Showing papers on "Microgrid published in 2004"

Microgrid: a conceptual solution

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". During disturbances, the generation and corresponding loads can separate from the distribution system to isolate the microgrid's load from the disturbance (providing UPS services) without harming the transmission grid's integrity. This ability to island generation and loads together has a potential to provide a higher local reliability than that provided by the power system as a whole. In this model it is also critical to be able to use the waste heat by placing the sources near the heat load. This implies that a unit can be placed at any point on the electrical system as required by the location of the heat load.

Design, analysis, and real-time testing of a controller for multibus microgrid system

Abstract: This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

Microgrid power quality enhancement using a three-phase four-wire grid-interfacing compensator

Abstract: This paper presents a three-phase four-wire grid-interfacing power quality compensator for microgrid applications. The compensator is proposed for use with each individual distributed generation (DG) system in the microgrid and consists of two four-phase-leg inverters (a shunt and a series), optimally controlled to achieve an enhancement of both the quality of power within the microgrid and the quality of currents flowing between the microgrid and the utility system. During utility grid voltage unbalance, the four-phase-leg compensator can compensate for all the unwanted positive-, negative-, and zero-sequence voltage-current components found within the unbalanced utility. Specifically, the shunt four-leg inverter is controlled to ensure balanced voltages within the microgrid and to regulate power sharing among the parallel-connected DG systems. The series inverter is controlled complementarily to inject negative- and zero-sequence voltages in series to balance the line currents, while generating zero real and reactive power. During utility voltage sags, the series inverter can also be controlled using a newly proposed flux-charge current-limiting algorithm to limit the flow of large fault currents between the micro- and utility grids. The performance of the proposed compensator has been verified in simulations and experimentally using a laboratory prototype.

DC microgrid based distribution power generation system

Abstract: This paper describes an autonomous-control method for a DC microgrid system having distribution power generators. This system consists of following five generation and control units; a solar-cell generation unit, a wind-turbine generation unit, a battery energy-storage unit, a flywheel power-leveling unit, and an AC grid-connected power control unit. The proposed control method intended for suppression of circulating current detects only the DC grid voltage. Each unit could be controlled autonomously without communicating each other. This method brings high reliability, high-flexibility and maintenance-free operation to the system. Experimental results from a 10 kW-prototype system verify the validity and effectiveness of the proposed control method.

H/sup /spl infin// repetitive control of DC-AC converters in microgrids

Abstract: This paper proposes a voltage controller design method for DC-AC converters supplying power to a microgrid, which is also connected to the power grid. This converter is meant to operate in conjunction with a small power generating unit. The design of the output voltage controller is based on H/sup /spl infin// and repetitive control techniques. This leads to a very low harmonic distortion of the output voltage, even in the presence of nonlinear loads and/or grid distortions. The output voltage controller contains an infinite-dimensional internal model, which enables it to reject all periodic disturbances which have the same period as the grid voltage, and whose highest frequency components are up to approximately 1.5 kHz.

Microgrid autonomous operation during and subsequent to islanding process

Abstract: Summary form only given. This paper investigates (i) preplanned switching events and (ii) fault events that lead to islanding of a distribution subsystem and formation of a microgrid. The microgrid includes two distributed generation (DG) units. One unit is a conventional rotating synchronous machine and the other is interfaced through a power electronic converter. The interface converter of the latter unit is equipped with independent real and reactive power control to minimize islanding transients and maintain both angle stability and voltage quality within the microgrid. The studies are performed based on a digital computer simulation approach using the PSCAD/EMTDC software package. The studies show that an appropriate control strategy for the power electronically interfaced DG unit can ensure stability of the micro-grid and maintain voltage quality at designated buses, even during islanding transients. This paper concludes that presence of an electronically-interfaced DG unit makes the concept of micro-grid a technically viable option for further investigations.

