Additional information about standby power for mission critical facilities

When utility power is interrupted, standby power system failure is not an option for mission critical facilities. Presenters from the March 19, 2015, webcast respond to questions left unanswered during the live event.


Dwayne G. Miller (left), JBA Consulting Engineers; and Kenneth Kutsmeda (right), Jacobs. Courtesy: JBA Consulting Engineers, JacobsRequirements of mission critical standby power systems transcend those of standard commercial projects, which are typically designed to merely comply with applicable building codes. Many times, high expectations of systems for mission critical facilities are influenced by the desire for increased levels of reliability and efficiency.

Dwayne G. Miller, PE, RCDD, AEE CPQ, CEO, JBA Consulting Engineers, Hong Kong, and Kenneth Kutsmeda, PE, LEED AP, Engineering Design Principal, Jacobs, Philadelphia, respond to questions that did not get answered in the live webcast event.

Question: What does ATS mean?

Kenneth Kutsmeda: Automatic transfer switch.

Question: Can a bank of capacitors be used to correct power factor (PF) problems in generators?

Dwayne Miller: This would be an unusual way to address a power factor issue particularly with a generator or small generation system. If the generator has a PF problem, the generator manufacturer needs to address it. For a large generation system with load induced power factor challenges, this might be a solution, however, I'd be very reluctant to attempt it, particularly with a variable load. If you have a steady state fixed load, you could use fixed capacitor banks to correct; however, I would recommend a system model be developed to understand how the capacitors will affect system impedance and system behavior during a fault. I typically avoid switched/automated capacitors due to transients associated with them. In the context of mission critical systems and standby power systems, adding capacitor banks could be a risky proposition, particularly with harmonic rich load (servers, UPS units, lots of VFDs, etc). In addition, power factor for mission critical facilities typically will be high (north of 95%) as a good portion of the loads are close to unity PF so capacitors for power factor correction are probably unnecessary.

Question: How does mission critical prime meet the unlimited run time stipulation set by Uptime Institute?

Kenneth Kutsmeda: For the record, Uptime Institute is a guideline and not a code requirement. Uptime Institute does state that "engine generators for Tier III and IV sites shall not have a limitation on consecutive hours of operations when loaded to N demand." ISO standby rated engine generators have a limitation of the total number of hours and therefore are not Uptime Institute certified. ISO prime rated engine generators do not have that limitation on runtime and as long as they are de-rated to N demand can be Uptime Institute certified. The mission critical or data center rating was developed primarily for marketing purposes. Different manufacturers have different limitations on their mission critical rating. In most case the manufacturer will have to review the loads and other aspects of the application to determine if the generator can run without limitation on hours. In the certification process, Uptime Institute will generally accept the generator if the manufacturer documents that there is no limitation on runtime. However, it is recommended you verify this with Uptime Institute before purchasing the generator.

Question: I have a facility that when we transfer to SEP (open transition), the input breaker for the online uninterruptible power supply (UPS) trips. The ATS vendor blames the UPS system and the UPS vendor blames the ATS system. Any suggestions?

Dwayne Miller: I will assume SEP stands for site emergency power. If you are transferring open transition, in rush from the UPS might be an issue. I'd also want to understand the transfer settings on the ATS. In my experience, the quickest way to get to the bottom of these sorts of issues is to measure and capture the event resulting in the trip. If you capture the 3-phase waveform that results in the trip, you will be in a much better position to determine what the problem is. I'm not sure what size your UPS is, but the input breaker would be another thing to review closely to see if you have adjustable settings. I would start at the UPS and work my way toward the source/generator to better understand how the generator, transfer switch, and UPS are acting as a system.

Question: If a legally required system's generator is placed in an outdoor walled area, can other types of equipment (such as chillers) be put in the same area?

Dwayne Miller: NFPA 110 addresses exterior locations in 7.2.2. The short answer is yes. National Electrical Code (NEC)-required clearance needs to be addressed as a minimum. In addition, the authority having jurisdiction (AHJ) for the specific location should be consulted as there are often local codes and ordinances that will create a more stringent requirement. The big issues for generators in enclosed areas are adequate clearance and clear path for air intake, radiator discharge, and exhaust. Walled areas can create problems with generator overheating and tripping off line due to over temp. Air-cooled chillers or other equipment that raise the temperature, equipment/obstructions that restricts airflow, the prevailing wind conditions, etc. all need to be reviewed to ensure the generator can run and operate properly.

Question: What is your recommendations for MV (15 kV) paralleled generators grounding for better reliability: Low-resistance grounding, high-resistance grounding, or hew hybrid (high- and low-) resistance grounding?

Kenneth Kutsmeda: Recently we have started to recommend the hybrid system; however, it is more expensive and requires more due diligence when setting up the system. In generally we would recommend low-resistance grounding systems for large redundant medium-voltage systems where limiting damage to the equipment during a ground fault is important but it is acceptable to take the system offline for a ground fault.

Question: What are the requirements for the fuel systems for serving NEC 700 and fire pumps loads?

Kenneth Kutsmeda: NEC Article 700 requires the alternate source of power have a minimum of 2 hours (full demand operation) of on-site fuel storage. If power is needed for the fuel transfer pumps to deliver fuel to the generator day tank, NEC 700 requires that the fuel pump be connected to the emergency power system. NEC 700 also states that the generator can't be solely dependent on a public utility gas system for their fuel supply unless acceptable by the AHJ where there is a low probability of simultaneous failures of power and off site fuel delivery.

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