Search

High-voltage direct-current (HVDC) transmission systems are considered an outstanding solution due to high electrical losses emerging from long-distance transmission. However, HVDC transmission lines (TLs) are vulnerable to lightning strikes. In this work, the Yunnan-Guizhou 500 kV HVDC transmission system is used as a case study to evaluate the impact of lightning strikes on DC-TL overvoltages, as no research studies have been conducted to assess the lightning transient behavior of DC-TLs. A comprehensive investigation of the 500 kV DC-TL transient performance during lightning strikes is performed, taking into account different technical aspects that have not been studied in detail by previous researchers. Additionally, analysis of the back-flashover phenomenon has not been conducted well in previous work, and results on the effect of changing the lightning strike current peak and tower grounding resistance on shielding-failure flashover are quite limited. The distributed-parameter model is used to represent the DC-TL using the electromagnetic transients program (EMTP), considering real parameters of shielding wires and DC towers to study the lightning impact in the case of back-flashover and shielding-failure phenomena. Lightning strike is applied to the shielding wire, and the impact of increasing the peak value of lightning current is investigated on the back-flashover occurrence. Moreover, the influence of tower grounding resistance variation on the transient overvoltages across the tower body and back-flashover phenomenon is evaluated. From the simulation results, increasing the lightning current peak and grounding resistance results in higher overvoltages across the tower body, which increases the probability of back-flashover. Additionally, the shielding failure of the TL is assumed, and the variation impact of the lightning current peak and grounding resistance on shielding-failure flashover is investigated. The results show that the impact of the lightning current peak has a more significant impact than the grounding resistance in the case of shielding-failure flashover. [ABSTRACT FROM AUTHOR]

Wind energy integration in power grids is increasing day by day to reduce the use of fossil fuels, and consequently greenhouse gas emissions. Using the pulse-width modulated (PWM) power converters in wind turbine generators, specifically in doubly-fed induction generators, results in generating a common-mode voltage (CMV). This common-mode voltage causes a flow of common-mode current (CMC) that leaks through the stray capacitances in the generator structure. These currents impose a voltage on the generator bearing which may deteriorate them. In the current work, an active common-mode voltage canceler (ACMVC) is developed to eliminate the CMV produced by a PWM converter. The ACMVC generates a compensating voltage at the converter terminals to eliminate the CMV with a subsequent reduction of the voltage stress on the generator bearing. This compensating voltage has the same amplitude as CMV, but opposite polarity. A simulation of the ACMVC model is performed using the PSCAD/EMTDC (Electromagnetic Transient Design and Control) software package. Results confirm the effectiveness of ACMVC in canceling not only the CMV but the CMC and bearing voltage as well. In addition, the relationship between the rise time of CMV and the peak value of CMC is investigated. [ABSTRACT FROM AUTHOR]

Hybrid DC microgrid clusters contain various types of converters such as BOOST, BUCK, and bidirectional DC/DC converters, making the control strategy complex and difficult to achieve plug-and-play. The common master–slave hierarchical control strategy makes it difficult to achieve accurate and stable system control. This paper proposes an accurate peer-to-peer hierarchical control method for the hybrid DC microgrid cluster, and the working principle of this hierarchical control method is analyzed in detail. The microgrid cluster consists of three sub-microgrids, where sub-microgrid A consists of three BUCK converters, sub-microgrid B consists of three BOOST converters, and sub-microgrid C consists of two bidirectional DC/DC converters. According to all possible operations of various sub-microgrids in the microgrid cluster, the top-, mid-, and bottom-level controls are designed to solve the coordination control problem among different types of sub-microgrids. In this paper, a hybrid microgrid cluster simulation model is built in the PLECS simulation environment, and an experimental hardware platform is designed. The simulation and experiment results verified the accuracy of the proposed control strategy and its fast plug-and-play regulation ability for the system. [ABSTRACT FROM AUTHOR]