Известия Томского политехнического университета: Инжиниринг георесурсов (Mar 2018)
Design of full order observer with real time monitoring of load torque for submersible induction motors
Abstract
The operation of electric submersible pump for oil wells is complicated by a number of factors, which include gas inclusions in the pumped liquid, the presence of asphalt and tar paraffin sedimentation, raising of sand and other abrasive particles, deposition of various salts on the working parts of submersible pumps, in particular, calcium carbonate CaCO3, calcium sulfate CaSO4, barium sulfate BaSO4 and sodium chloride NaCl. As the submersible pump operates under these conditions, the load torque gradually increases, with total decrease in the efficiency of the pump up to a partial or full jamming of the shaft. The increase in the load torque during the wedging additionally statically loads the walls of the tubing. The presence of gas inclusions leads to disturbance in the stationary of the flow of injected liquid, and as a consequence, additional stresses of alternating character appear in the walls of the tubing in a wide range of vibrations. In a number of cases the fatigue destruction of tubing and «downfall» of submersible equipment occur at the wellhead. Therefore, it is time to develop the methods and means for monitoring the load torque on the shaft of a submersible induction motor in real time. Direct measurement with the help of a torque sensor or the restoration of torque estimates by electrical measurements directly on the terminals of a submersible induction motor is not advisable for technical and economic reasons. The most promising is the development of a full order state observer, taking into account the properties of the submerged cable by measuring the currents and voltages at the output of the step-up transformer - at the input of the cable line. Setting up such an observer of the original structure is of scientific and practical interest. The aim of research is to develop and test the mathematical models of the original structure of the full-order state observer with the real time monitoring of the load torque for submerged induction motors feeding on a long submersible cable. The main material and studies are based on the use of the theory of full order state observers, numerical methods for solving systems of ordinary differential equations, numerical integration methods, automatic control theory, and the theory of signal filtering. Conclusion and outlines. The original structure of the full order state observer is proposed with on-line monitoring of the load torque on the shaft of a submersible induction motor power supplying from a long cable. For observer functioning, one need the information on the magnitude of currents and voltages at the input of the submersible motor cable, as well as signals on the estimates of parameters of the replacement circuit and the moment of inertia from the additional device, the parameter identifier (not discussed in this article). It is demonstrated that the structure of the observer provides the user with estimates of the orthogonal projections in the axes a [alpha], [beta] of the rotor flux coupling, the speed, the torque and the load torque on the shaft of the submersible induction motor in real time, both in steady-state conditions and in transient regimes: starting motor, on-off loading. The advantage of the observer is a high indicator of the quality of the evaluation with a small number of configurable parameters and rather simple setting. It is shown that the use of filter-post filters according to the Butterworth scheme improves the quality of evaluation of the load torque on the shaft of a submersible motor. The presence of signals evaluating the projections of the rotor flux linkage and the speed of the rotor makes it possible to recommend such an observer for electric drives made according to the scheme «frequency converter - induction motor». The studies shown that the integral errors of estimation during the observer working out of protracted transient processes are at an acceptable level: according to the speed estimation, they do not exceed 0,5 %, and according to the estimation of the load torque on the shaft no more than 20 %. The estimation error in steady-state regimes and in the absence of parameter variations is less than 1 %.
