What is Peterson Coil? - Its Function in Neutral Grounding

The Peterson coil is an adjustable iron-cored reactor connected between the neutral point and ground. It has the capability of producing resonance by turning the capacitance of the healthy phases during a line to ground fault. It is also known as an arc suppression coil or ground-fault neutralizer. The Peterson coil prevents the arcing grounds which leads to overvoltage on systems with an underground neutral.

During an earth fault, the arcing grounds are self-extinguished by the Peterson coil, and also in the case of sustained faults, it reduces the earth current to a low value so that the continuity of supply to the healthy phases is not interrupted.

Tappings are provided to tap the coil for a suitable value of inductive reactance depending upon the length of the transmission line and the capacitance to be neutralized.

Reactance of Peterson Coil :

The connection of the Peterson coil in resonant grounding for a 3-phase system is shown in the below figure.

Let the earth fault has occurred at point F in phase R. The fault current I_F starts to flow from point F to the ground and then to neutral N through the Peterson coil and back to the faulty phase. Fault current I_F flowing through the Peterson coil is given by,

I_F = V_ph/X_L

• V_ph = Phase voltage
• X_L = Inductive reactance of the peterson coil.

The line-to-earth capacitance of the healthy phases, phase Y and phase B is C_Y and C_B respectively. The voltage across the line to earth in a resonant grounded system during an earth fault is increased by √3 times because the system behaves as an ungrounded system.

Hence, if I is the charging current of phase to earth, then it becomes √3I per phase during an earth fault. The charging currents of the healthy phases are displaced by 120°. Therefore the resultant charging current is √3 times that of the charging current per phase,

I_C = √3.√3I = 3I

Where, the charging current per phase,

I = V_ph/X_C

• V_ph = Phase voltage
• X_C = Capacitive reactance of line to earth.

Therefore,

I_C = 3I

= 3(V_ph/X_C)

But in resonant grounding, for neutralization of arcing ground, the value of inductance L of Peterson coil is adjusted such that the fault current I_F becomes equal to the resultant charging current I_C.

The inductance L of the Peterson coil is given by,