Sensorless Start-up and Control of Permanent Magnet Synchronous Motor with Long Tieback
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- Institutt for elkraftteknikk 
The master thesis is a further investigation of the start-up procedure of a permanent magnet synchronous motor operated by variable frequency drive without position feedback via a long cable and transformers; as proposed in the specialization project, fall of 2013. The challenge is how to avoid large saturation in the transformers and motor while achieving the maximum possible starting torque with high cable resistance.In order to model and simulate the electrical system, the simulation software Simulink is used.A theoretical review is conducted in order to understand how to model system components, and how to start-up a permanent magnet synchronous motor while avoiding saturation in the transformer. Using the attained theoretical background, two permanent magnet synchronous motor Simulink models (with and without damper windings), an accurate pump load Simulink model, and three scalar control schemes which includes transmission system components for their control algorithms have been produced. In addition, a vector control scheme using field oriented control and extended Kalman filter for position estimation has been evaluated.A study case comprising of a permanent magnet synchronous motor operated by a variable frequency drive without position feedback via a long cable and transformers has been defined.The purpose of the study case is to study the defined control schemes, the permanent magnet synchronous motor's start-up procedure, and the electrical system's steady state behaviour. This include investigation of input parameter effects on the start-up procedure.The simulation results using the implemented pump load Simulink model shows that accurate load modelling affects the start-up procedure, as additional torque is required due to more rotational friction components.The simulation results of the study case show that the three proposed scalar control methods all are able to start-up the motor successfully, regardless of the initial rotor position. However, stability is not guaranteed at certain speed ranges due to the rise of small system disturbances. It has been shown that stability all through out the entire applied frequency range can be guaranteed by adding damping to the system. This is can be done by either adding transmission system components, motor damper windings, or a stabilization loop via frequency modulation.\peIt has been shown that cable length and applied frequency determines the accuracy of the transmission system voltage drop compensation algorithms of the proposed scalar control schemes. Increase in applied frequency and cable length increases the inaccuracy of the voltage drop compensation algorithms due to the ignored cable capacitance. Assuming that the maximum allowed voltage deviation from the required motor voltage during steady state is 0.1 pu, the longest cable length that can be used with the proposed scalar control schemes is 40 km. An exception is the open-loop scalar control scheme using constant voltage boosting, which can be used for cable lengths up to 20 km, due to its inaccurate calculation assumptions.Additionally, it has been shown that the breakaway torque and the reference frequency ramp slope used by the controllers directly affect the torque oscillations during start-up. Additional torque oscillations can be experienced if the breakaway torque is too high or if the reference frequency ramp slope is too low due to the lack of torque build up, which consequently makes the start-up time higher.The vector controller gave the best performance during load step tests due to its precise control of the rotor field. However, due to the requirement of rotor position feedback, position estimation is required. The investigated position estimation technique, extended Kalman filter is able to estimated speed and position with very little error. However, the initial rotor position is required by the extended Kalman filter algorithm in order to predict states properly.Simulation results show that the vector control scheme offers the lowest possible start time due to its high performance. However, due to the requirement of initial rotor position of the sensorless vector controller, it can not be used during start-up, due to inaccuracies of predicting rotor position at zero speed. This leaves either the partial and delayed open-loop scalar control scheme or the closed-loop scalar control scheme to be the most viable start-up control schemes; as both control schemes offer comparably low start-up times, start-up currents and voltage boosting; which will consequently affect the dimensioning of the transformer. The vector controller can then be implemented after the start-up procedure using the selected scalar control scheme, in order to obtain the optimal controller performance.