Harmonics and Interharmonics of Top and Mudpump Variable Frequency Drives in Drilling Rigs

Abstract

Drilling rigs are large loads temporary in nature. Nowadays, drilling of a well can be done in the range of a few hours to a few weeks. Given the cost and complexity of building, mobilizing and operating, and space constraints, oil and gas companies rarely own their drilling rig and contract out specialized drillers. Their rigs often are operated for decades and contain rugged Variable Frequency Drives (VFDs), most times of six-pulse thyristor bridge topology or SCR-driven DC motors. The dynamics of the drives show steep ramp-up/ramp-down, rich harmonic content, and a wide power factor range. To make matters worse, drilling operation is often conducted in isolated, weak systems, leading to large background voltage distortion when grid-connected. This paper presents simulations and measurements of top-drive and mud-pump VFDs in a large drill rig operating in Alberta, Canada. It was observed that the VFDs draw currents very high in harmonic and interharmonic contents. To further investigate the true impact of these harmonic emissions, the paper also quantifies the harmonic Attenuation Effect of the VFDs, which can lead to harmonic content overestimated, if ignored. Due to the rich harmonic emissions, care must be taken to appropriately size the interconnection transformer when grid connected, and this is covered in the paper as well. As expected, these loads must be managed accordingly by the electric utility to avoid issues to nearby customers.