A multiagent system for microgrids

Abstract: This paper presents the capabilities offered by multiagent system technology in the operation of a microgrid. A microgrid is a new type of power system, which is formed by the interconnection of small, modular generation to low voltage distribution systems. Microgrids can be connected to the main power network or be operated autonomously, similar to power systems of physical islands. The use of MAS technology can solve a number of specific operational problems: the small DG (distributed generation) units have different owners, so centralized control is difficult. Several decisions should be taken locally. Lack of dedicated communication facilities. Microgrids will operate in a liberalized market so the decisions of the controller of each unit concerning the market should have a certain degree of "intelligence". The local DG units besides selling power to the network have also other tasks: producing heat for local installations, keeping the voltage locally at a certain level or providing a backup system for local critical loads in case of a failure of the main system. These tasks reveal the importance of the distributed control and autonomous operation.

New grid scheduling and rescheduling methods in the GrADS project

Abstract: Summary form only given. The goal of the Grid Application Development Software (GrADS) project is to provide programming tools and an execution environment to ease program development for the grid. We present recent extensions to the GrADS software framework: 1. A new approach to scheduling workflow computations, applied to a 3D image reconstruction application; 2. A simple stop/migrate/restart approach to rescheduling grid applications, applied to a QR 3. A process-swapping approach to rescheduling, applied to an N-body simulation. Experiments validating these methods were carried out on both the GrADS MacroGrid (a small but functional grid) and the MicroGrid (a controlled emulation of the grid) and the results were demonstrated at the SC2003 conference.

Operation of a prototype microgrid system based on micro-sources quipped with fast-acting power electronics interfaces

Abstract: The paper presents experimental results from the operation of a prototype microgrid system, installed in the National Technical University of Athens, which comprises a PV generator, battery energy storage, local load and a controlled interconnection to the LV grid. Both the battery unit and the PV generator are connected to the AC grid via fast-acting DC/AC power converters. The converters are suitably controlled to permit the operation of the system either interconnected to the LV network, or in stand-alone (island) mode, with a seamless transfer from the one mode to the other. The paper provides a technical description of the system components and the control concept implemented, along with extensive measurement results which demonstrate its capability to operate in the aforementioned way.

An interaction problem of distributed generators installed in a MicroGrid

Abstract: This paper investigates an interaction problem that might be induced from various kinds of distributed generators installed in a MicroGrid. One of the most important features of MicroGrid is islanding operation. MicroGrid does not include large central generators and all distributed small generators have to share all loads existing in the MicroGrid. In this case, undesirable mutual oscillation problem might appear when MicroGrid goes into islanding mode. In this paper, MicroGrid itself and several types of distributed generator are modeled by using Matlab/Simulink. A typical case is simulated and examined by using the developed model.

Control of power electronic interfaces in distributed generation microgrids

Abstract: Technological advances and environmental pressures are driving the interconnection of renewable energy sources to the distribution network. The interconnection of large amounts of non-traditional generation however causes problems in a network designed for ‘conventional’ operation. The use of power electronics interfaces and the ‘bundling’ of micro-generation and loads into so-called Microgrids, offers a potential solution. Each Microgrid is designed to operate as a ‘good citizen’ or near ideal conventional load. This paper discusses the various elements of the new Microgrid concept and presents suggestions for some typical control strategies for the various system elements.

The CERTS microgrid and the future of the macrogrid

Abstract: The blackouts of summer 2003 underscored the dependence of western economies on reliable supply of electricity with tight tolerances of quality. While demand for electricity continues to grow, expansion of the traditional electricity supply system is constrained and is unlikely to keep pace with the growing thirst western economies have for electricity. Furthermore, no compelling case has been made that perpetual improvement in the overall power quality and reliability (PQR) delivered is possible or desirable. An alternative path to providing for sensitive loads is to provide for generation close to them. This would alleviate the pressure for endless improvement in grid PQR and might allow the establishment of a sounder economically based level of universal grid service. Providing for loads by means of local power generation is becoming increasingly competitive with central station generation for a number of reasons, four key ones being non-technical constraints on expansion of the grid, improvements in small scale technologies, opportunities for CHP application, and the ubiquitous nature of sensitive loads in advanced economies. Along with these new technologies, concepts for operating them partially under local control in microgrids are emerging, the CERTS Microgrid being one example. It has been demonstrated in simulation, and a laboratory test of a three microturbine system is planned for early 2005, to be followed by a field demonstration. A systemic energy analysis of a southern California naval base building demonstrates a current economic on-site power opportunity.

