Neutral earth connections:
Under normal conditions the LV neutral is securely connected to earth via a test link. There are stipulations regarding LV N→E links, mainly that they should be <20Ω. A separation of 8m from the MV earth is required. If however <1Ω is achieved a link between the MV and LV earths is fitted.
I’ve had UKPN try to pull a fast one with this. I asked about the <1Ω rule and was told don’t worry about that, then the penny dropped, they wanted our earth nest connected to their MV system.
For OH systems the same 8m rule applies.
This shows the reason for the separation of the electrodes. It reduces the possibility of crossover of a MV fault in to the LV system.
As sometimes happens in rural areas the feed to a house is direct from the OH transformer with no intermediate pole neutral/earth rod, therefore a PNB system is used. Neutral is earthed 8m away from the transformer pole but the connection is at the customers installation.
A friend from Scotland sent me this picture.
The earth is as big as the concentric cable, apparently it is a quite common practice in the wilds of Scotland.
There are a few exceptions to solid bonding.
For LV with a delta secondary if it requires earth reference can be connected to a center earthed high impedance zigzag transformer.
The top two drawings are for LV secondary, the center point of the zigzag is taken to either a choke or metal plate resistors to regulate the fault current.
The bottom drawing is for a HV secondary where the fault current is regulated by internal chokes.
For mining transformers earth fault current has to be limited to a maximum of 5A so a choke is used. All loads on these transformers are phase to phase with no neutral. As you can see these transformers have a 3.3KV OCB (VCB now) built in to the unit. As there is no neutral earth fault detection is by a CT and relay on the star point tap. This relay trips the 3.3kV there being no switchgear on the LV side.
Somehow the quarry had acquired two of these units, I was told “get them working, we're opening a mine”.
For HV the earth fault current needs to be limited. For a domestic supply you may be dealing with a couple of KA as the maximum. With a 11/3.3KV transformer the secondary output voltage to earth is 1905V, the earth fault current shoots way up in the KA range. If you consider most systems use the armored cable as the CPC (I’m not starting that argument again). I don’t know about a CPC, it’s going to be more like a fuse. The current is usually limited to between 500 to 1000A. Limited to 500A it’s still 952KW of heat going somewhere.
This is for a 33/11KV 20MVA Dyn11 transformer.
The star point is usually brought out to an external insulated terminal. From there to the N/E test switch followed by the protection C/T’s and finally the Neutral Earth resistor (NER). This resistor has to be able to absorb most of the heat generated until the fault is cleared by remote substations local E/F protection. If the fault persists the transformers own protection steps in and cuts the incoming supply. The NER can either be metal grids or an electrolyte filled tank with a central electrode.
The protection relays are for Restricted Earth Fault (R.E.F.), this detects an earth fault in the transformer secondary and tails only. The inverse EF. has the same function as an S type RCD. O/L protects the transformer O/P from believe it of not, overload.
These need a certain amount of maintenance such as topping up the electrolyte level with distilled water. I found one where the connection to the electrode had rotted through, the connection also supports the electrode. It had fallen off and was sat in the bottom of the tank! Now that’s what I call a floating neutral fault!
We had four 33/11KV 20MVA and four 11/3.3KV 4MVA transformers with liquid NER’s at the foundry. Just to give an idea of the size of these things.
Metal grid resistors are also used.
© Tony S