Design and control of a DC collection grid for a wind farm

In this thesis, the internal DC collection grid for a wind farm is investigated regarding the design, losses and dynamic operation for both normal operating conditions and for different fault conditions. The main advantage for the DC collection grid is the considerably lower weight of the 1 kHz transformers in the DC/DC converters compared to the equivalent 50 Hz transformers. For a wind farm with 48 2.3 MW wind turbines, and a DC/DC converter in each turbine as well as a main DC/DC converter for the whole wind farm, the losses for the DC system are 3 % of the transferred power, which is similar to the losses of a corresponding AC collection grid. For the dynamic control of the wind farm, the DC/DC converters control the power flow in the wind farm and thereby also the voltage levels for the 1.5 kV DC link in the turbine as well as for the 32 kV DC collection bus. For the limited bandwidth resulting from the switching frequency 1 kHz and the maximum voltage deviations of 5 % for the DC voltages, the required capacitances are 152 mF for the DC link in the wind turbine and 16 mF for the DC bus, both giving a stored energy corresponding to 74 ms transferred rated power. In the case of a fault in the connecting main grid, the output power from the wind farm must be decreased. Here, assuming that the excess power is dissipated in each turbine, the detection of the fault as well as the disconnection and the reconnection of the wind farm are investigated. The requirements for the HVDC link to avoid over voltages during a grid fault are stated and it is also shown that the reconnection can be done within 14 ms, which is well within the time specified in existing grid codes. Further, the behavior of the system during internal faults for the DC bus is investigated. Methods for detecting and finding the location of the faults are determined, and it is shown that a faulted part can be disconnected and the non-faulted parts reconnected within 300 ms without using fully rated DC breakers.

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