Solid Grounding or Effective Grounding - Working & Advantages

When the neutral of the system is directly connected to the ground with negligible resistance and reactance between them, then the grounding system is known as solid grounding or effective grounding.

A system is said to be solidly grounded when for all points of the system, the ratio of zero sequence reactance X₀ to positive sequence reactance X₁ does not exceed 3 and the ratio of zero sequence resistance to positive sequence resistance does not exceed 1 under all conditions and for any amount of generator capacity.

Consider a 3-phase power system with neutral solidly grounded and an earth fault on phase B as shown in the above figure. On the occurrence of earth fault over any phase, the phase to earth voltage of the phase on which the earth fault occurs becomes zero i.e., the potential of the neutral and phase B will be at earth potential. While the phase to ground voltages of healthy phases remains unchanged.

It will be seen that the fault current I_F will completely neutralize the capacitive current I_CF because I_F is in phase opposition to I_CF. In a solidly grounded system, the voltages of healthy phases under earth fault conditions will not exceed 80% of the line to line voltage. The phasor diagram is shown below.

For a fault, on phase B voltages V_RE and V_YE have still maintained at their (pre-fault) values. The capacitive currents flowing in the healthy phases are I_CR and I_CY will lead their corresponding voltages by 90°. Earth fault current I_F is a 90° lagging current (due to predominantly inductive impedance) with respect to phase to neutral voltage of faulty phase. The total capacitive current I_CF is a resultant of I_CR and I_CY.

Advantages of Solid Grounding :

• In solid grounding, the neutral is effectively held at earth potential.
• We have seen that the phase-to-earth voltage of the phase with earth fault becomes zero and it remains normal in the case of healthy phases. Hence, the equipment is to be insulated for phase voltage, which results in a saving in the cost of equipment.
• Due to phase opposition of resultant capacitive current I_CF with the fault current I_F. These two currents will nullify each other completely. Hence in solid grounding, the arcing ground phenomenon and overvoltage conditions cannot occur.
• When compared to other types of grounded systems, solidly grounded systems are less expensive for all operating voltages as the voltages of healthy phases in case of line to a ground fault do not exceed 80% of the line to line voltage, whereas in the case of other groundings the voltage of healthy phases rises to about 100% of line to line voltage.
• The solid grounding system permits the use of discriminative protective gears due to the availability of a large fault current between the fault point and grounded neutral.
• In solid grounding, the ground fault relaying is simple and satisfactory.

Disadvantages of Solid Grounding :

• In solid grounding, there will be high fault currents which may cause the system to become unstable.
• There may be a burning of circuit breaker contacts due to high fault currents.
• Interference in the neighboring communication lines will be greater due to high fault currents.