By Jennifer Eirich and Stephen Vechy
Hurricane Sandy was the worst storm to hit the Eastern U.S. in a century, resulting in 8.5 million power outages across 21 states-the highest outage total ever.
The outages were not the result of a massive power grid failure. Instead, it was the basic physical infrastructure-the poles, wires, submerged transformers and switchgear-that catastrophically failed under the storm deluge of wind and rain.
Studies show that power delivery systems are particularly vulnerable to storms and extreme weather events. The North American Electric Reliability Corp. (NERC) requires electric utilities to report events that cause power outages of more than 300 megawatts (MW) or affect more than 50,000 customers. A University of Vermont analysis of the NERC data revealed 933 outage-causing events from the years 1984 to 2006, with almost 44 percent of them because of extreme weather.
|On November 7, 2012, a Nor'easter began to impact the Mid-Atlantic and Northeast with strong winds, rain or snow and coastal flooding. Photo courtesy of NOAA.|
The U.S. Department of Energy (DOE) maintains its own database of grid disturbance events. A recent analysis of the DOE database by Lawrence Berkeley National Laboratory's Evan Mills shows an increasing number of outages from 1992 to 2010, dominated by weather-related events. According to Mills, some 78 percent of the reported 1,333 electric grid disruptions in the period were weather-related. The annual cost of outages across all U.S. business using Mills' estimate of 78 percent of outages as weather-related yields an estimated $35 billion to $55 billion for annual weather-related outage costs. Data suggests the trend of outages from weather-related events is increasing.
Many of our transmission and distribution systems, unfortunately, are out of date, inadequate, inefficient and need repairs. Electric utilities are responsible for upgrading these systems while simultaneously working to include smart grid enhancements.
"The truth is that much of the smart grid still relies on the same grid infrastructure we have today" said Tim de Chant, Nova Online blogger. "The distribution system may become more responsive, but physically, it won't be much different than it is today. That means when a substation is flooded or a tree knocks down a power line, the juice will stop flowing, just as it does today."
The American Society of Civil Engineers estimates the cost of upgrading the U.S. grid to meet future use is going to be $673 billion by 2020.
According to Smart Grid Consumer Collaborative's Consumer Voices report, consumers are willing to pay for some smart grid benefits. Among these are reliability and outage restoration after storms such as Hurricane Sandy. The smart grid, however, is not the panacea for system resiliency and reliability.
"The smart grid is no substitute for system hardening," said Mark McGranaghan, vice president of power delivery and utilization at the Electric Power Research Institute.
System hardening involves strengthening the distribution network to improve reliability with solutions such as underground utilities, stronger poles, new technologies, more aggressive tree trimming and smaller electric lines.
"During Hurricane Sandy, many of the failures occurred at the substations," said Jim Iverson, senior applications engineer for Cummins Power Generation. "So what is the weak link at the substation? Statistically, the number one reason standby generators fail to start is because of dead starting batteries. Over 80 percent of all starting failures are from this cause. The battery, therefore, is one of the essential building blocks on which to begin hardening the power distribution system."
This article presents various battery chemistries commonly used in switchgear applications and provides insight into which ones offer the best long-term reliability.
The electric utility switchgear application is typically characterized by a complex duty cycle that has changing loads throughout the discharge period. The duty cycle will typically have a high, short duration load in the beginning of the discharge, for applications such as breaker tripping, followed, for example, by one or more constant loads for applications such as control circuits, communications and monitoring circuits, or lights and alarms. These other loads could occur at various times in the duty cycle, depending on the needs of the customer.
Factors to Consider When Choosing a Battery
Following are performance factors to consider when comparing battery chemistries:
• One minute rate: Often, the first minute of the duty cycle, where critical breaker tripping operations occur, will be the determining factor in the size or capacity of the battery required. It is critical to review the sizing calculations for the particular site before selecting a battery based on ampere-hour rating alone.
• Design/operational life: It is important to understand the difference between warranty term and design life. European and North American batteries, for example, are specified on different specific gravity and operating temperature ranges. This must be taken into account when comparing the two because higher temperature and higher specific gravity will increase the grid corrosion rate, and, therefore, decrease the life expectancy.
• Shelf life: Batteries often are not immediately installed. If a freshening charge is then required before installation, this should be calculated into the procurement cost.
• Hydrogen evolution: Many substations were constructed years ago with a specific battery chemistry in mind. It is important to check the ventilation requirements because gassing rates may differ if using a different battery chemistry.
• Life cycle cost: When selecting a battery, the ampere-hour rating provides a gauge for estimating a specific workload within an established length of time. When factors such as design temperature and specific gravity increase or decrease operational life, however, overall cost is also affected and must be taken into account. This is especially true when comparing U.S. vs. European batteries, where ampere-hour ratings may be based on a different combination of factors.
|Genesis XE-TPPL batteries have proven to be ideal for backup applications because they are a smaller, lighter solution with a longer shelf life, longer service life, and a higher rate of efficiency than traditional technologies.|
Following is an overview of the various battery chemistries and their advantages and limitations for switchgear applications.
