Updated: Jan 19
In a resistance grounding the neutral point of supply system is connected to the ground through resistor. Adding resistance in between neutral and ground limits the fault current. The resistor connected between neutral and ground is known as neutral grounding resistor (NGR). The following are the advantages of resistance grounding.
The resistance grounding limits the fault current
It improves the system stability
It minimize the possibility of arcing ground.
The selection of resistance is very important for reliable grounding system. If the resistance value is selected low, then very high current will flow and it is just like solid grounding. If the resistance value is high, the limited fault current flow to the earth and system acts like a ungrounded system. The sufficient current must flow through the resistance to safe value and to detect the fault current for onward isolation of faulty section.
The resistance grounding is used where it becomes necessary to limit the fault current to safeguard the equipment against flow of large current above its rated full load current, The earth pit resistance is in order of 1 ohm to 1.5 ohms. If the secondary of 132/6.6 , FLC secondary 1400 A transformer is solidly grounded the earth fault current will be
= Neutral to Phase Voltage/ Resistance of earth pit
=(0.58 X 6.6 )/1
= 3.28 Kilo-Amps
= 3280 Amp.
This mammoth current flows through the equipment and can damage the equipment. In this case resistance of suitable value needs to be added in the neutral circuit to limit the fault current up to equipment maximum current rating. Now if we add 2 ohms in the neutral circuit, the earth fault current will be;
= (0.58 x 6.6)/(2+1)
= 1276 Amps
Thus, by adding resistance in the neutral circuit, it is possible to limit the fault current flowing through the equipment. The protection circuit is used to trip the upstream breaker to isolate the fault.
For LT supply 400 volts, solid earthing is used because the earth fault current magnitude is not so high which can damage the electrical equipment. The phase to neutral voltage of 44o volts system is 0.58 x 400 =230 Volts, the fault current is;
= 230/1 = 230 Ampers
Therefore, in 400 volts supply system, solidly grounding system can be used.
In a reactance grounding, reactance is connected between neutral and ground to limit the fault current.
The selection of the reactance is important for effective grounding. If reactance is very small it acts as a solid or effective grounding and if reactance is very large it acts as a isolated system.
In a solid grounding system the system reactance and the resistance both limits the fault current. In reactance grounding , the additional reactance added to the circuit nullifies the capacitive charging current. The reactance grounding is used for long transmission line or cable system. The current through the reactance during fault should be limited to 25% of 3 phase fault current in order to minimize transient overvoltages.