Showing papers on "Microgrid published in 2006"

Defining control strategies for MicroGrids islanded operation

Abstract: This paper describes and evaluates the feasibility of control strategies to be adopted for the operation of a microgrid when it becomes isolated. Normally, the microgrid operates in interconnected mode with the medium voltage network; however, scheduled or forced isolation can take place. In such conditions, the microgrid must have the ability to operate stably and autonomously. An evaluation of the need of storage devices and load shedding strategies is included in this paper.

Power Management Strategies for a Microgrid With Multiple Distributed Generation Units

Abstract: This paper addresses real and reactive power management strategies of electronically interfaced distributed generation (DG) units in the context of a multiple-DG microgrid system. The emphasis is primarily on electronically interfaced DG (EI-DG) units. DG controls and power management strategies are based on locally measured signals without communications. Based on the reactive power controls adopted, three power management strategies are identified and investigated. These strategies are based on 1) voltage-droop characteristic, 2) voltage regulation, and 3) load reactive power compensation. The real power of each DG unit is controlled based on a frequency-droop characteristic and a complimentary frequency restoration strategy. A systematic approach to develop a small-signal dynamic model of a multiple-DG microgrid, including real and reactive power management strategies, is also presented. The microgrid eigen structure, based on the developed model, is used to 1) investigate the microgrid dynamic behavior, 2) select control parameters of DG units, and 3) incorporate power management strategies in the DG controllers. The model is also used to investigate sensitivity of the design to changes of parameters and operating point and to optimize performance of the microgrid system. The results are used to discuss applications of the proposed power management strategies under various microgrid operating conditions

Autonomous control of microgrids

Abstract: Application of individual distributed generators can cause as many problems as it may solve. A better way to realize the emerging potential of distributed generation is to take a system approach which views generation and associated loads as a subsystem or a "microgrid". The sources can operate hi parallel to the grid or can operate in island, providing UPS services. The system will disconnect from the utility during large events (i.e. faults, voltage collapses), but may also intentionally disconnect when the quality of power from the grid falls below certain standards. Utilization of waste heat from the sources will increase total efficiency, making the project more financially attractive. Laboratory verification of the Consortium for Electric Reliability Technology Solutions (CERTS) microgrid control concepts are included.

High-Quality Power Generation Through Distributed Control of a Power Park Microgrid

Abstract: Inverters are a necessary interface for several forms of distributed generation (DG) and where they form a microgrid they have the potential to offer high power quality. The challenge is to coordinate the actions of a group of inverters so that they offer the level of power quality known to be possible from fast local control of a single inverter. The case examined here is a power park of several inverter-based DG in relatively close proximity. A basic requirement is that the inverters regulate the grid voltage and share the real and reactive power demands according to their ratings. In small girds with high proportions of nonlinear and unbalanced loads it is also important to actively control the waveform quality in terms of harmonics, transient disturbances, and balance. Further, it is important that these duties are shared equally between the units rather than having one master unit taking the lead in the voltage control function. A constraint faced in designing a sharing system is the limited bandwidth of signal communication even over distances of a few meters. A control method is proposed that separates the control tasks in the frequency domain. Power sharing and voltage regulation are controlled centrally and commands are distributed through a low-bandwidth communication link. Waveform quality functions are controlled in high bandwidth controllers distributed to each local inverter. Experimental tests on a grid of three 10-kVA inverters are used to show that the method fully exploits the inherent fast response of the inverters while also ensuring voltage balance even with extreme load imbalance. It is shown that circulating currents are avoided during steady state and transients

