Application of Direct Torque Control (DTC) in the Drilling System of Electric Drilling Tool

Drilling system with electric drilling tool is one of key drilling technologies for the development of electric-driven drilling in the days to come, but the characteristics of rock cutting by downhole electric motor driving bit is significantly different from that of rock cutting by conventional rotary table and TDS driving drilling string to rotate the bit. Downhole electric motor is highly effective with flexible control, which is incomparable for rotary table and TDS. However, due to the limit of downhole space size, downhole electric motor fails to reach the reserve of torque for rotary table and TDS; At the same time, the characteristics of rock cutting by bit is alternating load with high frequency and is greatly affected by the factors of the outside, such as WOB, RPM, and characteristics of rock cutting in the formation, which will make the situation worse for downhole electric motor with inadequate reserve of power; Thus it leads to the failure of realizing the running of bit at stable speed. So, to adapt to the characteristics of high frequency alternating load for the bit, control system of downhole electric motor is required to be provided with better performance of dynamic response. Based on the change of load characteristics, output torque of electric motor is timely adjusted to achieve the steady control of motor revolution speed. On the one hand, efficiency of rock cutting is improved, on the other hand, use life of bit is extended. This paper, through the comparison test of pilot performance for currently existing and more mature frequency converter of vector control mode without position, primarily verifies the mode of direct torque control (DTC) and control of permanent magnet synchronous motor by long-distance vector without position (1800 m) is achieved. Compared with conventional V/F ratio open-loop control mode, the higher performance of dynamic response is attained.