Short-term Effective Value Changes :
We can divide the short continuous change of the voltage-active value into three parts; These times are related to the processing times of the protective circuitry. The defined time and amplitude values of the short term changes according to IEEE 1159: 2009 are shown in Figure 1.

Figure 1. Short-term changes according to IEEE 1159: 2009

Short-term voltage variations; The starting and removal of the loads with high take-off currents are due to reasons such as disconnection in the power distribution system, short circuits. These changes are seen as swells, voltage pitting or voltage cuts according to the system condition and where the error occurred. The changes in time and amplitude current can also be classified as short-term disturbances.

If the supply voltage or the load current does not exceed 1 minute, the effective value is less than 0.1 pu. Power systems errors and hardware
defects can be the cause of downtime. The interruption is measured by the time the voltage amplitude is continuously below 10%. It is usually seen after tension pitting. Interruption of the protective devices may occur immediately after the voltage pitting in the transmission line. The duration of the interruption is dependent on the reclosing performance of the protective device.
The decrease of the effective value of the voltage between 0.1 pu and 0.9 pu within a period of one half-period to 1 minute is called sags. The definition of voltage pitting is not considered to be the level of pitting, but the residual voltage level. For example, voltage pitting of 80% means that the effective value of the voltage is 0.8 pu based on the rated operating voltage. Tension pitting may be caused by failures in the system as a result of high-current loads. An induction motor draws 6 to 10 times more than the nominal current at the time of take-off. Due to the system impedance, these high currents cause the voltage level to pit. In general, the period is defined as half a period to 1 minute. Since the change in rms value is not high in the tension pitting of short duration from the half-period, short-term pitting is classified as temporary.

It is called swells when the effective value of the voltage is over 1.1 pu in a period of 1 minute. The resulting voltage is indicated by the amplitude and its value is always greater than 1 pu. The tension is not as frequent as pitting. It is caused by malfunctions in transmission or distribution systems, as in the case of voltage pitting. It is usually seen in healthy phases during single phase-earth fault. However, it can also occur during the decommissioning of large loads and energization of large capacitor groups.
The intensity of the voltage rise varies depending on the condition of the fault, the system impedance and the ground. During a single phase-to-earth fault in a groundless system, the phase-to-earth voltage in the healthy phase rises to 1.73 pu. No voltage rise will occur as you approach an effectively earthed station. Because the distribution center transformer is usually connected to the triangle-asterisk (ı ı Y) and has zero impedance zero sequence for the fault current.