In this paper, a method is proposed to mitigate unbalanced voltage and reduce power losses due to neutral line current in a bipolar direct current (DC) distribution network by introducing a DC electric spring (DC-ES). The coupling relationship between positive and negative pole voltages are investigated given the resistance of neutral line in the bipolar DC distribution network. The coupling matrixes of the positive and negative pole voltages and the DC-ES output voltages are deduced. And this matrixes demonstrate that the DC-ES control system is composed of double inputs of pole voltages and double outputs of DC-ES voltages. The small-signal model of the bipolar DC distribution network with DC-ESs is derived further. Subsequently, the control system with double inputs and outputs is transformed into two independent control systems by introducing the decoupling control variables, thereby increasing its rapidity and anti-interference performance significantly. The proposed unbalanced voltage suppression method and decoupling control of DC-ESs are validated by simulations and experiments.

The use of bipolar direct current (DC) distribution systems introduces the possibility of unbalanced voltage (current). This situation increases the voltage deviations and power losses at each node owing to the presence of a neutral line current. This study proposes a method that mitigates the unbalanced voltage caused by constant power loads (CPLs) by DC electric springs (DC—ESs). A linearized model is used in the small-signal analysis of CPLs. On this basis, the coupling relationship between positive and negative pole voltages is analyzed. After DC—ESs are introduced, the positive and negative pole voltages and DC—ES output voltages are found to be closely related, thereby increasing the complexity of the control system. Therefore, a feedforward decoupling control block diagram is introduced on the basis of the small-signal model analysis of DC—ESs in a single node of the bipolar DC system. The control system is simplified, and different control loops can be independently controlled. Thus, the rapidity and anti-interference performance of the control system are greatly enhanced. Simulation and experimental results of the unbalanced voltage suppression in the bipolar DC system are used to verify the effectiveness of the proposed scheme.

The unbalanced DC loads between the positive and negative poles of bipolar DC distribution network leads to the increase of unbalanced current (voltage) and power losses. This affects the efficiency of DC power transmission and also influences the power quality of the DC load. In this article, an unbalanced DC load switching strategy based on automatic commutation switch (ACS) and genetic algorithm (GA) is proposed. The proposed method can automatically adjust the power supply polarity (PSP) of the DC load according to the unbalanced currents in the bipolar network. This article also introduces the configuration and topology of ACS in a bipolar DC distribution network. Furthermore, the ACS and its control diagrams are designed. The state of ACS is represented by the switch state vector, which corresponds to the gene sequence in the GA. Consequently, the calculation algorithm of switch commutation command (SCC) based on the vector gene coding strategy is proposed. In accordance with the optimal SSC, an online unbalanced-load switching strategy is designed. Finally, the simulation is built in MATLAB/Simulink to verify the effectiveness of the proposed method.

In dc microgrid, the cascaded dc system has been extensively employed. However, the negative impedance characteristics of the converter may induce system instability. This article proposes an optimized parallel virtual resistance (PVR) based active damping control to improve the stability of the cascade dc system in the dc microgrid. The advantage of this method is that it not only meets the closed-loop dynamic performance of the source converter (SC), but also meets the stability requirements of matching the input impedance of the load converter. The stability range of PVR under full load and its influence on the output impedance of SC in cascade system are determined. Additionally, a large-signal model is constructed by using state-space averaging method to investigate the open-loop stability in the cascaded dc system. The suggested control stability requirements are investigated by using the Lyapunov indirect approach, and a matching PVR is then adopted to ensure system stability. Finally, the performance of the control strategy is verified by establishing a MATLAB simulation and experiment platform.

Four-mode modulation (FMM) method is the most advanced for four-switch buck–boost converter among existing control strategies. But its efficiency is still not optimal. In this article, all FMM methods are analyzed, and the minimum inductor current root mean square value of each FMM method is obtained via the generalized Lagrange multiplier method. Under this circumstance, the optimal FMM methods are selected to form a control method that improves operating efficiency over the whole operating range. Experimental results and comparison with state-of-the-art methods demonstrate the effectiveness and superiority of proposed method.

The precise identification of fault locations in multiterminal dc (MTDC) transmission systems using traveling waves (TWs) is imperative for ensuring the secure and stable operation of power systems. The presence of wave impedance discontinuity in overhead-cable hybrid transmission lines (OCHTLs) causes significant changes in TW velocity. Traditional TW-based fault location methods struggle to adapt to these changes. This article introduces an innovative fault location approach tailored for OCHTL, with the primary aim of enhancing fault-ranging accuracy. Initially, a typical OCHTL equivalent model is established, and the propagation characteristics of the OCHTLs’ fault voltage TW (VTW) are analyzed. The variational mode decomposition-Teager energy operator (VMD-TEO) algorithm is then employed to determine the arrival time of the VTW wavefront. To optimize VMD parameters, the Kullback–Leibler (K–L) divergence method is applied, addressing modal aliasing. Additionally, we explore TW velocity attenuation characteristics across segments and introduce a frequency-modified algorithm for faulty segment identification. A high-precision fault location algorithm is then proposed based on two-terminal information, and its robustness and accuracy are validated through simulations using a PSCAD-built MTDC system with OCHTLs.