Can microgrids make a major contribution to UK energy supply

Abstract: Almost all the electricity is generated in the UK as part of a centralised power system designed around large fossil fuel or nuclear power stations. This power system is robust and reliable but the efficiency of power generation is low, resulting in large quantities of waste heat. The principal aim of this paper is to investigate an alternative concept: the energy production by small scale generators in close proximity to the energy users, integrated into microgrids. Microgrids - de-centralised electricity generation combined with on-site production of heat - bear the promise of substantial environmental benefits , brought about by a higher efficiency and by facilitating the integration of renewable energy sources such as photovoltaic arrays or wind turbines. By virtue of good match between generation and load, microgrids have a low impact on the electricity network, despite a potentially significant level of generationby interemittent energy sources. The paper discusses the technical and economic issues associated with this novel concept, giving an overview of the generator technologies, the current regulatory framework in the UK, and the barriers that have to be overcome if microgrids are to make a major contribution to the UK energy supply. The focus of this study is a microgrid of domestic users powered by small Combined Heat and Power generators and photovoltaics. Focusing on the energy balance between the generation and load, it is found that the optimum combination of the generators in the microgrid - consisting of around 1.4 kWp PV array per household and 45% household ownership of micro-CHP generators - will maintain energy balance on a yearly basis if suplemented by energy storage of 2.7 kWh per household. We find that there is no fundamental technological reason why microgrids cannot contribute an appreciable part of UK energy demand. Indeed, an estimate of cost indicates that the microgrids considered in this study would supply electricity at a cost comparable with the present electricity supply if the current support mechanisms for photovoltaics were maintained. Combining photovoltaics and micro-CHP and a smallbattery requirement gives a microgrid that is independent of the national electricity network. In the short term, this has particular benefits for remote communities but more wide-ranging possibilities open up in the medium to long term. Microgrids could meet the need to replace current generation nuclear and coal fired power stations, greatly reducing the demand on the transmission and distribution network.

Microgrids - Large Scale Integration of Microgeneration to Low Voltage Grids

Abstract: Microgrids comprise Low Voltage distribution systems with distributed energy sources, such as micro-turbines, fuel cells, PVs, etc., together with storage devices, i.e. flywheels, energy capacitors and batteries, and controllable loads, offering considerable control capabilities over the network operation. These systems are interconnected to the Medium Voltage Distribution network, but they can be also operated isolated from the main grid, in case of faults in the upstream network. From the customer point of view, Microgrids provide both thermal and electricity needs, and in addition enhance local reliability, reduce emissions, improve power quality by supporting voltage and reducing voltage dips, and potentially lower costs of energy supply. This paper outlines selected research findings of the EU funded MICROGRIDS project (Contract ENK-CT-2002-00610). These include: • Development and enhancement of Microsource controllers to support frequency and voltage based on droops. Application of software agents for secondary control. • Development of the Microgrid Central Controller (MGCC). Economic Scheduling functions have been developed and integrated in a software package able to simulate the capabilities of the MGCC to place bids to the market operator under various policies and to evaluate the resulting environmental benefits. • Analysis of the communication requirements of the Microgrids control architecture • Investigation of alternative market designs for trading energy and ancillary services within a Microgrid. Development of methods for the quantification of reliability and loss reduction. • Initial measurements from an actual LV installation.

A grid-interfacing power quality compensator for three-phase three-wire microgrid applications

Abstract: This paper proposes a grid-interfacing power quality compensator for three-phase three-wire microgrid applications with consideration of both the power quality of the microgrid and the quality of currents flowing between the microgrid and utility system. It is proposed that two inverters connected in shunt and series are used for each distributed generation (DG) system in the microgrid. In each inverter, both positive- and negative-sequence components are controlled to compensate for the effects caused by the unbalanced utility grid voltages. Specifically, the shunt inverter is controlled to ensure balanced voltages within the microgrid and to regulate power dispatches among parallel-connected DG systems, while the series inverter balances the line currents by injecting appropriate voltage components. A current-limiting algorithm is also proposed and integrated within the inverter control schemes to protect the microgrid from large fault currents during utility voltage sags. The proposed compensator has been tested in simulations and experimentally using a laboratory hardware prototype.

