Figure 14. Response on DFIG WECS starting on a DFIG based system.
From the results obtained, starting of SCIG based WECS first requires energization of a capacitor bank (at the 60th second) to supply its reactive power requirements. Without the capacitor bank energization, the SC as shown by the field current would be driven into overexcitation prompting action of the OEL. For the case of the starting of the DFIG based WECS, there is a sluggish response, but the SC is not driven into overexcitation and in contrast it behaves as a capacitive element reducing the operating level of SC field current.
3.3. Transformer Energization
Figs. 15 to 18 show the system response on transformer energization at different excitation levels for both the SCIG and DFIG based hybrid systems. Change of excitation level for the SCIG system is achieved by energizing a 0.5MVAr capacitor bank while for the DFIG based system it is achieved by setting the Q reference to 0.1pu on each WECS.