The impact of poor power quality and harmonics on the performance of an electrical power network

Abstract

Poor power quality has a negative impact on electrical protection systems, rotating machines, transformers, control circuits, electronics, and power electronics equipment. The demand from industries to use power electronics equipment leads to more poor power quality issues – especially relating to harmonics. Determining the level of harmonics in an electrical network has become a necessity, as most electrical equipment are susceptible to harmonics. Previously, electrical networks on the customer side consisted mostly of direct current and induction motors, which resulted in simple networks that were easy to model using various types of simulation software. Today’s electrical network is considered complex due to power electronic equipment such as variable frequency drives (VFDs), uninterruptible power supplies (UPSs), switch mode power supplies (SMPSs) and other electronics equipment. Power electronic equipment are primary source of harmonics and are also susceptible to harmonics. To protect the electrical network infrastructure, it is very important to identify the level of harmonics content in an electrical network, so that solutions can be developed to minimise the harmonic level to acceptable limits – as determined by power quality or harmonics standards.

This dissertation presents an analysis of the performance of an island electrical network for an offshore crude-oil drilling ship. A real-time digital simulator was used as a systematic analytical tool to study the level of harmonics in a reduced-scale electrical network model, used to represent the real drilling ship power network. The study objective was to evaluate the behaviour of the power network when direct on line (DOL) starters and variable frequency drives are used to run induction motors coupled with mechanical loads. The waveforms from a limited number of field measurements on the actual network are compared to those obtained from the reduced-scale real-time simulation model of the plant. The study reviews the theory and literature of power quality, generators, transformers, variable frequency drives, and induction machines, and focuses on poor power quality as contributed to by harmonics. The investigation was based on 12 pulse rectifiers for all variable speed drives, which are standard for drilling ships and other offshore installations – as they offer advantages in reducing the fifth and seventh harmonics. Both the field measurements and real-time simulation results in the dissertation indicate the presence of similar harmonic waveforms, and with comparable frequencies – but with different amplitudes. Unfortunately, the simulation results could not be closely matched to the field results, as most operating parameters that are needed for better representation of the plant in the simulation model, could not be obtained within the limited time available for field measurements. Nevertheless, the model developed could be used with a greater degree of accuracy to demonstrate the level of harmonics for an offshore drilling ship power network, provided all operating conditions’ parameters are available.