Abstract :
The present practice in thermal rating is reviewed with recognition of its defects and limitations, of which the most serious is its inability to relate the standard rating with the hottest-spot temperature (h.s.t.). Modern improvements in cooling reduce the h.s.t., but are circumscribed by these limitations. The multiflow principle allows the h.s.t. to be determined for each individual winding from the present standard temperature-rise test measurements. It is anomalous that forced-oil cooling¿a standard rating for many years-has never been defined. It is quite legitimate to install such a small oil pump that the term has no real significance. The proposed international recognition of directed flow is neither justified nor desirable. Increased and directed flow are alternatives, and must both be recognised. In natural cooling, small distribution transformers are underrated and large ones are overrated. The h.s.t. multiflow rating removes all these anomalies, and leads to a draft revision of the present UK national standard rating for both natural and forced cooling. The proposed new rating can be determined as simply as the present standard rating but with the full benefit of the hottest-spot temperature limit controlling the thermal life of the insulation, and hence of the transformer itself. This benefit applies not only to the guaranteed rating but is available also for other service loading conditions of concern to the user. These sometimes bear little relation to the standard, but widen the practical field of application
