DC microgrids that feature a simple and efficient integration with renewable energy sources and energy storage elements have drawn increasing attention in industrial applications. In this paper, a bidirectional Z-source circuit breaker with an O-shaped impedance network (abbreviated as O-Z-source circuit breaker) is proposed to guarantee the reliable operation of dc microgrids. By a simplified structure, the O-Z-source circuit breaker allows bidirectional power flow with fewer components compared with conventional Z-source circuit breakers. Increased operating power efficiency can also be obtained from the novel topology. Based on detailed mathematical models, design equations are derived for sizing the components in the O-Z-source circuit breaker. Moreover, a new concept of equivalent capacitance that is useful for estimating the minimum detectable fault ramp rate for Z-source circuit breakers is presented. A discussion over snubber circuits of the proposed breaker is also included. Finally, a laboratory prototype has been built to verify the analysis and design of the O-Z-source circuit breaker.

The dual-PWM adjustable speed drives (ASDs) inject interharmonics into the distribution grid, causing low-frequency oscillation (LFO) in the grid. This article investigates the common-modulation process of motor load torque oscillation and how it resulting in interharmonic components in the front end of the back-to-back PWM-based ASDs. Particular emphasis is given to the theoretical analysis of interharmonic-causing frequency aliasing and fundamental root-mean-square (rms) LFO. Besides, based on the Bessel PWM switching function and common-modulation principle, a model of interharmonics generated by the dual-PWM ASDs is established. The proposed model, which takes into account the motor operating conditions and load torque disturbance, predicts the input interharmonic frequency locations and voltage rms LFO pattern with respect to bridge crane dual-PWM ASD systems. Simulation and experimental results on dual-PWM ASDs and distribution grid in the Shanghai Yangshan Deepwater Port area verify the effectiveness and feasibility of the model and the theoretical analysis.

The grid‐connected dual‐PWM adjustable speed drives (ASDs) modulate the energy of the motor torque oscillation and inject it into the power system, potentially causing low‐frequency oscillation (LFO) in the grid. In this study, considering the motor torque oscillation and harmonic common‐modulation of the both‐side converters, the precise sequence impedance models are derived for dual‐PWM ASDs using the harmonic linearisation method. Based on the derived impedance of dual‐PWM ASD, the torque oscillation, which is equivalent to the time‐varying impedance, causes a series of discontinuities at the torque‐oscillation‐related frequencies on its amplitude‐frequency curve. Moreover, these discontinuous drops make it more likely to intersect with the amplitude‐frequency curve of the grid impedance, which may lead to LFO of the grid. Then, by applying the proposed impedance model and Bode plot, the stability of the grid‐ASD cascade system with motor torque oscillation is assessed. The conclusions of stability analysis demonstrate the exact mechanism that the mechanical system oscillations lead to the grid‐converter cascade system instability through the common‐modulation and coupling of the dual‐PWM ASD. Finally, experimental results on a 3 kW ASD system in the laboratory and measured results on the distribution grid in the Shanghai Yangshan Deepwater Port area verify the correctness and effectiveness of the proposed model and stability assessment.

DC link power supply systems that integrate power electronic converters are increasingly being adopted. In particular, emerging “source–load” systems, in which the DC link interfaces with converters, have attracted increasing research interest due to concerns about power quality and system stability. This paper addresses mid- and low-frequency oscillation issues in DC link voltage supplied induction motor drives (IMDs). It begins by constructing a multiple-input multiple-output (MIMO) state-space model of the induction motor. For the first time, the dq-axis control system is represented as an equivalent admittance model that forms two single-input single-output (SISO) loops. The PI controller and induction motor are integrated into the inverter’s input impedance model; Furthermore, the effectiveness and accuracy of the derived impedance model are experimentally validated under various operating conditions of the induction motor using a custom-built test platform. The experimental results offer a practical reference for system enhancement and stability evaluation.