Offshore wind power is attracting increased attention because of considerable wind resources, higher and steadier wind speeds, and smaller environmental impact. Recently, a current source converter (CSC)-based series-connected configuration is proposed and it is considered a promising solution for offshore wind farms as the offshore substation used in existing systems can be eliminated. However, such a CSC-based configuration has disadvantages in terms of size and weight, dynamic performance, cost, reliability, and efficiency. Therefore, this thesis proposes new configurations, modulation scheme, and control schemes to improve the performance of the CSC-based offshore wind farm.
First, a new configuration is proposed for the CSC-based offshore wind farm. Compared with existing CSC-based configurations, the new one is expected to be smaller in size and weight. Second, conventional space vector modulation (SVM) with fast dynamic response cannot be used for grid-side CSCs because of its high-magnitude low-order harmonics. To solve this issue, an advanced SVM with superior low-order harmonics performance is proposed.
Third, power balancing among series-connected CSCs is an important consideration for system reliability. The possible imbalance of power is investigated and quantitatively defined. A power balancing scheme is proposed, based on which equal power distribution among CSCs is ensured. Fourth, to lower the system insulation requirement of the CSC-based configuration, a bipolar operation is investigated. Compared with monopolar mode, bipolar mode gives lower insulation level, thus contributing to the system in terms of lower cost and higher reliability for a given power rating. In addition, an optimal dc-link current control giving higher efficiency is proposed for the bipolar system. Fifth, an optimized control strategy with reduced cost and improved efficiency is proposed for the CSC-based offshore wind farm. The nominal number of onshore CSCs is optimized, which reduces the cost on power converters. And an optimized bypass operation is introduced to onshore CSCs, which improves the efficiency of the system.
Finally, simulation and experimental results are provided to verify the performance of the proposed configuration, modulation scheme, and control schemes.