Outdoor Cabinets for Utility and Smart Grid Electronics
An energy management system consists of the metering systems, smart grid equipment, communication equipment connecting to smart meters and the intelligent grid, and back-office computer systems that enable these systems to communicate and work together.
An energy management system consists of the metering systems (smart meters), smart grid equipment (power line sensors, remote switches, and other equipment to improve service reliability and power restoration), communication equipment connecting to smart meters and the intelligent grid, and back-office computer systems that enable these systems to communicate and work together.
Much of the fixed-point communications electronics-including backup batteries-installed alongside transmission and distribution lines and substations will require protection from the elements, thermal management to maintain the proper operating temperature ranges, and security to prevent malicious access and tampering. In many cases, the installation of a thermally managed outdoor cabinet will be more cost effective and practical than a new building or shelter.
This article introduces the features offered with thermally-managed outdoor cabinets, including the thermal management systems, power efficiency, acoustic management, security and intrusion prevention. These features have an impact on the total cost of ownership over the life of the deployment-typically many years-and understanding and leveraging them can result in significant operational expense reductions, as well as increased reliability and longevity.
|Acoustic management becomes critical as electrical lines traverse residential neighborhoods.|
Most commercial electronic equipment is specified to operate in a -40F to 149F range. For every 18F temperature rise in electronic components, the mean time between failure (MTBF) will double. So maintaining equipment within the manufacturer's recommended temperature range reduces capital reinvestment on equipment by extending its operational life and improves the reliability of communications or smart grid infrastructure.
Thermal management systems are available in many technologies and performance ranges to accommodate the heat load from contained equipment and solar radiation. The thermal management system design must take into account the operating temperature range of the enclosed electronic equipment, the heat dissipation from that equipment, and the total heat load contribution of solar radiation from exposure to direct sunlight.
The four most common thermal management technologies uses for outdoor cabinet areas are as follows.
• Direct air cooling (DAC): A combination of vents, fans and filters, DAC systems are open-loop systems that bring outdoor ambient air into the interior of the enclosure for cooling purposes. Fan capacity can range from 50 to 1,000 cubic feet per minute (cfm) for enclosures. DAC systems are highly reliable and have a long life expectancy because the only moving parts are the fans to circulate air.
• Air-to-air heat exchanger (HEX): HEXs provide superior above-ambient temperature management and are closed-loop systems. Closed-loop systems prevent intrusion of any particulate contaminates, such as dust, moisture and humidity from entering the enclosure. HEXs are highly reliable and have a long life expectancy because the only moving parts are the fans to circulate air.
|Outdoor cabinets can be accessorized with cooling systems, side chambers and RF tops for antenna cable access.|
• Air conditioner (A/C): A/C offers the highest performance thermal management technology, able to support high heat loads and cool the interior of the enclosure far below ambient air temperatures. Most A/C units deployed with outdoor enclosures are closed-loop systems. Refrigerant within the A/C system absorbs thermal energy from within the enclosure and transfers it to the outdoor air.
• Thermoelectric cooler (TEC): TECs use the Peltier-Effect, where current is applied across two dissimilar materials and creates a temperature differential. TECs offer variable and scalable incremental cooling or heating in a compact form factor. TECs have high reliability and long life expectancy because the only moving parts are fans to circulate air. TECs are low maintenance and consume far less energy to operate than A/C.
Many external factors affect the thermal performance of an outdoor enclosure-primarily the range of seasonal temperatures and humidity, the thermal load produced by the sun, and the thermal load produced by the equipment. More than any other feature, the thermal management system could have the largest impact on the operational expenses.
Custom Engineered for Optimal Performance
When a manufacturer is presented with a new configuration for a cabinet, it engineers a thermal management solution that optimizes the thermal environment for equipment, and this design will minimize the total cost of ownership over the lifetime of the cabinet. Design factors that affect the design of the thermal management system may include:
• Equipment segregation into cooling zones: The cabinet and all of its affiliated accessories should be engineered to accommodate multiple temperature zones. This design method enables the cabinet to keep one zone at a different temperature than another. This capability is especially useful for cabinets that house both batteries and electronics. Batteries should be stored as close to room temperature as possible because any elevated temperatures substantially shorten cycle life. Electronics are more tolerant of higher temperatures and can comfortably operate between -40F and 149F.
• Intelligent controllers: A critical consideration when selecting a cabinet's thermal management system is an intelligent controller. An intelligent controller, in combination with multiple temperature sensors, should control the operating speed of the air-movers. These variable-speed controllers regulate the airflow, maintain a more even temperature within the cabinet, reduce energy consumption and reduce acoustic emissions outside the cabinet.
• Field upgrades: Thermal systems can be door, side or roof mounted. Door-mounted systems enable adjustment of the capacity of the thermal solution to match the deployed equipment. Quick-release hinges allow fast and efficient changes to the thermal management systems without removing the cabinet or electronic equipment from service. Door mounted systems can take advantage of more efficient thermal technologies as they become available.
Minimizing acoustic emissions from fielded cabinets is critical to harmonious relations with communities adjacent to your equipment. A good benchmark is the cabinet must not emit more than 65 dB-A at five feet from the cabinet (while under full operation). Several features can assist in minimizing acoustic emissions; sound absorbing material, variable speed fans and intelligent controllers, and multiple vents will diffuse noise in different directions.
Security and Intrusion Prevention
Security is a concern for any utility operator. Unauthorized access to equipment can lead to service downtime, or, at a minimum, loss of visibility to that station's status. Many features are available to provide uncompromising security.
• Interior door hinges: Hinges on all access doors should only be accessible from inside the cabinet and reside inside the exterior gasket area of the cabinet so they are protected from exposure to the elements. The hinge should not be accessible when the door is closed.
• Door latches and frame: All exterior doors should be locked with a multi-point latching mechanism to ensure all door corners and edges are secured. Any door latching mechanism should be in the cabinet interior. The latching system also should be captive so the latches effectively bond the door to the cabinet. The door latching hardware and mechanism should be capable of withstanding torque levels, typically 400 in-lbs, without physical distortion or loss of functionality.
• Firearms resistance: Penetration of the cabinet walls by projectiles can cause severe damage to the equipment. Industry conventions indicate outdoor cabinets must be capable of withstanding a point blank 12-gauge shotgun blast without penetration of the cabinet wall by any pellets. In addition, the cabinet should be resistant to a 22-caliber bullet fired from 50 feet and resistant to a 30-06 caliber fired from 50 yards.
|Zone cooling maintains electronics and batteries at different temperatures.|
Thermally-managed cabinets maximize equipment security and up-time, as well as extend service life by providing an optimal operating environment for communications equipment embedded within a utility's infrastructure. In addition, ongoing heating and cooling costs can be minimized by selecting the optimal mix of thermal management technologies. These features and capabilities have a major impact on the total cost of ownership over the life of the deployment, as well as increased reliability and longevity.