Protection of Microgrids During Utility Voltage Sags

Abstract: Microgrids are systems with clusters of microgenerators, which are installed for distributed power generation. When interfaced to the utility grid, microgrids are exposed to common utility power-quality disturbances. In particular, during utility-voltage sags, large line currents can flow along distribution feeders connecting the micro- and utility grids. To limit this flow of large line currents and, hence, protect the microgrids, this paper proposes two current-limiting algorithms, namely, the RL feedforward and flux-charge-model feedback algorithms, for controlling a series inverter connected between the micro- and utility grids during utility voltage sags. Both methods function by inserting a large virtual RL or L impedance in series with the distribution feeder to limit the line-current flow. Detailed descriptions, controller designs, and comparisons of both algorithms are presented. Phasor analyses of both methods are also presented to show how the inserted RL or L values can be optimally tuned to improve the inverter damping performance and minimize its injected voltages and circulating power under all operation conditions. Lastly, both methods have been tested in simulation and in experiments using an emulated laboratory microgrid system

A voltage based protection for micro-grids containing power electronic converters

Abstract: Protecting micro-grids containing micro-sources equipped with power electronics interfaces is a major challenge for engineers developing techniques to exploit renewable energy sources for electricity generation. Conventional techniques based on overcurrent protection have major limitations and new techniques have to be explored. The method described in this paper provides reliable and fast detection for different types of faults within the micro-grid. The micro-source output voltages are monitored and then transformed into dc quantities using the d-q reference frame. Any disturbance at the micro-source output due to a fault on the network will be reflected as disturbances in the d-q values. This disturbance is used to detect the fault and initiate the isolation the faulted section. Analysis and simulation results are presented for different types of faults within the microgrid.

Agent-Based Control Framework for Distributed Energy Resources Microgrids

Abstract: Distributed energy resources (DERs) provide many benefits for the electricity users and utilities. However, the electricity distribution system traditionally was not designed to accommodate active power generation and storage at the distribution level. The microgrid provides an effective approach to integrating many small-scale distributed energy resources into the bulk electric grid. This paper presents an agent-based control framework for distributed energy resources microgrids. The features of agent technology are first discussed. An agent-based control framework for DER microgrids is then presented. To demonstrate the effectiveness of the proposed agent-based control framework, simulation studies have been performed on a dc distributed energy system that can be used in a microgrid as a modular power generation unit. Simulation results clearly indicate that the agent-based control framework is effective to coordinate the various distributed energy resources and manage the power and voltage profiles.

Economic Analysis of Microgrids Including Reliability Aspects

Abstract: Recently, the new concept of microgrid (muG) has been emerging on distribution networks as a way to ease the integration of micro generation in LV networks and increase reliability. A muG is an association of a low voltage distribution network, small modular generation systems (micro-generators), loads and storage devices having some local coordinated functions. This entity can operate in two different modes: interconnected or emergency. In the first mode, the microgrid is connected with the distribution network, importing or exporting electricity and/or ancillary services. When in emergency mode, the microgrid operates isolated from the distribution network and uses local resources, changing from power control to frequency control and, if necessary, shedding load. A micro grid will only be established if its promoters achieve sufficient advantages that justify the incurred costs, namely the investment, operation and maintenance costs. The main purpose of this paper is to identify all the relevant costs and benefits and build a decision model for the situation, taking into account the regulatory framework, which is essential for the definition of some of the benefits. The paper also shows how to include in the evaluation the risk associated to the uncertainties in data and parameters. An illustrative example is included that shows a possible situation of equilibrium between global costs and benefits

Stability of a MicroGrid

Abstract: The demand for electricity continues to grow globally and the fonnation of MicroGrids is becoming a potentially attractive option to meet the expanding energy demands. A MicroGrid consists of a cluster of micro sources, energy storage systems (e.g. flywheel) and loads, operating as a single controllable system. A MicroGrid can be operated both connected to the main grid or as an island and must maintain its stability during both these modes of operation. This paper therefore investigates the important area of stability of a MicroGrid. An in-depth simulation study on stable operation of MicroGrids during faults is demonstrated and discussed in this paper with supporting PSCAD/ EMTDC results.

