operator to decrease demand or to increase system operating margins. "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute. Variations in the frequency Variations in the wave shape – usually described as harmonics Nonzero low-frequency impedance (when a load draws more power, the voltage drops) Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line) See also Static synchronous series compensator Dynamic voltage restoration Uninterruptible power supply Active filter References External links Electric power systems components