Hi,
DFIG has the ability to be controlled to deliver both reactive and active power. Lot of works consider that it operates at unity power factor.
What are the impacts or the challenges if it operates other than unity power factor? and how could it be used in PCC voltage control?
A wind energy conversion system using DFIG employs a reduced capacity converter capable of bidirectional power flow, connected to the rotor. The most commonly used control technique is a stator flux oriented vector control. With the d-axis aligned along the stator flux, injecting a positive d-axis rotor current improves the stator power factor. The mode of operation (subsynchronous or supersynchronous) is determined by the sign of the q-axis current. Thus by injecting rotor d-axis current greater than that required for magnetising, the stator can be made to supply reactive power to the grid, while the entire reactive power requirement of the machine is supplied from the rotor terminals.
The implications are:
1. The rotor converter capacity is generally around 30% of the rated power. Supplying the entire reactive power from the rotor side increases the rating of the rotor side converter.
2. Supplying reactive power is helpful for the connected grid and improves the voltage stability.
3. Even though the rotor side converter supplies reactive power, the grid side converter can be operated at unity power factor.
4. PCC voltage control is possible to some extent by this method depending on the impedances of the connected grid.
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