This paper deals with the estimation method for voltage quality problems of voltage sag and voltage swell caused by faults in a power system. Voltage sag is recognized as the most important power quality problem that causes malfunction and tripping of sensitive loads, causing enormous economic loss in modern industrial processes. Voltage swell is generally less than voltage sag, but repeated and cumulative voltage swells cause problems in the power supply section of sensitive loads, causing great damage. The most important cause of these voltage quality problems is a fault in the system. In particular, when an unbalanced fault such as a single line to ground fault occurs, voltage sag and swell may occur simultaneously. To date, most of the related studies have been conducted separately from voltage swells centering on voltage sags, and there is no study on the problem of the instantaneous voltage quality that can cause the voltage sags and swells simultaneously due to a single fault. Therefore, in this study, an effective evaluation method that considers both instantaneous voltage sag and voltage swell is proposed. The assessment of individual voltage sag and swell at the load of interest can be performed based on the concept of area of vulnerability. Also, the assessment of voltage quality problems that can occur simultaneously is carried out by determining the overlapping vulnerable areas of voltage sag and swell. In addition, an assessment method that reflects the operating characteristics of the protective relay system is introduced to evaluate not only voltage magnitude but also duration of voltage quality problems. Depending on the characteristics of voltage immunity, some loads are more sensitive to the duration of the voltage sag and swell. Since the durations of instantaneous voltage problems caused by system faults depend on the operation characteristics and the failure rate of protection systems, it is possible to evaluate voltage sag and swell durations by reflecting these characteristics. The proposed method can be effectively used to comprehensively grasp the long-term tendency of the instantaneous voltage sag and voltage swell at sensitive load points in power systems, and establish optimal system design and countermeasures against instantaneous voltage quality problems.