Cooling systems for HTS transformers : impact of cost, overload, and fault current performance expectations

In a future commercial marketplace HTS transformers will compete with oil-immersed copper transformers - highly developed, efficient and reliable. Although HTS can offer reduced weight, lower load losses, and perhaps current limiting capability, to have any chance of commercial competitiveness the total cost of ownership (TCO) taking into account purchase price, installation and lifetime operating costs, must be lower than the conventional alternative. In addition the HTS transformer in operation must meet regulatory and network operator expectations not easy to satisfy with high current density HTS wire and cryogenic operation. The cooling system for e.g. a 40 MVA 3-phase HTS transformer needs to provide for a total thermal load at 66 K of around 3 kW, dominated by current lead and AC loss. The TCO perspective shows that one of the commercially available cryocoolers options is clearly best for closed-circuit cooling systems in this application. A 3-phase vacuum-insulated glass-epoxy composite cryostat for an HTS transformer with warm iron core is perhaps the single most expensive component of the cryogenic system. A hybrid cryostat, with vacuum insulation around the cores and foam insulation around the transformer tank can provide adequate thermal performance at a fraction of the cost. ... mehrOverload capability without loss of lifetime is often counted an advantage of HTS transformers. However AC loss increases non-linearly with current, typically increasing by a factor of 10 for a doubling of current. Providing this amount of reserve capacity in a cryocooler can be prohibitively expensive. This, and the need for back-up cooling capacity during cryocooler maintenance or malfunction, has important consequences for system design and cost. For most grid applications HTS transformers will need to match the fault current and recovery performance of conventional transformers: to withstand the fault current drawn by a zero impedance short for 2 seconds to allow time for the grid protection system to isolate the fault, and then to recover to normal operation while carrying rated current. For HTS transformer windings immersed in subcooled liquid nitrogen the withstand time will be largely determined by the mass of the conductor. Recovery after the fault will depend on the boiling heat transfer characteristics of the winding, which can be seen as an important facet of cryogenic system design for transformers. Solid conductor insulation, rather than paper wrap, and subcooled operation provide for maximal heat transfer during recovery.