Your questions answered: Uninterruptible power supply (UPS) foundational learnings

Discover how uninterruptible power supply (UPS) systems play a vital role in ensuring the availability and protection of critical equipment and data during power outages and voltage fluctuations.

By Consulting-Specifying Engineer November 3, 2023

In today’s interconnected world, a reliable power supply is crucial for businesses and individuals alike. Uninterruptible Power Supply (UPS) systems play a vital role in ensuring the availability and protection of critical equipment and data during power outages and voltage fluctuations.

During a webcast on Sept. 27, presenters from Schneider Electric delved into the data associated with why a UPS is needed. In the session, presenters explored the power grid availability in North America and examined the staggering number of power outages that have occurred in the United States.

Additional questions were answered by Steven Boyles, Power Systems Engineer, Schneider Electric.

Question: What UPS series will provide the highest efficiencies?

Answer: Rather than answer a “series,” I would say that based on the designs, the design that drives the UPS modules to the highest power level will push the UPS into its “power range” where it is most efficient. Distributed parallel designs where the UPS is very lightly loaded will be least efficient.

Question: For small UPS’s, are there monitoring functions in the UPS which can be used to notify the customer of when the UPS is failing?

Answer: Yes, if a network management card is installed.  Most UPS manufacturers offer predictive analytics of some sort.

Question: When we install UPS in locations where there is dust and debris, what can we do to protect the UPS from becoming an air filter and overheating/failing?

Answer: There is not a great option for this. A UPS is really designed for cleaner environments. Exterior filtering would be preferred.

Question: What improvement are being made on batteries used for UPS’s?

Answer: Improvements include lithium Ion, nickel zinc and thin plate pure lead.

Question: What is percentage of harmonic is permissible?

Answer: The UPS presents <3% current THD on input, and <3% voltage THD on the output.

Question: When we have our power distribution system backup with standby generators and we have UPS’s in the critical power circuits, what do we need to do to minimize harmonics between the UPS and the generators?

Answer: You need to have at least some load on the UPS modules. Most manufacturers have no compatibility issues when UPS is at least 30% loaded.

Question: What is your recommendation for how to size a UPS for motor loads?

Answer: The best recommendation is to use VFD for motor starts to mitigate locked rotor currents, such that the UPS does not see the large in-rush and drive it to overload.

Question: “Some UPS’s are note unity power factor.”  I thought pf was determined by the load(s) connected, not by the power source.

Answer: UPS are designed with two loads, KVA and KW. If either approaches 100%, the UPS is fully loaded. Prior gen Schneider UPS were .9 pf capable on the output. So, i.e. 500KVA/450KW. If the UPS saw 450KW, it stated 100% load, even though the KVA might only be 475KVA or something.

Question: When we have a panelboard connected to the output of the UPS, should the panelboard use molded case circuit breakers for UPS power distribution or fuse blocks for UPS power distribution?

Answer: I always recommend circuit breakers over fuse blocks, but either will work

Question: What are the disposal requirements for LIB versus VLRA batteries?

Answer: They’re very similar. Most will require recycling of some kind. LiB is not fully mature in the recycling side, but by the time they need to be disposed of, there will be recycling methods.

Question: What about fire risk in battery technology? We have consulted with a client for whom LiB was a no-go for this reason. Are there any mitigations in UPS systems?

Answer: UL9540A large scale fire testing proves that LiB are fundamentally safe and that a fire within the cabinet does not propagate to even an adjacent battery module.

Question: Have you found difficulties properly disposing of lithium-ion or nickel-zinc batteries when they need to be replaced?

Answer: Both of these technologies are so new that the recycling side has not been fully developed yet.  I expect within the next 5-7 years, we’ll see recycling options.

Question: For large UPS modules, say 1MW built up from smaller kW units (e.g., 250kW), how does the UPS respond to failure of one unit?

Answer: One unit=250KW. In the case of 1MW UPS, if the load is less than 750KW, one unit fails, opens input/output contacts and removes from the load. The remaining power cabinets continue to carry the load, protected.

Question: In the case of N, N+1 and N+2, what is the cost impact considering various factors effecting reliability?

Answer: N is least reliable. N+1 is the industry standard, meaning a second UPS is installed as a spare, providing improved reliability.

Question: What is the difference between Iso-Redundant Configuration and N+1? Is it just that the secondary UPS system will take only 50% of the loads from the primary UPS system that malfunctioned?

Answer: Iso-Redundant is different in that the redundant UPS is in standby, or zero load, until the primary transfers to static bypass. In N+1 parallel redundant, both modules are carrying half the critical load all the time.

Question: When we have UPS’s in parallel for redundant UPS’s, do we need any communication between the UPS’s to keep their output in sync?

Answer: Yes. A comm cable is required between all units.

Question: For the static switches shown in your slides, are they used for performing maintenance on the UPS?

Answer: Static bypass switches are for emergency transfer to utility to keep the load powered if the UPS has a major failure and cannot carry the load on inverter. For maintenance, a transfer from static to maintenance bypass would be needed.

Question: Are static switches external to the UPS assembly? And are static switches make-before-break type of switches?

Answer: Static bypass switches are internal for most modern manufacturers. They are half of a static transfer switch. They transfer in less than 4 msec.

Question: What is the reason for the shift from centralized to distributed parallel architecture? Doesn’t distributed parallel architecture cost more?

Answer: No, not usually. Centralized static switches are large devices and are pretty expensive. This is mostly a moot point however, as almost everyone has gone to distributed, and we don’t offer centralized anymore.

Question: What is the maximum utilization factory (UF) of a UPS in a 2N configuration during normal operation?

Answer: 0.5.

Question: How do you isolate the parallel UPS’s from each other to prevent backfeeding a UPS that is out of service?

Answer: Breakers in and out in the up and downstream gear.

Question: What is static bypass and when does the UPS go into static bypass as compared to maintenance bypass?

Answer: Static bypass is for emergency operations when the UPS inverter can’t carry the load, and the UPS is trying to keep the load energized by transferring it to utility through the static bypass. Maintenance bypass is as stated to do maintenance, a planned event.

Question: Are the ventilation requirements for VLRB and lithium ion the same?

Answer: Ventilation is site and code specific. VRLA do not emit hydrogen unless they are overcharged, etc. Most jurisdictions require some minimal ventilation for VRLA. Lithium only emits gasses in similar conditions, and most jurisdictions again require some minimal ventilation.

Question: What are some of the certifications you see required from utilities as it relates to implementing UPS systems?

Answer: The UL listings are typically the concern. UL1778, UL1973, UL9540, etc.

Question: Do we need to keep our UPS in an environmental controlled space to maintain long life of the batteries?

Answer: Yes. This is recommended. 77F for most battery technology. Keep the UPS in the same environment.

Question: I’ve heard that for running arc flash and coordination studies, many consultants treat a UPS as if it is in bypass mode because it transfers to bypass so quickly. As opposed to trying to model something representing the input contactors and output contactors. Could you speak to that?

Answer: Yes, correct. The UPS is not a significant source of any fault current, and in a fault condition will nearly instantaneously transfer to static bypass to pass the fault through.

Question: What’s the difference between UPS and inverter?

Answer: UPS takes in AC, converts to DC, charges a battery source and then goes out to inverter. Inverters are typically specialized equipment with more of a DC source and inverter only. However, the terms get fuzzy in that there emergency lighting inverters, and we sell UPS modules for this application.