Reactance Grounding - Operation, Advantages & Disadvantages

The system in which the neutral point is grounded through impedance which is highly reactive is known as a reactance grounded system and the grounding is referred to as reactance grounding.

Necessity of Reactance Grounding :

If solid grounding is employed in the transmission lines whose operating voltage is 3.3 kV to 33 kV, then the fault currents will be high. In order to overcome this problem, either resistance grounding or reactance grounding is used.

Reactance grounding is used for circuits where the charging currents are high because by reactance grounding additional reactance is provided to the system, thereby increasing the lagging current due to which the capacitive currents are neutralized.

Reactance grounding lies between effective grounding and resonant grounding. In reactance grounding, the value of reactance is selected such that it must keep currents within the safe limit i.e., by reactance grounding the fault current is limited.

It also provides necessary phase opposition between capacitive ground current and the fault current. Whereas in the case of resistance grounding only fault current is limited without providing any phase opposition.

Depending upon the ratio of zero sequence reactance X₀ to positive sequence reactance X₁ the system is said to be solidly grounded or reactance grounded. If the ratio (X₀/X₁) exceeds three, the system is said to be a reactance grounded system and if this ratio is less than three, then the system is said to be a solidly grounded system. Even if the neutral of the system is effectively grounded and the ratio exceeds three, then the system is supposed to be a reactance grounded system rather than the solidly grounded system.

Reactance grounding is used for the lightly loaded transmission lines, underground cables, synchronous condensers, and circuits that have high charging currents. Reactance grounding ensures satisfactory relay operation and partial grading of equipment insulation. Also, the interference with communication circuits is reduced in reactance grounding when compared to that of solid grounding.

Reactance Grounding :

A 3-phase system provided with reactance grounding is shown in the below figure. In reactance grounding, the neutral N of the system is grounded through a reactance. Thus additional reactance is provided to the system, thereby increasing the lagging current due to which the capacitive ground currents are neutralized.

Here the fault current is dependent upon the reactance. Thus varying the reactance in the neutral to earth line, the magnitude of the fault current can be varied.

Advantages of Reactance Grounding :

• Reactance grounding provides satisfactory relay operation.
• In reactance grounding, arcing grounds are avoided.
• By using reactance grounding the transient ground faults will be converted into controlled current faults.
• If the system is reactance grounded, then the partial grading of the apparatus insulation is required.
• When compared to the solidly grounded system, the interference with communication circuits is reduced in the reactance grounded system.

Disadvantages of Reactance Grounding :

• In reactance grounding, the relaying device needs more fault current to operate compared to the fault current required in a resistance grounded system.
• In reactance grounding under fault conditions, very high transient voltages appear.

Due to these disadvantages, reactance grounding is not in common use.