Large-scale wind farms are usually integrated into transmission systems through interconnecting transformers. Applying traditional, steady-state, fundamental frequency-based differential protection (DP) strategies in these interconnecting transformers creates challenges. 1) The fault current of wind farms might be dominated by non-fundamental frequency components caused by the activation of their own protection systems during low voltage ride-through (LVRT). 2) The fault current contains low-order components that may affect the second harmonic restraint algorithm of the transformer DP, thereby blocking the differential relay for a long time. Therefore, a novel transient current similarity-based protection scheme is proposed to deal with these issues. The current protection scheme uses fundamental frequency and non-fundamental frequency characteristics and can therefore reduce the influence of LVRT to a minimum. The proposed method uses transient current (within 15 ms after fault inception) and ignores the features of steady-state fault current. In the proposed method, an improved Hausdorff distance algorithm is used to calculate the similarity of transient current signals at both ends of the interconnecting transformer. Simulation results demonstrate that using a common sampling frequency, the proposed protection scheme reveals current information, correctly identifies magnetizing currents and internal transformer faults in a short period, and achieves superior performance in cases of abnormal data, missing data, and noise.

A passive current sharing method for paralleled LLC resonant converters is proposed in this article, and the current sharing effect of paralleled common capacitor LLC resonant converter is analyzed. The passive current sharing method is achieved by adding coupled impedance between the LLC resonant converters, and the coupled impedance can be inductive, capacitive, infinitely small, and infinitely large. Among them, it is traditional paralleled LLC resonant converter without current sharing ability when the coupled impedance is infinitely large. First, the mathematical model of paralleled common capacitor LLC resonant converter with coupled impedance is built based on fundamental harmonic analysis. Second, according to the mathematic model, the current sharing effect of the paralleled common capacitor LLC resonant converter of different coupled impedance under different parameter difference is analyzed, it is easy to see the current sharing effect is improved after the couple impedance is added, and in general, inductive coupled impedance has a better current sharing effect for paralleled common capacitor LLC resonant converter. Last, the theoretical analysis results are verified by simulation and experimental results.

Ten-switch three-phase three-level inverter (TP-TLI) is receiving more attention owing to the reduced system cost compared to traditional TP-TLIs. However, it still faces serious neutral-point voltage (NPV) balance problems. This article proposes a novel virtual space vector modulation (NVSVM) with optimized neutral-point voltage control (NPVC) capability. A set of virtual vectors are selected to generate zero NP current. By analyzing the vector synthesis principle over the full modulation areas, a new control degree, which is utilized to optimize the NPVC capability and reduce line voltage total harmonic distortion (THD), is introduced. Besides, a dead time compensation method based on volt–second balance is also given to reduce the switching loss. The NVSVM not only improves the NPVC capability without dc offset and obvious ac oscillations in NPV under the full range of modulation index and power factor conditions but also reduces line voltage THD and the switching loss. Theoretical analysis shows that the available NP current for the proposed NVSVM is about 2.75 times greater at most than that for traditional VSVM. Simulation and experimental results are given to validate the effectiveness of the NVSVM and control strategies.

Variable switching frequency sinusoidal pulsewidth modulation (VSFSPWM) method does well in many aspects such as lower harmonic peak and losses compared with constant switching frequency sinusoidal pulsewidth modulation (CSFSPWM) for DCAC converters. This article presents a passive VSFSPWM (PVSFSPWM) method for spectrum improvement, filter inductor reduction, and losses reduction, obtaining similar output current quality as that of the CSFSPWM method. Because the PVSFSPWM method does not need any feedback information compared with the model-predictive-based method, it is an open-loop method and is called the passive method. In order to obtain the law of harmonic distribution, the mathematical expression of harmonic spectrum distribution is derived. The required filter inductor under the same current ripple and the losses between CSFSPWM and PVSFSPWM methods have been compared. Simulation and experimental results show that the PVSFSPWM method achieves excellent performance in EMI suppression, inductance, and losses reduction without impairing the output quality.

Two-stage single-phase inverter using dual-mode control can match wide input voltage variation range and increase efficiency. That is studied by many researchers. In addition, using dual-mode control can improve the line-to-line output voltage waveform to be more easily filtered into sine wave. Thus, the traditional method to determine the LC filter parameters is unreasonable and will cause over filtering. The redundant design of filter will bring unnecessary volume, weight and loss on inductor and capacitor of the inverter, while decrease the power density. To solve this issue, this article derives the line-to-line output voltage harmonic components calculation formulas for the conventional two-stage inverter and the dual-mode inverter by using double Fourier integral analysis. By comparing the harmonic components calculation results around switching frequency, a new LC filter parameters design method for dual-mode inverter is proposed. Through the proposed method, the LC filter parameters are declined by 50%, which significantly reduces the volume, weight and cost of the inverter under other conditions of constant performance. Calculation and experimental results based on a 1 kW prototype verify the validity of the proposed method.

For the conventional single-phase current source inverter (CSI), a large inductor is needed to stabilize the input current, which increases system volume, cost and losses. In this paper, a single-phase single-stage CSI with its operation principle is proposed. Different from conventional CSIs where only voltage-boost operation is supported, the input DC-link voltage of the proposed CSI can be higher than the peak value of output voltage (buck mode), and it can also be lower than the peak value of output voltage (boost mode). So, it is very suitable for occasions with a wide input voltage range. Moreover, the input inductor current can be balanced within per switching cycle using the proposed operation strategy. As a result, the input inductor and output filter capacitor are decreased. First, the derivation and operating modes of the CSI are given in detail. And then, the design guidelines of the components are presented, and a comparison with some existing CSIs has been made. A 1 kW prototype is also built and tested in the laboratory, the experimental results are presented to verify the effectiveness of the proposed CSI and control strategy.