There is general consensus that major changes lie ahead for utility grids worldwide. The goal for significant increases in renewable sources is universally accepted and making the workings of power networks much ‘smarter’ is growing as well. This added intelligence in the grid, coupled with growing penetration of variable sources (wind and solar), can create dynamic conditions that will be challenging for grid operators. In response to these concerns the interest in adding more energy storage to the grid has gained momentum in the last five years, particularly in the U.S.
Strong Storage Foundation
Many view the current interest in storage as a new phenomenon, although almost 20 GW of pumped hydropower storage was added to the US grid in the period from 1960 to 1990. This was done to provide off-peak base load to the growing fleet of nuclear power plants in the USA. Other countries like Japan and Germany have made major commitments to bulk storage using pumped hydropower and have goals of 15% and 10% for their total generation mix.
The vast size of the US makes commitments like this more challenging and utilities have begun to examine more types of distributed storage solutions throughout their networks.
The closer that stored energy is placed to customer loads, the greater the value to system reliability and load management. S&C Electric Company has worked closely with several major utilities to field 6 storage projects, which are summarized below.
Load Peak Shaving
One of the most logical uses of energy storage is to peak shave the load in a substation to postpone the capital expense of upgrading that station’s capacity for several years until overall load growth dictates that construction cost. Figure 1 shows the first megawatt scale sodium sulfur (NaS) battery installed in the U.S., which went into service in June, 2006.
Figure 2 shows the peak shaving ability of the NaS battery during the summer peak loads of 2006. The battery was connected on the load size of a 20 MVA transformer and kept the loading below the 20 MVA level, averaging 8 hours of peak reduction each day and charging over a 7-hour period during off-peak times.
Combing Smart Grid Goals with Intelligent Storage
After the success of the first peak-shaving demonstration American Electric Power (AEP) continued to team with S&C, the US Department of Energy (DoE) and Sandia National Labs to expand the storage applications and solve a greater variety of distribution feeder challenges. These goals include:
- To implement the DoE goal of utilizing storage technology as an integral part of establishing a ‘Smart Grid’;
- To quantify the viability of large multimegawatt batteries as a tool to mediate distribution substation and feeder issues; and
- To serve radially fed distribution feeder loads for several hours during periods when a long-term fault upstream of the battery necessitates ‘islanded’ operation on battery.
Beginning in 2008, three, 2 MW battery projects were launched in Ohio, Indiana and West Virginia to test battery power as a tool in the creation of ‘self healing’ distribution networks on zones where intelligent feeder switching and protective devices communicate in real time to isolate faults and restore service in seconds without waiting for SCADA commands from the utility control center.
By adding energy storage in the substation, this concept allows a fairly large area of a network to continue to operate as a self-powered island power grid in the event of loss of the station transmission feeder. In this application a 2 MW battery can power hundreds of customers who otherwise would be in the dark until crews restored service. Figure 3 shows one of three 2 MW systems installed for this application.
The Value Proposition for Islanding
Reliability is of great significance to the electric utility as it helps gauge its ability to provide consistent and dependable service to its customers. Electric utilities are required to provide electric service reliability data to their respective public utilities commissions.
Based on that data, they develop plans to maximize reliability across their electric grids. Numerous benefits accrue from the ability to intelligently island sections of the grid when a fault occurs, including:
- Improved Reliability Indices: Reliability indices such as CAIDI and SAIDI are standard measures of reliability used to determine the dependability of electric utility service. Islanding can significantly improve these indices as fewer customers will be without power, and the average duration of outages would be significantly reduced;
- Resource Optimization: Islanding allows prioritization of the restoration process by allowing limited human and physical resources to concentrate on non-islanded areas first. Islanding will save human and monetary resources as crews may not need to be dispatched overtime at night to islanded regions. Depending on the nature of the outage they can handle the restoration as part of their normal daily schedule; and
- Capital Deferral: Islanding can provide an immediate fix for a problematic network and allow more traditional solutions (station construction/enlargement, transmission extensions and/or distribution feeder enhancements) to be deferred until the grid can be re-designed to alleviate the problems.
With the benefits from islanding outlined above, the question then becomes: ‘What is the most practical way to actually achieve islanding?’ In analyzing a number of ways to proceed in solving several specific issues affecting three applications where islanding of stored energy might actually be justified, AEP considered two distinct methodologies for load management for achieving islanding of distributed resources:
Adaptive Dynamic Islanding
Adaptive Dynamic Islanding (ADI) will rely upon the utility’s ability to ‘turn on or turn off’ individual customer loads remotely through the use of advanced metering infrastructure (AMI). Once AMI is sufficiently deployed, the adaptive approach to dynamic islanding will become fairly practical to implement and control.
