NGR is a resistor used in the earthing of the system to reduce the magnitude of the fault current.
In addition to limiting the fault current, it ensures the protection of healthy phases against overvoltage conditions, helps in the efficient operation of protective devices and protects the power conductors against thermal disturbances caused by faults.
Therefore, the design and selection of NGR is very important to ensure the safety of equipment and personnel as well as the continuity of supply.
Before discussing the sizing of NGR, let us try to understand the basic nature of NGR and why it is necessary.
As the name suggests, NGR or Neutral Grounding Resistor is a resistor connected between neutral and ground to limit the fault current of the system.
Anyone with a basic knowledge of physics will understand that a higher resistance value leads to lower current levels in each circuit.
Isn’t it better not to connect NGR? That is, an ungrounded system, so that theoretically we have an infinite resistance (in practice very high impedance) between neutral and ground.
That is correct – by not connecting the NGR, the fault current is reduced to the lowest possible value in an ungrounded system, but this introduces a new problem, namely overvoltage.
Let us consider an ungrounded system for analysis.
Although there is no connection between the neutral and the ground, as shown in the figure above, the Ungrounded system can be considered as a system connected to the ground through its natural capacitance.
The insulator (various electrical equipment such as surge capacitors, cables, motors, etc.) acts like a dielectric material between two different voltage levels, namely the system voltage and the ground reference.
Hence, each system has its own natural capacitance due to its specific equipment. And if a system is Ungrounded, it can be understood as ground through the same capacitance.
In a healthy and balanced system, each three phase voltage is equal and 120 degrees apart and the neutral voltage is zero. Since the three leakage currents are capacitive in nature, they are in quadrant i.e. at 90 degrees and reach their respective voltages. These currents are also called capacitive charging currents. The equivalent system, its phasor diagram and the flow of currents can be seen from the figure below.
Since the capacitance in phase A is shorted due to the earth fault, the current Ia now flows through the earth path and is equal to the vector sum of the currents Ib and Ic. These currents, which are capacitive in nature, are still quadratic in their respective voltages, but their magnitude is now 1.732 times their nominal value (due to the increase in their terminal voltage).
The equivalent system, its phasor diagram and the flow of currents can be seen from the figure below. The voltage and current phasors of the faulty system are superimposed on the phasors of the healthy system (the faded phasors) for comparative analysis.
Unless the ground fault is removed at zero voltage, a certain DC offset voltage remains on the neutral. Since there is no discharge path, this offset voltage must remain on the neutral.
Stay with us to continue the article.
To connect with our experts, contact us at Contact Us
To view projects, visit the news section of the site News
