A three-phase sag generator for testing industrial equipment

A diesel powered three-phase standby 480 V, 15 kW, 60 Hz synchronous generator has been modified to give controlled three-phase voltage sags. These sags can be controlled in both depth and duration. A 486-based computer with a data acquisition system controls the generator and monitors the system under test. The custom software interface allows the user to select the depth and duration of the each sag. Typically, the user will select a series of sags in selectable increments down to a certain depth. The data acquisition system has both digital and analog inputs, so that data lines, AC and DC voltages and currents, contactors and relays, etc. can be monitored during the tests. The software interface enables the user to select allowable tolerance bands for the monitored signals. If one or more of the signals falls outside the allowable range, the test is suspended. The sag depth and duration that caused the equipment to “fail” the test is then recorded, After testing over a range of sag durations, a CBEMA-type curve is available for the system under test. The sag generator enables the user to systematically determine the sensitivity of the components of an installed process line. This will enable the user to determine cost-effective ride-through enhancement strategies or to test the effectiveness of currently installed equipment. Although this test generator is small, the basic concept can be scaled up to the 2 MVA range