Voltage and Cable Impedance Unbalance in Submergible Oil Well Pumps

Induction motors used with submergible oil well pumps are operated in harsh environments with extreme temperatures, and any additional motor heating caused by unbalanced currents must be minimized to extend insulation life. Since the pump is usually more than a mile deep, motor voltages may be´substantially different from the voltages measured at the surface, a situation further aggravated by the use of flat cable. The results presented correlate measured and calculated values of current and relate current unbalance to unbalances in surface voltages and cable impedances. Generally, cable insulation leakage and capacitance are sufficiently negligible that motor currents are virtually the same as currents measured at the surface; however, with excessive leakage or transformer connections permitting the cable armor to be used as a ground return, this may not be true, and attempts at surface current balancing could actually increase motor current unbalance. Finally, a method for flat cable compensation is presented which appears to minimize cable impedance effects for a fixed amount of voltage unbalance.