Intentional islanded operation of converter fed microgrids

Abstract: This paper develops a dynamic model of a converter fed islanded microgrid and proposes a control scheme to regulate its voltage and frequency. The model, which is formulated in an instantaneously synchronized reference frame, shows that the microgrid voltage depends on the real power balance between the converter and the load while reactive power balance determines the frequency. These results are used to synthesize a microgrid voltage and frequency control scheme that allow converters with standard inductor interface and dq frame current control to operate in intentional islanding model without using a frequency generator in the controller.

Decentralized Control for Parallel Operation of Distributed Generation Inverters in Microgrids Using Resistive Output Impedance

Abstract: In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac-distributed system is proposed. The paper explores the resistive output impedance of the parallel-connected inverters in an island microgrid. The control loops are devised and analyzed taking into account the special nature of a low voltage microgrid, in which the line impedance is mainly resistive and the distance between the inverters makes the control intercommunication between them difficult. In contrast with the conventional droop control method, the proposed controller uses resistive output impedance, and as a result a different control law is obtained. The controller is implemented by using a DSP board, which only uses local measurements of the unit, thus increasing the modularity, reliability, and flexibility of the distributed system. Experimental results are provided from two 6 kVA inverters connected in parallel, showing the features of the proposed wireless control.

System and method for controlling microgrid

Abstract: A system for controlling a microgrid includes microgrid assets and a tieline for coupling the microgrid to a bulk grid; and a tieline controller coupled to the tieline. At least one of the microgrid assets comprises a different type of asset than another one of the microgrid assets. The tieline controller is configured for providing tieline control signals to adjust active and reactive power in respective microgrid assets in response to commands from the bulk grid operating entity, microgrid system conditions, bulk grid conditions, or combinations thereof.

Validation of the CERTS microgrid concept the CEC/CERTS microgrid testbed

Abstract: The development of test plans to validate the CERTS microgrid concept is discussed, including the status of a testbed. Increased application of distributed energy resources on the distribution system has the potential to improve performance, lower operational costs and create value. Microgrids have the potential to deliver these high value benefits. This presentation focuses on operational characteristics of the CERTS microgrid, the partners in the project and the status of the CEC/CERTS microgrid testbed.

Benefits of power electronic interfaces for distributed energy systems

Abstract: Optimization of overall electrical system performance is important for the long-term economic viability of distributed energy (DE) systems. With the increasing use of DE systems in industry and its technological advancement, it is becoming more important to understand the integration of these systems with the electric power systems. New markets and benefits for distributed energy applications include the ability to provide ancillary services, improve energy efficiency, enhance power system reliability, and allow customer choice. Advanced power electronic (PE) interfaces will allow DE systems to provide increased functionality through improved power quality and voltage/VAR support, increase electrical system compatibility by reducing the fault contributions, and flexibility in operations with various other DE sources, while reducing overall interconnection costs. This paper examines the system integration and optimization issues associated with DE systems and show the benefits of using PE interfaces for such applications.

Microgrid field test experiences in Japan

Abstract: In Japan, three field tests of microgrid were started in 2005. The DER (distributed energy resources) capacity of each microgrid is 710, 750, and 2400 kW, respectively. A field test of voltage control for a distribution feeder connected with multiple PV systems was started in 2003. In this paper, at first proposed microgrid control system is outlined and then field tests outline is stated.

