5.2. Comparison between conventional and proposed control scheme
To validate the auxiliary controller and to demonstrate its significance, a test with conventional double closed-loop controller (without the auxiliary controller) has been carried out with the conventional four-switch converter, the results are presented in Fig. 10. The step-down operation from 200 kV to 100 kV is considered with a duty cycle value of less than 0.5. A short-circuit fault is applied at the output of the converter at 0.6 s. When a short-circuit fault occurs, the output voltage sharply decreases to zero. The double closed-loop controller forces the converter to turn-off rapidly by bringing the duty cycle D to zero. But as presented in the Fig. 10, the duty cycle increases again after about 10 ms because of the double closed-loop adjustment. This scenario makes the switches on and the current increases again. This leads to an oscillation in the circuit. Because the short-circuit fault is still in circuit, the output voltage vo cannot be recovered back to 100 kV, and the inductor current is maintained at a value higher than its rated value. This is an undesirable operation in HVDC systems as it might cause subsequent faults and destroy the whole system.