a b s t r a c tA technical and economic comparison is made between DC and AC collection systems of offshore wind farms. DC collection systems have the advantages of reduced weight and size of the DC cables and DC cables are free from reactive power compensation. The heavy 50/60 Hz transformers in the offshore transmission platform of AC collection systems can be replaced with smaller size medium frequency transformers in DC collection systems. However, the need for a high power DC-DC converter with high voltage transformation ratios and DC protection methods will remain a challenge for the DC collection systems. Also, DC collection systems do not necessarily reduce the power conversion stages compared to the AC collection systems even if HVDC (High Voltage DC) transmission is used to transfer the offshore wind power from the collection systems to the onshore grids. A cost assessment study verifies that the cost reductions achieved by the reduced size of the DC cables and offshore platform are outweighed by the cost of DC protective devices and DC-DC converters. This is because the length of the DC collection cables is relatively short compared to the long distance HVDC cables. The technical comparison supported by the simulation results shows that the total losses in the DC collection systems are higher than in AC collection systems. The effect of collection bus voltages on the losses is analysed for the DC collection systems.
This study presents an analysis of the energy curtailment caused by the DC series-parallel collection systems of HVDC connected offshore wind farms. Wind speed differences between the series connected wind turbines cause unequal voltages at the DC output of the wind turbines. This can lead to unacceptable over-voltage or under-voltage conditions. The over-voltage and under-voltage conditions on the turbine DC outputs can be avoided by curtailing the power outputs of the wind turbines, which will result in loss of wind power. The annual energy curtailment due to the over-voltage limits of turbine DC-DC converters is analysed for a 200 MW DC series-parallel wind farm. The impact of wake effects on the energy curtailment losses is quantified and demonstrated with a case study.
Voltage balancing among the series connected wind turbines is one of the main technical challenges of offshore wind farms with DC series-parallel collection systems. Due to wake effect considerations, the power generated among the series connected wind turbines does not remain the same and the series connection causes the output voltages of the turbines to be distributed in proportion to their power output. Power curtailment is required in order to prevent the turbines being subjected to over-voltage and under-voltage conditions. In this paper, a voltage adjustment procedure is proposed to eliminate overvoltage and under-voltage of wind turbines. Based on the voltage adjustment procedure and the power generation model of wind turbines, the energy curtailment of a wind farm is analyzed with the consideration of wake effect due to series connection. A case study on several DC series-parallel collection configurations is presented to analyze the energy curtailment of wind farms using the voltage adjustment procedure.
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