The Research Of PWM Rectifier Under Non Ideal Grids Conditions

Three-phase pulse width modulation (PWM) rectifiers have been used widely in industry applications due to its advantages of bidirectional power flow, sinusoidal line current, good dc-link voltage regulation ability. Various control strategies have been proposed, such as voltage-oriented control (VOC), direct power control (DPC) and model predictive control (MPC). The grid voltages are usually unideal which may be caused by single-phase load, grid impedance unbalances, voltage dip. Thus, a PWM rectifier designed for ideal grid conditions may exhibit an abnormal operating state under unideal conditions, such as harmonics in the dc output and odd harmonics in the input currents. Therefore, the control of three-phase PWM rectifiers under unideal grid voltages has become a worldwide concern.Unbalanced grid voltages condition is a special condition of distorted grids. Therefore, the strategies of three-phase PWM rectifier under unbalanced conditions were first in-depth studied. Because original pq theory has become the standard in the field of electricity, this paper proposes an improvement model precdictive direct power control (MPDPC) using power compensation based on original pq theory. No need of any positive and negative sequence extraction in the proposed MPDPC, and the proposed MPDPC can be implemented in the two-phase stationary coordinate system. The proposed MPDPC was proved by simulation and test results.However, the original pq theory was proved not suitable for unbalanced grid voltages. Therefore, this paper introduced an extended pq theory which was reported to be more suitable for unbalanced grid voltages than original pq theory. The advantage of extended pq theory was validated, then the theory was applied to DPC, a new switching table for proposed DPC under unbalanced grid voltages was established through analysis of the extended instantaneous power slopes. However, DPC presents large power ripples and high current harmonics due to the inaccuracy selection of voltage vector. To cope with the problems above, this paper proposes an improvement MPDPC. By selecting the best voltage vector minimizing a cost function consisting of power errors, much better steady state performance in terms of power ripples and current harmonics can be obtained. To obtain the best performance, the extended pq theory was applied to DPC-SVM. The desired converter voltage reference to achieve predictive deadbeat control of both active power and the extended reactive power is analytically derived. The performance of proposed DPC-SVM is better than the proposed DPC and MPDPC enven though at a lower sampling frequency, and all the proposed methods were validated by simulation and test results. The methods above could eliminate the negative effection which is brought by unbalanced grid voltages, but the grid voltages always contain low harmonics under actual conditions. Thus, a PWM rectifier designed for unbalanced conditions may exhibit an abnormal operating state under grid distortion conditions. Therefore, this paper proposes three methods to extract positive and negative sequence accuracy under distorted grid voltages; they are quick extraction method, cascaded delay signal cancellation, and infinite impulse response. This paper chooses cascaded delay signal cancellation through the comparison. We recaculate the original and delay signal by using the positive and negative sequence after extraction of positive and negative sequence accurary from the distorted grids. And we take the recaculated signals as control variables to combine the improvement methods which desgined for unbalanced girds and these methods for distorted grids were validated by simulations results.