Not completely understanding everything there but find myself slightly aroused please tell meant that only one? Something about a lady knowing thier stuff seems to do it, no idea why
With power transformers, one test done is to short out the secondary, and apply voltage on the primary, increasing it until you get to 100% full load current.
If this occurs at 10% of the rated voltage, then the transformer is said to have a "voltage impedance " of 10%
This actually means that if you put 100% voltage on a shorted transformer, you would get 100% Volts/10% impedance = 10x full load current.
10% = 10x
8% = 12.5x
5% = 20x
4.75% = 21x
And so on.
This means the maximum fault level through a transformer is limited predominantly by its own impedance - look at the rating of the transformer and you know the maximum fault level that could pass through the transformer.
In practice the level will be slightly lower than this due to cables, upstream transformers etc - but is a really good estimate from just the kVA and impedance.
So a 1MVA 11kv tx with 8% would be a maximum of:
52.5A (flc @ 11kV) x 12.5x = 656A @ 11kV
1445A (flc @ 400V) x 12.5 = 18kA @ 400V
In practice, much less than this, especially further from the transformer.
If you measure more current than this, the fault cannot be downstream of the transformer, so won't be cleared by any secondary side protection. It must be internal to the transformer, and you can operate the primary side protection instantly.
Generally distribution transformers have very simple protection - just 11kV fuses which like LV fuses have a single characteristic curve, but also have a small explosive charge, this operates a pin which is fired out of the fuse which trips all phases - I doubt it is this in this case as you don't generally overreach.
Time limit fuses are an older type of protection, which again I doubt it is this in this case as you don't generally overreach.
The most likely protection in this case would be an IDMT or IDMTL (InDependent Minimum Time [Lag]) relay with an additional instantaneous setting.
This type of protection has a characteristic curve (similar to a MCB) but which can be "moved around" in time (faster or slower), and in current level (higher or lower current) - like some MCCBs with electronic protection, but also you can change the actual shape of the curve itself (~4-5 selectable curve types).
The instantaneous protection is basically what it sounds like, another setting, independent of the IDMTL part where you can set a current value (above the maximum through fault level described above) where if it sees this it operates instantly. (You can also get time delay functions on this, but you wouldn't use them in this application)
For larger transformers or more important ones, there are other protection options available, I could bore anyone to death with the possibilities, but it's too long a post already, and you are probably already asleep!