Resistance VS Reactor Grounding 1

[unclebob]

Hi all! Most of the time, we use a resistance grounding on transformers to limit the line to ground fault current.
But yesterday, I have tested an oil transformer that was using a reactor on the X0. In my short experience in power
systems, I have seen many generators 13,8 kV grounded via a reactor but not a transformer. That xfo is a 50 Mva, 120 to 2.3 kV. Why using one type of grounding over the other?

Thank!

 

[RalphChristie]

It is either a resonant grounded system or low inductance grounded system.

Resonant grounding (Petersen Coil or Arc suppression coil)
It is a tuneable reactor connected in the transformer neutral to earth. The value of the reactance is chosen such that reactance current neutralises capacitance current. The current at the fault point is theoretical nil and unable to maintain the arc, hence the name. Normally used on systems above 15kV.

Low inductance grounding.
Normally reactance grounded systems is rarely used due to problems with overvoltages under arcing earth fault conditions, and the fact that the desired level of fault current higher is than the same size resistance grounded system. Therefore reactance grounding is usually not considered an alternative to resistance grounding.
However, to achieve the same value as the resistor, the design of a reactor is smaller and thus cheaper - maybe a possible option to use it.

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[jtkirb]

reactors are less expensive, take up less room, and give off less heat...in most cases.

There are other engineering uses, but as always, price is a major factor.

JTK