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.

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

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.

Although the LCL filter has a good performance of suppressing high-frequency harmonics, it may cause problems such as resonance oscillation and instability. For an LCL grid-connected inverter, the conventional capacitor-current-feedback type active damping method can suppress the resonant peak effectively, at the cost of additional current sensors. Under this background, a novel active damping control strategy based on an H∞ filter is proposed. The state space model and process noise model of the LCL filter are derived to solve the H∞ filter. The filter capacitor current can be estimated according to the information about grid current and the voltage at a point of common coupling, and feedback is further completed. The system damping is improved, and the estimation accuracy can be guaranteed even if parameter perturbations exist in the LCL filter. Simulation and experimental results show that the proposed method was insensitive to changes in the parameters of the LCL filter and the grid impedance, thereby verifying its feasibility and superiority.

In a certain environment where regional wind farms distribute irregularly, the traditional convolutional neural network prediction method cannot reflect the distribution states or influence relationship of regional wind farms, and it is difficult to accurately predict the wind speed. First, to solve this problem, the technology of graph convolutional networks is used for feature modeling, and the connected graph and weight matrix are established according to the topology of multiple wind farms and the cross-correlation coefficient of wind speed in each region. Second, depending on the time dynamic characteristics of wind speed at wind farms, an improved parallel convolution structure is used to obtain the correlation between wind speed series in multiple time periods at the same wind farm. Third, based on the spatial correlation and delay effect of wind speed at wind farms, the spatio-temporal characteristics of wind speed in different regions are aggregated by using a second-order aggregation method. Finally, the verification of data from one regional wind farm shows that the proposed method can extract the spatio-temporal characteristics and improve the performance of ultra short-term wind speed prediction for multiple wind farms on 0-4 h prediction scale.

Affected by factors such as wind speed and light intensity, wind power generation has characteristics of randomness, intermittence and large fluctuation, so its direct grid connection will cause damage to power grid. To realize a smooth grid connection of wind power and provide safe and reliable power supply to an urban rail transit system, a hybrid energy storage system composed of super capacitors and lithium batteries is proposed as a stabilizing measure. As the traction load of urban rail transit also fluctuates greatly, the hybrid energy storage system not only stabilizes the output of wind and photovoltaic (PV), but also stabilizes the traction load. The wavelet packet decomposition technology is used to decompose and reconstruct the traction load and wind and PV output power signals on multiple scales, the low-frequency wind and PV grid-connected power and medium-and high-frequency components are obtained, and batteries and super capacitors are used to absorb the medium-and high-frequency components, respectively. Aimed at the minimum comprehensive cost of hybrid energy storage system, the state-of-charge and power limit of the hybrid energy storage system are taken as constraints. The differential evolution particle swarm optimization algorithm with shrinkage factor is used to minimize the annual comprehensive cost of

To solve the instability problem caused by time-varying communication delay and uncertain faults in an isolated island AC microgrid, a novel robust hierarchical control method is proposed, which includes cascade current loop, voltage loop, virtual impedance and droop control loop. First, a robust controller based on adaptive backward integral non-singular fast terminal sliding mode control is designed in the current loop to adjust and track the current reference value under unknown bounded uncertainties and external disturbances. Second, the hybrid H2/H∞ control is used in the voltage loop, and the state feedback control law is used to generate the inner loop reference value to increase the robustness of the controller to disturbances, and sufficient conditions are given based on the linear matrix inequalities. Considering the unstable effects of time-varying delay (TVD), a distributed protocol based on consistency is adopted in the second control layer to improve the robustness of the controller against TVD. Third, droop control and virtual impedance loop are used to improve the system's power distribution accuracy. Finally, the performance of the proposed control method was evaluated by hardware-in-the-loop simulation, and its effectiveness was verified. Simulation results show that compared with the existing methods, the proposedmethod has advantages in transient response, steady-state performance and fault crossing capability under large and small signal disturbances.

The use of virtual synchronous generator (VSG) strategy for a photovoltaic (PV) inverter can achieve inertia and damping support. The traditional VSG cannot provide transient reactive voltage support for the system and cannot meet the demand for voltage regulation during the low-voltage ride-through (LVRT) periods. After the occurrence of a grounding fault, the VSG control of the PV inverter is switched to model predictive control (MPC). After the grounding fault is removed, the MPC is switched back to the VSG control strategy. To improve the target current tracking capability during the LVRT periods, an adaptive objective function is set in the MPC. MATLAB/ Simulink simulation and experimental results show that the PV VSG under the novel MPC has an LVRT capability. During the LVRT periods, the MPC current control is precise, and there is no transient current surge during the switching from the MPC to VSG control.

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.

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.