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Tony S

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.

 

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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.

 

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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.)

 

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