Grid power quality improvements using grid-coupled hybrid electric vehicles

Abstract: The paper examines the interaction between grid-coupled full hybrid electric vehicles (HEV) and the grid. The interaction allows the hybrid vehicle to travel a significant part of the time in all-electric mode. On the other hand, the grid coupling allows the grid to use the hybrid car as a controllable load and energy storage facility, enabling a higher penetration of variable generation units, such as renewable sources based on wind and photovoltaic cells. Experiments with a battery, grid connection and induction machine prove the feasibility of these grid-coupled hybrid electric vehicles. 1 Introduction The batteries currently used in HEVs are expensive and heavy, leading to a low specific energy content. Through the rapid development of batteries the cost per kWh diminishes while the specific energy content rises. As a consequence, larger amounts of energy can be stored in an economic way for the electric propulsion of HEVs and larger distances can be travelled in an all-electric “emission free” mode. An estimated 85 % of fuel (not energy) savings can be achieved if the vehicle has an all-electric driving range of 100 km [6]. In order to achieve this kind of fuel savings, the battery needs to be charged with electrical energy from the grid, as opposed to the current practice where a combustion engine powers an on-board electric generator to produce electric energy. The only requirement to make this happen is a grid coupling. This grid coupling makes a variety of interactions possible between a fleet of HEVs and the grid [1,5]. From the perspective of the hybrid fleet, the grid coupling enables a lower energy cost and a reduction in environmental pollution, on condition that the electric energy production itself is environmently friendly. From the perspective of the grid the fleet of hybrid electric vehicles can act as a controllable load to smooth grid load. Power electronics for the grid coupling, can provide reactive power to enhance the power factor of the installation and can be used as an active filter to eliminate harmonics. By injecting electrical energy in the grid, the HEV can be used as reserve power unit to set off the loss of a power plant, as replacement for peak power units, as part of a microgrid or as stand-alone mobile generator. Other possibilities such as peak-shaving are possible from a technical point of view, but lack the necessary economical feasibility.

Optimal Configuration Analysis of a Microgrid-Based Telecom Power System

Abstract: This paper presents a systematic microgrid-based telecom power system configuration analysis to establish a possible basis for an optimal design It compares several configurations using a value function that includes important telecommunication power plant characteristics, such as cost and availability The comparison yields guidelines on how best to configure the system Design choices studied include the system main bus voltage, redundancy policy, and the microsource configuration The analysis indicates that microgrid-based telecom power systems outperform traditional telecom power plants for medium to high-power sites, such as switching centers, and gives similar results for cell sites and low power transmission nodes Microgrid configurations with an approximate 156 V main dc bus voltage achieve a slightly higher value function than other evaluated options

Optimal sizing of distributed generators in microgrid

Abstract: Hybrid optimization model for electric renewables (HOMER), developed by National Renewable Energy Laboratory (NREL), enables economic analysis for single source and hybrid distributed energy resources (DERs). However, current version of HOMER does not support microgrid analysis. In this paper, economic analyzer for distributed energy resources (EADER) is developed. It finds minimum cost of energy (COE) and optimal mix of DERs with multiple sources and sinks. In addition to single source distributed generator (DG) and hybrid DG, EADER is also capable to analyze microgrid. EADER results are validated for single source DG and hybrid DG with results obtained from HOMER for the same systems. Further, a sample practical system from Western Maharashtra, India, is analyzed using EADER. The results which consider all practical constraints are presented and discussed.

Real-Time Analysis of the Control Structure and Management Functions of a Hybrid Microgrid System

Abstract: Due to their non-controllable nature Renewable Energy Source's (RES) participation in the network operation is currently rather deficient In this paper the microgrid concept is presented as an innovative solution to improve RES integration into the grid In this study an example of a hybrid microgrid composed of RES generators and a programmable support device together with a centralized management system is considered The microgrid can operate both in grid-connected mode and in islanded mode Its control structure as well as its management functions to assure successful performances in both operation modes and in the transitions between them are analyzed in detail and tested in real-time conditions with a physical control device

Micro-grid powered by photovoltaic and micro turbine

Abstract: In this paper, we propose to study the possibility of using a photovoltaic system combined with a high speed micro-turbine. This hybrid system can work as stand-alone system or grid connected system as it will be a part of a microgrid. Initially, we propose simple dynamic models of photovoltaic and micro turbine systems. Then, we carry out a comparison between simulations and measurements of the two systems. At last, simulation results show the effectiveness of the suggested hybrid system.

