Reducing electrical equipment downtime

Downtime is a common factor among industrial equipment. This article explains how to properly test for weak links, make retrofits to sensitive components, and how to minimize downtime when specifying equipment.

06/28/2001


Plant Engineering - January 2001

ELECTRICAL

Feature: Reducing electrical equipment downtime

Jeff Dougherty, Power Quality Engineer, Duke Power, Charlotte, NC

  • Downtime is the common factor among industrial equipment.

  • Equipment can be tested to identify weak links.

  • Retrofits can be made to sensitive components.

  • Downtime can be minimized when specifying equipment.

    • What is one factor all industrial equipment has in common? You probably guessed it based on the title of this article—equipment downtime. What would you think if I told you that you could reduce downtime when purchasing new equipment?

      Zero downtime is the goal of all equipment users. This concept means no lost production, idle labor, scrap material, costly clean up, added work shifts, missed deliveries, and late penalties—at least not from unplanned downtime. Preventive and predictive maintenance programs go a long way to reduce unproductive plant time.

      Likewise, maintenance programs on the electric utility's system can reduce the amount of unscheduled downtime. However, an electric utility cannot completely eliminate the voltage sags and power interruptions that cause equipment downtime.

      The electrical system
      The electrical system of the utility is much like that inside your plant. In most cases, the system is a combination of radial feeders protected by fuses and breakers. When an electrical fault or short circuit occurs, those fuses and breakers react to the overcurrent condition. As a result, a complete (and sometimes lengthy) outage, "blink" in the power, or voltage sag occurs.

      For example, during a voltage sag the lights dim when a large motor is started across-the-line. The large inrush of current reduces the voltage on the rest of the system and you see the effect on the lights.

      During a system fault, there is also a voltage reduction on the rest of the system. The severity of a voltage sag ( Fig. 1 ) depends on the electrical distance between the plant and the fault. Faults on high voltage transmission lines can result in voltage sags seen hundreds of miles away.

      The result is that faults do not have to be on the feeder that serves you to cause your equipment to shut down. The plant is affected by events anywhere on the entire electrical system, and you experience more voltage sags than complete power outages.

      Equipment downtime
      What happens when equipment experiences a sag in the voltage?

      Sometimes, nothing. If the sag is short in duration or shallow in depth, equipment may not even detect the change. Other times, equipment shuts down just as if the power was removed completely. All equipment shuts down for some level of voltage sag, but some equipment is much more sensitive.

      Have you ever had a piece of equipment mysteriously shut down while an identical machine beside it continues to operate? Again, the sensitivity of equipment or the ability to "ride through" a voltage sag varies.

      While a voltage sag may be illustrated as the waveform shown in Fig. 1, to you a voltage sag is a broken tool in a CNC machine, thread breaks in a knitting operation, or an extruder barrel that now has to be cleaned. Ultimately, if the equipment shuts down, a voltage sag equates to dollars in lost productivity.

      Equipment testing
      As previously mentioned, all equipment shuts down for some level of voltage sag. However, it is actually the components within the equipment that determines the voltage-sag ride-through capability of the entire process or machine.

      For example, an adjustable speed drive may be set to shut the process down if the voltage goes below 70% of normal. A motor contactor, however, may drop out if the voltage goes below 75% of its nameplate rating. Figure 2 illustrates the typical performance characteristics of several devices.

      In Fig. 2 , for any voltage/duration combination to the right and below the performance curve, the device shuts down. For example, a 65% voltage sag lasting ten cycles shuts the instrument power supply down, while a 70% sag lasting eight cycles does not.

      There are two ways to determine the sensitivity of equipment to voltage sags:

      • Install a voltage monitor at the equipment, and over time develop a history of voltage sags and instances of shutdown

      • Test the equipment with simulated voltage sags and monitor performance.

        • The second method is the quickest and most effective. A "sag generator" is used to test equipment and develop performance curves like those shown in Fig. 2 . Once the weak links are identified, retrofits can be made to make the most sensitive components less sensitive or less susceptible to voltage sags.

          As an example, testing conducted on a plastic extrusion process determined that the components in the emergency stop controls were the most sensitive—shutting the process down for a voltage sag to 75% of nominal lasting only three cycles. A constant voltage transformer (CVT) was incorporated into the machine for approximately $1000. The CVT improved the ride-through characteristics of the process and paid for itself by preventing just one unplanned shutdown.

          Specifying reduced downtime
          If equipment component sensitivities can be measured, and if retrofits are available to make the most sensitive components less sensitive, then the next logical step is to include ride-through performance in the specifications for new equipment. If you specify surge suppression for a new machine, doesn't it make sense to specify voltage sag suppression too?

          A typical scenario might be a company expanding its operation, thereby requiring new equipment. A performance curve could be included in the purchase specification for the new equipment stating that it must perform properly over this given range of voltage sag magnitudes/durations.

          The equipment manufacturer makes the necessary modifications to ensure proper operation. Many of these changes are readily available, but add initial cost to the equipment. Equipment manufacturers must be competitive with their prices and typically make modifications only when asked to do so. Upon delivery, the equipment can be tested to ensure performance. The result is that for a little more money up front, you have specified reduced equipment downtime.

          There are many challenges facing the industrial equipment user today. Reducing unplanned equipment downtime is probably one of the most important. While maintenance programs both inside and outside of an industrial facility will have a significant effect on downtime, there is now another way to increase productivity. Include voltage sag ride-through specifications when you order new equipment and further your progress toward a goal of zero downtime.

          —Edited by Jack Smith, Senior Editor,
          630-320-7147, jsmith@cahners.com



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