Standards offer utilities, customers guidance on power quality
Editor's note: The Energy Services Bulletin features real answers to real questions posed to our staff at the Energy Services Power Line. We hope you find it useful.
Question: We are looking for power quality specifications or guidelines for electrical distribution voltage. We know of the one produced by the American National Standards Institute, but that is plus or minus 10 percent. Is there another standard or guideline for power quality? An example of vulnerable loads from one utility customer was a hospital that has MRI equipment, X-rays, etc. We were told that it can't take "events" outside of normal limits. What are these normal limits? Are these specifications or guidelines different for different customers?
Answer: ANSI C84.1-1995 defines voltage levels, giving a range of service voltage and use voltages for different nominal system voltages. Generally, the service voltage is not supposed to deviate from nominal by more than 5 percent. This standard includes "soft" language that allows some unquantified deviation so long as it is not frequent.
The standard also gives direction on balance suggesting that unbalance be kept within 3 percent at no-load. But that is pretty loose. Power Line staff recommends that it be kept within 2 percent at no-load conditions. The unbalance is only going to get worse at the point of use when there are unequal single-phase loads.
Power Line staff are not aware of any standards for sags, swells, transients or harmonics that apply to what the utility should provide. The well-discussed IEEE 519 pertains to what the customer must do to control harmonics. Other facility- or customer-oriented standards are:
- Surge protection ANSI Standard C62.41
- IEEE Standard P1250: Guide on Equipment Sensitive to Momentary Voltage
- IEEE Standard P1100: Guide for Powering and Grounding Sensitive Equipment
Sags and third harmonics (which cause a reduction at the peak of the sine wave) are one of the big problems that interfere with sensitive equipment. In-plant sags are often caused by switching and starting large loads like large motors. Sags originating from the utility are most often caused by wind storms. Third harmonic problems are usually commercial, i.e. building phenomena, and rarely originate with the utility.
