As a common power electronic converter, inverters are widely used in various scenarios where DC power supplies supply AC loads, as well as in AC motor speed regulating frequency converters and other instruments. Among them, Y-source inverter (YSI) has a simple structure and high efficiency, but it cannot suppress the starting inrush current well, which is easy to produce leakage inductance peak voltage, resulting in device damage and limited voltage boosting capability. The split-source inverter (SSI) uses fewer passive components, and can achieve the increase of the modulation index and the boost duty ratio at the same time, so the voltage gain can be adjusted more flexibly. However, due to the non-ideal state of the components and the limitations of the topology, the actual voltage gain is limited. Thus, a new type of quasi-split source inverter (QSSI) is proposed. The design of the inverter is a combination of Y-source impedance network and split-source structure, and a voltage clamp unit is introduced to improve voltage gain and solve the problem of voltage oscillation caused by leakageinductance resonance. In addition, QSSI adopts dual-buck structure (DBIS), which effectively inhibits the shoot-through current of the inverter bridge arm, thus improving the reliability of the whole converter. The feasibility of the inverter is verified by theoretical analysis, simulation and experimental results.

A novel DC-DC converter is designed which is a high gain converter combining a dual switching converter, a coupled inductor and a switched capacitor Boost unit. The advantages of this converter are its large voltage gain, low voltage stress on the switching tubes, and the ability to ensure the continuous output current. This converter is made by integrating the switched capacitor and clamped branch with each other, which enhances the voltage input, reduces the voltage stress on the switching tube and also consumes the energy of the leakage inductor with the help of the clamped branch. The voltage input of the converter in continuous conduction mode (CCM), the voltage stress of the switching tubes and the current stress are analyzed in depth. Finally, a test rig is built with 12 V input, 228 V output and 200 W power rating to verify the correctness of theoretical analysis.

For an analog second-order Sigma-Delta modulation class-D power amplifier circuit, the continuous domain mathematical model and discrete time domain iterative mathematical model are constructed. For the power amplifier circuit, the continuous domain mathematical model is derived, which shows that the principle of the second order sigma-delta modulation circuit is self-excited oscillation based on positive feedback, so as to realize the output switching signal. Since the continuous domain mathematical model can only obtain the self-excited oscillation frequency, the continuous domain mathematical model is discretized to obtain the discrete time domain iterative mathematical model. This model can calculate the switching frequency value of any input signal and analyze the stability conditions of second-order Sigma-Delta modulation circuit. The experimental results show that the error between the theoretical switching frequency calculated based on the mathematical model and the experimental value is less than 14%, which verifies the correctness of the mathematical model.

Aiming at the problem of high harmonic content in the output current of traditional nearest level modulation in medium-sized modular multilevel matrix converter (M3C), the pulse width modulation (PWM) mode is introduced into the traditional nearest level modulation (NLM) strategy. And considering the reduction of switching losses and achieve the accurate conversion of submodule states, an improved nearest level pulse width modulation strategy for M3C is proposed to improve the harmonic characteristics of its output current. Compared with the traditional nearest level modulation strategy, the proposed strategy can effectively suppress harmonic components on the M3C output current under the premise of reducing the switching losses. The feasibility and effectiveness of the proposed strategy are verified by M3C simulation model and experimental platform.

Critical conduction mode (CRM), in which the switching frequency of Boost converter changes widely due to the input voltage and load, leading to an increase in switching loss and a decrease in overall efficiency. To solve the above problems, an optimal control strategy for the full power range efficiency of Boost converter is proposed. Firstly, the internal loop control of inductive current is introduced on the basis of the traditional variable amplitude control strategy, and the mathematical relationship between the peak of inductive current and the output power under discontinuous conduction mode (DCM) and critical conduction mode is derived, and the peak of inductive current is accurately controlled accordingly. Secondly, the optimal working mode is selected for the peak value of inductive current at different loads to reduce the current stress and improve the efficiency of the converter. Finally, a 300 W experimental prototype is built to verify the proposed control strategy. The results show that compared with the traditional method, the proposed optimal control strategy can reduce the current stress by at least 8.34% and increase the peak efficiency by 2.15%.

