High Performance Sensorless Controlled PMSM Drive with Long Cable for Subsea Applications
The increasing interest in subsea oil and gas as an energy source has led to a demand for highly efficient and reliable motors and their control. These motors are essential components of subsea applications like drilling, pumping, and boosting, which are necessary to extract natural gas and oil. The Permanent Magnet (PM) motors, known for their higher efficiency and high-power density, are increasingly preferred over traditional induction motors in these applications. It is important to consider that subsea systems, characterized by the inclusion of a sinewave filter, transformer, and long cable experience several control challenges. The voltage drops are a common issue across these components, while the passive elements of the sinewave filter specifically introduce magnitude & phase shifts, and the transformer is susceptible to core saturation problems. This dissertation presents innovative sensorless control strategies for PM motors to address the complexities associated with subsea system components. These strategies address the full spectrum of operational speeds while considering the constraints imposed by the sinewave filter, transformer, and the long cable. The sensorless starting of a PM motor is one of the key control challenges under heavy loads due to resistive voltage drops across the system components and transformer core saturation. In this thesis, an enhanced Volts-per-Hertz (V/Hz) control strategy that compensates for voltage drops and avoids transformer core saturation, ensuring reliable startup even under substantial loads is proposed. Moreover, the issue of temporary reverse speed at startup is another challenge that can cause the loss of synchronism. This has been addressed by incorporating an Initial Position Detection (IPD) methodology paired with V/Hz and a voltage compensation technique to accurately estimate the rotor's initial position to start the motor without speed reversal. Additionally, the accuracy of the closed-loop sensorless vector control highly depends on the system parameters. To address this, a Model Reference Adaptive System (MRAS) based online parameter estimation technique is incorporated to adjust the control variables while the motor is in operation. A multi-loop sensorless vector control technique is also employed to mitigate the effect of the sinewave filter, ensuring the system's stability. The final control scheme relies on High-Frequency Signal Injection (HFSI) based position estimation to start the motor in a closed-loop vector control from zero speed. The HFSI-based estimation technique must also account for various challenges associated with the sinewave filter and cable. As a low-pass sinewave filter is connected at the inverter terminal, which necessitates the careful selection of the injection frequency to prevent loss of signal information. The effectiveness of these control strategies for PM motors is validated through controller hardware in the loop (C-HIL) real-time simulations using Typhoon HIL-604 and Texas Instruments digital signal processor.