Research On Technology Of Starting And Protecting Of Medium-Voltage Downhole Asynchronous Motor

In today's oil industry, because the restrictions of oil well itself, relying on natural blowout already can't satisfy the requirements, therefore drilling for oil ESP with equipment for artificial lift has become an important method. In many parts of ESP, the motor is an important component. Due to its advantages of simple manufacturing, reliable operation, high capacity, and good maintainability in harsh conditions for a long time and stable working, asynchronous motor is chosen in all the system of ESP. Because of working in thousands of meters under the ground, power cable may cause the voltage drop, so the low voltage cannot satisfy the rated load conditions of the normal starting requirements. What`s more, under the condition of direct starting, the current will reach the 4-7 times rated and even higher, the grid and the unit system itself can be damaged.Research on the process of starting and stopping, and the method of protecting is mainly included in the following aspects:Firstly, the working principle and speed sensorless measurement methods are given through the analysis, continuous measurement of the thyristor voltage can realize the measurement of speed.Secondly, soft starter for the medium-voltage downhole motor is designed and debugged. High-voltage thyristor of starting device costs higher, this paper proposes the method of constituting series thyristor valves using low-voltage thyristor, gives the answer to the question of dynamic and static voltage equalizing, designs the trigger mode based on the alternative current source, realizes the isolation between main circuit and trigger circuit.Third, deep study on the soft-starting and soft-stopping process is introduced. Through the analysis of power factor angle and the fluctuation of current and torque in the end of starting, the closed-loop control based on the feedback system of the power factor angle is raised, it can solves the problem above.Fourth, the protection in the running process is also analyzed, according to the characteristics of the downhole motor, arithmetic for protecting is given, the program of soft-starting and running is designed based on DSP LF2407. Finally, the application of direct torque control theory in the soft-starting process is deeply discussed, and the control strategy is simulated.