Medium-voltage PWM indirect vector controlled induction motor drive with a long motor feeder

This thesis studies two types of medium-voltage (MV) vector controlled PWM induction motor drives for long motor feeder applications, namely voltage source and current source based drives. As rectifiers and inverters are the main elements in any drive system, both are investigated, but with more emphasis on the inverter side. MV PWM current source rectifiers (CSR) usually use selective harmonic elimination as a modulating strategy which provides good harmonic reduction at a low switching frequency. The rectifier performance does not meet the IEEE 519 regulation concerning harmonics injected into the supply current. Two solutions are presented. First, line side passive filters for harmonic mitigation and power factor improvement. Second, shunt active power filters based on p-q theory, with a step-down high-frequency transformer. The presented solutions are compared to highlight their features and limitations. Several topologies can be used for PWM CSRs. A detailed study of the selection criteria for PWM CSR semiconductors is presented based on losses, physical size, and number of series devices. Utilization of a PWM voltage source inverter (VSI) drive system in long motor feeder applications leads to motor terminal over-voltage problems. Surge filters are used to mitigate this phenomenon. Among the various types of filters, the motor terminal RC filter and the inverter output RLC filter are common. A detailed investigation is presented of the effects of these filters on the VSI drive system. PWM current source inverter (CSI) drive systems are characterised by their motor friendly voltage behaviour and controlled short circuit capabilities. Therefore, a proposed indirect vector controlled PWM CSI drive is presented for long motor feeder applications. Performance comparison between PWM VSI and CSI drive systems for long motor feeder applications is performed to clarify features and limitations of each system. Motor voltage is needed for speed estimation but measurement of voltage/speed is impractical for long motor feeder drives. A model reference adaptive system speed estimator is proposed with a novel remote motor voltage calculator that depends on the inverter voltage and feeder parameters. Offset and drift problems that occur during the flux estimation process are solved by a proposed DC-offset eliminator.