Corona rings can improve the insulation performance of insulator strings because of their electric field grading property or can decrease the insulation performance due to their arc clearance reduction property. The resulting insulation performance of the string which is under the influence of those two conflicting effects depends on the corona ring parameter settings. Therefore, a detailed experimental work is required to determine the insulation performance for different types of test voltages under different ambient conditions. This paper presents the results of experimental studies regarding the effects of corona rings on the insulation performance of 380 kV non-ceramic V-insulator strings. Those effects and flashover routes were determined for lightning impulses, switching impulses and AC voltages. Corona ring diameter, corona tube diameter, and mounting height were selected as the ring design parameters. Experimental studies were performed for the strings including corona rings of different design parameters as well as for the ring free strings. Test results were discussed from discharge development physics point of view and optimal configuration was determined for the strings.