This paper describes a transformer-based voltage sag generator (VSG) suitable to evaluate the susceptibility of electrical equipment to voltage sag. The built VSG utilized one auto-transformer and two solid state relays (SSRs) to provide nominal voltage and sag voltage to the load. The switch statuses of two SSRs are controlled by nominal voltage and sag voltage duration signal produced by electronic circuits. The VSG operating result shows that it enables effective control of sag magnitude, duration, beginning and ending points on output voltage wave. If needed, it can work as a voltage swell generator and a voltage interruption generator. By supplying high voltage (HV) transformer from the primary side, the VSG also can provide HV sag, swell, and interruption. The presented VSG is easier to set up in the lab, and the construction cost is much lower than buying VSG products at the current market.