Repetitive Control of DC-AC Converters

Abstract: This paper proposes a voltage controller design method for dc-ac converters supplying power to a microgrid, which is also connected to the power grid. This converter is meant to operate in conjunction with a small power generating unit. The design of the output voltage controller is based on and repetitive control techniques. This leads to a very low harmonic distortion of the output voltage, even in the presence of nonlinear loads and/or grid distortions. The output voltage controller contains an infinite-dimensional internal model, which enables it to reject all periodic disturbances which have the same period as the grid voltage, and whose highest frequency components are up to approximately 1.5 kHz.

The MicroGrid: using online simulation to predict application performance in diverse grid network environments

Abstract: Improvements in networking and middleware technology are enabling large-scale grids that aggregate resources over wide-area networks to support applications at unprecedented levels of scale and performance. Unfortunately, existing middleware and tools provide little information to users as to the suitability of a given grid topology for a specific grid application. Instead, users generally use ad-hoc performance models to evaluate mappings of their applications to resource and network topologies. Grid application behavior alone is complex, and adding resource and network behavior makes the situation even worse. As a result, users typically employ nearly blind experimentation to find good deployments of their applications in each new grid environment. Only through actual deployment and execution can a user discovers if the mapping was a good one. Further, even after finding a good configuration, there is no basis to determine if a much better configuration has been missed. This approach slows effective grid application development and deployment. We present a richer methodology for evaluating grid software and diverse grid environments based on the MicroGrid grid online simulator. With the MicroGrid, users, grid researchers, or grid operators can define and simulate arbitrary collections of resources and networks. This allows study of an existing grid testbed under controlled conditions or even to study the efficacy of higher performance environments than are available today. Further, the MicroGrid supports direct execution of grid applications unchanged. These application can be written with MPI, C, C++, Perl, and/or Python and use the Globus middleware. This enables detailed and accurate study of application behavior. This work presents: (1) the first validation of the MicroGrid for studying whole-program performance of MPI grid applications and (2) a demonstration of the MicroGrid as a tool for predicting the performance of applications on a range of grid resources and novel network topologies.

Design, Analysis, and Real-Time Testing of a Controller for Multibus Microgrid System

Abstract: This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

Validating and Scaling the MicroGrid: A Scientific Instrument for Grid Dynamics

Abstract: Large-scale Grids that aggregate and share resources over wide-area networks present major challenges in understanding dynamic application and resource behavior for performance, stability, and reliability. Accurate study of the dynamic behavior of applications, middleware, resources, and networks depends on coordinated and accurate modeling of all four of these elements simultaneously. We have designed and implemented a tool called the MicroGrid which enables accurate and comprehensive study of the dynamic interaction of applications, middleware, resource, and networks. The MicroGrid creates a virtual Grid environment – accurately modeling networks, resources, the information services (resource and network metadata) transparently. Thus, the MicroGrid enables users, Grid researchers, or Grid operators to study arbitrary collections of resources and networks. The MicroGrid includes the MaSSF online network simulator which provides packet-level accurate, but scalable network modeling. We present experimental results with applications which validate the implementation of the MicroGrid, showing that it not only runs real Grid applications and middleware, but that it accurately models both their and underlying resource and network behavior. We also study a range of techniques for scaling a critical part of the online network simulator to the simulation of large networks. These techniques employ a sophisticated graph partitioner, and a range of edge and node weighting schemes exploiting a range of static network and dynamic application information. The best of these, profile-driven placement, scales well to online simulation of large networks of 6,000 nodes using 24 simulation engine nodes.

Development of sources and a testbed for CERTS microgrid testing

Abstract: The development of the CERTS microgrid concept is discussed, in addition to discussion of modifications necessary to the controls of microsources (microturbines, for example) that are necessary for operation in the CERTS microgrid. Also described is a testbed that will be constructed to test the CERTS microgrid concept.

