The effect of Variable Speed Drive (VSD) waveforms on electrical motors is generally well understood; however, typical industrial solutions fail to adequately protect specialized ESP systems, as ESP motors cannot be drive-duty rated. This paper examines VSD/ESP systems, discusses how switching transients and high frequency content may manifest themselves via the carrier frequency of the VSD, addresses the harm those transients may cause to ESP systems, and provides solutions that can mitigate related ESP failures.
Dismantle, inspection, and failure analysis of the VSD-powered ESP equipment indicated that there was an underlying electrical issue with these systems, thus comprehensive power quality studies were conducted. Data was collected on unfiltered VSD outputs, after the installation of Sine Wave (line to line) filters, and after the installation of Common mode (line to ground) filters. Current and voltage data was collected using Fluke 1750 power quality analyzers, as well as Tektronix THS3024 and Yokogawa PX8000 oscilloscopes with appropriate high frequency test probes. Spectral analysis (also known as harmonic analysis, or frequency domain analysis) was then performed on the collected data, from which corrective actions were determined.
Spectral analysis indicated that even on sine wave filtered systems, there was always significant common mode current present. While these currents contribute relatively little to Total Harmonic Distortion between 20th and 73rd harmonic (dependent on carrier frequency), it was found if their individual resultant amperage was greater than only 0.3 amps L-G, ESP failures would occur. Failures were caused by bearing fluting; feedthrough and penetrator arcing; stator end turn failures; skin effect heating of cables; and MLE, pothead, or splice insulation failures. Due to the specialized application of ESP's, motor design changes have not been readily implemented to improve their resistance to high frequency electrical content. Therefore, the only reasonable solution is to improve VSD output power quality to conform to IEEE 519-2014 specifications in the high frequency realm, with special attention paid to reducing harmonic content beyond the 20th harmonic. The installation of well-designed sine wave and common mode filters provides the most cost effective means to correct poor power quality in ESP systems, thus both types of filters were installed in the case presented. Adding these electrical filters resulted in the ESP mean time between failure to typically double, but most importantly, resulting failure modes were attributed to other non-electrical root causes.
During testing, an inspection of ESP wellhead cable utilizing airborne ultrasonic scanning tools was performed. The operator detected the distinct sound of an inverter over 1000' away from the VSD that was powering the ESP. As filters were installed, the sound decreased and was ultimately not detectable. It is believed that with enough research, the use of airborne ultrasound may provide a novel, non-invasive means to detect harmful high frequency electrical signals in ESP systems.