The grid-forming(GFM) converter is one of the main components of high-permeability power electronic equipment, and its fault ride-through(FRT) capability is an important basis for ensuring the stable operation of a power system with a high degree of power electronics. On this basis, an FRT strategy for GFM converter is proposed, which not only considers the hardware constraints(i.e., current constraints) of the converter, but also can keep it running in protected mode under symmetric and asymmetric faults. First, the FRT-related problem of the GFM converter is analyzed in detail. Then, an appropriate FRT model and the corresponding control method are established. Finally, the effectiveness of the proposed method was verified by power-hardware-in-the-loop simulation and experiment. Results show that compared with a grid-following converter, the proposed control method can guarantee the instantaneous injection of reactive current when the GFM converter fails to prevent the overcurrent problem, and the GFM converter can still operate fault-tolerant under serious fault conditions.

With a substantial increase in the proportion of power electronic grid-connected devices in a power system, the output characteristics on the power supply side are obviously different from the output characteristics of traditional power supply represented by synchronous generators. The fault analysis of power electronics dominated power systems, especially the network asymmetrical fault analysis, faces new challenges. However, the existing fault analysis methods basically do not realize the time-varying amplitude/frequency characteristics of the devices' internal voltage, and the available time-varying amplitude/frequency symmetrical components method based on time-varying amplitude/frequency signals is just a set of mathematical decomposition formulas without explicit physical connotation as a support. Under this background, the characteristics of the relationship between the three-phase instantaneous values formed by the positive-and negative-sequence time-varying amplitude/frequency rotating vector are analyzed, and the

Based on the two-stage (AC/DC and DC/DC) structure of a DC charging pile for electric vehicles(EVs) and through the analysis of the functional requirements of the two-stage circuit structure, a MATLAB/Simulink simulation model is built with a main circuit which consists of a Vienna circuit as the front-stage AC/DC rectifier and a Buck-Boost circuit as the rear-stage DC/DC voltage conversion circuit. Simulations are performed with load set as a pure resistive load and the PNGV model of a battery, respectively. Simulation results meet the requirement of parameters such as output voltage stability accuracy, output voltage ripple and input current harmonics, which proves that the model can be used to analyze the power grid loaded by EVs and promote the study on the impact of EVs on power grid.

Aimed at the disadvantages of a traditional AC-AC variable frequency converter such as too many power devices, complex control and low power factor, a control method for an AC-AC variable frequency circuit with continuous output frequency based on the plugged pulse AC-AC variable frequency circuit control principle is proposed. The circuit topologies and working principle of a three-phase to three-phase converter and a six-phase to three-phase converter are presented in detail. The simulation models of these two variable frequency speed control systems are established in a MATLAB/Simulink simulation environment, and the simulation results are consistent with the theoretical analysis. Finally, prototypes were fabricated, and the effectiveness and feasibility of the proposed novel control method for variable frequency speed control system were verified by simulation and experimental results.

Under certain parameters, a permanent magnet synchronous motor (PMSM) will exhibit nonlinear chaotic behavior, which is mainly manifested in torque and speed oscillation, resulting in unstable system performance. Under this background, a sliding mode control (SMC) experiment was carried out on an actual motor platform. After a reasonable process of data, the corresponding system phase diagram was drawn, and it was compared with that without SMC, thereby verifying the chaotic phenomenon from another point of view. Based on the analysis model of chaotic motion of PMSM, the chaotic dynamic behavior of PMSM was studied theoretically by using the stability theory and equilibrium point properties. It was found that the experimental results were consistent with the numerical simulation, which verifies the existence of chaos and the correctness of theoretical analysis and shows that SMC has a better inhibitory effect on the chaotic phenomenon.

Aimed at the problem that the traditional virtual inertia control method cannot restore the DC bus voltage to its rated value at the frequency recovery stage, a virtual inertia control method using a high-pass filter is proposed. MATLAB/Simulink simulation results show that the changes in DC bus voltage can reflect the microgrid frequency in real-time, and the novel virtual inertia control strategy can ensure the frequency stability in the entire changing process. At the frequency recovery stage, the virtual inertia is reduced to accelerate the frequency recovery, providing sufficient time margin for subsequent frequency adjustment. The high-pass filter virtual inertia control method can restore the DC bus voltage to its initial value and maintain the DC bus voltage stability of the new energy storage microgrid without changing the inertia support of the super capacitor energy storage.

