Fig. 7.
Classical double closed-loop control scheme.
It can be noticed from Fig. 7 that when the converter operates at the rated conditions, the control loops produce the rated and D. However, this control scheme cannot distinguish the normal operation and recovery period from the faulty operation of the transmission lines and thus produces gating signals even during the short-circuit faults. In other words, this controller cannot interrupt the short-circuit faults completely. This is an undesirable characteristic of the classical double closed-loop controller.
3.2. Proposed control scheme
To overcome this problem, a novel auxiliary controller is proposed as shown in Fig. 8 to be used in conjunction with the classical controller. The auxiliary controller performs zero crossing detection on the duty cycle, and when the duty cycle value falls below zero, it sends a trigger signal to a two-port switch. With the trigger signal being applied, the two-port switch produces the zero duty cycle value throughout the operation of converter. When the fault is completely isolated/cleared, the trigger signal can be disabled to restore normal operation. This algorithm is applicable for the proposed converters and also for the conventional converters