# Power Quality Impact and Its Assessment: A Review and a Survey of Lithuanian Industrial Companies
## 2 Literature Review
### Economic Impact
When testing the immunity to RMS voltage variations, the results should be presented in the voltage-duration plane by drawing a curve that separates zones where equipment operation remains uninterrupted and where it is interrupted with all the consequences that follow. The goal is to draw these curves for each device and, when necessary, for a group of them by evaluating all influencing factors. The severity of voltage sags depends on various factors, such as voltage sag depth, duration, shape, energy, and point on wave. Further, it depends on the device and its operating mode: for example, [28] demonstrates that the immunity of the tested LV three-phase induction motor (746 W) depends on its loading. A case study on residual voltages at the terminals of induction motors after a three-phase short circuit at various grid nodes is presented in [2], and if their immunity curves were known, it would be possible to predict the probability of their work continuity which directly correlates with the reliability of the technological process and financial losses. Next, Figure 3 presents several immunity test results of the LV single-phase equipment found in [24; 29; 30]: it can be seen that the most sensitive of the examples given is variable-frequency drive, the operation of which is interrupted when the voltage drops below 0.8 p.u. Contrary to other appliances, electric motor recovery after a voltage sag is followed by irnush current that causes another voltage sag which not only interferes with the recovery but also can affect other loads. The presented examples are subject to the limitations of inductive reasoning: these results are case-specific and, at present, the number of such studies is clearly insufficient for generalization. To continue, the first question that arises is what should be the benchmark for the voltage sag immunity requirements. Naturally, there is a desire for the equipment to be fully resistant, but this is limited by cost and other real-world factors. At the moment, there are several universal reference curves established for end-use equipment, of which the most well-known probably are the SEMI F47, ITIC, and IEC 61000-4-11 (identical to IEC 61000-4-34). It is noteworthy to mention that the opposite of these curves is fault-ride-through requirements for power-generating modules--PQ phenomena are dangerous not only for end-use equipment but also for generators by posing a risk of their disconnection, which negatively affects the reliability of electric power systems as well as hinders electricity trading. Plotting the curve is a qualitative assessment, thus the second question that arises is how to quantify the severity of voltage sags. For this, it is necessary to introduce various indexes that would give the weights to the regions of the voltage-duration plane. For example, [31] does this by creating the feature vector with five--grid energy loss, voltage sag value, voltage sag duration, equipment tolerance (by using the rectangular curve as a reference), and equipment compatibility (by using the SEMI F47 and ITIC curves as a reference)--indexes, and justifiably criticizes other sources that uses only either one or two indexes, because such approach perhaps cannot be completely objective and reasonably reflect the situation. Among these sources is IEEE Std 1564-2014 [32]--this guide for voltage sag indexes provides the methodology for voltage sag severity evaluation by using the SEMI F47 curve as a reference. SEMI F47 is a globally used standard (originally published in 2000, updated in 2006), which defines minimum voltage sag immunity requirements by setting out the limits that the equipment needs to tolerate without creating any process upsets or shutdowns. Equipment is not classified by type, but the primary focus of the specification is semiconductor processing equipment (e.g., etching, film deposition, photolithography equipment, etc.)