Improving power quality

Design options for industrial controls can provide built-in robustness to voltage sags. The controls already have capabilities installed and don’t require after-the-fact voltage sag mitigation for defense against the effects of voltage sags.

Parts 1 and 2 of this article series explained and described the origins of industrial process sensitivity to power quality (PQ) events, or variations in the electrical power supply – specifically, voltage sags – as well as retrofit methods of mitigating those variations and help sensitive processes remain operating despite those events.

Don’t assume battery-based UPS systems are needed; there are other technologies.

Effective use of existing PQ standards

Since 1999, organizations have addressed the relatively new effects of already existing PQ phenomena on industrial processes – such as the SEMI F47 standard for voltage sag immunity. In 1999, the Semiconductor Equipment and Materials Organization (SEMI) published the SEMI F47 standard to address the deficiencies in the 1996 Information Technology Industry Council (ITIC) performance curve – to which semiconductor manufacturing equipment was designed to function normally at the time. The ITIC Curve (lower part) is shown as a green line in Figure 1.

EPRI’s studies in power quality informed the SEMI organization as to the severity of most commonly occurring voltage sags causing costly process upsets to semiconductor manufacturing equipment. The SEMI F47 Standard provided the modifications to the ITIC curve (the red lines shown in Figure 1) judged sufficient to allow semiconductor manufacturing process components and the processes to ride through these most common magnitudes and durations of voltage sags.

To be compliant with the standard, semiconductor control components or entire processes had to ride through those areas above 50% of nominal voltage from 0 to 0.2 seconds, above 70% from 0.2 to 0.5 seconds, and above 80% from 0.5 to 1 second (later extended to 2 seconds). The SEMI F47 standard only addressed single- and two-phase (line-to-line) voltage sags — not three-phase sags, which allowed significant improvement to the response of semiconductor manufacturing processes to voltage sags.

However, the SEMI F47 standard, while it could be used for other industries, targeted the semiconductor industry specifically. Growing concern about three-phase voltage sags and the need for a standard addressing all industrial processes led the Institute of Electrical and Electronics Engineers (IEEE) to introduce the IEEE Std 1668 for general industry in 2017. This standard combines the same robustness requirements as provided by the SEMI F47 standard for single- and two-phase voltage sags with an additional requirement for three-phase voltage sags as shown in Figure 2 (green line).

Thus, industrial control components – or an entire process – may be required to comply with this standard as part of the purchase request.

Use of robust dc power supplies, ac components

All control circuit components are not the same with respect to robustness to voltage sags. No one manufacturer “has the corner on the market” of robust control components. Individual models are more robust than most, however.

Starting in 1999, EPRI’s Power Electronics Application Center (known at the time as EPRI-PEAC) and continuing via EPRI and other test labs today, many ac powered components (contactors, relays, sensors, controllers, and dc power supplies) have been certified to meet the SEMI F47 standard.

Figure 4: Example of a universal input DC control circuit design. Courtesy: EPRI[/caption]

Adjusting ride-through parameters on motor-drive systems

Many modern adjustable speed drives (ASDs) are designed with various parameters which may be changed in order to provide robustness to voltage sags – some electronic soft-starters also may have changeable settings.

The simplest parameter may be a time delay; indeed, some ASDs have a 2-second time delay as part of a parameter for power loss. If the power does not return within 2 seconds, the drive shuts down. Fortunately, most voltage sags end well before 2 seconds. This parameter may or may not be enabled out of the box, however. For most drives with these parameters, the buyer must make the change as these have arrived disabled.

Often, the drive shuts down for dc bus undervoltage due to a voltage sag. Other parameters may involve actions such as kinetic buffering where the rotational energy of the spinning motor may be used to maintain the ASD’s dc bus voltage until the end of the sag. A coast parameter allows the drive to release control of the motor, thus preserving the dc bus voltage until the return of power whereupon another parameter allows the ASD to catch a spinning motor at its then rotational speed and then ramp the motor back to operating rpm.

These helpful parameter rarely share the same names among manufacturers and may not even be close to one another in the parameter list; however, a search of the ASD manual can identify them. Some parameters also will list related parameters necessary for the change to be effective.

Overall conclusions

This 3-part article identified voltage sags as the main culprit involved with industrial process shut down incidents – the 120-volt control circuit being most sensitive to voltage sags. Most voltage sags have sag magnitudes (voltage remaining) at or above 50% of nominal with durations at 0.5 seconds or less. Within the control circuit, several individual components common to most controls could be causing the controls, and thus the process, to shut down. These components are often the PLC, its I/O, dc power supplies, ac “ice cube” relays and contactors, and adjustable speed drives.

Allowing the control circuit and the process to ride through voltage sags involves mitigating one or more component’s sensitivities to voltage sags or propping up the voltage of the control circuit for the duration of the voltage sag.

While the latter may be accomplished using a battery-based uninterruptible power supply (UPS), these require regular maintenance checks and full battery replacement after about three to four years for lead-acid batteries. Newer technologies such as lithium-ion-based UPS units tout battery lifespans out to the seven to eight-year range.

In contrast, “battery-less” mitigation options exist that need little or no maintenance, operate better in elevated temperatures, and do not require battery replacement. These may operate for 10 to 15 years.

Control circuits may be designed to be more robust by requiring the incorporation of voltage sag standard-compliant components – SEMI F47 or IEEE Std 1668, or by requiring through the purchase order the entire process be compliant to those standards without requiring a battery-based UPS to maintain. The controls may be most robust by using only dc components supplied by a three-phase, universal-input dc power supply operated at the highest input voltages in its range, and at less-than full load.

By keeping the control voltage up, the RUN signal to an ASD may be maintained, yet the drive may shut down for dc bus undervoltage. Most modern drives have adjustable parameters that may allow the drive to ignore the voltage sag for a period of time until full input voltage returns if enabled. These parameters may not be enabled by the manufacturer.

Several methods of improving the voltage sag sensitivity of industrial controls from mitigation to complete design may be followed so the industrial process may be made more robust to the effects of voltage sags.

Mark Stephens is principal project manager; Alden Wright is technical leader,  Electric Power Research Institute (EPRI). Edited by Chris Vavra, associate editor,  Control Engineering, CFE Media,  cvavra@cfemedia.com.

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Keywords: power supplies, voltage sags

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ONLINE extra

EPRI explains on related topics:

– Video: Control circuits may be designed to be more robust by requiring the incorporation of voltage sag standard-compliant components – SEMI F47 or IEEE Std 1668.

– Power quality concepts article series.

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