The Role of Distributed Generation in Power Quality and Reliability

electronics and to cause malfunctions of digital equipment. Swells can damage device power
supplies and/or cause resets to occur.
Overvoltages and Undervoltages
When sags and swells last for longer than 2 minutes they are classified as over or under voltage
conditions. Such long duration voltage variations are most often caused by unusual conditions
on the power system. Out of service lines or transformers sometimes cause undervoltage
conditions usually lasting less than one or two days. Voltage can also be reduced intentionally in
response to a shortage of electric supply. These brownouts have occurred in New York during
peak summer periods. Brownouts can cause overheating in constant speed motors due to the
increase in current density as well as the problems with electronic equipment that occur with
short-term voltage variations. Most utility voltage regulation problems are the result of too much
impedance in the power system to supply the load; customers at the end of feeders suffer the
most from low voltage. Under heavy load conditions, the voltage can drop. Longer-term
variations can usually be corrected by changing the tap settings on a load tap changing
transformer.
Problems on the utility grid can cause higher than nominal voltages long enough to adversely
affect facilities. This situation might happen because of problems with voltage regulation
capacitors or transmission and distribution transformers. The utility does have overvoltage
protection, but at times these devices do not respond fast enough to completely protect all
equipment downstream. 9
There are two basic solutions to utility-based overvoltage problems, each represents a different
philosophy. The first philosophy is to protect the facility from these disturbances at the expense
of uptime. In other words, if an overvoltage occurs, shut down the power. This is done through
devices that sense the voltage level, and if they exceed a preset limit, they operate, opening the
circuit. The second philosophy is to protect the facility, or at least sensitive parts of it. This
approach maintains uptime while providing controlled voltage regulation. Voltage Regulators
are employed to stabilize the voltage under an assortment of power disturbances including
overvoltage. Many times, voltage regulation is only one part of a device's capability. An
uninterruptible power system, or UPS, is one example. When utility power is available, the UPS
regulates the voltage to sensitive loads. Should the source power fail, the UPS provides back up
power.
Voltage regulators, or devices that incorporate voltage regulation, may be used at key
distribution sites within the facility such as the service entrance, the main distribution panel, or
the computer room panel. The most common types of voltage regulators, however, are small,
portable units used to protect one piece of equipment or one sensitive system. Utilities also use
voltage regulators on the distribution lines to maintain the voltage drop from substation to the
end of the circuit feed.
9 Utility power systems are designed to maintain voltage with the +10% to -10% voltage range for the most part.
Over and under voltage and frequency equipment is designed to protect equipment for disturbances that result in
voltage excursions outside of this range.
Appendix A A6 The Role of DG in Power Reliability