The virtual synchronous generator (VSG) simulates the characteristics of synchronous generators to provide inertia and damping, but inevitably introduces the rotor oscillation characteristics of synchronous generator. Especially in VSG parallel operation systems, when the system is disturbed, it is prone to cause power oscillation problems, which may even damage power electronic equipment. To address this issue, based on state feedback and pole-placement theory, a power oscillation decentralized coordinated suppression strategy for VSG parallel systems based on transient damping enhancement is proposed. By constructing a combination of angular frequency high-frequency component compensation and electromagnetic power high-frequency component compensation, the transient damping of the system is improved, and power oscillation is suppressed without affecting the system steady-state characteristics. Then, the stability of the proposed control strategy is demonstrated by constructing a Lyapunov function. Finally, the effectiveness of the proposed method is verified through simulation.

With the increasing proportion of renewable energy in the grid, the overall damping and inertia of the grid decline, which brings challenges to the safe and stable operation of the grid. Virtual synchronous generator (VSG) technology, by simulating the characteristics of synchronous generator, brings damping and inertia support to the grid. However, it inevitably brings problems such as active low-frequency oscillation and active steady-state deviation. Through the establishment of VSG model, the oscillation mechanism of VSG is analyzed. The small-signal model of active power reveals the principle of low-frequency oscillation and steady-state deviation of active power caused by moment of inertia and damping coefficient. In order to suppress the low-frequency oscillation of active power and eliminate the steady-state deviation of active power, an adaptive VSG control strategy based on power dynamic region division is proposed. According to the different power change rates, the system is divided into the acceleration region and the deceleration region, so as to adaptive control the rate of increase or decrease of the moment of inertia and the damping coefficient, so that VSG has good dynamic characteristics in different working modes. The proposed strategy can effectively suppress the active low-frequency oscillation of VSG, reduce the steady-state deviation of the active power, and improve the active power support capability of VSG. Finally, the system simulation model is built in MATLAB/Simulink, and the correctness of the proposed strategy is verified.

Under weak grid conditions, the grid-connected inverter (GCI) commonly employs a type-II Phase-Locked Loop (PLL) system, which is prone to interaction with the grid impedance, leading to instability issues characterized by low-frequency oscillations. A traditional solution aimed at enhancing GCI stability by reducing the type-II PLL's bandwidth. However, this approach compromises the dynamic performance of the PLL and its adaptability to a wide range of grid impedance variations. To address this, the traditional Type-II PLL was replaced by Type-I PLL, significantly improving the stability of GCI in weak grids without degrading the PLL's dynamic performance. Furthermore, to tackle the phase-locking error issue in type-I PLLs during significant grid frequency fluctuations, an adaptive feedforward method for frequency control is introduced. This enhancement transforms the type-I PLL into a quasi-type-I PLL, eliminating phase tracking errors and enhancing the stability and dynamic performance of GCI in weak grid environments. Both simulation and experimental results confirm the validity and efficacy of the proposed methods.

The traditional parameter design methods based on gain margin (GM) and phase margin (PM) usually result in a low bandwidth of the phase-locked loop (PLL). This means that the fast dynamic of the PLL is sacrificed to improve system stability. Furthermore, the values of GM and PM are of only minor help in determining the maximum available power of a grid-connected inverter (GCI) system. Determining the maximum available power often requires complex multiple simulations or experiments. An operation-margin-based PLL parameter design method is proposed to solve the above problems. A small-signal model of GCI system considering the dynamic characteristics of current loop and PLL dynamics is established. Then, an open-loop transfer function model in complex vector form that can characterize the system is obtained. Based on this model, the three-dimensional control parameter domain under different operating point variables is plotted and the influences of output current and grid voltage on the value of control parameter are analyzed. Finally, the simulations and experiments verify the effectiveness and correctness of the proposed control parameter design method.

During the black start process of a renewable energy station, traditional grid-forming converters usually output symmetrical three-phase currents. At this time, the asymmetrical loads within the station will lead to unbalanced three-phase voltages, thereby triggering power fluctuations between the inverter and the grid, increasing the risk of black start failure. In order to achieve voltage balance during the black start process, the three stages of the black start of a renewable energy station based on the grid-forming energy storage converter are analyzed. By combining the connection mode of the asymmetrical load, it studies the mechanism of voltage unbalance during the zero-to-rise voltage stage considering the constraints of the three-phase three-wire topology. Aiming at the grid-forming characteristics of the energy storage converter, a black start voltage balance strategy based on model predictive control is proposed. This method does not require positive- and negative-sequence separation control, and can effectively suppress the negative sequence voltage generated by the asymmetrical load, and reduce the voltage unbalance factor. The hardware-in-the-loop experiment verifies the effectiveness of the strategy proposed, which can provide a new technical reference for the black start of renewable energy stations.