Abstract
As the statistical analysis showed, the majority of voltage dips are asymmetrical (81% with 16 occurrences in one and a half months). The conducted modeling made it possible to establish the following. The minimum voltage protection reacts to a voltage drop in at least one of the phases below 70%. Asymmetry does not affect the correct operation of the protection. With a symmetrical voltage drop below 70%, the protection disconnects the motor power supply. With an asymmetrical voltage drop in one of the phases below 70%, the disconnection also takes place. Current relay protections are protection against short-circuit currents, but they also react to voltage asymmetry, for example, overload. According to the modeling results, in our case, current relay protections (maximum current protection, current cutoff) do not operate in asymmetrical modes not associated with the occurrence of short circuits directly in the area of their responsibility, since the setting values of these protections are many times higher than the nominal value. But there are changes in current with voltage asymmetry. The settings for current cutoff and maximum current protection were adjusted from the starting current. In asymmetric modes, the change in current affects the operation of the overload protection, which disconnects the motor power supply when the rated current is exceeded by 20% after a time delay. Thus, asymmetric modes, including asymmetric voltage dips, change the current values. This affects the correct operation of current protection. The greatest impact is on overload protection. The impact on current cutoff and maximum current protection is smaller due to significantly higher values of current settings. Therefore, the asymmetry of voltage dips must be taken into account when adjusting overload protection in order to maintain the stability margin of the electrical system.
Keywords
relay protection and automation, asynchronous motor, electric drive, voltage dips, asymmetric modes, short circuit, oil pumping station, current cutoff, maximum current protection, overload protection, undervoltage protection, simulation
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