A Novel Investigation of Power Quality Effects on Premature ESP Failures

Abstract

Abstract Electrical Submersible Pumps (ESP) are an essential artificial lift method in oil fields when the reservoir pressure is insufficient to drive the well fluid to the surface naturally. One of Mubadala Petroleum's operated fields in particular experienced several ESP failures from downhole ground faults, which resulted in a run life much shorter than the typical average run life experienced in similar applications. As such, power supply quality is investigated to understand whether it could accelerate ESP system breakdown. Commonly, a Variable Frequency Drive (VFD) is employed to optimize operations as the reservoir conditions change. However, VFDs tend to generate transient and harmonic distortions as a result of electronic switching. Measurement tools, such as a power quality analyzer and an oscilloscope, are connected to VFD output to measure its power quality. Partial Discharge (PD) is considered to be a possible factor contributing to electrical failures. As such, a PD tool is connected to the medium voltage supplied to ESP to determine if any PD exists. The investigation of power supply quality focuses on the percentage of Total Harmonic Distortion (%THD) at three different locations in the system; input and output of VFD, and output of the step-up transformer. Since there are no international standards in place to regulate the power quality output from the drive, different VFD models create different levels of output harmonics. Our results show that two different models can give different levels of %THD, both in terms of voltage and current. However, acceptable levels are observed for all of the designs tested, ranging from 2-15%. Power transient and voltage spikes were also measured during well startup and shutdown period, and indicated no significant differences between VFD models, and no issues of concern. However, significant levels of partial discharge, as measured in nano-coulombs, are observed at the medium voltage measuring point through to the downhole cable connections. It indicated the possibility of electrical anomalies, which can result in cable insulation failure. The level of partial discharge on each specific connector piece can be performed further during the Dismantle, Inspection, and Failure Analysis (DIFA) process after the equipment is pulled out of the well. The primary impact of high harmonic voltage distortion in ESP power systems is excessive heating, which can accelerate insulation resistance deterioration. Partial discharge can also contribute to insulation failure of downhole connectors and cable splicing points. This paper provides a systematic approach to investigate the power quality system supplied to downhole ESP equipment, and the application of a partial discharge measurement method, which is new to the ESP industry.