Resistor vs. Reactor – Which Transformer Neutral Grounding Method Would You Choose?

Transformer neutral ground
Transformer neutral grounded through a reactor a.k.a. inductor.

Short-circuit faults involving earth produce high fault current magnitude especially when the transformer neutral ground circuit is solidly grounded. Why?

The neutral ground circuit in the transformer provides the return path for the fault currents. To limit this current, impedance – either in the form of a reactor or a resistor – is installed in the neutral circuit. See the figure below.

Ground Fault Current Flow
Ground Fault Current Path

The table below provides the reasoning for choosing between resistor or reactor as the impedance in the transformer neutral circuit.

Resistor in the transformer neutral ground circuit

Transformer neutral ground - Resistor

Image courtesy: Postglover

Reactor in the transformer neutral ground circuit

Transformer neutral ground - Reactor

Image courtesy: Trench

Pros

 
  1. It uses the resistance of the design material to limit the fault current.
  1. It is universally applied without any limitation. In other words, having a resistor in the neutral does not affect system dynamics.
  1. It uses inductive reactance produced by the alternating current to limit the fault current.
  1. It is well suited for applications that require several thousand amps to flow through it for a short duration. Limiting current to low magnitudes creates problems. Read Cons section for the why.

Cons

 
  1. It is expensive to build since it must have enough mass to absorb the fault current energy—the price increases with the increase in fault current it can handle and time rating.
  1. It is typically applied in systems where there is a desire to limit the ground fault current to a magnitude that is 25% to 60% of the three-phase fault current (refer IEEE 142 – Reactance grounding). If the ground fault current is limited to less than 25% of the three-phase fault current, then the neutral winding can be subjected to transient over-voltages (explained below). Therefore, a surge arrester should be installed with the reactor to achieve the reduction in fault current and prevent transient over-voltages from damaging the neutral winding of the transformer.

Transformer neutral ground impedance and transient over-voltages

Transient over-voltages are produced by arcing faults, not surges. The over-voltage occurs when the arc strikes due to a line-to-ground fault and charges the system capacitive reactance. When the arc momentarily extinguishes, the charge needs to dissipate. When the neutral ground resistor is used as the impedance, its resistance is usually less than the capacitive reactance, thereby allowing the voltage to discharge. However, when the reactor is used and when its reactive impedance is high (to limit ground-fault current to less than 25% of the three-phase current), the voltage cannot discharge. As the arc re-fires, the charge can continually build, thus creating the over-voltage.

Summary

If it is desired to limit the fault current to a really low magnitude using the actual resistance then a resistor is recommended. On the other hand, if several thousand amps of fault current is permissible in the system then the reactor is recommended. In either case, the reactor can be an economical solution. Keep in mind, we are talking about shunt air-core reactor in the transformer neutral for current limiting purpose. Series reactors, however, are expensive. The price of any neutral ground impedance device increases with the increase in the continuous current rating (for reactors), impedance rating, and time rating.


Are you interested in sizing an arrester based on system grounding? See below cheatsheet. Need full details that go with this? Check this PEguru article on arresters.

Cheatsheet for calculating lightning surge arrester ratings
Cheatsheet for calculating lightning surge arrester ratings

Aleen Mohammed

Aleen Mohammed

Registered Professional Engineer | MSEE
Skillset: Substation Design | Power System Studies | Arc Flash Study
DISCLAIMER: ALL VIEWS EXPRESSED IN THIS ARTICLE ARE MINE ALONE.

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I could not find anywhere information on the calculations to arrive at 25% or 60%. IEEE 142 only states X0=10X1 for 25% and X0=3X1. Do you have any idea on how to arrive to those numbers?

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