Using Medium Voltage Drive MVD for Preheating the Induction Motor IM of Electric Submersible Pump ESP to Extend its Deepwater Run Life
Authors: Rojas Marisela , Merlino Andrew , Martinez Ricardo , Li Yong 
Published: October 2018

This paper provides the validation test results of preheat sequence applied to induction motors at two Test Facilities and offshore application for operation in the Gulf of Mexico.

Although the objective of preheating Induction Motors (IM) is to lower the viscosity of the lubricant oil by 2 orders of magnitude (from 1000 cP to 10cP) for extending Electric Sumersible Pump (ESP) run life, this paper is exclusively focused on motor preheating results.

The motor is energized with low voltage at a frequency of 120Hz maintaining the voltage low enough in order to keep the supplied shaft torque under the system's breakaway torque; thus the shaft never spins. The Medium Voltage Drive (MVD) is a Variable Frequency Drive output power determines heat rate that is adjusted to obtain temperature slope of 1°F/min specified by the project.

The motor is modeled electrically and magnetically through Finite Element Analisys (FEA) to estimate its power losses; the motor internal temperatures can be predicted by the Motor-CAD (Computer-Aided Design) thermal model which is calibrated by winding resistance change and skin tempeperature measurement.

The systems for validation were: First test facilities: 1500hp Induction Motor coupled to a pump and driven with a 2500hp MVDSecond test facilities: 1500hp Induction Motor coupled to a dyno and driven with a 2500hp MVD.Offshore: Five 1500hp ESPs driven with 2500hp MVD each.

The results at first and second test facilities and offshore in the Gulf of Mexico demonstrate the preheat sequence can be successfully implemented in the field by using existing MVD with little software changes in order to apply low voltage at 120Hz without spinning the rotor. The stator current and induced current on the rotor make motor internal temperature (including lubricant oil) to rise achieving different temperature slopes. Temperature slopes vary in function of applied motor current (there was no need of overpassing motor nominal current on any test), motor thermal capacity, initial motor temperature, and external temperature.

All tested motors are very similar and was found that Keeping heating power at around 34kW, winding temperature rise can be achieved at a rate of 1.52°F/min at an initial temperature of 38°F and 1.2°F/min at an initial temperature of 148°F. Temperature rise rate at the motor air gap (actually filled with oil) and bearings location can also be predicted by the motor thermal model.

The required preheating time is previously calculated to reach less than 10cP viscosity of lubricant oil to guarantee safe startup without the occurrence of bearing spin; otherwise bearing friction torque overcomes the T-ring retaining torque causing bearing(s) damage.

When the need of preheating the induction motor of electric submersible pumps installed in deepwater applications was identified, there was no clear means to make it possible. This was the first time that concept was applied and successfully implemented in the field.

A second milestone was to preheat the motor with the MVD without adding equipment. Among five potential methods for preheating the motor, the selected scheme worked as expected with minimum MVD software changes.