Resonant grounding or arc suppression coil grounding
In the arc suppression coil method of grounding,the arcing-ground danger has been eliminated and the system is approximated to the isolated neutral system,in which one or two healthy phases continue to supply power and complete shutdown on the system is avoided till the fault was located and isolated.
This type of method operates on the principle that if an inductance of appropriate value is connected in parallel with the capacitance,the fault current can be reduced considerably or even it can be neutralized.Thus the magnitude of inductance of the coil depends upon the capacitive currents flowing into the ground capacitance.
An arc suppression coil is also known as peterson coil or Ground fault neutralizer,is an iron core tapped reactor connected in neutral to ground connection. The reactor is provided with tappings so that it can be tuned to the system capaciatnce. The function of arc suppression coil is to make the arcing ground faults self extinguishing and in case of sustained faults,to reduce ground fault current to a comparatively low value so that the system can be kept in operation with one line grounded.
On occurrence of a ground fault(say on phase B),a lagging reactive current flows from the faulted phase to the ground and returns to the system through the inductive coil.Similarly,capacitive currents also flow from healthy phases to ground.The lagging fault current IF and leading capacitive current Ic(phasor sum of ICR and ICY) are almost in phase opposition.By choosing the appropriate value of inductance L of the arc suppression coil the two currents can be made almost equal so that there is no current through the ground fault and so there will be no arc.The combination of neutral reactance L and line capacitance C acts as a parallel resonant circuit.
If Vp is the line-to-neutral voltage,
For balance condition,
Features of arc-suppression coil grounding
- During a single line-to-ground fault,the voltage across healthy phases rise to line-to-line value.
- Continuity of supply can be maintained for long periods with one line grounded till the faulty section is isolated.
- During sustained ground fault,the current is a small fraction of capacitive ground fault current.Burning and heating effects are reduced to minimum and damage to the equipment is limited.
- Transient ground faults are suppressed.More than two-third of such faults are cleared without necessitating the operation of the circuit breaker.
- Voltage gradient on ground surface in the vicinity of fault is greatly reduced and so the danger to the life in the proximity of fault.
- Inductive interference to neighboring communication circuits is lesser in magnitude but duration may be long.
- Relaying needs special provision and is usually restricted to indication.
- During normal operation,the arc suppression coil has little effect on balance of system voltages to ground and losses occurring in it are negligible.
- Arc suppression coil grounding reduces the line outages from ground faults to 20 to 30 per cent of those obtainable with other types of grounding.
- This method is mostly used in radial lines as this avoids the construction of duplicate circuits for maintaining service contiunity and maintain overall economy.
This method of neutral grounding is usually confined to medium voltage overhead transmission lines which are connected to the system generating source through power transformers.
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