Microgrids

Caterpillar is deploying a 750-kW microgrid on the island of Guam—a challenging deployment environment because of the island power grid and extreme weather phenomena. To address these challenges, the microgrid will include a rapid solid-state switch to protect the microgrid from grid disturbances.

NREL collaborated with Caterpillar to test a prototype utility-scale energy storage inverter and microgrid controller. Microgrid operation was validated in a power hardware-in-the-loop experiment using a programmable DC power supply to emulate the battery and a grid simulator to emulate the Guam grid-tie point. The validation scenarios included grid disturbances approaching 1 MW.

NREL developed a PV-battery-diesel hybrid power system for the U.S. Army Rapid Equipping Force and the Expeditionary Energy and Sustainment Systems to provide power to forward operating bases. The cornerstone of the hybrid power system is the Consolidated Utility Base Energy (CUBE) system. The CUBE provides the power conversion, distribution, and protection necessary to integrate various power sources and was built from the ground up to provide a flexible platform that can be modified to meet specific needs.

The CUBE was tested to demonstrate fuel savings as well as power quality relative to a baseline diesel-generator-only system. Additional tests were performed to demonstrate CUBE power quality during load steps, mode transitions, and a black start. Results demonstrated the ability of the CUBE to provide comparable load step response as a diesel generator, to maintain high power quality during transitions from diesel generator as a grid-forming unit to CUBE as a grid-forming unit and vice versa, and to provide high power quality during a black start onto a load.

NREL has developed a cyber-physical test bed to investigate the complex interactions among emerging microgrid technologies such as grid-interactive power sources, control systems, and communication platforms and bandwidths. The cyber-physical testbed consists of three major components for testing and validation:

• Real-time models of a distribution feeder with microgrid assets integrated into a power hardware-in-the-loop platform
• Real-time-capable network simulator-in-the-loop models
• Physical hardware, including inverters and a simple system controller.

On this platform, several load profiles and microgrid configurations were tested to examine effects on system performance with increasing channel delays and router processing delays. Testing demonstrated that the controller's ability to maintain a target grid import power band was severely diminished with increasing network delays and laid the foundation for future testing of more complex cyber-physical systems.

NREL supported the development and acceptance testing of a microgrid battery energy storage system developed by EaglePicher Technologies as part of an effort sponsored by U.S. Northern Command. The three-tiered, 300-kW/386-kWh grid-tied system is capable of providing grid stabilization, microgrid support, and on-command power response. The three tiers of batteries are lithium-Ion, nickel cadmium, and lead acid configured to deliver an appropriate balance of available energy and power. The system is installed in a microgrid test bed at NREL's Energy Systems Integration Facility with load banks that emulate microgrid critical loads and a programmable AC power supply that emulates the grid tie. It is being tested to demonstrate its ability to provide voltage support, frequency support, arbitrage, peak shaving, and microgrid critical load service.

NREL is supporting Honeywell on a Department of Defense Environmental Security Technology Certification Program 1-MW microgrid demonstration at the Navy's Pacific Missile Range Facility in Hawaii. NREL assisted with the initial design and installation of the energy management system in 2013, which enabled the installation to dispatch more PV generation while avoiding power export to the utility. The system will be upgraded by reconfiguring the onsite electrical distribution system to allow for an operating microgrid that leverages all onsite generation equipment and maximizes the footprint served. The microgrid includes conventional generation (diesel-fueled reciprocating engine generators) as well as solar PV (multiple distributed arrays ranging from 50 kW to 260 kW). The installation also has an energy management system that uses batteries and advanced monitoring and control technology to dampen short-duration swings in solar PV production.

The Microgrid Cost Study is focused on identifying the costs of components, integration, and installation of existing U.S. microgrids and project cost improvements and technical accelerators over the next five years and beyond. This information can be used to develop research and development agendas for next-generation microgrids that provide cost-effective, reliable, and clean energy solutions. This project will provide insight, transparency, and standardization in the reporting of microgrid costs and identify market segment differences for future cost reductions across microgrid applications

NREL supported Raytheon on a Department of Defense Environmental Security Technology Certification Program microgrid demonstration at Marine Corps Air Station Miramar. The project included integration of a central controller with PV inverters, a zinc bromide flow battery energy storage system, utility service entrance equipment, metering, and building electrical loads. The goals were to demonstrate energy security, provide islanding capability, and reduce energy costs.

Microgrid functionality was initially tested at NREL's Energy Systems Integration Facility in 2014 using a Parker battery inverter, AE PV inverters, and programmable DC power supplies to emulate the battery and PV arrays and a programmable AC power supply to emulate the grid-tie. Grid-tied and islanded operation of the fully installed, high-penetration system at Miramar was demonstrated in December 2015 and again in June 2016. As a result, the project team received the 2016 Environmental Security Technology Certification Program Project of the Year Award for Energy and Water.

NREL is collaborating with the San Diego Gas & Electric Co. to model a microgrid in Borrego Springs, California, and evaluate how a microgrid controller with advanced functionality would perform there. Researchers are constructing a scaled model of the microgrid by employing power and controller hardware to represent the distributed energy resources—including a large PV plant, energy storage systems, and diesel generators— while other circuit components are virtually represented in a model on real-time digital simulators. They are then interfacing an actual microgrid controller to the power hardware and to the virtual components to test the system's performance, particularly with regard to disconnection and reconnection of the microgrid to the utility.

NREL is running this model on a combination of local real-time digital simulators and real-time digital simulators at San Diego Gas & Electric's Integrated Test Facility, with the simulators connected remotely. This will allow San Diego Gas & Electric to use NREL's power hardware capabilities remotely.

NREL and San Diego Gas & Electric are also working to understand the use of grid-forming inverter functionalities in a microgrid setting. NREL will install grid-forming inverters in its Energy Systems Integration Facility and perform power hardware-in-the-loop experiments to understand the support these inverters provide to the microgrid.