The Institute for Electrical and Electronic Engineers (IEEE) defines power quality as “the concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment” (Std. 1159-2019, Annex C, Glossary, p.92). This definition addresses two important voltage issues regarding power quality: 1) powering equipment to operate properly; and 2) compatible with the facilities' electrical system.
In recent years, more compatibility issues have surfaced from the lack of attention to voltage. Steady-state voltage requirements of electrical equipment must always be considered (e.g. ±5% of nominal between 120V to 480V). However, for this discussion, good engineering practice of electrical system design dramatically changes the electrical performance.
For example, a potato chip manufacturer experienced poor product quality at a new plant location. This location was built inside an existing building already serviced with 120/208V 3-phase power. The problem came when the 480V 3-phase potato chip machines were added. In order to power the machines, a 208V/480V delta-delta step-up transformer was installed. Once in service, the downstream 480V equipment suffered from low voltage, and the 120V controls and computers suffered from voltage sags — attributed to the manufacturing of potato chips. The best engineering practice is to supply the facility with 277/480V 3-phase, buffering the 120/208V load from power quality events.
In another example, applying the incorrect voltage class equipment to the supplied service voltage caused frequent shutdowns. A medical office building was adding split-system air conditioners on the rooftop. The air conditioners were not working properly during high outside temperatures. It was discovered that the units were rated at 208V/240V single-phase; the building was supplied with 120V/208V 3-phase service.
At first glance, there may not seem to be any correlation. Nevertheless, a deeper look into the air conditioner minimum and maximum voltage revealed the units require 197V to 253V. The service voltage can be ±5% of 208V (197V to 218V) at the electric revenue meter, plus the allowable voltage drop of 5% within the building means the voltage could be in spec as low as 187V at the point of utilization (equipment). This is much lower than the air conditioner minimum voltage requirement.
Simply matching the voltage of the equipment and the service will help to avoid potential power quality problems. Additionally, running equipment at the low end of the voltage range will lead to more susceptibility during voltage sags.
Glenn is a power quality consultant for EC&M. He can be reached at [email protected].
