NGRs or Neutral Grounding Resistors are used to limit the fault current in a generator or a transformer during earth faults.
In star connected 3 phase equipment such as a generator or a transformer, the star point is grounded. In systems where the star point is directly grounded, known as solid earthing, there is a chance of heavy currents in the windings during an earth fault as the net resistance is only the soil resistance.
This heavy current, in the order of hundreds of amperes, can damage the windings. Hence, a series resistance is introduced in the star point. This increases the net resistance in the event of an earth fault and limits the current. This resistor is known as the Neutral Grounding Resistor(NGR).
The current flowing in the Neutral Grounding resistor can be monitored. This can be used to activate the Earth Fault Relay. It is generally mounted with a Current Transformer.
NGRs can be broadly classified into two types.
Low Resistance Grounding, which limits current to more than 100A and
High Resistance Grounding, which limits current to generally 5 to 10 amperes.
The Choice of the rating of the NGR is made taking into account different factors such as the rating of the machine, overall fault level of the system and the system capacitance.
The rating of the machine should be kept in mind while deciding on the NGR value. Fault current should be limited to a value that can be safely handled by the machine. It is also essential that the fault feeds enough current which can be sensed by the earth fault protection. Too high a value of the NGR would the limit the current to a very low value which will not be able to activate the earth fault protection.
The system capacitance is another factor which should be kept in mind. A 3 phase star connected system forms capacitances with the ground. In the event of an earth fault, this capacitance can get charged with the line voltage and can cause transient overvoltages. The value NGR should be chosen in a manner which will permit a let-through current and enable the capacitances to discharge.
Disadvantages of Solidly Grounded Systems
Grounding Resistances - An introduction