what is neutral grounding? why is it required? what is
neutral grounding resistor and how is its value calculated??
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Answer Posted / pankaj rajput
Explanations given by Mahmud are very well in line with
your query. I am just adding something more...
Due to line capacitance (between phases & earth),
intermittent arcing may take place during single phase to
ground fault in an Isolated Neutral system. This phenomenon
is called Arcing Grounds and should be avoided to have
better equipment safety & power quality.
SQRT 3 times of normal capacitance currents of healthy
phases flows between the fault location & ground. This
current is oscillatory in nature and creates arc. Also the
voltage of healthy phases will become line voltage because
the faulty phase is connected to ground.
Hence system star point should be grounded.
- Henereunder is one more clarification to Mahmud's
explanation...
Impedance grounding is applicable only to MV systems. LV &
HV system star points are generally solidly grounded i.e.
grounding without impedance.
The reason of solid grounding in LV system is that the
amount of earth fault current is very less e.g. for a
single phase to earth fault in a 415V 3 phase system, the
earth fault current is only 240A (considering earthing
resistance 1 Ohm). Whereas in 11kV system, the amount of
fault current will be 6351A. Hence resistance/impedance is
added to reduce the current to a safe value e.g. a 1250 Ohm
resistance in 11kV system limits the fault current to 5A.
The reason of solid grounding in HV/EHV systems can be
explained as below:
- Usually these are medium & long transmission lines. Hence
the line capacitance is considerably high.
- The capacitance current of the healthy phases flows in
reverse direction to earth fault current (predominantly
inductive current) fed by the faulty phase. Therefore it
nullifies/reduces the flow of earth fault current. Hence no
impedance is required to limit the earth fault current.
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