Abstract
A speed-sensorless state-feedback controller for induction machines (IMs) with LC filter is proposed. The speed and state estimation is based on a speed-adaptive observer, requiring only the measurement of the filter input currents. The motor currents are controlled by a state-feedback controller with prefilter and integral control action, in order to achieve fast and asymptotic set point tracking. Observer and controller gains are calculated offline using linear quadratic regulator (LQR) theory and updated online (gain-scheduling) in order to attain stability and improve controller performance in the whole operation range. Implementation aspects, such as discretization of the control system and reduction of computational effort, are taken into account as well. The proposed control scheme is validated by simulations and experimental results, even for critical operating conditions such as speed zero-crossings. It is shown that the overall control system performs very well under various load- and speed conditions; while its tuning remains simple which makes it attractive for industrial application such as geothermal electric submersible pumping (ESP) systems.
Highlights
In medium-voltage (MV) variable-speed drive applications with long power cables such as geothermal electrical submersible pumping (ESP) systems [1], an inverter output LC filter is often employed between voltage source inverter (VSI) and induction machine (IM) as to (i) decrease voltage deflection at the motor terminals, due to impedance imbalance between the cable and the motor, and to (ii) reduce steep voltage slopes which might damage the motor insulation and bearings due to high capacitive discharge [2,3,4]
The testbench comprises a 3 kW induction machine and load machine, both equipped with position encoders, a torque sensor, a custom-built LC filter, 2-level VSIs and the dSPACE real-time system.The modular dSPACE system runs on a DS1007 processing unit, with a DS5101 module for the pulsewidth modulation (PWM) generation, a DS2004 A/D module, and a DS3002 encoder board
The control system can be utilized for a wide range of high/medium-voltage applications with long cables where an LC-filter is mandatory to (i) minimize bearing life time and (ii) to eliminate high dv/dt, overvoltages, cable ringing or motor overheating
Summary
In medium-voltage (MV) variable-speed drive applications with long power cables such as geothermal electrical submersible pumping (ESP) systems [1], an inverter output (load) LC filter is often employed between voltage source inverter (VSI) and induction machine (IM) as to (i) decrease voltage deflection at the motor terminals, due to impedance imbalance between the cable and the motor, and to (ii) reduce steep voltage slopes which might damage the motor insulation and bearings due to high capacitive discharge [2,3,4]. It can be seen that, in the no load case, the filter current (providing reactive power only) is heavily dam√ped near ωp = 0.78 p.u., which corresponds to the self-excitation frequency ωR2 = 1/ CfLs, i.e., the frequency at which reactive power is mainly exchanged between the stator inductance and the filter capacitance. The resonance peak is identified at ωR1 = 10.24 p.u., which corresponds to its designed value It can be deduced from the phase plot that the filter load changes from a slightly inductive load to a predominantly capacative load in-between the two resonant peaks, while the machine load remains inductive until the filter resonant frequency is reached. The cable, on the other hand, may be neglected in the control design, since its impact is negligible due to the LC filter, except for the resistive part which may be added to the stator resistance of the IM [1]
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