Resonant grounding is also known as arc suppression coil grounding as it uses an arc suppression coil in the neutral earthing circuit i.e., the neutral of the system is connected to the ground through an adjustable iron cored reactor known as arc suppression coil. It is a special case of reactance grounding. A three-phase system with resonant grounding along with an earth fault at point F in phase R is shown in the below figure.
When the arc suppression coil with the appropriate value of inductance L of the coil is connected in parallel with the capacitance of the system, then the fault current IF flows through the coil, and the capacitive current IC between un-faulted phases and earth will be in phase opposition with each other.
If this inductance L is adjusted such that the fault current IF flowing through the coil and the capacitive current of the system becomes equal, then the resultant current will be zero and arcing grounds will be extinguished.
Therefore the reactance of the arc suppression coil is adjusted in such a way that resonance occurs. Hence, the grounding is referred to as resonance grounding. In resonant grounding, during an earth fault, the arcing grounds are self-extinguished with the help of an arc suppression coil, and also in the case of sustained faults, the continuity of supply to the healthy phases is not interrupted.
The arc suppression coil is also known as the Peterson coil or ground-fault neutralizer. Tappings are provided to the arc suppression coil in order to tap the coil for a suitable value of reactance depending upon the length of the transmission line and the capacitance to be neutralized.
The reactance of the coil is calculated from the expression,XL = XC/3 ΩThe induction of the coil is given by,
Where,- L = Inductance of the coil
- XL = Inductive reactance of the coil
- XC = Capacitive reactance of the system
- C = Line to earth capacitance
- f = Frequency.
The coil is rated at maximum earth fault current and even if the earth fault current exceeds the rated value of the coil then the circuit breaker (CB) shown in the figure which is normally open, will close after a certain time lag because of which the fault current will flow through the parallel circuit bypassing the arc suppression coil and the coil will be protected.
This type of grounding method is generally used in transmission lines operating at medium voltage levels connected to the generating station through intervening power transformers. The resonant grounding reduces the supply interruption due to transient line-to-ground faults and chances of the developing line to ground fault into two or three-phase fault are also reduced.