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 (a shunt and a series) are used for each distributed generation (DG) system in the microgrid. For 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 (balanced or unbalanced). The proposed compensator has been tested in simulations and experimentally using a laboratory hardware prototype.

Optimal selection of on-site generation with combined heat and power applications

Abstract: While demand for electricity continues to grow, expansion of the traditional electricity supply system, or macrogrid, is constrained and is unlikely to keep pace with the growing thirst western economies have for electricity. Furthermore, no compelling case has been made that perpetual improvement in the overall power quality and reliability (PQR) delivered is technically possible or economically desirable. An alternative path to providing high PQR for sensitive loads would generate close to them in microgrids, such as the Consortium for Electricity Reliability Technology Solutions (CERTS) Microgrid. Distributed generation would alleviate the pressure for endless improvement in macrogrid PQR and might allow the establishment of a sounder economically based level of universal grid service. Energy conversion from available fuels to electricity close to loads can also provide combined heat and power (CHP) opportunities that can significantly improve the economics of small-scale on-site power generation, especially in hot climates when the waste heat serves absorption cycle cooling equipment that displaces expensive on-peak electricity. An optimisation model, the Distributed Energy Resources Customer Adoption Model (DER-CAM), developed at Berkeley Lab identifies the energy bill minimising combination of on-site generation and heat recovery equipment for sites, given their electricity and heat requirements, the tariffs they face, and a menu of available equipment. DER-CAM is used to conduct a systemic energy analysis of a southern California naval base building and demonstrates a typical current economic on-site power opportunity. Results achieve cost reductions of about 15% with DER, depending on the tariff. Furthermore, almost all of the energy is provided on-site, indicating that modest cost savings can be achieved when the microgrid is free to select distributed generation and heat recovery equipment in order to minimise its overall costs.

Hardware Development of a Laboratory-Scale Microgrid Phase 2: Operation and Control of a Two-Inverter Microgrid

Abstract: This report summarizes the activities of the second year of a three-year project to develop control software for microsource distributed generation systems. In this phase, a laboratory-scale microgrid was expanded to include: (1) Two emulated distributed resources; (2) Static switchgear to allow rapid disconnection and reconnection; (3) Electronic synchronizing circuitry to enable transient-free grid interconnection; (4) Control software for dynamically varying the frequency and voltage controller structures; and (5) Power measurement instrumentation for capturing transient waveforms at the interconnect during switching events.

N-phase interface tracking method utilizing unique enumeration of microgrid cells

Abstract: A method for tracking N fluid materials and their associated interfaces during simulated fluid flow is disclosed. A microgrid cell methodology is embedded on a regular macrogrid to subdivide and then tag fluid materials in a computational system preferably using a prime numbering algorithm. The motion of microgrid cells is tracked based on local velocity conditions, rectifying small anomalies by a coupled evaluation of local volume fraction fields and global mass conservation. Volume fractions can be calculated at any time step via an evaluation of the prime locations so that average cellular density and viscosity values can be regularly updated.

Effects of a carbon tax on microgrid combined heat and power adoption

Abstract: This paper describes the economically optimal adoption and operation of distributed energy resources (DER) by a hypothetical California microgrid consisting of a group of commercial buildings over an historic test year, 1999. The optimisation is conducted using a customer adoption model (DER-CAM) developed at Berkeley Lab and implemented in the General Algebraic Modeling System (GAMS). A microgrid is a semiautonomous grouping of electricity and heat loads interconnected to the existing utility grid (macrogrid) but able to island from it. The microgrid minimises the cost of meeting its energy requirements (consisting of both electricity and heat loads) by optimising the installation and operation of DER technologies while purchasing residual energy from the local combined natural gas and electricity utility. The available DER technologies are small-scale generators (< 500 kW), such as reciprocating engines, microturbines, and fuel cells, with or without combined heat and power (CHP) equipment, such as water and space heating and/or absorption cooling. By introducing a tax on carbon emissions, it is shown that if the microgrid is allowed to install CHP-enabled DER technologies, its carbon emissions are mitigated more than without CHP, demonstrating the potential benefits of small-scale CHP technology for climate change mitigation. Reciprocating engines with heat recovery and/or absorption cooling tend to be attractive technologies for the mild southern California climate, but the carbon mitigation tends to be modest compared to purchasing utility electricity because of the predominance of relatively clean central station generation in California.

