Search Results (6)

Developing reliable protection systems is critical for the advancement of medium-voltage direct-current (MVDC) grids. This paper highlights the significance of fault detection in MVDC grids, especially in ensuring the reliability and efficiency of renewable energy systems. This paper provides a comparative analysis of fault detection algorithms, including overcurrent, undervoltage, rate of change of current (ROCOC), rate of change of voltage (ROCOV), differential, and inductor voltage derivative methods. The performance of these algorithms is quantified by metrics such as the detection speed, accuracy, and sensitivity under diverse scenarios. The authors assess these algorithms within a multi-terminal MVDC grid designed for renewable hydrogen production, evaluating the detection speed across various fault types (bus and link faults) and conditions, including the variation in the fault location and resistance. The results reveal that the UV, ROCOC, ROCOV and LIVRD methods achieve detection speeds as high as 0.01 ms, outperforming other techniques under low-resistance fault conditions. By using uniform fault scenarios, we identify the most effective algorithms for rapid fault detection, aiming to enhance the protection strategies for MVDC grids. These findings underline critical performance differences between methods, guiding the design of tailored protection schemes that address specific fault challenges in renewable-powered grids. Additionally, the practical implications of these findings for designing resilient protection schemes in renewable-powered grids are discussed. Simulations are conducted using PSCAD/EMTDC V4.6 software, ensuring the consistency and accuracy of the performance comparison. The insights gained provide a concrete understanding of each algorithm’s trade-offs, enabling informed decisions when selecting optimal fault detection methods to ensure MVDC grid reliability. Full article

This manuscript introduces a novel multilevel middle point clamped (MMPC) DC-DC converter and its associated switching scheme aimed at maintaining the desired medium-voltage DC (MVDC) collector grid within offshore all-DC wind farms. Building upon previous work by the authors, which proposed an all-DC structure serving as a benchmark system, this study explores the application of the MMPC DC-DC converter within this framework. Within the all-DC wind generation system, a 9-phase hybrid generator (HG) integrated into the wind turbine is linked to the MVDC collector grid through an AC-DC stage, which is a passive rectifier. This passive rectifier offers elevated voltage ratings and protection against back power flow. The conventional neutral point clamped (NPC) converter concept has been thoroughly investigated and expanded upon to develop the proposed MMPC DC-DC converter. The proposed MMPC DC-DC converter integrates boosting capabilities, facilitating the connection of the generator’s rectified voltage to the MVDC collector grid while regulating variable rectified voltage to a fixed MVDC collector grid voltage. The MVDC collector grid is further interconnected with high-voltage DC (HVDC) through a DC-DC converter situated in an offshore substation. This paper further provides a comprehensive overview of the proposed MMPC DC-DC converter, detailing its operational modes and corresponding switching schemes. Through an in-depth examination of operational modes, duty cycles for each switch and mode are defined, subsequently establishing the relationship between rectified input voltage and MVDC output voltage for the MMPC DC-DC converter. Utilizing the middle point clamped architecture, this innovative converter offers several advantages, including low ripple voltage, a modular structure, and reduced switching stress because of the multilevel voltage and the incorporation of a hard point, which also facilitates the capacitor voltage balancing. Finally, the effectiveness of the proposed converter is evaluated via simulation studies of a wind turbine conversion system utilizing two cascaded MMPC DC-DC converters operating under variable input voltage conditions. The simulations confirm its efficacy, supported by promising results, and validating its performance. Full article

This paper presents a complete review of MVDC applications and their required technologies. Four main MVDC applications were investigated: rail, shipboard systems, distribution grids, and offshore collection systems. For each application, the voltage and power levels, grid structures, converter topologies, and protection and control structure were reviewed. Case studies of the varying applications as well as the literature were analyzed to ascertain the common trends and to review suggested future topologies. For rail, ship, and distribution systems, the technology and ability to implement MVDC grids is available, and there are already a number of case studies. Offshore wind collection systems, however, are yet able to be implemented. Across the four applications, the MVDC voltages ranged from 5–50 kV DC and tens of MW, with some papers suggesting an upper limit of 100 kV DC and hundreds of MV for distribution networks and offshore wind farm applications. This enables the use of varying technologies at both the lower and high voltage ranges, giving flexibility in the choice of topology that is required required. Full article

A new bi-directional circuit breaker is presented for medium-voltage dc (MVDC) systems. The Y-source impedance network topology is used to implement the breaker. The current transfer function is derived to show the frequency response and the breaker operation with the high frequencies. Mathematical analysis is achieved with different conditions of coupling among the breaker inductors. The minimum level of the magnetic coupling is determined, which is represented by the null condition. The effect of the turns-ratio on this condition is investigated as well. The breaker is designed with two types of fault conductance slope rates. The Y-source breaker is simulated, and the results verify the breaker operation during the fault condition and the load change. The results also demonstrate the effect of the coupling level on the minimum values of the source current when the fault occurs. Based on the expected fault type in the MVDC systems, the proposed breaker is developed to interrupt the overcurrent due to any of these fault types. A protection scheme is proposed for a 12-bus, two-level micro-grid, where the Y-source breakers are used in the bi-directional zones. The results verify the ability of the breaker to conduct and interrupt the current in both directions of the power flow. Full article

Offshore wind farms with good wind quality are exponentially increasing. To take advantage of this, the offshore wind farms and the grid are connected using the MVDC. In the event of a grid fault, the traditional wind generators and the MVDC are disconnected from the grid fault to protect the devices. However, the wind generators and the MVDC must support the recovery of the grid fault because the disconnection of large capacity wind farms will cause a grid collapse. To prevent this problem, the LVRT requires maintaining the connection between the wind generators and the grid to contribute to the recovery of the grid fault. In this situation, the DC voltage of the MVDC rises due to the unbalanced power of the input and output. Several methods have been proposed to suppress the DC voltage rise of the MVDC. Among various methods, the CR is an effective method to suppress the DC voltage rise of the wind generators and the MVDC. However, the conventional CR designs only consider rated voltage and system capacity. Therefore, this paper proposes the parameter estimation of the CR considering the important factors. The proposed method is verified by PSCAD/EMTDC. Full article

Nowadays, the efficient and reliable protection and location schemes for MMC-MVDC (Modular Multilevel Converter-Medium Voltage Direct Current) grid are few. This paper is the first to propose a scheme to not only protect the feeders and the busbar, but locate the segments in MMC-MVDC grid. To improve the reliability, this paper analyzes the transient characteristics of the pole-to-pole fault and then obtains the characteristic frequency band. Based on S-transform, STCFB (S-transform characteristic frequency band) Phase of fault component is utilized to construct the identification criterion for faulty feeder and faulty segment. The whole scheme can be divided into three steps, namely, protection starting criterion, faulty feeder and busbar protection criterion, and faulty segment location criterion. Firstly, the current gradient method is utilized to quickly detect the fault and start the protection device. Secondly, the non-unit protection criterion on busbar and feeders is proposed according to STCFB Phase of the voltage and current fault component. Thirdly, according to the STCFB Phase on both sides of the feeder segment, the faulty segment can be located. A radial MMC-MVDC distribution network model was built in PSCAD/EMTDC software to evaluate the performance of the protection and location method. Simulation results for different cases demonstrate that the proposed scheme has high accuracy, good adaptability and reliability. Full article