Investigation and mitigation of temporary overvoltage caused by harmonic filters for variable speed drives

Abstract

The introduction of variable speed drives (VSD’s) has become very popular in industrial operations, as it allows for smoother and more efficient operations of different processes. The installation of a VSD in an electrical system reduces the operational costs and if well implemented, it prolongs the life span of electrical equipment in the power system. Despite the fact that the use of VSD’s offers these benefits, the main disadvantage is that its operations produce harmonics. However, the implementation of harmonic filters to eliminate these harmonics can cause temporary overvoltage during the filter switching which can negatively affect the systems operations. Temporary overvoltage and its mitigation are a matter of concern; thus, this study is aimed at investigating and mitigating the temporary overvoltage caused by the harmonic filters used to control the operations of variable speed drives. The investigations conducted comprised of analytical modelling, computer simulations, and case studies. For all these aspects of the study, four harmonic filters were modelled, simulated, analysed, and were also implemented for the case studies. Firstly, was the analytical investigation where a network of VSDs of with and without filters was designed, modelled, and simulated to ascertain the harmonics produced. Secondly, was the computer simulation of the network in the ATP software. Thirdly, were the case studies, in which the harmonic analyser was installed for three different industries to measure the harmonic distortion and the power quality of the system. From the results, it is found that the 3rd and 5th harmonic orders were the highest in all three studies conducted. Harmonic filters may be used to reduce harmonic distortion to levels detailed in IEEE 519-1992 standards. The single-tuned, double-tuned, CType, and high pass filters were proposed for the purpose of mitigation. According to the results from the simulation, the double-tuned, and single-tuned filters were the most successful in mitigating the 3rd and 5th harmonics. Overvoltage’s for both energization and de-energization of the harmonic filters were found to be greater than the power frequency withstand limit. To avoid the undesirable effects that overvoltage has on the system, pre-insertion resistors, surge arresters, and controlled switching was considered for the mitigation of the overvoltages produced during the switching of the harmonic filters. Based on the simulation results, the surge arrester greatly mitigated the overvoltage caused by the energization, followed by the use of a pre-insertion resistor. Controlled switching was the least effective mitigation method in all three case studies and is therefore not the best choice to limit the temporary overvoltage.