Abstract
Voltage sags have become a major issue of power quality for industries in recently years. For the adjustable-speed drive systems, the energy charged in the dc bus capacitor is not enough for the load and cause the dc bus voltage to be drop down when voltage sag. If the dc bus voltage low enough could cause under-voltage fault and inflict significant losses with production process interruption. Therefore, in order to solve this problem, we can use the properties of active front-end converter that has bidirectional power flow, full control of dc bus voltage, and makes the ac side current nearly sinusoidal waves. It helps to provide ride-through capability for the voltage sag. This thesis proposes an auxiliary front-end converter for the adjustable-speed drive systems. At the normal operation, it provides active filter for the harmonic current and energy regeneration capability for the motor load. Upon detection of a voltage sag, the auxiliary front-end converter operates to maintain a regulated dc-link voltage. Using positive and negative synchronous reference frame transformation, the input voltage will be separated with positive- and negative-sequence magnitude that be used to calculate the positive- and negative-sequence current command, and then transfer to stationary frame to accomplish current control. This controller can not only eliminate the 120Hz voltage ripple in the dc-link under unbalance voltage sag but also maintain a regulated dc bus voltage. Finally, the effectiveness of the proposed scheme and control method is verified by simulation and laboratory test results.