## 1 Introduction  
Power quality issue is becoming critical for the network operators and electrical equipment manufacturers in the last few years. The electronics-based switching devices are burgeoning and escalating the current harmonics emission level in the electric supply system. With the enhancement in power electronics, the efficiency of domestic, commercial, and industrial electronic equipment has been boosted with a substantial reduction in their size. Almost every modern electrical equipment, such as personal computers (PC), battery chargers, household appliances, large commercial and industrial electrical systems encompasses a converter based power supply. These power supplies incorporate rectifiers and nonlinear components, thus polluting the distribution system with current harmonics. Therefore, current harmonic estimation is critical for power quality assessment. Probabilistic modelsfor current harmonic estimation are based on the measurements of the electrical loads [1] and most of the non-linear loads can be modeled as different switch-mode power supplies [2]. However, the time-dependent stability of the current harmonics generated by the power supplies may alter the outcome of such estimations. This paper deals with the deviation in the current harmonics magnitude and phase angles of switch-mode power supplies over time under constant load and input voltage. The direct current (DC) power supply takes input from the utility or local DC source such as a battery. It maintains an interminable voltage level within the designed current limits and can be regulated or unregulated. The regulated power supplies uphold the output voltage close to the desired nominal value for the variations in voltage, load current, and temperature [3]. The switch-mode power supplies (SMPS) are regulated power supplies and contain alternating current to direct current (AC-DC) converters. Because of the nonlinear components in the SMPS, they are the source of power quality problems and feed current harmonics in the network. These current harmonics prompt voltage distortion and the power factor reduction in the network. The current harmonics and voltage distortion cause overloading of the transformers. Their lifespan is reduced, and reliability is compromised. Excessive harmonic emission overheats electrical appliances and causes additional noise. Moreover, protection equipment, such as relays and circuit breakers, can also malfunction. Apart from polluting the network, these harmonics also affect the performance of SMPS itself [4]. Therefore, it is essential to estimate the impact of current harmonics emission in the network added by nonlinear power supplies. However, various uncertainties are associated with the power system that may affect the power quality estimations. Supply voltage variation and load operating modes can alter the harmonic emission in the grid [5]. The components present inside various electrical appliances may provide a variation because of material properties [6]. The aging of the components also affects the output [7]. The variation in power system operating conditions also results in variation of the network impedance [8; 9]. However, uncertainties in the measurement of power quality indices like thermal stability of the components and transients are mostly ignored in the harmonic estimation models available in the literature. Since SMPS are widely used in industrial, commercial and domestic loads, their collective impact could be devastating. Household loads in a residential distribution grid can be categorized into the linear load and the electronic load. However, electronic appliances are responsible for the majority of the current harmonic emission. These devices can be modeled as SMPS loads. Household appliances are categorized based on the SMPS circuit topologies in [2]. Consequently, for an accurate power quality assessment, every uncertainty during measurements of SMPS should be taken into account. Several studies are available related to harmonic emission associated with the SMPS load.