain.

A New Topology Pattern for AC-DC Mixed Microgrid

In order to realize the modular design of the microgrid, this paper proposed a new modular topology for the AC-DC mixed microgrid. In that topology, the AC microgrid unit and the DC microgrid unit were packaged together by the back-to-back converter. The battery-supercapacitor hybrid energy storage system was connected to the DC bus of back-to-back converter. By the reasonable design on the battery-supercapacitor hybrid system, the energy storage system could supply the rapid power and energy support for the microgrid spontaneously. The mathematical model and the control algorithm of that microgrid topology were studied. By the simulation analysis, it can be concluded that AC-DC mixed modular microgrid topology could operate steadily on both the grid-connected mode and the isolated mode. Furthermore, we can conclude by the simulation that the designed modular microgrid could operate uninterrupted when the microgrid topology switched from the grid-connected mode to the isolated mode. The seamless switching became the natural property for the modular microgrid. As a result, the modular microgrid topology can be considered as a usual power/load module to realize the friendly power interaction with the power grid.

A Fault Clearing Method in Converter-Dominated Microgrids With Conventional Protection Means

The integration of converter-interfaced distributed generation in microgrids has raised several technical issues, including the successful operation of protective devices during faults. The protection issue is associated with the lack of large current injection during a fault, due to limits imposed by the semiconductor switches. This paper proposes a fault-detection and clearing control strategy method for symmetrical and asymmetrical line faults in a looped microgrid. The protection devices are simple overcurrent devices with the same settings, due to the looped microgrid topology. The proposed method is applied without using any kind of physical communication. The fault is detected by measuring indirectly the microgrid impedance. After the fault identification, the distributed energy resources (DERs) adjust their control in order to inject a current proportional to the measured microgrid impedance, according to a droop curve. This means that the DER closer to the fault injects a relatively larger current, achieving by this way a selective coordination of the protection means. The effectiveness of the proposed control strategy is evaluated through detailed simulation and experimental tests.

A decentralized impedance-based adaptive droop method for power loss reduction in a converter-dominated islanded microgrid

Abstract As the integration of Distributed Energy Resources (DERs) in modern grids is increasing, the microgrid concept has been established. In the case of converter-dominated microgrids without communication links, the droop control method is mainly adopted in the primary control level. Normally, the economic operation of the microgrid is incorporated in the secondary control level, requiring communication among the DERs and a central controller. On the contrary, this paper proposes an adaptive droop control method based on local measurements, which achieves a power sharing with reduced power losses within the islanded microgrid. The droop coefficients are adjusted by calculating the microgrid impedance, sensed by each DER. Therefore, the remote DERs calculate a large impedance and inject lower active and reactive power, reducing the line losses. The location and the size of the DERs and the loads can be arbitrary. Another advantage is the absence of the virtual impedance control, as the power decoupling is implemented inherently by the adaptive droop coefficients. The effectiveness of the proposed control strategy is verified by extended simulation results in comparison with the accurate sharing method for different microgrid topologies.

Secondary Frequency and Voltage Control of Islanded Microgrids via Distributed Averaging

In this work we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive trade-off between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances or loads. The distributed architecture allows for flexibility and redundancy, and eliminates the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.

A multi-agent decentralized energy management system based on distributed intelligence for the design and control of autonomous polygeneration microgrids

The autonomous polygeneration microgrid topology has been developed in order to cover holistically needs in a remote area such as electrical energy, space heating and cooling, potable water through desalination and hydrogen as fuel for transportation. The existence of an advanced energy management system is essential for the operation of an autonomous polygeneration microgrid. So far, energy management systems based on a centralized management and control have been developed for the autonomous polygeneration microgrid topology based on computational intelligence approaches. A decentralized management and control energy management system can have important benefits, when taking into consideration the autonomous character of these microgrids. This paper presents the design and investigation of a decentralized energy management system for the autonomous polygeneration microgrid topology. The decentralized energy management system gives the possibility to control each unit of the microgrid independently. The most important advantage of using a decentralized architecture is that the managed microgrid has much higher chances of partial operation in cases when malfunctions occur at different parts of it, instead of a complete system breakdown. The designed system was based on a multi-agent system and employed Fuzzy Cognitive Maps for its implementation. It was then compared through a case study with an existing centralized energy management system. The technical performance of the decentralized solution performance is on par with the existing centralized one, presenting improvements in financial and operational terms for the implementation and operation of an autonomous polygeneration microgrid.

