As the wireless industry continues to evolve, carriers continue to roll out new services to differentiate their networks and create brand loyalty. Fig 1, on page 31, illustrates the changing network architecture of the wireless market. The latest networks capture wireless data via the Internet from enterprise systems with content from hosted services and applications enabled by advanced broadband capabilities. To achieve this level of service, carriers' electrical systems must deliver availability equal to the public switched-telephone network's uptime measurement of 99.999%. Of course, power supply and protection play significant roles in developing infrastructures that support maximum availability (see the sidebar, below). In this article, I'll discuss some of the best methods available that help wireless carriers meet today's reliability requirements.
Transient voltage surge suppression (TVSS) devices are often the first line of defense against power disruptions. TVSS technology provides continuous distortion limitations at every point of the sign wave, whether it's a surge or spike. This type of filtration is one of the few methods used to eliminate ring waves, which are produced by rapid drops or increases in power supplies.
Individual TVSS units may be applied to electrical distribution panels to protect both linear and nonlinear loads from harmful transients and electric line noise. These units are designed to limit power spikes on voice and data lines and eradicate lesser power aberrations. They also feature detection circuits, which use resident indicator or status contacts to provide notification for all modes of eminent failure.
Hybrid Power Protection
Communication networks of the past consisted primarily of voice analog equipment, which relied mainly on DC power. In contrast, today's data systems typically require an AC power source. Sending data over wireless networks requires a mix of voice and data equipment that depend on both AC and DC power and protection.
There are a number of protection configurations for data and voice networks. These may include the use of a DC power supply with inverters for AC loads, a DC power supply and an AC UPS for AC loads, or an AC UPS with rectifiers for DC loads. Standby generators as a backup source of power also are a viable alternative to long battery-backup times.
For critical networks, a hybrid distributed redundant power strategy may be the best configuration. A hybrid system uses distributed redundant DC rectifier systems supplied from large, dual-redundant AC UPS systems. Small, self-contained DC rectifier systems along with AC power distribution units (PDUs) can be located throughout a facility to supply either AC or DC power to the load equipment.
Configurations of this kind provide considerable reliability by using dual redundant AC UPSs with redundant power distribution paths. A superior level of fault tolerance is obtained without any single points of failure in the AC or DC power system. Any device within the AC or DC power system can fail or undergo service without interruption. Redundant generators also can be deployed for reliable power during sustained utility outages.
For instances where an economical solution is needed, a line-interactive UPS offers power conditioning against sags, spikes, surges, and brownouts, as well as battery backup. While this UPS topology may be cost-effective, it will not protect against all power problems, including power-system short circuits, frequency variations, harmonics, and common-mode noise.
True online double conversion provides complete isolation from problems originating from utility or generator power. As illustrated in Fig. 2, power is continuously supplied to the batteries via the rectifier at the same time the inverter is providing a constant source of conditioned AC power to the load. This ensures an instantaneous transition during power outages.
As a practical alternative, power systems that include generators can greatly reduce the number of large, heavy batteries required for critical systems. For carriers that have hundreds of sites in a geographic area, an economical option involves the use of portable generators that can be dispatched as required. Portable units offer substantial savings and eliminate the need for obtaining permits for fixed-site generators.
A common dilemma that occurs during power outages involves the start-ups of loads on a generator. Generator start-ups cause output to fluctuate, thereby triggering line-inter-active UPSs to cycle to battery. This repetitive cycling can induce the battery to completely discharge, which significantly degrades battery life.
Equally problematic is the generator instability that occurs when the UPS load is transferred to the generator. The UPS load transition causes the generator power output and frequency to sag, which causes the UPS to go back to battery. Once the UPS senses consistent generator output, it switches the load back to the generator. If generator output dips again, the process repeats itself.
For a power protection strategy that includes generators, a double-conversion UPS is optimal. It can support major fluctuations in power supply frequency while providing synchronized, steady output power — without switching back and forth to battery.
Incoming AC power is rectified to DC power to supply the UPS's internal DC bus. The output inverter takes the DC power and produces regulated AC power to support vital equipment loads. Batteries connected to the DC bus are float charged during normal operation. When the input power is beyond the normal operating parameters, the batteries provide power to support the inverter and critical load.
Power and Cooling
For complete infrastructure support, power protection must be just as reliable as cooling systems. During power outages, UPSs supply power to vital computers and components. However, most UPS systems do not have enough power to run cooling systems.
The temperature inside a data center may quickly rise because the built-in delays on HVAC equipment restart after emergency power generators start up. During this delay, temperatures can escalate to the point where the reliability of critical systems is compromised. The integral relationship between power and cooling illustrates the importance of a comprehensive approach to the planning and design of infrastructure support systems that address individual communication networks and environments.
The Need for Redundancy
A power protection configuration that offers redundancy, both in systems and on the component level, is optimal for critical wireless networks. N+1 redundancy offers considerable reliability and can be economical and easy to deploy. Maximizing uptime necessitates dual-bus architectures, with redundancy throughout, to eradicate points of failure and support both planned and unplanned system maintenance. To eliminate downtime, a dual-bus configuration provides a mirrored system, which may be electronically tied together. In the event that one of the buses goes down, the duplicated system is available to seamlessly transition the load.
The latest monitoring technologies include out-of-band access, which uses a modem or Internet connection, and in-band monitoring, which works through an internal network link. The majority of network monitoring systems provide management and control of infrastructure supports using either simple network management protocol (SNMP) or http access.
Some monitoring capabilities allow end users to run trending and analysis reports. In many situations, these reports can help trace the underlying cause of an event by looking at chronological documentation, which is supported by equipment messaging and event/status logs that identify the time and cause of a problem.
The growth of the wireless industry will continue as carriers introduce new and improved services to attract and retain customers. Carriers equipped with infrastructures that offer reliable power for critical data will hold the keys to customer satisfaction and loyalty. For many carriers, a power protection strategy that maximizes availability is the crucial element in preparing for future generations of wireless networks.
James Hall is the telecom market manager for Liebert Corp., in Columbus, Ohio. You can reach him at [email protected].
The Elements of Availability
Individual UPS modules, static transfer switches, and other power distribution equipment must be incredibly reliable, as measured by field-documented mean-time-between-failures (MTBFs). In addition, system elements must be designed and assembled in a way that minimizes complexity and single points of failure.
A UPS must be able to protect critical loads from a full range of power disturbances.
System designs must permit concurrent maintenance of all power system components, supporting loads with a redundant configuration that allows for failure or service of duplicate systems.
Systems must have fault resiliency to cope with a failure of any power system component without affecting the operation of the critical-load equipment. Furthermore, the power distribution system must have fault resiliency to survive load faults and human error.