Reactive Power Characteristics And Control For Matrix Converter

Abstract:Matrix converter is a kind of green power electronic converter without intermediate energy storage link, featured by sinusoidal input and output currents, bidirectional energy flow, controllable input power factor as well as a compact design, and it has found many applications in adjustable speed drives, renewable energy generation, power quality control, and so on. In some applications, expect for the desired high-quality output voltages, matrix converter is also required to obtain the input reactive power freely as much as possible. However, due to the lack of the energy storage link, the input and output sides of matrix converter would interfere with each other, the input reactive power cannot be regulated independently as that of PWM converter. Besides, the reactive power characteristics and control methods are very complex, which are related to modulation algorithm, voltage transfer ratio, and load properties and so on. Hence, this paper will focus on the characteristics and control issues of the input reactive power for matrix converter. The main works may include:For the traditional matrix converter topology, the reactive power features under several conventional modulation methods are studied, and the related control ranges and constraints are analyzed and compared, which show that the control ranges of input reactive power are limited. Thus, a reactive power-extended modulation method based on mathematic construction is proposed, which eliminates the dependence of reactive power on the load power factor by constructing a reactive modulation matrix. The proposed method is compared with the optimum-amplitude method and indirect space vector modulation (SVM), in terms of the input reactive power under different operating conditions, which shows that the proposed method can obtain the maximum input reactive power over most situations.For the two-stage matrix converter (TSMC) topology, the DC-link voltage should always be positive, which leads to a limited control range of input reactive power. Thus, a space vector-inversion method is proposed to realize SVM, which uses opposite vector to replace the original vector. This method can compensate the reactive power limitations to an extent. To further improve the reactive power control properties, a reactive power-extended modulation strategies based on space vector synthesis is presented for the two-stage matrix converter. The operating principle of this method is:first, the output voltage formation and input reactive current formation are completed separately; then, they are combined to form new output reference vectors, with the help of the space vector-inversion method; finally, the combined SVM is used to obtain the desired output voltage and input current. Compared with the existing modulation strategy based on pulse merging, the proposed method has lower computational effort and better reactive power control capability over most situations.Consider the flexible DC-link of the two-stage matrix converter, a novel topology with static synchronous compensator (STATCOM) function is proposed to enhance the input reactive power properties. First, the topology is introduced as well as its working principle, and the related modulation strategy is presented based on the indirect SVM. With this scheme, the input reactive power dependence on the output current can be removed, which would overcome the topological drawback of the limited reactive power control range.As for the matrix converter-based wind energy conversion system (WECS) with doubly-fed induction generator (DFIG), the low voltage ride through (LVRT) issue is also related to the reactive power control. For this issue, a novel scheme based on topology reconfiguration of TSMC is proposed, which adds an auxiliary switching circuit in the DC-link of TSMC. Under the normal network, the auxiliary switching circuit does not work; during the grid faults, the proposed topology would be reconfigured into a current source STATCOM and a general voltage source inverter. With this scheme, the WECS could stay connected to the grid, and provide reactive power to support the grid voltage.