Nickel Cadmium: Nickel Cadmium (NiCd) designs consist of a nickel hydroxide positive electrode and a cadmium negative electrode in an electrolyte solution of potassium hydroxide, resulting in a nominal voltage of 1.2V. The traditional design is pocket plate, but, over the years, the development of sintered plates, fiber plates, plastic bonded electrode and foam plates have brought refined attributes for specific applications. In these and certain other stationary applications, NiCd batteries may prove to be more cost-effective than lead acid.
Advantages of NiCd designs include:
• High power delivery for short duration applications,
• Works well in extreme temperature applications,
• Tolerates abuse including over-discharge, and
• Long shelf life.
Limitations of NiCd designs include:
• High self-discharge rate;
• More complex to charge and maintain full charge, making it increasingly difficult to come back online after a disturbance;
• Memory effect where certain NiCd batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged; and
• Cost to recycle.
Flooded Lead Acid: Flat-plate flooded lead acid batteries are the principal plate design found in stationary applications throughout North America, including electric utility/switchgear applications. The flat plate consists of a grid structure of lead calcium or lead antimony. The grid serves the dual purpose of being the electrical conductor and the mechanical support for the lead dioxide positive plate material and spongy lead active material of the negative plate. The flat plate design has proven to be a robust, cost-effective design. It is a flexible design, in which plate characteristics-such as thickness, alloys, wire radius and placement-can be modified with relative ease to create a battery design that is optimized for a particular application (such as float service, cycle service, long duration, high rate, general purpose, etc).
|PowerSafe EC lead-calcium valve-regulated lead-acid (VRLA) batteries are engineered to meet the power needs of the utility market and challenging switchgear utility applications. Its post-seal design allows for natural plate growth without compromising the seal.|
Lead Antimony vs. Lead Calcium
The lead antimony design offers excellent cycling characteristics and can provide up to 1500-1800 deep discharge (80 percent depth of discharge) cycles. There are, however, several operational drawbacks of antimony designs when used in float applications. These include high water consumption, high float current, antimony poisoning of the negative plate and a high self-discharge rate. The net result over time is increasing float current (energy cost to charge the battery and heat generation), increasing grid corrosion rates (accelerating the aging process), increasing gassing rates (including hydrogen evolution) and increasing water consumption (more maintenance). Calcium, on the other hand, provides low maintenance properties, and lead ternary alloys (lead-calcium-tin) are the prevalent mix used in lead calcium alloys.
Advantages of flooded lead acid batteries include:
• High reliability,
• Long life,
• Easily recyclable,
• Well-understood technology,
• Economical solution, and
• Services a broad range of applications.
Limitations of flooded lead acid batteries include:
• Reduced life in high temperatures, and
• Requires watering/maintenance
Tubular Lead Acid: The tubular plate consists of lead spines that act as the conducting medium, surrounded by active material that is encased in a fiberglass tube to retain the active material. This type of design is prevalent in European lead acid battery designs. The alloy used in these plates is antimony-based to facilitate the casting of the lead spines. Recently, European designs that use these types of plates (OPzS cells) have increasingly been marketed to the traditional stationary float applications-such as utility/switchgear, as a potentially lower cost solution.
Advantages of tubular lead acid batteries include:
• Fast charge; and
• Efficient, long life.
Limitations of tubular lead acid batteries include:
Thin-Plate Pure Lead (TPPL): Pure lead plate designs have a low corrosion rate because they do not have impurities or alloy additives in the pure lead. This leads to a long float life expectation, in many cases more than 20 years. They also have a low gassing rate, which reduces water usage and reduces maintenance costs. TPPL batteries can provide considerable energy density improvement over NiCd and flooded lead acid battery technologies. TPPL batteries can easily be charged and recharged, minimizing downtime. A trusted and proven technology, they are being embraced for similar applications in telecommunications as well as multiple military applications, including starting batteries for tanks, silent watch applications and submarine microgrid/hybrid applications. TPPL batteries offer an excellent solution for switchgear applications.
Advantages of TPPL batteries:
• Long life;
• Easy to get back online;
• No need to water, reduced maintenance;
• Completely recyclable;
• High energy density ; and
• Can handle high temperatures.
Limitations of TPPL batteries:
Hurricane Sandy was just one of an increasing number of weather events expected to impact vulnerable U.S. utility transmission and distribution networks in the years to come. As a result, utility designers are faced with a two-fold challenge: to enhance their systems with new, more responsive smart grid technology while hardening their infrastructure to withstand future attacks from Mother Nature. The hardening process begins with an essential building block-the backup switchgear battery. It is recommended that careful consideration be given to the attributes of the various battery chemistries. Lead acid technology, especially TPPL, is a preferable solution for most applications-offering long-life, strong performance, and low maintenance requirements that offset the somewhat higher investment.
About the authors: Jennifer Eirich is the marketing manager, Utilities, at EnerSys. Eirich received her Bachelor of Science in Chemical Engineering from Pennsylvania State University. Stephen Vechy is marketing director for EnerSys.