Search

The medium-voltage grid emulator is gaining popularity for testing grid-code compliance of large-capacity converters for renewable energy resources. The cascaded H-bridge converter based on active-front-ends is a promising candidate for high-power grid emulators owing to its high modularity and extendibility. However, the cascaded H-bridge topology-based grid emulator suffers the undesired and unpredictable large dc-link voltage ripple under output voltages with multiple-frequency components, e.g., when emulating the grid voltage flickers and harmonics. In this article, the dc-link voltage ripple characteristics under multi-frequency output conditions are analyzed and a dc-link ripple suppression method adopting harmonic current injection is proposed. The injected current reference is generated by reconstructing the dc-link output current and added to the original active-front-end current control loop. Compared with the existing control methods, e.g., proportional-integral-resonant (PIR), the proposed method has a good voltage ripple suppression effect, especially for multi-frequency outputs. Furthermore, the proposed idea does not need complicated calculation or extra control loops and provides a feedforward reference for the current control, leading to a good dynamic response of dc-link voltage control. Finally, the performance of this suppression control scheme is verified on two 1200 V@40 kW power electronics building blocks of a cascaded H-bridge converter.

In this paper, a sub-module capacitor voltage control (SCVC) is proposed to alleviate the energy balance restriction in the alternate arm converter (AAC). Considering that the arm inductor has a great impact on the energy exchange in AAC, the current commutation modulation is demonstrated firstly in this paper to extend the operating range. Then, the current commutation process for the alternate arms is further explored and derived by analytical expressions. During the commutation process, the current through the turning-on arm is rising, while the contralateral arm current is reversely freewheeling through the diodes of director switches (DSs). Moreover, the mechanism of the capacitor voltage sag caused by commutation process is clarified, which provides the fundamental to design the controller. In this manner, the sub-module capacitor voltage control (SCVC) strategy is further provided to compensate the capacitor voltage sag and achieve the normal operating in the four quadrants. Finally, the simulation and experimental results are both performed to verify the effectiveness of the presented modulation and control strategy in the different conditions.