Figure 8. SCIG based system response on pure inductive load pickup with 90% level of excitation.
There is significant rise in the field current as expected and above the 1.05pu OEL activation limit for inductive loads of 1MVAR and above. This prompts the OEL to reduce the AVR reference voltage in turn reducing the terminal voltage of the SC and in turn of the system. The allowable steady state minimum voltage is 0.9pu which is when the ride through strategies that are meant for short term voltage sags are activated. The level of excitation can be lowered to 50% by adjusting the SC AVR or by energization of more capacitor banks. Although lowering of the AVR reference would lead to lowering of the excitation level, the system voltage would also decrease, and this is not desirable. It is thus preferred to energize a capacitor bank as it would lower the excitation level and at the same time increase the voltage. In this case, a 1.4 MVAr capacitor bank was energized (not shown but energized at 10th second). This drove the SC to operate in underexcitation mode, so as to pick up more reactive load. Fig. 9 show the results obtained: