Providing 99.9999% service reliability in the increasingly decentralized access network is a challenge to all carriers. As network architectures have evolved, more network critical electronics are distributed in outside plant cabinets, controlled environment vaults and huts and in many cases are not connected to central office or headend facilities by a copper carrier. Consequently, the power architecture has evolved toward smaller local plants integrated into the OSP infrastructure.
While power electronics have been designed to achieve high levels of reliability in this environment, the same cannot be said for the energy storage media: the batteries. During the last decade, much has been invested in valve-regulated lead-acid (VRLA) battery design effort, charging system design and monitoring and prediction algorithm technology to overcome the problems. Recently, developers have explored more “exotic” technologies such as flywheels, fuel cells and micro-turbines. However, the wisdom of seeking a simple solution draws one back to the dream of a maintenance-free passive storage device that does not add moving machinery or fuel-based systems to the network.
The key criteria that produce high reliability in the CO include:
Reliable, long-life energy storage devices: Flooded lead-acid batteries are dependable and viable in the CO, achieving useful lifespans of 20 to 40 years
Good maintenance: Routine maintenance practices are well-understood and practical in the CO, both for the flooded cells and engine-generator sets
Good facility management: On-site personnel in the CO monitor loads, equipment condition and provisioning requirements on a regular basis
Generating capacity to cover emergency conditions: Although rare, power failures lasting longer than eight hours do occur. The centralized nature of the office permits the deployment of large diesel engine/generator sets to cover this eventuality
To reproduce these winning conditions in the remote environment, the industry needs to address each of these elements in the specific context of the OSP environment.
The quest for a battery technology capable of reproducing the reliability of CO flooded cells in the outside environment has lasted for many years. The need to operate at extremes of temperature from -40 C in winter to 65 C when baking in the sun, without down-rating reserve capacity, has proved to be an insurmountable obstacle for most technologies. Also, energy density has become more critical in remote terminals, where space is at a premium, along with environmental and personal safety, as installations have moved into public or customer premises locations.
The same considerations — hostile environments, low maintenance, long life, low volume and weight, safety under all foreseeable conditions — are prevalent in the developing market for energy storage in hybrid and electric vehicles. A number of battery technologies have been developed for mobile applications, among them nickel metal hydride, lithium ion and lithium polymer. At the 1999 Electric Vehicle Symposium, the lithium-metal-polymer (LMP) battery was cited as the most promising of these technologies in optimizing the relevant criteria. The U.S. Advanced Battery Consortium chose to support the development of this technology as the long-term electric vehicle solution.
Field trials of LMP technology in stationary applications already are in progress in several locations. Expected lifespan is in the range of 20 years. Operating characteristics are optimum at 60 C, and integrated internal temperature management provides more than 80% of battery capacity even at temperatures of -40 C. The LMP battery is one-third the volume and one-fifth the weight of equivalent energy batteries using other technologies.
Any regular maintenance in the OSP environment is too much maintenance. The exponential growth in the number of remote terminal locations, combined with the drive to reduce operating expenses and personnel deployment, has many operators practicing reactive rather than preventative maintenance.
This contradicts the dictates of experience with VRLA batteries in the OSP environment. Under regimes of controlled temperature and regular monitoring and maintenance, batteries can reach an average lifespan of seven years. Without proactive care, expected lifespan can be as short as two years.
Zero-maintenance batteries can be achieved with a solid battery chemistry such as LMP. The absence of liquid electrolyte eliminates traditional concerns of out-gassing or freezing under extreme temperature and discharge conditions. The elimination of lead in plates and posts avoids corrosion-related issues and metal flow in cable connections. Construction of battery modules in integral 48 V blocks eliminates the series connection of modules to achieve the required 48 V potential and the risk of a single module failure producing a chain reaction of potentially catastrophic failure in cells connected in series.
Plant record maintenance and power load monitoring were routine in the CO and permitted battery investment control by increasing capacity on an as-needed basis. These practices are much less common in management of remote terminals. Here, initial provisioning of ultimate battery capacity has been practiced, resulting in higher initial investment. Without ongoing load monitoring, inconsistent or unknown reserve capacity exists. Extra batteries installed to provide for future growth may reach the end of their calendar life and be replaced before ever being justified by cabinet load conditions. In addition, enhanced features such as DSL may increase the power load and reduce effective backup reserve time to critical levels, exposing customers to loss of service on AC failures.
Linear, predictable battery characteristics — combined with local intelligence and remote monitoring capability in the battery system — can permit a just-in-time approach to power provisioning in the outside plant. Advance alarms can signal when available battery reserve descends to close to the eight-hour reserve objective because of load or other factors. This data can be integrated into existing operation surveillance systems for management by the operator.
Remote terminal cabinetized engine/generator systems are available that offer around 100 hours of maintenance-free operation when connected to a pipeline fuel source. The decision to invest in this emergency protection is dependent on service criticality, probability of long power outages and dependability of designed battery reserve time. An outage in excess of eight hours can be expected on average once every four years. In urban areas, the probability of outage is approximately once every 40 years. The permanent installation of engine/generator sets at each remote terminal would be expensive and add a complex piece of mechanical equipment to manage in the OSP network.
The use of power system intelligence to provide accurate, timely information on the state of reserve capacity can make the logistics of emergency portable generator deployment more manageable and effective, and can permit this to be part of the outsourced responsibility of a power service provider.
The provisioning of power evolved as a natural extension of the telecom network architecture based on COs and copper distribution. It is a less obvious fit to the evolving architecture based on distributed hubs interconnected by noncurrent carrying media — fiber optics or radio waves. The concurrent shift of focus of telecom core technologies from electro-mechanical devices to electronic, optical and data processing elements increases the gap between the key expertise the telecom operator values in its technicians and the expertise required to maintain power equipment. A paradigm that includes power system components that are inherently more reliable and an appropriate separation of the power system elements from the telecom equipment could reposition the supply and guarantee uninterruptible 48 VDC bulk power as a power utility function, supplied to the telecom operator by a third party.
The evolution of the communications network toward a more decentralized architecture with ever-growing numbers of remote terminals requires a reassessment of the power system infrastructure that can provide the high standard of availability customers expect. The existing power infrastructure is an extrapolation of technologies and practices traditionally applied successfully in the CO; these have produced less than acceptable results in the harsher, unmanned OSP environment.
Advanced battery technologies such as LMP are available today that can extend CO-grade reliability to remote terminals. The expected use of these same technologies in the automotive market can be expected to quickly drive down the manufacturer's cost and experience curve.
Evolution of the telecom architecture and technologies is making the skill set and interests of telecom operators less compatible with their own internal provisioning and maintenance of uninterruptible bulk power sources. The availability of new power system technologies permits the consideration of novel paradigms for outsourcing the responsibility for 99.9999% reliability power.
Michael Davis is a management consultant for Davis Consulting, Hudson, Quebec. His e-mail address is [email protected]