Modulation Methods And Stability Analysis For Matrix Converters

As a promising solution for power conversion, the matrix converter (MC) continues to be a hotspot in the field of the power electronics in the past three decades. The major cause for inter-ests in MCs lies in that it boasts many advantageous features, such as bi-directional power flow, controllable input power factor, low harmonic contents and absence of bulky storage elements. These superior performances can be used to replace the traditional back-to-back converters to control and distribute the power energy efficiently, especially in the applications like Wind En-ergy Conversion Systems (WECS), Adjustable Speed Drives, More-Electric Aircrafts.Owing to the absence of intermediate energy storage elements, the outputs of MCs are in-fluenced easily by abnormal input voltages. To this end, this paper proposes a generalized vector modulation with feed-forward compensation capacity based on the space vector representation of the switch-state transfer matrices. To simplify the calculation of the modulation duty-cycle matrix, both the singular value decomposition (SVD) and the space vector modulation (SVM) techniques are used to render clear physical meanings to the modulation process, and to synthe-size the desired variables without complex computations. Using this approach, the duty cycles for switch combinations can be calculated online to decouple the low order harmonics in the in-put side from the outputs. Besides, the geometrical perspective provided by this method makes the optimization on the switching sequence convenient. The simulation results verify the valid-ity of the proposed method in maintaining the outputs sinusoidal and balanced disregarding the abnormal input voltages.Further, this paper presents a new SVM strategy for MCs to reduce the common-mode voltage (CMV), based on the same SVD method. The reduction is achieved by using the switch state configurations that connect each input phase to a different output phase, or the switch state configuration that connects all the output phases to the input phase with minimum absolute voltage. These two types of configurations always produce lower peak CMV than the others, especially the former ones that result in zero CMV at the output side of MCs. They are integrated in the switching sequence by equivalent decomposition and combination using the relationships among different switch states found by the SVD method. Consequently, no additional hardware is required. In comparison with the existing SVM methods, this strategy has a very similar software overhead and calculation time. Simulation and experiment results are shown to validate the effectiveness of the proposed modulation method in reducing not only the peak value but also the root mean square value of the CMV.However, when the MCs are tightly regulated by either modulation strategies or control algorithms, its outputs are exposed to tend to be a constant power load (CPL). In this paper, the stability of a matrix converter system, supplying a three-phase constant power load (CPL), is analyzed. The model of the whole system, with its input filter, power supply, and modu-lation strategy taking into account, is established by state average method and then linearized around the steady-state operation point. Furthermore, the CPL, which is introduced by either the feed-forward compensation of the modulation or the strictly feedback control, is linearly approx-imated as negative impedance. Subsequently, the stability of the small signal system is evaluated by investigating the poles of the transfer function, the eigenvalues of the Jacobian matrix and the characteristic loci of the return ratio matrix. These three distinct techniques congruously confirm that the negative impedance of the CPL is detrimental to the damp of the system filter and may be the root cause that leads the under-damped system to unstable. The analytical pre-diction, reinforced by the simulations, provides insight into the oscillation phenomenon at the input side, and serves as guidelines for designing the damped filter and close-loop control of the whole system.