## 1 Introduction  
Voltage sag (US English) or voltage dip (British English) is a decrease in RMS voltage to a range between 0.1 p.u. and 0.9 p.u., and it can be caused by grid faults or high current demand (such as the inrush current required for electric motors starting or transformers energizing). According to the main North American power quality (PQ) standard IEEE 1159-2019 [1], the shortest possible duration of voltage sag is 0.5 cycles (60 Hz). The shortest duration in the proposed voltage sag classification table in the main European PQ standard EN 50160:2010 [2] is 10 ms (i.e., 0.5 cycles of 50 Hz). According to the main PQ measurement standard IEC 61000-4-30:2015 [3] and the main voltage sag indexes standard IEEE 1564-2014 [4], the requirement for voltage sag measurement is one cycle RMS value refreshed every half-cycle. Voltage sags have remained a critical PQ issue to industrial loads over the past two decades [5,6]. This PQ event can occur at every power system node, with a frequency varying from several times to hundreds of times per year [4]. The sag can trigger a disconnection of at least some electric motors, and a manufacturing process will be interrupted since all elements in the chain are interdependent. Electric motors can be disconnected due to the following reasons: (1) when undervoltage relays are triggered; (2) due to relatively long-lasting insufficient power supply (which is essential to create a rotating magnetic field). These disconnections increase the outage time of factories, leading to huge economic loss. However, interruptions to the operation of critical equipment remain the most dangerous consequence. Some instances of loads that are critical for public and occupational health and safety (also for environmental protection) are listed in [5]: fire-fighting pumps, nitrogen loops (pumps) for chemical inerting, cooling loops in nuclear power plants, nuclear waste repositories, and oil refineries. The Fukushima Daiichi nuclear disaster, when three operational boiling water reactors (a type of light-water nuclear reactor) were shut down after detecting an earthquake, can be cited as an example. Since the electric power supply system had been damaged (by the earthquake), the diesel generators were automatically switched on for decay heat removal systems--it was vital to supply the pumps for coolant circulation through the reactor cores (after fission had ceased). However, one hour later, the tsunami (caused by the same earthquake) flooded these generators, which consequently caused the nuclear disaster (because cooling was lost) [7,8]. Despite an earthquake being a force majeure, the example clearly illustrates the importance of power supply interruption for critical pumps. Since voltage sag can be a dangerous event, various requirements are introduced (are attempted to be introduced) for equipment immunity. Although the detailed and critical analysis of these requirements is outside of this paper's scope, they are worth mentioning to highlight once more the importance of voltage sags for both end-user loads and power plants. Firstly, in the case of end-user equipment, the main standards (curves), included in the normative references of IEEE Std 1564-2014, are IEC 61000-4-11 and SEMI F47: the equipment must tolerate voltage sags in the area above the set curve, i.e., continuously operate without interruption. SEMI F47 sets stricter requirements than IEC 61000-4-11. Secondly, in Commission Regulation 2016/631 [9], the European Union electricity network code on requirements for grid connection of generators, fault-ride-through requirements are set for power-generating module types B, C, and D. Please note that Commission Regulation 2016/631 states that \"fault-ride-through capabilities in the case of asymmetrical faults shall be specified by each TSO\". In general terms, all generators are (an effective) measure for voltage sags mitigation (including both rotary machines and converter-based), thus it is very important to avoid their disconnection during grid faults.