Reduced converter topology for integrated wind and small‐hydro energy generation system

This study presents a robust and cost-effective microgrid topology for integrating two commonly available wind and small-hydro renewable energy sources. The proposed topology has two generators, one of them is self-excited squirrel cage induction generator driven by a small-hydro turbine and other one is permanent magnet brushless DC generator driven by a small-scale wind turbine. The proposed system is integrated using a voltage source converter and a boost converter which are maintaining voltage and frequency, mitigating power quality problems and performing mechanical sensorless maximum power point tracking, respectively. A battery bank is also used to balance the load and supply power mismatch and to ensure the supply for critical loads. The proposed control algorithm is tested and verified on a developed laboratory prototype of the system. Test results verify that voltage and frequency of the system are regulated and the power-quality-related issues are also resolved in this microgrid.

Demand response impacts on off-grid hybrid photovoltaic-diesel generator microgrids

Hybrid microgrids consisting of diesel generator set(s) and converter based power sources, such as solar photovoltaic or wind sources, offer an alternative to generator based off-grid power systems. The hybrid approach has been shown to be economical in many off-grid applications and can result in reduced generator operation, fuel requirements, and maintenance. However, the intermittent nature of demand and renewable energy sources typically require energy storage, such as batteries, to properly operate the hybrid microgrid. These batteries increase the system cost, require proper operation and maintenance, and have been shown to be unreliable in case studies on hybrid microgrids. This work examines the impacts of leveraging demand response in a hybrid microgrid in lieu of energy storage. The study is performed by simulating two different hybrid diesel generator—PV microgrid topologies, one with a single diesel generator and one with multiple paralleled diesel generators, for a small residential neighborhood with varying levels of demand response. Various system designs are considered and the optimal design, based on cost of energy, is presented for each level of demand response. The solar resources, performance of solar PV source, performance of diesel generators, costs of system components, maintenance, and operation are modeled and simulated at a time interval of ten minutes over a twenty-five year period for both microgrid topologies. Results are quantified through cost of energy, diesel fuel requirements, and utilization of the energy sources under varying levels of demand response. The results indicate that a moderate level of demand response can have significant positive impacts to the operation of hybrid microgrids through reduced energy cost, fuel consumption, and increased utilization of PV sources.

A Novel Method of Fault Location for Single-Phase Microgrids

This paper presents a novel fault location method for single-phase microgrids. In order to locate a fault, a feature specific to the fault location is found, namely the maximum oscillation magnitude of the transient voltage signal induced by the fault. Our theoretical study and extensive simulations demonstrate that there is an approximated linear relationship between the maximum magnitude of the transient signal observed by a sensor, and the distance between the sensor and the fault location. Based on the discovered relationship, microgrid topology and sensor location information, we have designed an algorithm capable of locating the fault in the single-phase microgrids. The proposed fault location method has been implemented and validated through simulations in Electro-Magnetic Transients Program and MATLAB. The average localization error is less than 10% in the evaluation results, which manifests the significance of the novel method, as there is little research done for fault location in single-phase microgrids.

A Dynamic Data-driven Approach for Operation Planning of Microgrids

Abstract Distributed generation resources (DGs) and their utilization in large-scale power systems are attracting more and more utilities as they are becoming more qualitatively reliable and economically viable. However, uncertainties in power generation from DGs and fluctuations in load demand must be considered when determining the optimal operation plan for a microgrid. In this context, a novel dynamic data-driven application systems (DDDAS) approach is proposed for determining the real-time operation plan of an electric microgridwhile considering its conflicting objectives. In particular, the proposed approachis equipped with three modules: 1) a database including the real-time microgrid topology data (i.e., power demand, market price for electricity, etc.) and the data for environmental factors (i.e., solar radiation, wind speed, temperature, etc.); 2) a simulation, in which operation of the microgrid is simulated with embedded rule-based scaleidentification procedures; and 3) a multi-objective optimization module which finds the near-optimal operation plan in terms of minimum operating cost and minimum emission using a particle-filtering based algorithm. The complexity of the optimization depends on the scaleof the problem identified from the simulation module. The results obtained from the optimization module are sent back to the microgrid system to enhance its operation. The experiments conducted in this study demonstratethe power of the proposed approach in real-time assessment and control of operation in microgrids.

