| Abstract |
In modern decades, the Doubly-Fed Induction Generator (DFIG) is considered as a suitable electrical generator which can be used in stand-alone Wind Energy Conversion System (WECS) as it can operate at variable speed with constant output Voltage and Frequency (V and F) and its connected power converters have low cost.
Induction Motor (IM) loads like farm irrigation pumps, ventilation fans used in livestock housing, air compressors and small pistons are used in the agricultural remote areas. As known, IMs which are directly connected to the supply draw several times (6 times) their rated motor current with low power factor which causes a supply voltage dip. This voltage dip causes a reduction of the motor torque during start-up period and repeated tripping for relays especially in separated power systems.
In a specific installation in which the major load is a 3-phase IM and directly online start-up method is used, the generator rating, such as synchronous generator, may be sized up to 8–9 times the capacity of the motor power. Hence, high cost is required for the electrical system equipment such as transformers, cables and circuit breakers as they require to be scaled to the value higher than the required value at steady-state condition to sustain the motor start-up requirements. Several methods have been employed to mitigate the voltage sag caused by the start-up process for IMs in order not to exceed the permissible values for IM loads. While, the usage of those methods to maintain the supply voltage at acceptable levels is considered as expensive solutions.
When the DFIG is used to supply dynamic loads such as IMs, the dynamic interactions studies between the IM loads and DFIGs are needed, especially during the start-up period. Few researches focused on the effect of starting current of dynamic loads on the operation of stand-alone DFIG are founded.
The presented control methods in these researches did not treat the large start-up current drawn by IM. Furthermore, the control strategies are tested by directly connected IM of small power rating compared to DFIG power rating. Also, the larger power rating of IMs are not tested and the motor generator group is not scaled.
In this thesis the stand-alone DFIG is controlled to maintain the stator output V and F at rated value when supplying an IM within DFIG rating limits. Also, the maximum power point tracking control is applied to improve the performance of the system. The capability limits of DFIG system is obtained at rated stator output V and F. The DFIG rating according to the requirements of the directly connected IM is determined. Furthermore, the problems associated with the start-up period of directly connected IM are solved without using any additional starting method by controlling the rotor side converter connected to the DFIG to produce variable stator output V and F while V/f ratio is kept constant. Finally, the DFIG rating according to the requirements of connected IM when using constant V/f control method is re-scaled.
The new control method contributed to increase the DFIG capability limits and the system efficiency, consequently the sizing of generator power rating to fed certain power rating of IM is reduced. |
