Figure 9. SCIG based system response on pure reactive load
pickup with 90% level of excitation.
Significant improvements are observed and now 2 and 1MVAr inductive
loads can be picked up with the SC in underexcited mode. The OEL acted
swiftly on picking up of the 4MVAr load. This led to significant drop in
voltage which would lead to system collapse. The action on the pickup of
the 2MVAr load was minor and the voltage still remained within
acceptable technical limit. It is thus recommended for the system to
have 2MVAr as the limit for the SCIG based hybrid system.
A similar exercise is conducted for the DFIG based hybrid system. Fig.
10 shows the results obtained at 100% excitation level. As observed,
the pickup of even the smallest inductive load block prompted the OEL to
reduce the voltage but not to low levels as compared to the SCIG based
system. This is due to rotor control with the q component of rotor
current re-adjusting to maintain the voltage to within acceptable
limits. Also, the inductive load is picked up by both the DFIG and SC.
On lowering the level of excitation to 50%, by adjusting the DFIG
reference reactive power so as to drive the SC in underexcitation mode,
Fig. 11 shows the results obtained. Significant improvements are
observed and now for 2, 1 and 0.5MVAr blocks, the voltages do not drop
below 1pu. The OEL still acted significantly on picking up of the 4MVAr
load. It is thus recommended for the system to have 2MVAr, similar to
SCIG based system, as the limit for the DFIG based system.