Parallel NER’s (Neutral Earth Resistors)
Like generators, transformers can be run in parallel but it’s not good practice. At times needs must and once again prospective fault currents become a problem. Ignoring line to line faults for now, line to earth can be controlled. As shown in neutral earth connections #1 the NER is effective at limiting current for a single (islanded) unit.
Close the secondary bus-section switch you not only parallel the transformer phase outputs but also the NER’s, reducing the total fault limiting resistance by a half and therefore doubling the prospective fault current. See where this is going? bigger and better bangs if things go wrong.
A system working normally at a primary substation will have the 11kV bus-section open and each NER looking after its own transformer and outgoing network.
In transformer overloading I mentioned the limits imposed on DNO transformers depending on cyclic loading. Sometimes a transformer has to be taken out of service for maintenance and so its partner has to take up the load.
In exceptional circumstances transformers will be run in parallel. One NER is switched out of service keeping the prospective earth fault currents within design levels. (I’ve seen a 11kV joint fail to earth and I’m glad I wasn’t near it.)