With the large-scale development and utilization of renewable energy sources such as photovoltaics and wind power, inverters based on droop control are widely used in distributed generation systems and microgrids. Sequential impedance modeling is one of the important methods to assess the stability of a multi-inverter parallel system and optimize the control parameters. However, there is currently limited research on the sequential impedance modeling of droop-controlled inverters. Therefore, a detailed investigation was carried out. First, the steady-state operating point of a droop-controlled inverter was determined, and the inverter response was analyzed by introducing positive- and negative-sequence small signal disturbances. Second, the sequential impedance model was derived from the inverter response to characterize the inverter’s impedance properties. Subsequently, the accuracy of the sequential impedance model was verified through PSCAD simulations. Finally, the effects of key parameters such as droop coefficients and the cutoff frequency of a low-pass filter in the power calculation link on the inverter output impedance were analyzed and verified through experiments. Results demonstrate that the proposed sequential impedance model has a good performance in analyzing the droop control strategies, providing valuable insights for further optimization and practical implementation of the strategies.

The grid impedance exerts a serious impact on the stability of grid-tied inverters, and its accurate estimation is conducive to realizing the high-performance adaptive control of grid-tied inverters in weak grid conditions. The PQ variation method is a typical method of online grid impedance estimation. However, research shows that there exists a relationship of nonlinear function between real grid impedances and their estimation results, which may affect the estimation results. The traditional PQ variation method has shortcomings in the grid impedance estimation accuracy due to ignoring the influence of steady-state operating point fluctuations before and after the variation. Therefore, from the two aspects of error compensation with fixed phase-angle difference and estimation value correction with offline look-up table, two improved estimation schemes considering the operating point fluctuations are proposed respectively to achieve an accurate grid impedance estimation under weak grid conditions. The detailed theoretical analysis and test results show that compared with the traditional PQ variation method, the proposed grid impedance estimation method can achieve a higher degree of accuracy.

To address the problem of power quality deterioration in a charging pile’s front-stage AC-DC rectifier under frequency fluctuations of public power grid, a current inner-loop control method with frequency adaptation capability is proposed based on the application of a load virtual synchronous machine (LVSM) control strategy. First, a small-signal model of the LVSM frequency loop is established, and analysis results show that the virtual frequency will follow the grid frequency that deviates from the fundamental frequency. Second, a frequency-adaptive quasi-proportional-resonant controller with reduced computational complexity is designed for the current inner-loop to suppress grid-connected current harmonics injection into the grid. Finally, simulation results show that the proposed controller can actively respond to changes in grid voltage and frequency while improving the power quality during the regulation process, which verifies the theoretical analysis.

In power systems, unbalanced and nonlinear loads might lead to voltage and current imbalance and harmonic distortions, which will affect the operation of some critical equipment. To improve the power quality of a system consisting of multiple grid-forming (GFM) converters, it is necessary to reasonably share the load current’s fundamental negative-sequence and harmonic component while reducing the voltage imbalance and harmonics at the point of common coupling (PCC) as much as possible. To solve this problem, a power quality control strategy for GFM converters based on unified unbalanced/harmonic voltage-current droop is proposed in this paper. By establishing a unified droop relationship of fundamental negative-sequence and harmonic component between PCC voltage and output current, unbalanced and harmonic current is shared in accordance with the capacity of each converter while suppressing imbalance and harmonics of PCC voltage. The proposed method can be applied to multiple converters in both the islanded and grid-connected modes without extracting the fundamental negative-sequence or individual harmonic sequences separately. Compared with the existing control methods, this method is simpler and easy to implement in embedded controllers. The design scheme for control parameters based on closed-loop the pole analysis is discussed in detail. Moreover, through a comparison with the existing methods, it is shown that the proposed method has superior dynamic performance and less computation resources. Finally, the effectiveness of the proposed control method was verified by experimental results.

With the significant integration of transportation and energy fields, the electrified and "green" energy consumption in transportation is an important technical solution for achieving the targets of carbon peak and carbon neutrality. The use of low-temperature energy as coolants for high-temperature superconductor (HTS) cables can simultaneously transport the electric energy and fuels, which can improve the energy transmission efficiency and reduce the overall costs for large-scale renewable energy transmission. For a 10 km transmission distance, a 1.5 kV/10 kA liquid hydrogen (LH₂) energy pipeline structure for railway transportation is designed. Meanwhile, the effect of the arrangement angle of positive and negative bipolar HTS tapes on their critical currents was investigated by establishing a finite element simulation model of HTS cable. The maximum critical current was about 15 240 A in the parallel arrangement. Finally, the transmission losses of HTS cables and conventional copper cables with the same capacity (i.e., 15 MW/10 km) were compared. Results show that the transmission loss of 1.5 kV HTS cable was only 15% of that of 10 kV conventional AC cable, and it was also slightly lower than that at the 35 kV voltage level. The findings in this research can provide a feasible technical solution for LH₂ HTS energy pipelines to be used for large-capacity combined hydrogen-electricity transmission.

