Characterization of transients due to capacitor banks in low voltage systems

The scarcity of literature addressing transients due to capacitor bank switching is obvious. Hence, this study was conducted with three objectives in mind. First, to characterize capacitor bank switching transients generated in a low voltage power system with selected inductive and resistive load. During the energizing and de-energizing processes, capacitor banks generate and inject severe transient overvoltage and inrush current into the system that will result in several issues. Although there are several studies conducted on capacitor bank switching transients in medium voltage systems, there are only a few studies done on the same phenomena in low voltage systems. Previous literature shows that damage due to capacitor bank switching transients is higher in low voltage systems than that in medium and high voltage systems. Characterization of transient voltage and current in low voltage systems due to switching of capacitor banks is an important process, needed to provide solutions to the issues, which has not been done comprehensively so far. Second, to simulate the experimental power system to generate capacitor bank switching transients and validate the results with experiments. Also, it was intended to compare the simulation results by experiment in real low voltage systems in addition to calculate the specific transient energy generated by capacitor bank switching. Third, to find the response of available surge protective device in mitigating capacitor bank switching transient effects. Power System Computer Aided Design software package has been used in this study to simulate the energization of capacitor bank switching in specific low voltage systems, with and without employing mitigation devices. Besides, the study employed experimental measurements of quantities such as peak transient over voltages, inrush currents, and transient duration for each case in five step shunt capacitor banks. Analysis of voltage and current waves has been carried out to extract acceptable capacitor switching times by observing the transient voltage and current. The findings showed that there is a severe current for the given loads due to switching of capacitor banks which tends to propagate in the system which may damage the capacitor bank itself and affect the end users. There are increases of transient peak values of current and voltage waveform due to the capacitor size and short rise time. In this case, long duration of transient may lead to high loss of energy in the system. The transients that were generated by both simulation and experimental technique were compared, and found to be slightly different owing to the environmental effects and level of accuracy of the measuring devices. Surge Protective Device meant for lightning transient protection are not efficient in their present form in mitigating transients generated by capacitor switching. It was shown that typical levels of the transient’s impulse current magnitude range from 116.9A to 163.1A, and from 113.4 A to 165.5A for simulation and measurement results, respectively when connected to the single phase measurement. It was also shown that the transient current for the three phase connected measurement is ranged from 116.2 A to 166.6A and from 117.7 A to 180.3 A for simulation and experimental results, respectively. Hence, curtailing of the specific energy and the energy dissipation of transients by means of filtering or attenuating devices has been strongly recommended. The comparison of the outcomes clearly shows that there are only minor differences between the simulation results and the experimental results in real low voltage systems. Severe currents that are produced during capacitor operations in power systems could be harmful to insulation, and capacitor bank control and equipment. The time of occurrence of the transient in the nominal 50Hz voltage waveform due to capacitor switching is a deciding factor for the safe operation of the system and equipment. The application of Surge Protective Devices, in their present form, will be insufficient, as they could not significantly reduce transient inrush current and voltage in low voltage system. This is proven by both experimental and simulation results. Depending on the time position of the transient in nominal voltage waveform and the transient polarity, its effects may vary. The worst scenario is the transient occurring at the peaks of the nominal voltage with the polarity as same as that of the nominal voltage waveform. Furthermore, there is a need to look for electronic switching to reduce the duration of the transient, which indirectly will reduce the effect of transients to the systems. The results can serve as a guidance for manufacturing technologists as well as electrical and electronic engineers in addressing and developing capacitor banks, thus solving transient switching issues for low voltage systems.

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