Resistance Grounding - Working, Phasor Diagram & Advantages

The system in which the neutral point is grounded through a current limiting device (resistor) is known as a resistance grounded system and the grounding is referred to as resistance grounding. This method is used when it becomes necessary to limit the earth fault current.

The resistor used in this grounding may comprise either wire or water column resistance and the value of resistance used is generally higher than the system reactance so as to limit the power loss in the resistor due to earth fault.

The value of resistance required for grounding purposes increases with the increase in operating voltage in order to limit the short circuit current during an earth fault. The resistance grounded system with earth fault at point F in phase B is shown above and its phasor diagram is shown in the below figure.

Where,

• I_C = Capacitive current
• √3I_C = Capacitive current per phase
• 3I_C = Resultant capacitive fault current
• I_F = Fault current.

The current I_F, I_BR, and I_BY will be flowing through fault point F in phase B. The phase angle of the fault current will depend upon the impedance at the fault point. The capacitive currents I_BR and I_BY will lag the voltages V_BR and V_BY respectively by 90°. The fault current I_F is resolved into two components one is the reactive component I_X and the other is the resistive component I_R.

The resultant capacitive fault current will be in phase opposition with the reactive component at fault point F. This resultant capacitive fault current can be neutralized by adjusting the value of resistance R to a sufficiently low value. Resistance grounding is used for low voltage short-length overhead lines, as the charging currents in these lines are small.

If the earthing resistance is of low value (when I_rea is equal to I_C), the fault currents are high and the system condition approaches the solid earthing. On the other hand, if the earthing resistance is of higher value so that I_rea is less than I_C then the system conditions approach that of the ungrounded neutral system with the risk of transient overvoltages occurring.

The value of resistance is so chosen that at the time of an earth fault on any phase, a current equal to the full load current of the largest alternator or transformer feeding the system, flows in the earth connection. This will keep the overvoltages within limits which can be easily handled by the equipment and switch gears.

Advantages of Resistance Grounding :

• In resistance grounding, the arcing grounds can be minimized by adjusting the connected resistance to a suitable value.
• Resistance grounding improves the stability of the system, as the power dissipation in the grounding resistance reduces the accelerating power.
• Because of the presence of earthing resistance, the ground-fault current in a resistance grounded system is comparatively small with respect to the solidly grounded system. Hence, the inductive interference with the neighboring circuit is also reduced.
• In a resistance grounded system, the transient ground faults will be converted into controlled current faults.
• A low value of resistance in resistance grounding permits the use of discriminative types of protective gears.

Disadvantages of Resistance Grounding :

• During an earth fault, the system neutral will almost be invariably displaced, because of which there is a need for lightning arrestors, and also the insulation of the equipment for higher voltages is required resulting in an increase in the cost of the system.
• Resistance grounded system is more expensive than a solidly grounded system.
• In a resistance grounded system, there is a large amount of energy loss for the dissipation of fault energy.