New method for assessing the disturbance level of a distribution system

Few electric utilities go beyond current IEC and ANSI requirements regarding the thresholds of harmonic distortion, voltage unbalance, overvoltages and voltage dips on their distribution lines on a full-time basis. Technical document IEC 1000-2-1 stipulates that those threshold values should be respected at least 95% of the time, while the draft of revised ANSI standard 519 states that specified values may be exceeded for short intervals. Thus, during certain periods of time no limits are specified; hence the need for some means of characterizing disturbances and defining thresholds that are better adapted to modern power systems. The authors therefore propose a new approach based on the thermal energy accumulated by the equipment affected by power system disturbances. In addition to a redefinition of the steady-state value, an energy-related index is given for better identifying the nature of fluctuating system disturbances