This paper presents a hybrid seven-level converter based on T-type converter and H-bridge cascaded suitable for low-voltage and high-power-density applications. The operation principles and conduction paths are analyzed comprehensively. Voltage balance control of the floating capacitors is the key point of this paper. Mathematical expressions of capacitor voltage balance conditions are deduced based on sinusoidal pulse width modulation (SPWM). The results show that floating capacitor voltages are only balanced when the modulation index is below 0.82 under SPWM. In order to enlarge the modulation index ensuring the capacitor voltage balance, space vector modulation (SVM) is also applied to regulate floating capacitor voltages taking advantage of redundant switching vectors. Meanwhile, in SVM, there are no additional control requirements for the capacitor voltage. Taken both SPWM and SVM into consideration, the limit to the range of operation for this seven-level converter is presented, which is a function of the modulation index and the power factor. The merit of dc voltage utilization improvement in this topology is also analyzed based on simulation results. Simulations and experimental results under different modulation index and RL load are presented to verify the modulation strategies illustrated in this paper.

The Modular Multilevel Matrix Converter is a direct AC/AC topology and promising in the medium-voltage high-power application. However, the current M3C prototypes mainly adopt the centralized control structure which may hinder its modularity and flexibility. Based on current distributed control architecture, a new one and corresponding design principles for M3C are proposed to realize both the distribution of control tasks and improve the expandability. And in order to reduce the dependency of real-time communication network, a time-dimension decoupling strategy based on voltage decoupling and current coupling control algorithm is employed, in which, the voltage loop are divided into the input side, output side and inner circulating decoupling models which are executed in the system controller, while the real-time current loop is established on the coupling abc coordinate frame in each local controller. In order to reduce the influence of communication delay, a novel scheme to replace direct AC current reference with DC power value and double synchronization phase information is adopted. The controller parameters of current loop are also designed carefully after frequency decomposition. Finally, an experimental prototype is constructed to verify the feasibility of proposed control scheme after a series tests of steady-state and dynamic operations.

The modular multilevel matrix converter (M3C) is introduced and applied to a single stage AC/DC solid state transformer (SST), which uses only one concentrated medium frequency transformer (MFT) for isolation. As a result, the transformer insulation design is simplified significantly and higher power density could be achieved. Meanwhile, a novel decoupling transformation based on two internal three-phase circulating systems is proposed to improve previous method. Through this transformation the unbalanced capacitor voltage components can be divided into four independent dimensions, which correspond to four kind of capacitor voltage ripple frequency. And a DC-side vector model is then established to calculate the required circulating current reference for capacitor balancing. Besides, under the application of SST, the capacitor voltage ripple suppression strategy via injection of extra circulating current is proposed. The amplitude and frequency index are optimized to obtain the best effect. Therefore, the submodule capacitor volume can be reduced to achieve higher power density. Simulation and experimental results are also provided to verify the theoretical analysis.

Interior permanent magnet synchronous motor (IPMSM) with back electromotive-force (EMF) based sensorless field-oriented control (FOC) is widely used in medium-high speed applications. Unfortunately, the back EMF is too small to be estimated accurately during low speed operation. Hence, the current-frequency ( I–f ) control with controllable current vector is used for startup. However, the conventional I–f control is hard to realize the tradeoff between startup speed and load capacity, suffering high-acceleration and high-load startup failure. In this article, a current vector angle controller for sensorless IPMSM startup is proposed to adjust speed-up-stage current acceleration and constant-speed-stage current amplitude with load automatically. The proposed angle controller guarantees the current vector angle close to the maximum torque per ampere angle, which contributes to large startup torque, high operating efficiency and smooth transition from I–f startup to sensorless FOC. Furthermore, the robustness against parameters variation of angle controller is evaluated quantitatively. These theoretical advantages are verified in experiments with a 380 V, 1.5 kW IPMSM. In particular, the experimental results demonstrate that the proposed I–f control realizes full-load startup even with ${\rm{ \pm }}$ 50% flux linkage and ${\rm{ \pm }}$ 30% inductance variation. Furthermore, the startup acceleration auto-adjusts with load to reconcile startup speed and load capacity.

Since the voltage stress on the power switches is only half of the input voltage, three-level (TL) converters have been widely used in high voltage applications. To ensure normal operation of TL converters, the voltages on the input divided capacitors and the blocking capacitor must be essentially balanced. In practice, asymmetry of the main circuit and the drive circuit may result in unbalance of these voltages, leading to higher voltage stress on the power switches and other problems. This paper proposes a novel Pulse-Width Modulation (PWM) method with better voltage-balancing ability, which is implemented by interleaving two traditional PWM methods. First, the mechanism of voltage unbalance is analyzed. The new PWM method decouples the blocking capacitor voltage from the voltages of the input divided capacitors, and ensures that the former voltage is always kept around its ideal value, which can greatly simplify the sampling circuit and the voltage-balancing control scheme. Then, a voltage-balancing strategy aiming to adjust the phase shift of the drive signals only is given. Finally, experimental results are presented to verify the effectiveness of the proposed method and voltage-balancing strategy.