Where we go for NGR for grounding? LV/MV/HV or depends on Fault level?
NGR = Neutral Ground/Earth Resistor, connected between neutral and earth. Can you be more specific where do you want to use it, the application and your voltage level? And what do you want to achieve with the NGR? Limitation of the earth-fault current? Reduced touch and step potentials?
Normally on transformers:
LV - under 1000V - solidly earthed
MV - between 1000V and ?33000V? - depending on the application, but can be solidly earthed, earthed through resistors or impedances.
HV - above 33000 - Solidly earthed
How we choose NGR?
What do you want to achieve with the NGR? Limitation of the earth-fault current? Reduced touch and step potentials? Depending on what you want to achieve you have to decide to which level you want to reduce the earth current, and depending on that you'll choose the size of the resistors. The resistors have also to be rated to carry that current for a certain time.
My application is to limit the earth fault current. Actually my question is that why not we use NGR in LV or HV, why only in MV? Is there any Specific reason to select NGR/Solid earthing?
Advantages of solid earthing:
Neutral held effectively at earth potential.
Phase-to-ground faults of same magnitude as phase-to-phase faults so no need for special relays
Cost of current limiting device eliminated.
Size and cost of transformers reduced by grading towards neutral point.
Lightning arresters need not exceed 80% of line voltage. Disadvantages of solid earthing:
Most system faults are phase-to-ground, severe shocks are more considerable than with resistance earthing.
Third harmonics tends to circulate between neutrals.
High degree of damage at fault point and possible damage to feeder equipment.
High step and touch potentials.
Low initial cost but higher long-term equipment repair cost
Advantages of resistance earthing:
Limits electrical and mechanical stresses on system when an earth fault occurs.
Lesser degree of damage at fault-point and usually no damage to feeder equipment.
Reduced step and touch potentials. Disadvantages of resistance earthing:
Higher initial costs.
Full line-to-line insulation required between phase and earth.
Lightning arresters rated at least for 100% line voltage for 100seconds
Higher initial cost, but lower long-term equipment repair cost.
HV-systems is solidly earthed, because the size and cost of transformers is reduced by grading insulation towards neutral point.
Thus: The bigger the voltage, the more insulation you have to use. More insulation needs more space and means more cost. If you degrade the insulation towards the neutral, you save on insulation, but then you have to use solid earthing.
MV - depending on what you want to achieve.
LV is solidly earthed to ensure that the likeihood of the presence of any voltage above normal appearing in the LV-circuit is reduced to a minimum.
