According to international standards, power quality disturbances (PQD) is classified in agreement with amplitude or proportion, bandwidth and duration. In this paper an additional PQD classification is proposed, consisting in classifying PQD according to their occurrence in random or permanent disturbances. Random disturbances are those whose times of occurrence cannot be determined in advance (surges, sags, swells, etc.) and permanent disturbances are those present in electric distribution systems continuously (harmonics, inter-harmonics, voltage fluctuations, flicker, etc). This paper shows a method for calculation of permanent disturbances indices based on measurements, distribution system modeling and Monte Carlo perfect simulation, a method to simulate PQD with random behavior is proposed