Fault resistance causes conventional distance protection to operate incorrectly, threatening the safe operation of power system. The immunity to fault resistance is improved by utilizing fault-point voltage, measured current, and setting impedance to construct the virtual measured voltage. Then, a new distance protection scheme, known as protection scheme I, is presented to identify in-zone and out-of-zone faults by comparing the amplitudes of the original and virtual measured voltages. Then, the original and virtual measured voltages are improved by a special voltage phasor in phase with the fault-point voltage phasor. Based on the improved original and virtual measured voltages, another distance protection scheme is presented, namely protection scheme II. The two proposed protection schemes are immune to fault resistance and load current and operate well under all types of faults. However, protection scheme II has higher sensitivity to fault detection compared with protection scheme I. Simulation results verify the correctness and effectiveness of the two proposed schemes.

This article proposes a power relay system to wirelessly power multiple loads via capacitive coupling. The intermediate capacitive relay unit is designed to power the load as well as enhance the power transfer capability, which contains two receiving plates and two transmitting plates. It is proven in this article that the capacitive coupling between the receiving and transmitting plates in the same relay unit can be eliminated by being placed perpendicularly with the help of the proposed split-inductor-based compensation network. A general mathematical model of the multiload capacitive power relay system is established. The L compensation circuits are employed to compensate the first and the last relays, while the LCL compensation circuits are designed to compensate the intermediate relays. Thus, the constant current output can be obtained for each load without affecting each other when neglecting the parasitic resistances. Additionally, the load current and efficiency variations versus the load resistance, coupling coefficient, and quality factor are thoroughly analyzed. Finally, a three-load experimental prototype is constructed to verify the feasibility of the proposed multiload capacitive power transfer relay system. The maximum efficiency can reach 86.1% at a power level of 37 W.

This paper presents a modified power supply system based on the current alternating current (AC)-fed railways with neutral zones that can further improve the eco-friendliness and smart level of railways. The modified system complements the existing infrastructure with additional energy-storage-based smart electrical infrastructure. This infrastructure comprises power electronic devices with energy storage system connected in parallel to both sides of each neutral zone in the traction substations, power electronic devices connected in parallel to both sides of each neutral zone in section posts, and an energy management system. The description and functions of such a modified system are outlined in this paper. The system allows for the centralized- and distributed-control of different functions via an energy management system. In addition, a control algorithm is proposed, based on the modified system for regenerative braking energy utilization. This would ensure that all the regenerative braking energy in the whole railway electrical system is used more efficiently. Finally, a modified power supply system with eight power supply sections is considered to be a case study; furthermore, the advantages of the proposed system and the effectiveness of the proposed control algorithm are verified.

A novel unidirectional three-phase multilevel rectifier composed of star-connected three single-phase topology based on five-level flying capacitor dc–dc converter is proposed, which can be widely applied to many industrial applications, especially for the front-end of multilevel converters for ac adjustable motor drives with open-end winding connection. In addition to not requiring transformers (either line frequency or high frequency), compared to other high-voltage rectifiers, this proposed topology features fewer active switches with lower voltage stress, a high-voltage common dc bus, high input power factor, and low input current harmonics. In this article, a comprehensive analysis to the basic operating principles including the topology configuration, the steady-state modeling, and phase-shifted SPWM modulation is presented. A hybrid control strategy is studied, including the input power factor control, the dc-link voltage balancing control, and the flying capacitor voltage balancing control with balanced and unbalanced loads. Finally, the experimental results are presented to validate the feasibility and correctness of the proposed solution.

In this article, a novel load-independent capacitive power transfer (CPT) system with multiple constant voltage (CV) outputs is proposed. The capacitive repeater unit is designed to enhance the power transfer capability, which contains four plates where two plates are used to receive power from its previous unit and the other two plates transfer power to the next unit. Electric coupling between the receiving and transmitting plates in the repeater unit can be eliminated by placing them perpendicularly and employing the split-inductor matching network. The load is connected with each capacitive repeater unit so that multiple loads can be powered simultaneously. Aluminum oxide ceramic is placed between the two adjacent units to enhance their electric coupling. The $L$ -LCL network is adopted to compensate each capacitive repeater unit. Thus, CV outputs can be achieved for all loads, which ensure the independent operation of each load. The output voltages and efficiency are analyzed when considering the parasitic resistances. Finally, a three-load experimental setup is built to verify the effectiveness of the proposed system with an efficiency of 93.21%.

High-frequency and high-magnitude common mode voltage (CMV) generated by the operation of transformerless multilevel converters is one of the main causes for decreasing the lifespan of ac motors. In this article, the principle of CMV generated by rectifier and inverter stages of a recently proposed transformerless unidirectional three-level converter for ac motor drives is studied, a modified carrier-based pulsewidth modulation strategy for both CMV elimination and neutral point (NP) voltage balance is proposed. The strategy can effectively eliminate the CMV of the system by modifying the pulse voltage generation and replacing the phase of the carrier signal of the two active switches in the three-level power circuit according to the polarity of each phase current. Moreover, by superposing the NP voltage balance control signal onto the reference voltage of those selected active switches (whose switching action has no impact on the CMV) in the rectifier stage, the NP voltage balance control is integrated into the CMV elimination. The merit of this method is that it does not require the complex calculation of phase shift position and has little impact on the multilevel voltages of the rectifier and inverter stages. The experiments verified the effectiveness of the method.