TNC

First a caution - HV distribution is not my thing. But I have had a fair amount to do with gensets, large UPS and so on in the fairly recent past. Take what follows with a pinch of salt, it may contain errors.


I'd expect at the least an HV  earth electrode for the transformer core and chassis,  though that may well not connect to anything on the secondary side. That is to trip off the HV on a primary winding to core or casing sort of earth fault. (at the other end of the HV line, the effect of a jumbo RCD is done by looking at current transformers and balance, or if the supply is small, or old, then perhaps just expulsion fuses.

 

I'd also expect some part of the secondary side to be earthed, if not at the transformer then in an adjacent building. If this is not done the secondary voltage may well be the intended 230 or whatever between the wires,  but with both sides floating off at some fraction of the HT voltage relative to ground, via transformer inter-winding capacitance and perhaps insulation leakage, and you would never know until it was too late.

IT does not work in that sort of case without the equivalent of an earthed screen between the primary and secondary.


The LV earth may well have to be some distance from the HV one, if there are concerns about voltage rise (high HV side PSSC, and poor ground conductivity perhaps)

Un armoured transfomer tails are a risk if they are vulnerable to strke, but if the route is made very clear, or mechanically well protected, maybe not an immediate concern.


It is sometimes done that what is in effect an earth fault relay on the LV side is used to operate a breaker on the HV side. Others will be better placed to comment on the detail.


If adding earth fault protection, until you can measure the leakage in normal use, it is common to start sizing for 0.1% up to about 0.5% of the main current, perhaps  with twiddle options for a factor of 3 either way after commissioning. (which sort of figures with a 30mA RCD on a 30A final circuit,  compared  to  300mA  at the origin of the 100A per phase on a modest farm.)


How big is the transformer and what sort of vintage ? Funnier things were done in the dark ages.

First a caution - HV distribution is not my thing. But I have had a fair amount to do with gensets, large UPS and so on in the fairly recent past. Take what follows with a pinch of salt, it may contain errors.


I'd expect at the least an HV  earth electrode for the transformer core and chassis,  though that may well not connect to anything on the secondary side. That is to trip off the HV on a primary winding to core or casing sort of earth fault. (at the other end of the HV line, the effect of a jumbo RCD is done by looking at current transformers and balance, or if the supply is small, or old, then perhaps just expulsion fuses.

 

I'd also expect some part of the secondary side to be earthed, if not at the transformer then in an adjacent building. If this is not done the secondary voltage may well be the intended 230 or whatever between the wires,  but with both sides floating off at some fraction of the HT voltage relative to ground, via transformer inter-winding capacitance and perhaps insulation leakage, and you would never know until it was too late.

IT does not work in that sort of case without the equivalent of an earthed screen between the primary and secondary.


The LV earth may well have to be some distance from the HV one, if there are concerns about voltage rise (high HV side PSSC, and poor ground conductivity perhaps)

Un armoured transfomer tails are a risk if they are vulnerable to strke, but if the route is made very clear, or mechanically well protected, maybe not an immediate concern.


It is sometimes done that what is in effect an earth fault relay on the LV side is used to operate a breaker on the HV side. Others will be better placed to comment on the detail.


If adding earth fault protection, until you can measure the leakage in normal use, it is common to start sizing for 0.1% up to about 0.5% of the main current, perhaps  with twiddle options for a factor of 3 either way after commissioning. (which sort of figures with a 30mA RCD on a 30A final circuit,  compared  to  300mA  at the origin of the 100A per phase on a modest farm.)


How big is the transformer and what sort of vintage ? Funnier things were done in the dark ages.