Reliability Stipulated Microgrid Architecture Using Particle Swarm Optimization

Abstract: Distributed energy resources have been receiving increasing attention over the past decade as alternatives to centralized generation. In the next few decades, the penetration of microgrids is expected to grow dramatically. The bulk of DERs will be integrated into existing distribution systems. To maximize the economic and reliability benefits of these installations, distributed systems will evolve into networked 'microgrids'. This paper describes a rational method of developing micro-grid architectures in a manner that maximizes the economic benefits of deploying the resources. The method consists of a particle swarm optimization (PSO) formulation that seeks to make the most economical connections that will satisfy the stipulated reliability criteria. This paper describes the method and its implementation, and illustrates it by means of examples. The validation of the method using a rigorous dynamic programming approach, is also outlined

Distributed Generation Placement for Optimal Microgrid Architecture

Abstract: This paper describes a method for siting of distributed energy resources (DER) within the framework of an optimal microgrid architecture. An optimal microgrid architecture is characterized by minimum cost interconnection, sizing, and siting of DER subject to stipulated global and local reliability criteria. This paper addresses the siting aspect of optimal microgrid architecture. The siting problem considers factors like deployment costs and savings gained by the use of combined heat and power (CHP). The problem is formulated as one of nonlinear programming and simulated annealing optimization is applied. This paper presents the development and implementation of the method, and demonstrates it using a six bus test system

Analysis of voltage regulation problem for a 25 kV distribution network with distributed generation

Abstract: This paper introduces a distribution network benchmark for investigation of alternative distribution system planning approaches. The benchmark system is part of a 25-kV distribution network serviced by a Canadian utility company and includes a significant amount of distributed generation (DG) sources. The benchmark is used to study under-voltage problems experienced by the utility company which are associated with interaction between DG units and the conventional voltage regulation apparatus. Analytical models of the network and line voltage regulators are developed. Control modes of the voltage regulators and performance during reverse power flow conditions are also discussed. Alternative planning approaches to mitigate the reported problems are introduced and their effect on voltage profiles and power flows are investigated.

A Probabilistic Search Method for Optimal Resource Deployment in a Microgrid

Abstract: The benefits of distributed energy resources (DERs) can be exploited by developing cost efficient deployment strategies. System planning based on stipulated reliability criteria leads to robust architectures which can meet the consumers' requirements. This paper presents a method for determining siting, sizing and optimal mix of resources from the available types of DER in a microgrid. The method recognizes the impact of Combined Heat and Power (CHP) to meet the thermal load. The paper presents a dynamic programming formulation and demonstrates its application on a six bus test system.

Field Tests of a Microgrid Control System

Abstract: A microgrid control system using multi-agent technologies is proposed. In this control system, operation planning is realized based on generation and load forecasting by using artificial neural network and fuzzy systems. Unit commitment of generations includes start/stop of power generations and energy storages. Load following function is accomplished by an adaptive control system based on conventional PI control scheme. To include, in a flexible manner, new generations or loads into the microgrid, multi-agent technologies might be applied. In this paper, at first proposed Microgrid control system is outlined and then field tests outline is stated.

Load fluctuation analysis of commercial and residential customers for operation planning of a hybrid photovoltaic and cogeneration system

Abstract: The Japanese government has set targets for increasing the contribution from renewable energy sources, such as wind power and photovoltaic (PV) generation. The fluctuating power of renewable energy sources, however, might degrade power quality such as voltage and frequency. To increase renewable energy near the demand side, a microgrid that utilizes controllable prime movers such as gas engines to compensate fluctuating demand and output of renewable energy is proposed here. First, measured load data of an office building and an apartment building were analyzed to clarify trends of daily load curve, periodograms, and residual demand. These analyses revealed the appropriate time length for operation planning and the economic load control problems of a hybrid generation system. The results of the optimal operation planning problem indicate that the proposed operation and control method is effective for energy-efficient operation of microgrids.