Your questions answered: Critical power: Transformers, UPS and switchgear in mission critical facilities

Engineers should know the design concepts for selecting and sizing transformers, uninterruptible power supplies (UPS) and switchgear to help achieve energy efficiency in mission critical facilities (data centers, hospitals, etc.).  

Questions not answered during the live event on May 9, 2019, are answered here. 

Presenters:  

• Kenneth Kutsmeda, PE, LEED AP, engineering manager, Jacobs, Philadelphia 
• Richard Vedvik, PE, senior electrical engineer and acoustics engineer, IMEG Corp., Rock Island, Illinois

Question: Are the International Fire Code restrictions specific to lithium batteries or all types of batteries? 

Vedvik: As mentioned in the presentation, the IFC section reference applies to all battery types. It is the most restrictive for high energy-density battery chemistries, such as lithium Ion.  

Question: Are you still upsizing a genset feeding an uninterruptible power supply by 125% to 150%? Is there a rule of thumb? 

Kutsmeda: The overcurrent protection device serving a uninterruptible power supply should be sized according to the topology for charging, bypass and overload characteristics. With changing technologies, applying a “rule of thumb” is no longer prudent. When designers are faced with multiple options, each option should be evaluated.  

Upsizing the generator is based on compensating for the input current distortion of the UPS. Older UPS systems had 30% or greater distortion. This limited the amount of UPS load you could place on a generator and therefore required upsizing the generator. The newer insulated-gate bipolar transistor technology and filters have reduced that input current distortion to below 5%; therefore, that limitation or upsizing is no longer required. Most manufacturers still recommend limiting the UPS load on the generator to 80% of the generator capacity. 

Question: Would power factor capacitor and harmonics mitigation equipment help in reducing line losses? 

Vedvik: Because a lagging power factor causes an increase in current, correcting for a lagging power factor will minimize losses in the feeder. Taking steps to reduce the side effects of low power factor or harmonics will also reduce line losses.  

Question: How much is maximum load recommended to load UPS capacity? 

Kutsmeda: It really depends on your business case on what type of load you are serving. The general rule of thumb is no more than 90% to 95% to allow for fluctuations in the load. Vendors state that you can load the UPS to 100%, because the UPS have overload capabilities to cover short load fluctuations. 

Question: Have you considered the effect of K-rating on transformer impedance? 

Vedvik: Due to the allowable length of the webcast, we could not go into detail on K-rating options and strategies. One of the side effects of nonlinear loads is additional eddy currents and heat generation. K-rated transformers are designed to reduce heating and achieve this with lower resistance (oversized) neutral windings.  

Question: Does using aluminum for large feeders help efficiency? 

Vedvik: The energy losses discussed are based on resistance in the wiring and connections. The calculations demonstrated can be performed with any wiring material for which resistance per unit length is known. The added resistance per unit length for aluminum feeders when compared with copper can result in additional losses. 

Question: The implementation of high-efficiency motors contributes to savings, especially on continuous operation process (usually about 300 days/24×7). Is this something the industry is taking in count as an energy savings solution? 

Vedvik: Motor efficiency should be considered, along with harmonic content and power factor when wanting to minimize energy losses in the electrical system. The designer can present a life cycle cost analysis to justify additional costs for premium efficiency motors.  

Question: How do you see NFPA codes and standards affecting the future of data center batteries? 

Vedvik: NFPA appears to be focused on safety, which is a good thing. As battery chemistry and design topologies increase safety and mitigate thermal runaway, I would expect codes to loosen constraints.  

Question: In the last example, did the client opt to run both UPS in high-efficiency eco mode continuously? 

Kutsmeda: No. Like many owners, after spending the money on the UPS systems, putting them in bypass was not accepted. They do have dual medium-voltage utility service with one being a dedicated connection to a high-tension substation adjacent to the property. Because of the reliability of this service, they are considering putting one side on eco mode.  

Question: In your case study slide regarding power usage effectiveness, what was the cooling design used? 

Kutsmeda: This case study was performed on a facility built in 2014, so the PUE numbers are a little dated. It is intended to show the savings of the UPS system in eco mode, not the entire system. The facility has a chilled water plant with cooling towers and water side economizers (not operating during the test). Computer room air handling units blow air under the raised floor with a ceiling plenum return. Higher density rows have hot aisle containment. 

Question: In the case study without power distribution units, it was highly efficient, but at 415/240 V there would have been more than the typical 5 kA short-circuit current rating at the rack plug strips. How was that addressed? 

Kutsmeda: We were able to keep the fault current at the cabinet level to below the 5 kA rated rack power strip. Used single module UPS systems with smaller bypass feeders instead of large multimodule systems. Used smaller substation transformers for reduced let through current. We used remote power panels at the end of the rows with individual circuit conductors (not overhead busway, which the client did not want) running up to ceiling over and then down to add impedance. 

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