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.