Neutral Earthing Resistors (NERs) are employed in AC distribution networks to limit the current that would flow through the neutral star point of a transformer or generator in the event of an earth fault. The rating of the NER is chosen so that the fault current is limited to that necessary to operate the protection relays within the required time.
Please note that, the resistance value is set at the manufacturing stage and remains constant throughout the life of the NER. This value is unaffected by changes in ambient temperature, unlike the water resistance.
IEEE 32 allows certain tolerance of resistance due to variation of ambient parameters. It's never possible for a resistance to remain constant in the event of change of ambient temperature. NER's normally are punch grid type or formed SS.
These material resistance value too go change w.r.t temperature.
Now question is why we use NER? and what to earth fault current we should restrict by NER? Any one have any standard? I know some engineering practices, some reference documents where some recommendations are available. But no standard as of yet. Its also difficult for a standard to ascertain, since every equipment have their own E/F current withstand capability.
Neutral Earthing Resistors (NERs) are employed in AC distribution networks to limit the current that would flow through the neutral star point of a transformer or generator in the event of an earth fault.
The following parameters are needed to specify the earthing resistor:
U : Rated voltage line to line voltage or system voltage (kV)
If : Rated fault current in A; the limit of earth fault current differs from Utility/Authority to another, subject to the network parameters, insulation levels of the equipment and the protection system.
Ts : Rated time (Sec) - Duration of the fault
Resistance value:
R = U/If * √3
When the flow of an electric current through a resistor is relatively short, dissipation is negligible and the heating temperature of that resistor depends on its capacity to store the electric energy (i.e. its heat value itself) in proportion to the mass and specific heat of the material used.
' If : Rated fault current in A; the limit of earth fault current differs from Utility/Authority to another, subject to the network parameters, insulation levels of the equipment and the protection system.'
You said the above, what's the guide line? who gives the fault current value, what are the basis and how they established the basis? All comes in as an array of Q.
It's a practice, some uses 500A, some 300 200 etc. all are applicable, and you have to check the sensitivity of protection system, relay range etc and how the protection system is designed.
I believe I made myself clear. Again, each Utility/Electrical Authority has its own research and studying department. This department is responsible for studying the performance of the current network, analyzing the faults & identifying the causes, planning for future addition/upgrading of existing network, specifying the short circuit levels, insulation levels of the different equipment, etc. Accordingly, the Authority regulations and specifications should be followed.
NER
Re: NER
