Voltage sag and harmonic pollution have a serious impact on the medium-voltage premiumpower-supply-required park represented by optical semiconductors, data storage, precision instruments and biopharmaceuticals. Which makes existing equipment require higher voltage capacity of power electronics, higher economic cost and compensation performance. In this paper, an Inductive Hybrid Unified Power Quality Conditioner (IH-UPQC), which is characterized as high integration, reduced DC-link voltage and satisfactory harmonic isolation of transformer, is presented to realize the bi-directional power quality improvement for medium-voltage premium-power-supply-required park. First, the three-phase equivalent circuit and mathematical model are established to reveal the mechanism of harmonic suppression and voltage regulation. Based on the theoretical analysis, the compound control strategy of series and shunt Hybrid Active Power Filter (HAPF) as well as the synchronous method sliding Goertzel discrete Fourier transformation-pre-filtering Phase Lock Loop (PLL) are designed. Then, the main circuit components design including Double Resonant Passive Filter (DRPF), L-C-R low pass filter and the special designed filtering winding for implementing inductive filtering method are described. Finally, the performance of IHUPQC is validated in 10kV distribution network by MATLAB/Simulink. The simulation results demonstrate that the IH-UPQC is effective to perform grid voltage regulation, load harmonic current suppression and reactive power compensation.

The practical applications of current PQDs classification and identification algorithms are limited due to their often poor recognition of disturbance types outside the training sets. To deal with this issue, a novel classification and identification method for complex PQDs based on confidence-enhanced guided multi-label learning (CEGML) is proposed in this paper. Initially, the fully convolutional network (FCN) is employed to extract deep and shallow features, which are subsequently fused. In addition, the concept of the Simple Parameter-Free Attention Module (SimAM) is combined with the Gated Recurrent Unit (GRU) to design the Synchronized Recurrent Attention Model (SRAM), enhancing the extraction of key features and the model’s ability to fit labels. At the same time, a linear layer is utilized to predict the confidence level of each label within the PQDs. Lastly, a confidence label is designed to differentiate between single disturbances and multiple disturbances, and a confidence enhancement factor is set to elevate the confidence levels of each label in the case of multiple disturbances, enabling the network to accurately identify more complex combinations of disturbance types beyond the training sets, even with limited and basic training data. Simulation and practical test results demonstrate that the PQD classification and identification method based on CEGML proposed in this paper effectively recognizes complex disturbance types not included in the training set, achieving higher identification accuracy than existing methods.

The three-phase three-wire neutral-point-clamped shunt active power filter (NPC-SAPF), which most adopts classical closed-loop feedback control methods such as proportional-integral (PI), proportional-resonant (PR) and repetitive control, can only output 1st–25th harmonic currents with 10–20 kHz switching frequency. The reason for this is that the controller design must make a compromise between system stability and harmonic current compensation ability under the condition of less than 20 kHz switching frequency. To broaden the bandwidth of the compensation current, a Lyapunov stability theory-based control strategy is presented in this paper for NPC-SAPF. The proposed control law is obtained by constructing the switching function on the basis of the mathematical model and the Lyapunov candidate function, which can avoid introducing closed-loop feedback control and keep the system globally asymptotically stable. By means of the proposed method, the NPC-SAPF has compensation ability for the 1st–50th harmonic currents, the total harmonic distortion (THD) and each harmonic content of grid currents satisfy the requirements of IEEE Standard 519-2014. In order to verify the superiority of the proposed control strategy, stability conditions of the proposed strategy and the representative PR controllers are compared. The simulation results in MATLAB/Simulink (MathWorks, Natick, MA, USA) and the experimental results obtained on a 6.6 kVA NPC-SAPF laboratory prototype validate the proposed control strategy.

To deal with asymmetrical grid faults, grid-following inverters are required to output positive- and negative-sequence currents to regulate the voltage at the point of common coupling. Owing to the negative-sequence current injection, the stability mechanism of the dual-sequence synchronizations becomes more complicated under asymmetrical scenarios. In this article, a dual-sequence dynamic coupling model, which takes into account both positive- and negative-sequence current couplings as well as power angle difference, is proposed. Based on this model, not only the existence conditions of positive- and negative-sequence equilibrium points are identified, but also the transient process stability conditions of the operating points are determined, resulting in a complete transient stability judgment foundation during asymmetrical grid faults. It is pointed out that couplings between the positive- and negative-sequence synchronization processes bring about more transient synchronization instability. Simulation and experimental results verify that the proposed synchronization stability analysis based on the dual-sequence dynamic coupling model has higher accuracy for inverters under asymmetrical grid faults compared with the previous methods.

The grid-forming inverter (GFM), which can generate frequency and voltage independently, is a necessary grid-connected device for new energy-high-penetration grid. But there is a risk of deviation from the steady-state operating point and thus lose synchronization in the presence of large disturbances due to the grid faults. The active and precise allocation of a new stable point under large disturbance requires the online estimate of grid impedance. However, previous grid impedance estimation methods rely on small-signal systems, which do not allow for real-time estimation in the presence of large disturbances. To fill this gap, this paper establishes the kinetic equations for the voltages and currents at the point of common coupling (PCC) in the α-β frame according to the natural response excited by large disturbances, and propose a non-intrusive online estimation method based on this model to fit the large-signal grid impedance using the recursive least-squares (RLS). The experimental results show that the proposed method can estimate the impedance within 0.3 s after the grid impedance change, and the estimation accuracy is 95%. When the GFM is losing synchronization, the proposed method can still estimate the grid impedance, which can then be used to actively configure a steady-state operating point.