The use of variable-speed drive units (VSD) specifically designed for use with electrical submersible pumps (ESP) in the recovery of crude oil has been well established. The main advantages of such drive systems is greater pumping flexibility, soft start operation and automatic speed control. The variable frequency power sources that supply the asynchronous induction motors used with the pumps are generally static inverters This type of equipment Inherently produces non-sinusoidal current and voltage waveforms containing high-order harmonics. The operational life of the downhole equipment in the arduous conditions found in the oil industry has been limited. Although operating conditions can vary from field to field it has been found that a significant proportion of ‘failures’ were caused by electrical problems in the power delivery system involving the cable and connectors Possible contributory factors in these premature failures may be due to the hostile environment (high temperatures and pressure, adverse well liquids and gases) or mechanical damage incurred during installation and service. These factors in combination with the inherent electrical characteristics of the drive system may have a weakening effect on the electrical insulation properties of the components leading to eventual breakdown.
This contribution reports on an investigation of the electrical waveform characteristics of inverter-powered ESP systems under various mechanical load conditions. The primary purpose of this research is to enable users to select elements of a system to precise parametric criteria, thus enabling enhanced operational times with the attendant cost benefits. Such research will match theoretical and practical results, creating a more scientific address to determination of the required system parameters. The Harland Simon Group plc are sponsoring this project in conjunction with Southampton University A generalized computer program has been developed to simulate the steady-state electrical performance of the drive system. In support of the theoretical study, an experimental test rig has been constructed enabling testing of the drive system above ground. This gives the facility of monitoring the voltage and current at key points in the system. Primary interest is in voltage waveforms at points not normally accessible in real operating systems, such as the motor terminals and selected positions in the connecting cable run. As will be seen from the equipment specifications below, the lengths of cables can be several thousands of feet.
A typical ESP drive system consists of a variable-speed controller (e g current source inverter), the output of which is connected to a step-up transformer. The h.v. output of the transformer is connected to the motor via three lengths of cable. short length of surface cable to the wellhead, the long length to the ‘packer’, and a second short section from the ‘packer’ to the motor. For the purpose of this study the following equipment was considered.