Adaptive dynamic islanding promises the ability to treat each customer load as an island given sufficient development/deployment of AMI technology. Once a sufficiently high level of AMI penetration is reached, every customer’s electric meter can be controlled remotely. Thus certain critical loads such as hospitals, police stations and firehouses could be given priority in the event of electrical power outages. In similar fashion, it would be possible for less critical customer loads to be ‘cycled’ during the interruption in order to share the available power and ‘socialize’ the benefits from islanding while spreading the inconvenience of interruptions equitably within the island.
Since the penetration of AMI devices was not sufficient to allow implementation of Adaptive Dynamic Islanding (at least in the areas where it could potentially be justified), AEP looked for a practical alternative to provide the needed level of load control over a section of the network.
Discrete Dynamic Islanding
Discrete Dynamic Islanding describes the ability to connect/disconnect discreet sections or zones of the grid (feeder sections) instead of individual customers, as in the adaptive dynamic islanding scheme described above. Thus each section of the grid that becomes islanded will include several residential and/or commercial buildings.
This type of islanding is made possible through the use of advanced communication and control systems that employ distributed intelligence spread among the feeder’s sectionalizing and protective devices that then communicate directly with each other to automatically isolate faults and restore service to un-faulted line sections. This method of islanding proved to be easier and quicker to implement as the core technology and the requisite hardware was currently available. AEP decided to implement this method in order to more quickly evaluate the overall benefits of islanding technology.
Figure 4 shows the discrete (zonal) approach to load control and management based on total load in each feeder section at the time of an outage and the magnitude of stored energy available from the battery. These two variables are managed based on the projected time for feeder restoration.
The three islanding projects installed in the AEP grid have performed well and met the objectives from network improvement in each location.
Wind-to-Battery Storage Project
One of the most significant battery storage projects undertaken in the USA began in 2007 with the goal of building a ‘test bed’ demonstration system for detailed testing of a battery integrated with an actual wind farm.
Xcel Energy constructed a small-scale demonstration using a 1 MW, 6-hour NaS battery system installed near a 13 MW wind farm as shown in Figure 5.
The goal of the project was to monitor performance for at least one year and investigate the cost-effectiveness of the storage device and develop methods and procedures to evaluate other types of energy storage technologies in the future. The focus of the project dealt with five primary categories for evaluating the benefits of storage:
- Basic Generation Storage (Time Shifting): Scheduled moving blocks of energy from off-peak to on-peak;
- Economic Dispatch: Tested using spot energy prices in the market to determine the dispatch of stored energy for maximum value;
- Frequency Regulation: Tested the use of the battery to follow a frequency regulation signal derived from changes in the Area Control Error (ACE) for the MISO market;
- Wind Smoothing (Ramp Rate Control): Tested the effectiveness adjusting the battery charge and discharge rates to effectively smooth the rate of charge of wind power flowing into the utility grid; and
- Wind Leveling (Steady Output Control): These tests adjusted the percentage of storage to the size of the wind farm by lowering the farm’s total wind capacity to show effectiveness of load leveling with the storage capacity at 10%, 20% and 100% of the wind farm capacity.
The project provided a wealth of data regarding optimizing energy storage and wind power. The project analysts recommend more testing using the latest wind forecasting programs to further improve the overall performance into the utilities’ daily business operations as the penetration of renewables continue to grow.
Storage Value in the Future
These early storage projects conducted in utility distribution systems have been vital in raising the awareness of the benefits of storage for integration of renewable energy sources. The success of the projects have caused more utilities to move forward with projects of their own; and, helped prompt DoE to fund nearly 500 MW of storage projects through the stimulus bill in 2009. Storage is now poised to become an integral part of the nation’s electricity grids over the next two decades.
About the Author:
Bradford P. Roberts is the Power Quality Systems Director for the Power Quality Products Division of S&C Electric Company, which specializes in low- and medium-voltage power protection systems.
Roberts was Chairman of the IEEE Power Engineering Society’s Emerging Technologies Committee and Executive Director of the Electricity Storage Association (ESA), and was Chairman of the Board. He has been a member of the ESA Board for 10 years.
He is currently a member of DoE’s Electricity Advisory Committee and Chairman of the Energy Storage Sub-committee.
Renewable Energy Focus U.S. Issue 2, September/October 2010.