Tech Tip 13
Sensitive Equipment Can Increase Production Losses
- Adjustable Speed Drives Trip Off-line
- Programmable Logic Controller Shuts Process Down
- Lights Blink
- Utility Customer Perceives Poor Reliability
- New Sensitive Electronic Equipment
- Short Circuits on Electrical System
- Specify Less Sensitive Equipment
- Reduce Problems on the Electrical System
- Apply Mitigation at the Equipment
Sensitive electronic equipment can increase production losses and generate complaints about declining reliability. Plant personnel notice the lights blink just as production quits. They think they had a power interruption when, in fact, it was a voltage sag. A few considerations when purchasing equipment can prevent thousands of dollars in production losses and frustration from overly sensitive to voltage sags.
Voltage sags are brief reductions of the system voltage. A sag may last from one hundredth of a second to more than a second. The magnitude of a sag is commonly between eighty (80%) percent and ninety (90%) percent of the normal operating voltage.
Voltage sags are a common part of the electric supply system. A short circuit on one part of the system may cause voltage to sag many miles away. Even faults on a neighboring utility may cause sensitive processes to quit. Short circuits in the plant or in a nearby plant may also cause nuisance shut downs from sags.
Your local Cinergy company can perform a sag analysis to determine the magnitude and frequency of sags to be expected from the utility system.
PROCESS EQUIPMENT SELECTION
Production losses increase dramatically with small increases in the equipment sensitivity. Consider two different brands of equipment that do the same job. Brand A will shut down if the voltage sags to 70% or lower. Brand B is more sensitive shutting down for sags to 90% or lower voltage. The shaded areas in Figure 1 show a hypothetical example of the areas of vulnerability for the two brands of equipment and how the sensitivity translates to production losses. When a fault (short circuit) occurs anywhere within the area of vulnerability, the equipment will shut down due to the resulting voltage sag. In this case, Brand B is much more vulnerable and may suffer 6 times more production losses than Brand A.
Another consideration is how long these devices can ride-thru a voltage sag. About eighty (80%) percent of all voltage sags are gone within 0.2 seconds due to the operation of the utility's protective equipment. If this sensitive equipment can maintain the load for 0.2 seconds during a voltage sag, the performance will be greatly enhanced.
Careful selection of new equipment can reduce production losses. Selection should not be based purely on initial cost since the cost of production losses may be very significant. The additional cost of a less sensitive device may be recovered through avoided production losses. Specifications for equipment should include voltage sag survivability to reduce down time and financial losses.
Figure 1: Area of vulnerability for sensitive equipment.
Mitigation for existing systems could include a UPS (uninterruptible power supply), CVT (constant voltage transformer), or a motor-generator set for a number of loads to provide sag ride-thru. The optimal location for the UPS or CVT would be at the controls of the sensitive equipment such that a smaller more economical unit can be purchased.
RULE OF THUMB
Voltage Sags cannot be eliminated, but here are a few rules of thumb to take care of most voltage sag problems. Undervoltage trip setting should be 70% or less of the equipment rated voltage. Power Loss ride through should be at least 0.2 seconds. Equipment manufacturers can provide information on Auto Restart and/or Flying Restart options and their applications.
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.