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Volume 8 is bushings, tap changers, cooling, and the joints between them. This module introduces those interfaces. It does not select a bushing technology, a tap-changer type, or a radiator area. Those choices are paid modules, and they have to match the insulation level, the current, and the cooling stage already fixed in the enquiry.
Three interfaces, three jobs
A bushing takes a lead from the oil, or from the air around a dry winding, out to the terminal without letting the voltage track over a short surface. At distribution voltages a solid bushing is usual. At higher voltages a condenser-graded bushing shares the voltage across many layers. The test tap on that kind of bushing is how capacitance and dissipation factor are watched later. The bushing is a leading place for a serious failure because it stores energy and it sits in the tank cover. Specifying “a bushing” without a voltage class, a current, a creepage duty, and a technology is not yet a specification.
A tap changer changes the turns ratio. Off-circuit, it is moved only with the transformer isolated. On load, it moves in service, inside a defined range and step. The enquiry has to say which winding is tapped, whether the changer breaks current, the number of steps, and whether rated power holds across the range. IEC 60214-1 is the tap-changer standard. It is not, by being cited, a complete tap specification.
Cooling moves the heat of no-load loss and load loss out to air or water. IEC 60076-2 assigns a four-letter code for a liquid-immersed unit: the internal liquid, how that liquid moves, the external medium, and how that medium moves. IEEE practice uses its own letter code and its own temperature-rise limits. ONAN and ONAF are not two names for the same rating. The higher power exists only in the stage whose fans or pumps are running.
The interfaces have to agree
A bushing current below the winding current, a tap changer on the wrong winding end, or a cooler valve that can be left shut between the tank and the radiator are interface errors. They are not visible in the ideal turns ratio. The paid component modules open each device. This module asks you to write the interface down: voltage, current, cooling stage, and who may operate the tap changer.
- A bushing has a voltage class, a current, a creepage duty, and a technology. One of those missing is an incomplete line.
- Say whether taps are on load or off circuit, on which winding, and at which end.
- Read the cooling code as four decisions. Do not treat a forced stage as the self-cooled rating.
- IEC and IEEE letter codes are not a translation of each other. Use the code in the contract.
Cutaways, condenser grading, and diverter operation stay on the paid plan.
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