Consulting-Specifying Engineer - August 2008 - (Page 30) M/E Roundtable is designed to meet some high level of complexity and statistic, but you need a doctorate to figure out how to operate and maintain it—you will face problems. Ultimately, however, it comes down to a cost/risk benefit analysis. KOSIK: Having a very robust electrical distribution system serving the critical load will do nothing if other systems, such as cooling, ventilation, exhaust, and water supply systems that are necessary for the mission to be successful, do not have equally robust system topologies. It usually comes down to the age-old adage of the weakest link in the chain. SPEARS: End users define the levels of reliability required. We now see regular requests for “six nines” designs. More commonly, though, four to five nines is the design goal. The following types of enterprises are driving all of us toward the “higher nines” targets: • Life safety/public safety (airlines, communications, public health) • Defense and government • Applications where operational loss would interrupt commerce (banks, retail, online commerce) • Applications where operational loss would interrupt revenue streams. CSE: What are some of the more successful design schemes you see that guarantee maximum uptime in mission critical facilities? COTTULI: The install, building, and commissioning phases of the project cycle are critical to success in terms of achieving a system’s maximum impact against a design intent. The commissioning phase particularly is important because it represents the point where the system can be put through all of its operational procedures at rated load. Taking the time in this phase is vital to understanding the system behavior and in turn ensuring the capability to support the load. KOSIK: There really is no guarantee against an interruption to the operations, even with the most robust mechanical and electrical design scheme. This is based on one critical fact: Humans are the single greatest reason that critical facilities have outages. This is why it is important that the systems are not overly complicated to operate regardless of the required uptime, especially during times when a component has failed or during a maintenance operation. This is when stress levels are high and people can make mistakes. At times like these, having clear, easily understandable, and intuitive control and monitoring systems will go a long way to help avoiding unintentional mishaps that can cause a loss of operational continuity. SPEARS: Dual powered/multi-corded information technology (IT) gear allows the use of dual bus, or A/B bus powering architecture, and this is becoming generally accepted as the most reliable scheme for tier III/IV requirements. Its benefits are best realized in newer facilities, where all of the equipment is dualcorded. In legacy installations, the A/B bus arrangement requires the additional complexity of inline static switches and the need to synchronize independent generator and UPS systems. A carefully designed system will include provisions to minimize the need for series switching, and enhance the ability to perform concurrent maintenance, and allow for battery and load bank testing. RENER: From a system design scheme, I have seen multiple power distribution units (PDUs) serving dual corded racks as well as PDUs with multiple sources (transfer switches) resulting in multiple, independent pathways to the end-use equipment. Equipment selection and design can only go so far. I believe the biggest reliability issues depend on human factors where independent acceptance testing and commissioning, and operator training play a major role. CSE : What specific power quality issues do you encounter in designing power systems for mission critical facilities? KOSIK: It is not a power quality issue as much as it is a power efficiency issue. Without careful analysis and planning, highly reliable power and cooling systems also can become highly inefficient and use unnecessary high amounts of electricity. This is happening because of the inherent efficiency points that each piece of power and cooling equipment will have. An example of this is when multiple, large centrifugal chillers are used as a part of a cooling plant design. Generally, these chillers will have an optimal efficiency at approximately 70% of the total load. So if the plant configuration is 2N or N+2, it is very important to use control strategies that allow the chillers to run efficiently but also ensure that there is no loss of cooling in case of a failure or during a maintenance procedure. RENER: No. 1 issue: getting reliable accurate information out of the utility provider. Very few utilities companies have detailed, accurate power quality information on their power services at a site. The second issue is getting reliable power nameplate information to do load calculations for the planned data equipment. Nameplate data provides peak information and often does not provide enough information on normal usage. The peak numbers are needed to plan the distribution system, but normal usage data are needed to effectively evaluate the power required at the facility service level. Many times equipment has yet to be selected, or is not even available from the manufacturer, during the early planning and design phases of the mission critical facility. Power quality, especially harmonics, within the facility also is a significant issue. COTTULI: Power quality problems engineers typically encounter include brownouts and blackouts. As the demand on the utility grid continues to climb on aging utility infrastructures, these occurrences are expected to increase. Consequently, customers 30 Consulting-Specifying Engineer • AUGUST 2008
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.