Matrix and output control, the MC output performance

Matrix converter (MC) can realize AC-AC power conversion in a single stage without large DC-link energy storage elements 1. After four decades of continuous research 1-4, MC is finally reaching its commercialization. Yaskawa Electric Corporation has launched two series of MC products, U1000 and FSDrive-MX1S, which are aimed at lowvoltage (200-400 V) and medium-voltage (3.3-6.6 kV) applications separately. These products feature high efficiency, high power density, and power regeneration capability, verifying the advantages of MC over conventional AC-AC converters.
Due to the direct coupling of input and output control, the MC output performance is sensitive to input voltage disturbances (e.g. unbalance and distortions) 5-7. To prevent the disturbances from degrading output performance, instantaneous input voltage can be measured to correct the duty cycles in each sampling period 8-11. This kind of control method is effective even under severely disturbed input voltage 11. However, it could reduce the system stability, which has been widely proved in literature 12-16. In 12, rigorous stability analysis was performed based on the variable state average model of the whole system, which showed that the maximum voltage transfer ratio of MC could not reach its intrinsic limit (i.e. 0.866) due to the stability issue. In 16, the stability analysis method based on impedance ratio criterion was presented. It is revealed that measuring input voltage deteriorates MC stability through introducing negative input impedances.
For a practical MC, stabilization method is indispensable. Compared with the passive stabilization which parallels a damping resistor with the filter inductor 17, 18, modifying control algorithms to actively stabilize MC has the ability to work with a very weak source (e.g. a generator). Therefore, it has attracted attention from many researchers 11-13, 16, 19-22. To date, the most comprehensive active stabilization method is the general constructive method proposed in 16. This method modifies the output voltage reference in motoring mode and the input power factor angle in regenerating mode. The additional control signal mainly comes from the input voltage through a digital high-pass filter. This constructive method can reveal the essence of many other methods, including the ones presented in 12-13, 19. In 11, 2022, the stabilization methods are based on directly modifying
the input modulation signals, but they are also equivalent to modifying the output voltage reference 21. In 11 and 20, the additional control signals are derived from the filtered input voltage in stationary frame, while in 21 and 22 they come from the source current derivative. The object of active stabilization methods is to address the stability issue of real-time duty cycle correction, while the correction is to eliminate the effects of input voltage disturbances. However, the performance of active stabilization methods under input voltage disturbances has rarely been studied in literature, which is the motivation of this paper.
Contributions of this paper are twofold. First, the performance of the traditional active stabilization method is evaluated quantitatively, considering input voltage unbalance and distortions. The analysis result shows that it deteriorates output power quality by generating low-frequency harmonics in the additional output voltage reference. Second, an improved method is proposed. Resonant controllers are utilized to eliminate the extra low-frequency harmonics, so as to maintain good output power quality without degrading the stability performance. Discretization of the improved method, consideration of input frequency variation and the influence on input power quality are presented. The rest of this paper is organized as follows. Section II presents the performance evaluation of the traditional active stabilization method. Section III introduces the principle and equations of the improved method in detail. Section IV shows the experimental verification. Section V draws the conclusion.

II. PERFORMANCE EVALUATION OF THE TRADITIONAL ACTIVE STABILIZATION METHOD

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A. Control Block Diagram The typical active stabilization method based on input voltage filtering is the general constructive approach proposed in 16. Hence, this method is taken as an example in this paper. Nevertheless, the presented evaluation and improvement are also applicable to other methods based on input voltage filtering. The MC and control block diagram of MC with this method is shown in Fig. 1. As it is shown, this method realizes a virtual damping resistor Rvd at the input side of MC. In the motoring mode, the reference amplitude uom* of output voltage is modified with an additional control signal del(uom)* generated from the instantaneous amplitude uim of input voltage.

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