Your questions answered: Power for industrial facilities

Unanswered questions from the July 26 webcast are answered by Zia Salami with CDM Smith

By Zia Salami, Ph.D. August 11, 2022
Zia Salami, Ph.D., CDM Smith

Power insights

  • Learn more about power for industrial facilities in this Q&A with Zia Salami.
  • Uninterruptible power supply system are critical in these facilities, as highlighted in this information.

Many questions arose during the July 26, 2022, webcast about power for industrial facilities. Zia Salami, Ph.D., Management Specialist – SME: Electrical Power System, CDM Smith, Charlotte, North Carolina, answered several of them here.

Power systems for industrial, manufacturing and other energy-intensive production facilities requires careful design and coordination. These facilities need to maintain power (including maintaining power quality) to eliminate product losses. Increasingly, engineers frequently specify uninterruptible power supply-type systems for large portions or entire facilities.

Some projects require backup power, in the form of generators and lithium-ion batteries. Incorporating the uninterruptible power supply system into these buildings requires electrical engineers to understand the load requirements and specific functions served by these power supplies.

How does adding mechanical ventilation loads as critical loads at the UPS output impact the performance of the UPS?

Mechanical ventilation loads (critical pumps, fans, compressors) would add considerable load to the UPS, requiring very large batteries. If no standby generator, loads should have VFDs to reduce inrush and strain on the batteries.

In general, how long can a facility rely on a UPS system?

There is no general rule of thumb. If the system has a standby generator or if only computers that can be shut down (controlled), only 5 to 10 minutes of battery capacity may be necessary. Battery capacity always depends on total load, application and required duration.

I never specified a rotary UPS. What applications may be suitable for rotary UPS?

A rotary UPS (i.e., a M-G set) is applicable only for very brief periods — seconds, not minutes. When the motor de-energizes from loss of grid or short voltage dip/interruption, the generator can maintain output based on the total inertia of the motor, generator and sometimes a flywheel.

During the presentation, the presenter covered load flow studies. Courtesy: Consulting-Specifying Engineer

Does the limit of 150% apply to regenerative VFDs? These will inject power back to the system.

In general, power electronic device will control the fault to a specific limit. Of course, the limit may change based on equipment type and design.

Can harmonics on a system, generated by VFDs, fool a UPS into thinking the power quality is unacceptable (e.g., zero-crossing error) and go to battery?

That is possible if UPS will go to bypass or battery based on power quality/harmonic distortion issue. This depends on the UPS front-end, specs/limits, and controls.

How do we select the UPS topology needed?

If the topology is referring to offline/standby, online/double conversion and line-interactive, as was discussed in the presentation, therefore:

  • Offline/standby is the most basic, good for applications like home computers, printers or scanners.
  • Online/double conversion is the most reliable and offers the best protection, making them ideal for motor applications or situations where you can’t afford any transfer time. Best uses are for data centers or intensive care units.
  • Line-interactive would be suitable for light sag and surges, plus it has lower electric consumption. If you want a reliable, efficient and cost-effective UPS, the line-interactive is the way to go.

Should a UPS have an external maintenance bypass switch?

In general, yes, so that maintenance and/or testing can be performed on the UPS components.

The presenter mentioned five minutes as the minimum time for standby generation to come online — why?

Depending on standby type, a standby generator takes time to come up to speed and warm from cold-start.

Should the health of the UPS system be monitored by the DCS or SCADA system?

Yes. That would be ideal if possible.

With regard to UPS rating: What is the largest load that I can connect to the UPS assuming it is all electronics (no AC, lighting, etc.)? In other words, if I have a 100 kW UPS, can I connect 100 kW of electronic load to it or do I have to reduce it by x%?

It’s never a good idea to size (capacity) a UPS to exactly the connected load. In general, we should consider full connected loads while charging battery in parallel including load growth margin (e.g., 10% to 20%). In addition, DC system/battery run time should size long enough until restore power, safe system shutdown, or backup data for example in case of data center.

Should UPS be maintenance-tested at regular intervals? If so, once, twice or other times per year?

Annual testing is typical, but manufacturer’s recommendation(s) should be followed.

For protection of highly critical loads that must be maintained during loss of off-site power via UPS and standby generation, what critical response time (e.g., milliseconds) or cycles should be required to avoid trip and what is feasible by current technologies?

To me, the critical response time depends on equipment requirements and capabilities to withstand during power transfer/dead band (no power). I believe, the current technologies, especially UPS system, are able perform this function in real fast time (e.g., milliseconds).

Why is the output capability not based on kW rather than kVA? Batteries are DC — they do not speak kVA.

In general UPS size is based on kVA and DC system either kW or amp-hours. However, UPS kVA rating is typically close to UPS kW rating since the UPS power factor is close to unity (1.0 p.u.). The UPS efficiency may vary between 85% to 95% that will impact the amount of require input current to the UPS system (i.e., lower efficiency more require input current).

Does a UPS contribute to fault current on the input VAC bus? How?

As I mentioned in the presentation, the UPS will contribute short circuit to AC output and DC system based on specific percent of the output AC FLA or rated DC FLA (e.g., 150%)

What are the best standards or guides for DC arc flash analysis as IEEE and NFPA do not address or provide?

IEEE 1584-2018 has mentioned several publication references in Section 4.12 in order to provide some guidance for DC incident energy calculation. For example, ETAP uses 3 different methods to perform DC arc flash calculation:

Can a UPS with a pulse width modulated output be used to supply a VFD load?

A UPS can be used to power a VFD, but manufacturer(s) of both should be consulted. All UPSs and VFDs are PWM.

What will be a recommended practice for the UPS and battery systems?

There are several major recommended practices that we need to pay attention, but the most important is the proper sizing of the UPS system induing the battery and required runtime based on specific loads and its application.

Any comments on load flow batteries from either speaker?

Not clear on this question, however, we need to perform DC load flow study when systems are fed from batteries to determine kW, current, voltage, etc. when is possible.

Have you had experience with super-capacitor backup UPS? Can you speak to their capabilities?

Not much experience with super-capacitor backup UPS, but super- or ultra-capacitor UPSs (i.e., no battery) are specially intended for applications that are sensitive to power disturbances lasting only a few cycles up to (perhaps) one minute.

Is there a recommended textbook on UPS systems?

You can try this: “UPS System Design Handbook” with ISBN 0646168576, 9780646168579

Can you shed light on the prospect of super-capacitors or ultra-capacitors as potential alternative to battery storage?

As mentioned, not much experience with super-capacitor backup UPS, but super- or ultra-capacitor UPSs (i.e., no battery) are specially intended for applications that are sensitive to power disturbances lasting only a few cycles up to (perhaps) one minute.

How is UPS specification take into account critical motor loads?

UPSs can power critical motors (choose wisely), but to avoid or mitigate inrush current and potential UPS shutdown:

  • Keep the motors small in relation to total UPS load.
  • Oversize the UPS battery if possible (and therefore the UPS battery charger).


Author Bio: Zia Salami, Ph.D., Management Specialist – SME: Electrical Power System, CDM Smith, Charlotte, North Carolina.