On battery-less autonomous polygeneration microgrids: Investigation of the combined hybrid capacitors/hydrogen alternative

The autonomous polygeneration microgrid topology aims to cover holistically the needs in remote areas as far as electrical power, potable water through desalination, fuel for transportation in the form of hydrogen, heating and cooling are concerned. Deep discharge lead acid batteries are mostly used in such systems, associated with specific disadvantages, both technical and environmental. This paper investigated the possibility of replacing the battery bank from a polygeneration microgrid with a hybrid capacitor bank and more intensive utilization of a hydrogen subsystem. Initially commercial hybrid capacitors were tested under laboratory conditions and based on the respective results a case study was performed. The optimized combination of hybrid capacitors and higher hydrogen usage was then investigated through simulations and compared to a polygeneration microgrid featuring deep discharge lead acid batteries. From the results it was clear that it is technically possible to exchange the battery bank with a hybrid capacitor bank and higher hydrogen utilization. From the economic point of view, the current cost of the hybrid capacitors and the hydrogen components is high which leads to higher overall cost in comparison with deep discharge lead acid batteries. Taking into account, though, the decreasing cost prospects and trends of both the hybrid capacitors and the hydrogen components it is expected that this approach will become economically competitive in a few years.

Experimental investigation of the performance of a reverse osmosis desalination unit under full- and part-load operation

Sea water reverse osmosis (SWRO) desalination constitutes a successful technology for covering the potable water needs of islands and coastal regions. SWRO units can be combined with renewable energy technologies such as photovoltaic and wind generators. Conventional small scale SWRO units are not often combined with energy recovery devices; however, these devices can decrease drastically the energy consumption of the SWRO units. Furthermore, in the literature there are references which prove that the operation of a desalination unit in part-load conditions can result in lower specific energy consumption compared to full-load operation. This paper presents the simulation and the experimental investigation under full- and part-load conditions of an existing SWRO desalination unit (50 mS/cm feed water) in order for the desalination unit to be optimally utilized in a polygeneration microgrid topology which uses advanced energy management algorithms. The experimental operation of the SWRO Unit in part-load conditions is achieved by varying the speed of the motor—pump assembly, the pressure and the flow rate of the feed water. During the evaluation of the measurements result, an optimum operating window in the range of 40–57 bar was drawn regarding the operation of the SWRO desalination unit in part- and full-load conditions. More specifically, in this pressure range the average value of fresh water production was 60 L/h with an acceptable fresh water electrical conductivity of 550 μS/cm, and with a specific energy consumption range from 6.1 to 7.7 kWh/m3. With these results the operational parameters of the polygeneration microgrid energy management system can be optimized.

Research on Electric Power Control Technique for Stable Operation of DC Micro-Grid

Distributed generation (DG) into DC micro-grid can save a lot of converter equipments compared with AC micro-grid,.So,with the popularity of DG,DC micro-grid technology also has get great development.However,the key technology of DC micro-grid is not yet mature.This paper is based on shanghai DC micro-grid demonstration project of Sate Grid Smart Grid Research Institute.Firstly,we analysis the advantages and disadvantages of AC,DC and AC/DC hybrid micro-grid.Then, after comparison and analysis of DC micro-grid topology,we put forward system scheme and corresponding control strategy.The simulation results show that, the proposed control strategy is feasible,simple,real-time and effective.This design scheme is applicable for radiant topology and would provide guidance to the development direction of DC micro-grid project later.

A Comparison of Microgrid Topologies Considering Both Market Operations and Reliability

This article investigates and compares various types of microgrids in the literature. The comparison is concerned with the economic operation of the micro-sources employed in different topologies and the reliability of the end-users. Also, the effect of each component on the performance of the overall microgrid is discussed in detail, e.g., the individual and centralized power converters, etc. The numerical simulation of a case study shows that the single AC and three wire network microgrid types are predominant over two others, namely single DC and hybrid AC-DC networks. The AC microgrid shows its benefits in market operations, while the three-wire network takes advantage of customizing loads as its reliability preferences. The DC microgrid is the worst case, suffering from high power losses and poor reliability.