Research On Vector Control And Low Voltage Ride-Through Technologies Of Cascaded High Voltage Frequency Converters

With the development of information technology、motor drive technology and power electronics technology, performance of inverter continues to increase. Now it is more and more widely used. At present, study on structural theory and application method of frequency converter has become a hot issue in the field of electric transmission. In this paper,6kV power unit cascaded high-voltage frequency converter has been comprehensive designed, and been researched on the aspects of principle and the control mode, structure of the system, control strategy and implementation of special control method in the case of power failure.First of all, this paper analyzes the main circuit topology, modulation method and power unit design principle of high voltage inverter, establishes 18 power units of frequency converter model, phase shifted carrier signal generating model and power unit pulse generating model, which make the output voltage of the inverter have characteristics of high switching frequency and multi steps, and realize accurate wave form modulation.Secondly, designs the speed sensorless vector control system, derives closed loop vector control system based on rotor flux orientation; analysis The principle of three kinds of flux observation model, compares the results to determine the optimal observation scheme; and on this basis, the improved speed estimation model of induction motor is derived. Using the PSCAD to simulate, it is find that the current regulator and speed regulator are obtained the desired response, the actual value can be a very good tracking command value; In the aspect of speed estimation, design of lead correction compensation and speed filter, make the system get better estimation results in low and high speed conditions, and the overall performance of has been greatly improved.Finally, considering the grid voltage falls caused by grid line fault or large motor starting, will bring influence to normal operation of the inverter, designs the low voltage ride through (LVRT) link. Through the DC closed loop to regulate the current instructions given in vector control, change the output frequency so that the motor rotor energy back to the DC bus, ensure the DC bus voltage in the normal working range, and avoid the low voltage trip.