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In this work, the process of designing a wind power plant composed of; doubly fed induction generators, a static compensator unit, mechanically switched capacitors and on-load tap changer, for voltage control is shown. The selected control structure is based on a decentralized system, since it offers very fast system response to grid disturbances and lower sensitivity to the grid impedance, and moreover, this control structure releases the central control of having grid impedance estimation techniques or adaptive control methods. The difference between the required reactive power and the one supplied by the doubly fed induction generator wind turbines is overcome by installing a reactive power compensator, i.e. a static compensator unit, which is coordinated with the plant control by a specific dispatcher. This dispatcher is set according to the result of the wind power plant load flow. To release the operation of the converters during steady-state disturbances, mechanically switched capacitors are installed in the wind power plant, which due to their characteristics, they are appropriate for permanent disturbances compensation. The mechanically switched capacitors are controlled to allow the reactive power operation of the converters, in steadystate, within a maximum band of 10 %. It is clear that an on-load tap changer system will help to keep the stator voltage close to its nominal value, but the action of the mechanically switched capacitors is badly influencing the on-load tap changer line drop calculation. It is proposed to coordinate the on-load tap changer system with the main substation control. By exchanging the information related to the mechanically switched capacitors switch status, the current injected from the mechanically switched capacitors can be removed from the total measured current at medium voltage, thus reducing the tap moving operations. Finally, due to the wind power plant reactive power is sized for maximum active power level, it is expected that a big amount of reactive power remains unused most of the time due to the wind power generation characteristics. Hence, a VAr reserve concept system is proposed and applied, thereby the grid can be benefited from this extra available reactive power. |