Simulation substation batteries often work under discontinuous operation conditions, which will result in capacity regeneration of batteries during their performance degradation. The degradation of batteries shows nonstationary and random characteristics, leading to a low prediction accuracy for the remaining useful life(RUL). Aimed at the problem of RUL prediction of batteries with capacity regeneration, a prediction method is proposed based on variational mode decomposition(VMD) and bat optimized kernel extreme learning machine(Bat-KELM). First, VMD is employed to decompose the battery state-of-health(SOH) time series into overall degradation components and capacity regeneration components. Then, Bat-KELM is used to construct prediction models of each component, so that the prediction accuracy of component trend is improved. At last, the prediction results of all components are blended together to yield the accurate battery SOH prediction results as well as the RUL results. The proposed method is applied to the analysis of battery degradation instance data, and results show its superiority in terms of prediction accuracy compared with the KELM and VMD-KELM models.

In the traditional power system, the synchronous generator independently forms the internal voltage amplitude/frequency, which is connected to grid to establish the system voltage. However, the establishment of grid voltage at present is increasingly dependent on renewable energy equipment. Under the phase-locked synchronization, the grid-connected converter needs to detect the grid voltage to form the internal voltage, which seems to be different from the synchronous machine that independently forms the internal voltage and further establishes the system voltage. On this basis, the mechanism of internal voltage amplitude/frequency formed by the current control of the phase-locked synchronous converter is studied, and it is explicitly stated that the internal voltage amplitude/frequency is uniquely determined by current, which is essentially the same as the synchronous generator independently forming the internal voltage to establish the system voltage. In this paper, based on the control structure and the nature of forming the AC instantaneous value, it is explained that the converter output is essentially the internal voltage amplitude/frequency. Then, starting from the closed-loop dynamic process of equipment and network, the redundant relationship between terminal voltage and current is clarified. After the input current is determined, the internal voltage amplitude/frequency can be uniquely determined accordingly. Finally, combined with the simulation analysis, the correctness of uniquely determining the internal voltage amplitude/frequency by input current is demonstrated.

Aimed at the problems of a traditional dual-active-bridge (DAB) converter such as large switching loss, large circulating current, narrow range of load variation and low operating efficiency, a novel power control method for DAB converter based on variable inductance and phase shift (PS) angle is proposed, in which the variable inductance and PS angle are taken as the main control parameters to improve the operating efficiency of the DAB converter in a wider range of load variations. In addition, the transfer function of the DAB converter is linearized to improve the practicality and convenience of the controller. Because the device saturation is controllable in the proposed method, the core size can be reduced to optimize the converter size. Finally, the effectiveness and superiority of the proposed method were verified by experiments. Results show that under the condition of maximum PS angle, the inductance variation significantly affected the power transmission of the DAB converter, and its overall operating efficiency was about 5% higher than that of the traditional DAB converter under both the light and heavy load conditions.

The traditional phase-locked loops (PLLs) in a VSC-HVDC system such as a synchronous reference frame PLL (SRF-PLL) and a dual second-order generalized integrator PLL (DSOGI-PLL) cannot accurately track the positive-sequence voltage phase of grid fundamental wave under non-ideal power grid, which will cause different degrees of phase-locked error, affect the control performance of VSC and reduce the system stability. To solve this problem, an improved DSOGI-PLL scheme is proposed. First, the attenuation characteristics of SOGI-QSG at different frequencies are analyzed according to the Bode diagram, and the limitations of DSOGI-PLL applications are obtained. Then, based on the voltage of harmonic grid, the internal model of repetitive control is introduced on the basis of DSOGI to realize the real-time tracking and regulation of harmonic signals, thus suppressing the harmonic voltage interference. At the same time, considering the DC bias and voltage frequency fluctuation of power grid, a method of DC bias elimination and frequency adaptation is proposed to realize the adaptive phase tracking of power grid. Finally, the superiority of the proposed strategy was verified through a comparison between the simulation and experimental results.