Fuel consumption minimisation of a microgrid

Abstract: A cost optimisation scheme for a microgrid is presented. Prior to the optimisation of the microgrid itself, several power sharing schemes between two generators are compared. Then, the fuel minimisation of a small power system with a variety of power sources is discussed. The optimisation of a small power system has important differences with respect to a large system which falls into the traditional economic dispatch problem. Among the most important differences may be the presence of a local heat demand which adds another dimension to the problem. The small power system considered in this paper is a microgrid that consists of two reciprocating gas engines, a combined heat and power plant, a photovoltaic array and a wind generator. The optimisation is aimed at reducing the fuel consumption rate of the system, constrained to fulfil the local energy demand (both electrical and thermal) plus a certain minimum power reserve and penalised for any heat produced in excess of demand. The solution of the optimisation problem strongly supports the concept of having a dedicated communication infrastructure between the power sources.

Control of inverter-based distributed generation used to provide premium power quality

Abstract: Previous work has demonstrated that inverter-based distributed generation has large potential to provide premium power quality. This paper reviews how solutions for some of the key technical issues might be formed. More specifically, distributed control and microgrid ride-through control are discussed. Two specific examples are included to complete the discussion.

Small Signal Stability Analysis of the Integrated Power System - MicroGrid Model

Abstract: We propose a method of small-signal stability analysis of power systems with microgrids. The methodology is based on the development of an integrated model in quadratic form and subsequent development of the transition matrix of the overall system. The eigenvalues of the transition matrix provide the small-signal stability properties of the system. An example system is analyzed with the proposed methodology. The results clearly identify the interactions between the power system and the microgrid. As the number of DERs increase in the system, so does the number of natural frequencies of oscillation of the overall system. The paper also identifies issues that need further investigation.

Behavior of two capstone 30kW microturbines operating in parallel with impedance between them

Abstract: This report describes the tests conducted to determine the behavior of two Capstone 30 kW microturbines connected in parallel with some impedance between them. This test was meant to simulate the operation of two microturbines at nearby customer facilities. This arrangement also constitutes a simple microgrid. The goal of this test was to investigate if any voltage and power instabilities exist between the two microturbines. Two test sequences were conducted. The first test sequence operated two microturbine/load bank pairs using manual control of the microturbine and load bank setpoints. The second test sequence used the Capstone Load Following mode of operation to control generation levels of one of the microturbines. The two microturbine/load bank sets were connected together through a 300 foot long, four conductor, #12 cable so th at the impedance between them would cause up to a 5 percent voltage drop (depending on the load balance between the two sets). Data was collected from both Capstone microturbines, two power quality instruments and a power monitor. The tests showed there were no instabilities in the microturbines' voltage or power output as long as care was taken not to overload either unit. A voltage drop of almost 5 percent was observed between the two microturbines at the highest loadings. This soft connection between the two microturbines did not cause problems. Basic protective functions of the microturbines avoided unintentional islanded operation, but probably would not be sufficient or desirable for normal microgrid operations. Some simple automatic load sharing could be accomplished by using the Load Following mode of operation of the microturbine. This ability was demonstrated during the second set of tests. Load sharing works as long as all loads are kept within operating limits of the two microturbines. Use of the load following mode of the microturbine seemed to work fairly well. To improve the responsiveness of the load following, a faster pulse rate would need to be obtained from the kilowatt-hour meter for the expected loads. The faster pulse rate would allow a shorter averaging period in the microturbine which would make it respond more quickly. Revision of the microturbine operating software would be desirable so that both microturbines could share load and voltage regulation duties in the microgrid. Once these abilities were integrated into the microturbines, the functions would need to be verified in a set of lab tests and then checked in actual field operations. Additional protection functions would also need to be integrated into the microgrid so that a fault on the microgrid would not drop all load and generation.