Current microgrid designs integrating distributed generation and renewable energy sources require large-scale energy storage, typically in the form of batteries, to enable a high power quality transition to islanding. These energy storage systems, however, are prohibitively expensive and will slow the application of microgrids at U.S. military installations. This project had two main objectives:

  1. Demonstrate the ability to operate a microgrid with less expensive power storage instead of large-scale energy storage.
  2. Demonstrate that the renewable energy with small-scale power storage can maintain power quality in islanded mode with minimal use of the generators during non-optimal (e.g., cloud covered) periods.

Technology Description

This project demonstrated a power optimized storage approach to microgrids that replaces today’s approach of long-term energy storage with (legacy) generators primarily off-line and intermittent renewable sources like photovoltaic (PV). The technologies employed are power delivery optimized storage, transiently rated inverters, integration with legacy generator controls, and microgrid compatible inverters for PV.

A 400 kilowatt (kW) microgrid application employing power optimized energy storage, transient rated storage inverter, microgrid enabled PV inverters, and a relatively high percentage PV energy source component as well as modified legacy natural gas (NG) generator control was successfully demonstrated at Fort Sill, Oklahoma. The microgrid load and some of its auxiliary equipment is an air conditioning chiller, presenting a variable load up to 350kW. The major components of the power optimized microgrid and the associated three phase power connection scheme are depicted below.

Demonstration Results

The demonstrations were successful and showed that power optimized storage, in conjunction with NG generators and renewables, can support an islanded microgrid without loss of power quality, staying within Institute of Electrical and Electronics Engineers (IEEE) Standard 1547 voltage and frequency limits. This includes the case of an unintentional island, where grid is lost and generators were off. The storage system powers the load until generators go online, with generator synchronization being faster due to the stable bus provided by the storage system. The demonstrations also showed that high penetration PV along with power optimized storage can power an islanded microgrid and supplement generators while maintaining a stable voltage bus.

Implementation Issues

Compared to energy optimized storage, the power optimized storage system proved to be 33% of the cost and 13% of the physical volume. This will enable greater acceptance and penetration of microgrids, as energy storage is typically the most costly required new equipment for a high performance microgrid. The system’s smaller footprint provides future owners greater flexibility in storage system installation location.

For large inductive loads, reduced voltage starting techniques should be used where possible, to reduce transients within the power system and maintain line voltage above IEEE 1547 minimums. Any future operators must be identified early in the deployment process so that they acquire a more complete understanding of the technology, its intended use, and future maintenance issues. When using parallel racks of batteries, isolating a “bad” rack is more efficient than keeping it in the microgrid, as the Battery Management System calculation of state of charge is based on all the racks.