Single-Phase Voltage Sag Generator Based on Three Random-Fire Solid-State Relays with Programmable Angle of Inception for IEC 61000-4-11 Testing

Voltage sags are the most frequent power quality disturbance on low-voltage networks, and equipment immunity is verified against IEC 61000-4-11:2020. Laboratory-grade generators are expensive, and low-cost alternatives based on zero-crossing solid-state relays cannot control the angle of inception, a parameter that changes the drop-out time of an electromagnetic contactor by a factor of 6.5. This paper presents a single-phase sag generator built from three random-fire solid-state relays driven by an Arduino MEGA 2560 R3. Two relays commutate between the mains and a variable autotransformer; a third, in series with a transition resistor, bridges the switchover, removing the dead half-cycle of two-relay zero-crossing topologies. Over 576 measurements, the generator reproduces the angle of inception across 0–175° modulo 180°, and, therefore, equivalent to the full cycle for a polarity-insensitive load, with an expanded uncertainty of ±1.33° (k = 2), and the dead interval falls from 17.5 ms to 0.009 ms. Four of the six level–duration conditions prescribed for a class 3 environment are reproduced within tolerance; the two sub-cycle interruptions are not, the shortest sag being three half-cycles. Three limitations are quantified: the recovery instant is not programmable, being fixed by the current zero of the outgoing branch; it shifts by the load impedance angle under reactive loads; and non-linear loads were not tested. A design inequality for the transition resistor bounds the deliverable load power, and the firmware is open source.