Power quality and factory automation

A case study involving monitoring power quality disturbances at a representative plant and identifying the disturbances that disrupt production is described. The sensitivities of representative electronic control equipment to the identified disturbances were measured and then projected to form a plant disturbance threshold. For the monitoring effort, six disturbance analyzers were installed at four voltage levels extending from the utility 40 kV station to 120 V control power in an individual machine tool. Voltage sags were the only disturbance to directly cause lost production and were the most common disturbance at 68% of the total number of events recorded. Two programmable logic controller (PLC) transfer lines and a computerized numerically controlled (CNC) lathe were tested with a sag generator to determine the sensitivities of the equipment. The most sensitive components required the voltage during a sag to drop to less than 80-86% of rated to malfunction, whereas the least sensitive required the voltage to drop below 30% of rated. From the test results, the calculated sag threshold at the utility feed to the plant to disrupt production was 87% of the nominal voltage for more than 8.3 ms.>