Speed Sensorless Vector Control of Permanent Magnet Wind Power Generator – The Redundant Drive Concept
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The reliability of high power drives can be increased using a generator/motor structure comprising multiple independent stator modules or segments each having an own drive inverter. In case that one of the inverters fails, the operation can continue by other inverters assuming that there is no failure in generator windings. In order to increase the drive system power level, several smaller inverter units can be used instead of a one larger one. Wind power generators can operate long periods at reduced power due to the lack of sufficient wind. Hence, it is important that the generator operates at high efficiency also when operating below the nominal load point. The optimal efficiency is obtained only when the generator control is functioning in an optimal way, which is ensured by using the optimal vector control and the MTPA optimization which keeps the minimum stator current and the optimal stator flux level. Wind power generators are a fast increasing application area requiring high power levels. It is a suitable application for the speed sensorless vector control described. Also, a principle of the closed loop control using an absolute encoder feedback was shown for the comparison. The test results with a small laboratory test bench confirmed the validity of the control methods and the idea of the redundancy. At the moment, several large scale 3.8 MW permanent magnet generators comprising three stator segments are in operation in windmills. However, the applications of redundant drives are just not limited to the wind power production but the idea can be used everywhere requiring redundancy. For example, the same drive principle is used also in large 800 kW ski-lift motors comprising 8 stator segments and inverters.
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