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Voltage source converter (VSC)-based high voltage DC (HVDC) transmission is considered the future of offshore power transmission. This paper aims at providing a reliable VSC-HVDC transmission system architecture between offshore wind farms and onshore grids. In this paper, a large-capacity, low-speed flywheel energy storage system (FESS) based on a squirrel cage induction machine is applied in parallel with the VSC-HVDC at the grid side converter. The FESS is dedicated for surge power (due to power flow imbalance during fault) absorption instead of being dissipated in the form of resistive losses. Since the duration of these surges is relatively small, it has been shown that the flywheel can effectively mitigate this problem. In addition to the fault ride-through support during fault conditions, the FESS is employed for power leveling functionality during normal operation. The performance parameters of the proposed approach are investigated via both simulation and experimental results. A 132-kV, 100-MW HVDC system is simulated using MATLAB/Simulink during normal and fault conditions. The proposed architecture is substantiated experimentally through a scaled down test rig with a 2-kW FESS. [ABSTRACT FROM AUTHOR]

Converting existing ac transmission lines to an extended multiterminal three-wire bipolar HVDC system can be considered a cost-effective way to interconnect dispersed offshore wind farms to onshore ac grids instead of building new bipolar HVDC systems. To automatically coordinate between different converters in a multiterminal HVDC (MTDC) system, droop control techniques have been adopted as an effective means without the need for fast communications between units. The droop control design is mainly dependent on the line resistances. This paper shows that the equivalent resistance of a three-wire bipolar system changes based on the operational mode. The modification to droop control design of an MTDC equipped with a three-wire bipolar system is then presented to tackle this resistance variation with the operating condition. Two types of MTDC systems are considered in this work, namely, radial parallel and meshed parallel systems. Different simulation studies have been conducted to validate the results of the presented analysis. [ABSTRACT FROM PUBLISHER]

Power generation through wind is expected to play a major role in the world's future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology. [ABSTRACT FROM PUBLISHER]

Renewable energy sources, such as offshore wind farms, require high voltage gains in order to interface with power transmission networks. These conversions are normally made using bulky, complex, and costly transformers and high-voltage ac–dc converters with unnecessary bidirectional power flow capability. Multiple modules of single-switch single-inductor dc–dc converters can reach high gains without transformers in these applications due to low semiconductor conduction loss in high-power devices. This paper describes a new approach for high-gain high-voltage dc–dc converters using multiple modules of single-switch single-inductor transformerless converters. Results for low-voltage experimental prototypes show gains of up to 29 p.u. and demonstrate the potential of the approach as high-gain dc–dc converters for offshore wind farms. This paper then demonstrates the viability of multiple-module converters compared to a conventional high-voltage dc converter and a theoretical full-bridge converter due to fewer devices and valves, comparable isolation levels, and ease of interleaving for increased reliability. [ABSTRACT FROM AUTHOR]