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Content Type:Q&A

Question:

What effect does poor power quality have on sensitive equipment? A utility customer complains that his equipment trips off when brief sags or momentary outages occur.

Answer:

Here is some general information on power quality and sensitive equipment, as well as specific information on the particular issue you mention. The power quality issues that are liable to affect sensitive equipment are:

  • Harmonics
  • Spikes
  • Electromagnetic interference (EMI)
  • Poor grounding and bonding practices
  • Swells
  • Sags
  • Momentary outages

Harmonics can come in from the utility but they are usually generated from within the facility. If, when a power analyzer is attached to the customer’s transformer, secondary and current harmonics are of the same order as voltage harmonics but much larger, the source of the problem is the customer’s own loads. Voltage harmonics can—but do not usually —cause tripping of modern electronic equipment unless they are quite severe. More likely problems are reduced efficiency and increased heat loss in motors, overloaded neutrals, increased heating in cables and transformers, and sometimes tripped or blown power factor capacitors.

Spikes have not been worsened by the increase in electronic loads with the possible exception of some notching (negative spike) injection from SCR power semiconductors. Surge protectors have become more prevalent with the implementation of more vulnerable electronic equipment. Spikes can blow components in the power supplies of sensitive equipment.

EMI usually manifests itself as interference with audio or video equipment or control systems with sensitive sensors. It can be caused by high speed switching or electronic equipment like PWM adjustable speed drives and poor shielding of cables and certain equipment.

Grounding is solidly connecting equipment to the earth potential, and bonding is solidly connecting equipment to other equipment that is supposed to be at the same potential. It is always a potential problem if it is done incorrectly. It is less forgiving when there are modern high speed switching devices or high frequency apparatus in use. Many strange things can happen as a result of poor grounding. It can cause failures and misoperation of electronic equipment, bearing failures in motors and driven loads, and of course electrical shock hazards.

Swells usually occur from slow acting or faulty voltage regulating equipment either on the utility side or the customer side of the meter. Most customers do not have this sort of voltage regulating equipment.

Sags occur when heavy loads are started, most notably large motors starting across the line. They can also occur when there are brief faults on the utility line, like contact with tree branches or other faults that quickly clear themselves. These sometimes become momentary outages when protective devices begin opening and re-closing.

Momentary outages are literally just that. They can be relatively harmless to much equipment, but disastrous to computer and control systems that are not protected by an appropriate UPS. For motor-driven systems they can sometimes be more of a problem than an outage that sustains long enough for a motor to come to a stop.

Advanced Energy in Raleigh, NC, has researched the sag and momentary outage issue as it affects motors and controls. See "How to ride through sags and outages." They determined that motors handle sags much better than momentary outages. With the sag, if the motor power contactor stays engaged, there is no loss of synchrony with line frequency, the motor draws higher current to compensate, and resumption of full voltage can be quite uneventful. When there is a momentary outage or a sag sufficiently low enough to let motor controller contacts open, there can be trouble. The trouble occurs if power is resumed while the motor is still coasting and generating back EMF that may be out of sync with the reapplied line voltage. Advanced Energy has documented severe torque shocks from reapplication of power. This could do mechanical damage to the connected load or damage the product being handled. Advanced Energy developed a product that addresses this and that can be installed in controllers of motors with critical or sensitive loads. It controls the moment of contact reclosure to occur when line voltage and motor back EMF are in phase.

The internal power supply or external controller for every piece of equipment should be designed to take appropriate action in the event of sags or momentary outages. In motors, this usually means power and control contactors should hold in through an appropriate number of cycles of sag down to an appropriate minimum voltage. After power is completely interrupted or contacts open, the controls may either be designed to automatically resume operation or trip and await manual reset where safety issues or damage to a process can occur from uncontrolled restart or restart before the motor has coasted to rest. There will certainly be sags and momentary outages of duration from fractions of an AC cycle to several cycles, seconds, or minutes. The momentary events are much more frequent than outages that last several seconds or more. Controls must be specified and programmed to do the right thing for sags of any magnitude and duration, and for outages of any duration.

Very critical equipment needs an uninterruptible power supply (UPS). Light to intermediate loads are often protected by battery-backed-up UPS systems. There are generator/flywheel devices for loads that are both large and very critical. They can keep power up for several seconds or even minutes until onsite diesel generator sets have time to operate.

A convenient starting place for articles on power quality is the magazine EC&M and its predecessor Power Quality. There is a search box for specifying key words. This website also has a Power Quality section that you can access by clicking Power Quality from the top menu bar. A good Power Quality Primer is provided by the Copper Development Association.

Other resources: U.S. DOE. Energy Efficiency & Renewable Energy Program. Advanced Manufacturing Office: Issue Focus: Unbalanced Voltage, Winter 2005 and Spring 2005 .

Topic: Motor Systems--Operation and Maintenance
Topic: Utility Companies--Transmission/Distribution
Topic: Electrical Systems--Power Quality
Topic: Operation/Maintenance--Motor Systems
Sector: Commercial
Sector: Industrial
Sector: Utility
Content Type: Q&A
Keywords: poor grounding and bonding practices, momentary outages, spikes
ID:  4503