To solve the problems in a high-permeability regional power grid such as reduced moment of inertia, decreased system frequency regulation capability and lower frequency stability, the frequency regulation control of wind turbines is added on the basis of energy storage frequency regulation. Through the analysis of the influence of wind power permeability (δ) on the system, an adaptive wind-storage coordinated primary frequency regulation control (PFRC) strategy considering the influences of state-of-charge (SOC) and δ is proposed from the perspective of reducing the battery loss and prolonging the energy storage life. This control strategy is realized through the coordination of energy storage adaptive PFRC considering SOC and frequency regulation dead-zone and the wind power adaptive PFRC strategy considering δ, which can effectively improve the system stability and the frequency regulation effect of the power grid. Under the step disturbance, the lowest point of frequency drop changes from 0.33 to 0.07. Under the continuous load disturbance, compared with the single energy storage frequency regulation, the frequency quality under the proposed control strategy is changed from 3.966 to 1.676, and the SOC quality is changed from 0.064 07 to 0.016 48. The data is smaller, and the change in the SOC of energy storage is gentler, which can save the configuration of energy storage capacity and reduce the loss of the energy storage unit. Therefore, a better frequency regulation effect is achieved to maintain the stable operation of the system. Finally, the effecttiveness of the control strategy is verified by building a regional power grid model in MATLAB/Simulink.

A two-layer optimization algorithm based on a microgrid load fluctuation suppression strategy is proposed, aiming to address the uncertainties in renewable energy generation and random load fluctuations. Through the modeling of operating states of generation devices and power allocation modes, the upper-layer model mainly considers the micro-resource output power constraints and related constraints on microgrid operations, so as to ensure the scheduling priorities of generation devices while reducing the operating cost of microgrid. The lower-layer model minimizes the equivalent load fluctuations in microgrid while using the values predicted by LSTM-Attention neural network to further constrain the output power of generation devices. The two-layer model is transformed into a single-layer model by applying the KKT condition, thus reducing the complexity in solving the model. Two different load scenarios are designed, and the two-layer optimization algorithm is used to perform simulations under the two scenarios, respectively. Simulation results show that the proposed algorithm can dynamically adjust the output power of generation devices and improve the consumption proportion of renewable energy. In addition, it can change the microgrid’s operating state according to different load scenarios and thus mitigate the impact of load fluctuations on microgrid operations.

To realize the collaborative control of power mutual assistance, voltage frequency support and state-of-charge (SOC) of energy storage batteries between multiple microgrids, a multi-port interconnected current source converter suitable for the interconnection of microgrids is proposed. Through the introduction of the average SOC of the microgrid group into the active command control loop of a central controller and the addition of a capacity weight factor to the loop, the dynamic equilibrium of energy storage SOCs of three microgrids under various working conditions is realized. In addition, with the consideration of frequency exceeding of the microgrid that may occur during the SOC adjustment process, an SOC-frequency collaborative control strategy that takes into account the frequency performance of the microgrid is put forward, and the frequency operation is guaranteed to be within a specified range on the basis of realizing the SOC equilibrium of the microgrid group. Simulation results verify the correctness and effectiveness of the proposed control strategy.

Self-synchronized voltage source inverters (SSVSIs) have attracted much attention for their capability to improve the voltage and frequency stability of power systems. To solve the problems of energy storage state-of-charge (SOC) imbalance, DC-side voltage fluctuations and AC-side output power oscillations during the parallel operation of PV-storage SSVSIs, the dynamic characteristics of AC- and DC-side power response of a parallel system of PV-storage SSVSIs are analyzed at first. Then, a cooperative control method for the parallel system is proposed, which realizes the dynamic balance of energy storage SOC by considering a cooperative allocation strategy for energy storage SOC and improves the dynamic characteristics of DC-side voltage through the feed-forward control of energy storage power. In addition, the AC-side output power oscillations are suppressed by damping enhancement control. Finally, the effectiveness of the proposed control method was verified by simulations and experimental results based on RT-LAB.

To mitigate the fluctuations in the output from micro-sources within a half-bridge converter series Y-connection microgrid (HCSY-MG) grid-connected system and ensure both the balance of the sum of DC-side voltages in each phase and the balance of three-phase grid-connected current, a distributed hybrid energy storage system (HESS) charge-discharge optimal control strategy based on improved proximal policy optimization (PPO) is put forward. With the consideration of grid-connected current in the HCSY-MG system and the characteristics of the distributed HESS, the key system variables affecting the grid-connected current are identified, and the optimal topology for integrating the HESS into the system is determined. Subsequently, by combining the characteristics of the series-connected system, the charge-discharge problem of the distributed HESS is transformed into a Markov decision process within the framework of deep reinforcement learning. Meanwhile, to address the challenge of determining the entropy loss weight in the PPO method, an improved PPO method is proposed, which balances the agent’s convergence and exploration capabilities. Finally, the typical operational data of a renewable energy generation site is used as a case study, and the feasibility and effectiveness of the proposed control strategy is validated.