The Under-Frequency Load Shedding (UFLS) scheme is an emergency measure in place to prevent system collapse in the event of a large generation deficit relative to demand at any moment in time. However, the effectiveness of traditional UFLS schemes may be compromised as network power flows change due to the growth of distributed generation (DG) and a reduction in system inertia. This study has shown some of the issues affecting the UFLS protection system with rising installed capacities of DG in the Great Britain power system. Simulation results show how the effectiveness of the scheme is challenged with current and future expected DG installed capacities and the risk of over shedding demand due to the scheme's current settings is evaluated. Many proposed improvements to traditional load shedding schemes are based on upgrades to network monitoring and communication systems, which are not yet fully available. This study evaluates more readily available solutions, including relocation of under-frequency relays and reducing the time delay of the scheme, which may provide interim improvements to the UFLS scheme, based on a case study in GB.
This paper compares the performances of three different Low Voltage Fault Ride-Through (LVFRT) techniques for Wind Energy Conversion Systems (WECS). The comparison aims to identify the most effective technique for alleviation of adverse impacts of AC faults on WECS electrical and mechanical parts, which include DC voltage rise and generator over-speed. The comparison is based on a criticalqualitative review of existing literature on the selected LVFRT techniques, which are further supported by quantitative substantiation using simulations. The major findings of this comparative study are highlighted, with emphasis on metrics, which account for practical implementation, hardware, cost, and complexity issues. They are important to assess the overall effectiveness of the techniques evaluated. Although practical and commercial limitations exist, the initial findings suggest that an energy storage solution would be suitable for the enhancement of LVFRT for WECS in future power networks, and if the stored energy is utilised correctly, offer further attractive benefits.
The series-connected offshore wind farm (SC-OWF) is a promising offshore wind generation solution to mitigate the need of centralized offshore high-voltage/power converter stations. Predominantly, researchers have focused on the steadystate operation and control of SC-OWFs, without considering the system-level characteristics and ability to ride-through dc side and ac network faults. This paper proposes an enhanced system for SC-OWF applications with fault-resilient capability, where comprehensive circuit configuration and protection strategies are articulated to minimize the negative effects caused by various types of dc and ac faults. For the offshore wind farm architecture, a grouping scheme is adopted where a substation based on disconnectors and diodes is proposed to realize prompt fault bypass/isolation and protection functions in the event of offshore system faults. Additionally, an onshore fault-tolerant modular multilevel converter (MMC) with modified dc-system-oriented control is employed to enable smooth and secure operation under steady-state and fault conditions. The proposed SC-OWF system is quantitatively substantiated by time-domain simulations where four ac/dc fault cases are considered, and the results consolidate the feasibility of the proposed configuration and control, indicating fault resilience of the SC-OWF system. Additionally, size, weight and cost estimations of the proposed offshore substation are presented and compared to a conventional MMC offshore station, to further highlight the merits of the proposed solution.
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