Factors Influencing the Insulation Coordination of Transformers

The following conclusions have been reached: 1. The time lag characteristics of major insulation are practically the same for both positive and negative waves. 2. The impulse ratio of major insulation is practically 2.2. 3. The present standard A.I.E.E. tests for the 34.5-kv class and above provide coordination with the proposed coordinating gaps. 4. A.I.E.E. tests below 34.5-kv class are too low and should be increased. 5. Greatest security is obtained if the surge flashover of the bushing is only slightly greater than that of the coordinating gap. A margin of 10 per cent is suggested as the most desirable bushing level.