Figure 12 . Response of SCIG and DFIG based system to rapid wind speed changes.
From the voltage and frequency observations, it is observed that the DFIG based system responded better to rapid wind fluctuations compared to the SCIG based system of which its frequency goes above the acceptable technical limit of 50.5Hz. The pitch regulation of the SCIG is much slower as compared to the DFIG based system. The DFIG has an additional control system in form of the MPPT that enables it to track and optimize the available windspeed with the turbine characteristics, thus aiding in pitch control. With regards to the flicker measurement, the SCIG based system operated beyond the acceptable technical limit of 1 while the DFIG remained below 1 during the 5-second period. Although this study does not cover flicker mitigation, an active voltage controller, for example a STATCOM, is required to counter the flicker especially for the SCIG based system.
Harmonic assessment and acceptable harmonic limits have been formalised by IEEE standard 519 [33]. The measurement window is recommended to be over a 10 cycle period. The technical limits for a voltage signal at 33kV are 5% for THD and 3% for any single harmonic. Table 1 below shows the THD for both SCIG and DFIG based systems for a period of 4 seconds.
Table 1. THD for both SCIG and DFIG based Hybrid System.