Robert H. Lasseter

Smart Distribution: Coupled Microgrids

The distribution system provides major opportunities for smart grid concepts. One way to approach distribution system problems is to rethinking our distribution system to include the integration of high levels of distributed energy resources, using microgrid concepts. Basic objectives are improved reliability, promote high penetration of renewable sources, dynamic islanding, and improved generation efficiencies through the use of waste heat. Managing significant levels of distributed energy resources (DERs) with a wide and dynamic set of resources and control points can become overwhelming. The best way to manage such a system is to break the distribution system down into small clusters or microgrids, with distributed optimizing controls coordinating multimicrogrids. The Consortium for Electric Reliability Technology Solutions (CERTSs) concept views clustered generation and associated loads as a grid resource or a “microgrid.” The clustered sources and loads can operate in parallel to the grid or as an island. This grid resource can disconnect from the utility during events (i.e., faults, voltage collapses), but may also intentionally disconnect when the quality of power from the grid falls below certain standards. This paper focuses on DER-based distribution, the basics of microgrids, possibility of smart distribution systems using coupled microgrid and the current state of autonomous microgrid technology.

Integration of distributed energy resources. The CERTS Microgrid Concept

LBNL-50829 Consortium for Electric Reliability Technology Solutions White Paper on Integration of Distributed Energy Resources The CERTS MicroGrid Concept Prepared for Transmission Reliability Program Office of Power Technologies Assistant Secretary for Energy Efficiency and Renewable Energy U . S . Department of Energy Energy Systems Integration Program Public Interest Energy Research California Energy Commission Prepared by Robert Lasseter, Abbas A k h i l , Chris Marnay, John Stephens, Jeff Dagle, Ross Guttromson, A . Sakis Meliopoulous, Robert Yinger, and Joe Eto A p r i l 2002 The work described in this report was coordinated by the Consortium for Electric Reliability Technology Solutions, and funded by the Assistant Secretary of Energy Efficiency and Renewable Energy, Office of Power Technologies of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 and by the California Energy Commission, Public Interest Energy Research Program, under Work for Others Contract No. B G 99-39.

Distributed Generation Interface to the CERTS Microgrid

This paper focuses on the energy storage system and the power electronic interface included in microsources of the CERTS microgrid. CERTS stands for the Consortium for Electric Reliability Technology Solutions. The consortium was formed in 1999 to research, develop, and disseminate new methods, tools, and technologies to protect and enhance the reliability of the U.S. electric power system and efficiency of competitive electricity markets. To provide the plug-and-play feature and the power quality requirements of the CERTS microgrid, all microsources regardless of their prime mover type must have a unified dynamic performance. This necessitates attaching an energy storage module to some or all of the microsources. The storage module is attached to the prime mover through a power electronic interface that couples the microsource to the microgrid. Details of the energy storage module, the power electronic interface and the corresponding controls are described. Performance of an example microsource, which includes a synchronous generator, a storage module and an electronic interface, is studied. Dynamic performance of the example microsource when operating in the CERTS microgrid is evaluated based on digital time-domain simulations in the EMTP-RV software environment. Effectiveness of the storage module, the electronic interface and the corresponding controls in enhancing the microsource performance is verified.

Real-World MicroGrids-An Overview

Microgrids are networks of small, distributed electrical power generators operated as a collective unit - a system of energy systems. The range of hardware and control options for Microgrid operation are reviewed. The paper summarizes and highlights the operating principles and key conclusions of research and field trials to-date. An overview is given on demonstration projects for Microgrids which have been, and are being, constructed.

CERTS Microgrid Laboratory Test Bed

The CERTS Microgrid concept captures the emerging potential of distributed generation using a system approach. CERTS views generation and associated loads as a subsystem or a “microgrid.” The sources can operate in parallel to the grid or can operate in island, providing uninterruptible power-supply services. The system can 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. CERTS Microgrid concepts were demonstrated at a full-scale test bed built near Columbus, OH, and operated by American Electric Power. The testing fully confirmed earlier research that had been conducted initially through analytical simulations, then through laboratory emulations, and finally through factory acceptance testing of individual microgrid components. The islanding and resynchronization method met all Institute of Electrical and Electronics Engineers Standard 1547 and power-quality requirements. The electrical protection system was able to distinguish between normal and faulted operation. The controls were found to be robust under all conditions, including difficult motor starts and high impedance faults.

Statcom controls for operation with unbalanced voltages

Voltage sourced static VAr compensators such as the Statcom need to be able to handle unbalanced voltages. Mild imbalance can be caused by unbalanced loads while severe short-term imbalance can be caused by power system faults. A synchronous frame voltage regulator is presented that works even when three phase symmetry is lost. This regulator addresses voltage imbalance by using separate regulation loops for the positive and negative sequence components of the voltage. The proposed regulator allows the Statcom to ride through severe transient imbalance without disconnecting from the power system and, further, to assist in rebalancing voltages. The regulator maintains sufficient bandwidth to perform flicker compensation. The controllers performance is simulated for a Statcom in a model distribution system where it is subjected to a severe single line to ground fault and a rapidly varying three phase load.

Providing premium power through distributed resources

The modern industrial facility depends on sensitive electronic equipment that can be shut down suddenly by severe power system disturbances. A large number of these disturbances on the power system are a result of line faults which can cause momentary voltage sags. This results in equipment malfunctioning and high restart cost. This papers describes the control of distributed resources as a solution to such problems. In particular the focus is on systems of distributed resources that can switch from grid connection to island operation without causing problems for critical loads.

Dynamic models for micro-turbines and fuel cells

Distributed resources (DR) include a variety of energy sources, such as micro-turbines, photovoltaics, fuel cells, and storage devices, with capacities in the 1 kW to 10 MW range. Deployment of DR on distribution networks could potentially increase their reliability and lower the cost of power delivery by placing energy sources nearer to the demand centers. By providing a way to by-pass conventional power delivery systems, DR could also offer additional supply flexibility. The trends in technology points toward smallness, under the 500 kW level. An example are the small gas fired micro-turbines in the 25-100 kW range that can be mass produced at low cost. Fuel cells are also well suited for distributed generation applications. There are two basic classes of micro-source systems; one is a DC source, such as fuel cells, photovoltaics, and battery storage, the other is a high frequency AC source such as the micro-turbine which needs to be rectified. Power electronic interfaces introduces new control issues and new possibilities. This paper discusses micro-turbine and fuel cell models, and the instanteous power issue. An inverter model is also presented.

Dynamic response of a thyristor controlled switched capacitor

This paper computes the small signal dynamic response of a thyristor controlled series capacitor system for use in flexible AC transmission system control design. The computation includes the effects of synchronization and the nonlinearity due to thyristor switching. Eigenvalues of the small signal dynamic response are computed and used to study the dynamic response of the Kayenta system using different methods of synchronization and a closed loop control. >

The operation of diesel gensets in a CERTS microgrid

In this paper the operation of diesel engine-driven wound-field synchronous generator sets as distributed generators (DGpsilas) is studied. The objective of this work is to develop the modeling and control framework for such gensets to enable their operation in a distribution system that contains multiple DGpsilas including inverter-based sources. The paper presents experimental results for the interaction of conventional gensets with inverter-based sources in a microgrid test system. From the test results it is observed that there is significant circulating reactive power between the sources as well as frequency oscillations caused by the response of the conventional genset controller. A new controller for the genset is proposed that alleviates these issues and enables the various sources to share power and maintain power quality within the system. The operation of the new controller is demonstrated using simulation results.