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It is essential that the utility delivers electric power at the consumer terminals with variation in electrical quantities within the specified limits. With the widespread emergence of industrial automation, most of the machinery involved in process industries, critical manufacturing and complex communication networks acts as nonlinear loads and have detrimental effect on the power quality of the grid (Jayaprakash et al., 2013). Moreover, with the proliferation of power electronics devices in other sectors further adds to the power quality issues (Sant et al., 2013). The current and voltage harmonics introduced in the grid by these nonlinear loads can cause malfunction in protection devices and control algorithms of plant and machinery (Sant et al., 2013). For desired plant operation, without any interruption in operation or damage to the machinery and plant, it is important that supply voltage is regulated within the specified values (Sadigh & Smedley, 2012).
One of the serious challenges faced by any industrial plant, is the sag and swell in supply voltages that can cause the protection system to isolate the plant leading to loss of production (Rauf & Khadkikar, 2015). The voltage sag is mainly caused by (i) energising of power transformers, (ii) starting of large induction motors, (iii) energizing of long transmission lines, (iv) disconnection of highly capacitive banks, and (v) line faults (Kavitha & Subramanian, 2017). The main causes of voltage swell are (i) disconnection of large loads, (ii) open conductor faults, and (iii) load shedding (Kavitha & Subramanian, 2017). According to the standard IEEE-519-2014, voltage sag refers to 10–90% reduction in the rms value of supply voltage over the duration of half cycle to one minute with the power frequency being maintained constant (IEEE Std 519-2014, 2014). On the other hand, this standard refers to voltage swell as 10–80% increase in supply voltage over the duration of half cycle to one minute with power frequency being maintained constant (IEEE Std 519-2014, 2014). The sag and swell in supply voltage can cause overloading, reduced efficiency, loss of production, etc. (Kavitha & Subramanian, 2017). Generally, the occurrence of voltage sag does not cause damage to the insulation directly. However, overloading of machines or equipment during the voltage sag leads to higher copper losses and excessive heating. As a consequence, the insulation may get damaged. Excessive heating on account of overloading due to voltage sag can be observed in cables, windings and power semiconductor switches. The occurrence of sag can lead to overheating of electric motors and nonlinear loads such as switched mode power supplies may necessitate repairs (Kavitha & Subramanian, 2017). Hence, it is necessary for the industrial plants to ensure mitigation of sag and swell in supply voltages.