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Volume 11 puts the transformer in a network: relays, trip paths, and the settings that have to ignore magnetising inrush and still see a real fault. This module names the jobs. It does not set a relay, and it does not pick a curve. Settings are a later, paid module, and they are only as good as the current transformer locations and the tank devices the scheme assumes are there.
Electrical protection and the tank devices
Differential protection compares current into the transformer with current out of it. An internal fault produces a difference. A through-fault, ideally, does not. Overcurrent protection sees magnitude. It is simpler and it is slower to tell an internal fault from a heavy through-load or a downstream fault. Restricted earth fault looks at residual current on one winding. These are different zones. A scheme that says only “full protection” has not said which zones exist.
On the tank, a gas-operated relay in the pipe to the conservator responds to gas accumulation and to a sudden oil surge. A pressure-relief device responds to a sudden rise of pressure in the tank. A winding-temperature image and an oil-temperature indicator start fans and, at a further step, alarm or trip. They are not substitutes for the electrical relays. A shut valve between the tank and the conservator leaves the gas-operated relay blind while every other check can still look normal. That valve position is part of the protection, not a maintenance detail.
Inrush is a magnetising event
When a core is energised, the flux may not start from zero. For a cycle or more the core can saturate and the magnetising current can be many times the rated current, rich in second harmonic, and larger on one phase than another. Differential protection that treats every current difference as a fault will trip on energisation. Restrain it with a harmonic check or an equivalent method the relay actually implements. Do not “solve” inrush by raising the differential setting until a real fault is also ignored. The size of inrush depends on the point on the wave, the remanent flux, and the source impedance. It is not a single number you can copy from a similar rating.
- Say which zones exist: differential, overcurrent, restricted earth fault, and the tank devices.
- A gas-operated relay needs an open pipe to the conservator. A shut valve removes it.
- Inrush is not an internal fault. The restraint has to ignore it without blinding the relay.
- Do not copy a setting from another rating. Source impedance and remanence move the current.
Worked settings and coordination with the network stay on the paid plan.
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