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 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 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.
What is Peterson Coil?
Let the earth fault has occurred at point F in phase R. The fault current IF 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 IF flowing through the Peterson coil is given by,
IF = Vph/XL
  • Vph = Phase voltage
  • XL = Inductive reactance of the peterson coil.

The line-to-earth capacitance of the healthy phases, phase Y and phase B is CY and CB 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,

IC = √3.√3I = 3I
Where, the charging current per phase,
I = Vph/XC
  • Vph = Phase voltage
  • XC = Capacitive reactance of line to earth.
Therefore,
IC = 3I
= 3(Vph/XC)
But in resonant grounding, for neutralization of arcing ground, the value of inductance L of Peterson coil is adjusted such that the fault current IF becomes equal to the resultant charging current IC.
What is Peterson Coil?
The inductance L of the Peterson coil is given by,
What is Peterson Coil?

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