Advantages of using NEX(vs NER) and when NEX should be used instead of NER

[TimDta]

In substations, industrial installations, mine sites etc neutral earthing resistors(NER) are used. However, in many installations, neutral earthing reactors(NEX) are used. I was told that for a couple of projects, NER was deleted from the project drawings and NER was replaced with NEX. Qa)What could be the reasons for replacing NER with NEX in project drawings, Qb)What are the advantages of using NEX(vs NER) and Qc)When NEX should be used instead of NER?

 

[PersonalProfile]

Far from an expert, though:

a. Can be selected to (relatively) limit fault current / potential.
b. Refer above, though has limitations (inrush current associated with "energising" the reactor).
c. Depends on the situation.

There is piece meal information available online and or detailed information in various text.

Regards,
Lyle

 

[pwrtran]

I assume that the supply to the mine sites is a 3-wire distribution system as most mining loads are 3-phase loads. When a single L-G occurs on such a system, the neutral reactor supplies a current which is in opposite direction of the capacitive arcing current, in other words it compensates the capacitive fault current by 90 degrees, as the result it can quickly distinguish the arcing and prevent the arcing re-initiates again. So it reduces the lost time.

 

[DTR2011]

I believe the NEX is also referred to as a Petersen Coil. There is some discussion on this forum Petersen Coil

 

[cuky2000]

_{The Historical Development of Neutral-Grounding Practices
In the early attempts to minimize the ground-fault current in solidly grounded medium-voltage systems,copying the utility practice of using neutral reactors was found to be impractical since, in order to control the transient-overvoltage problem inherent in ungrounded systems, the untuned grounding reactor had to limit the ground-fault current to not less than 25% of the prevailing three-phase short-circuit current. For example, in systems with a typical available three-phase fault current of 20,000 A, reactance grounding would require the minimum ground-fault current to be 5,000 A, an unacceptable value for industrial grounding purposes. E. Owen}

For LV and MV applications In the USA the neutral impedance grounded is dominated by using resistance grounded. This perhaps could be attributed by experience and studies in have indicated that this inductive reactance to ground resonates with the system shunt capacitance to ground under arcing ground fault conditions and creates very high transient temporarily overvoltages (TOV).

General Guide to Apply Neutral Reactance Grounding:
1. Not recommended for low ground fault current.
_{The available ground-fault current should be at least 25% (X0 = 10X1) and preferably 60% (X0 = 3X1) of the three-phase fault current to prevent serious transient overvoltages. Available is considerably higher than the level of fault current desirable in a resistance grounded system.}
2. Usually not considered an alternative to low-resistance grounding.
3. Typically reserved for applications where there is a desire to limit the ground-fault duty to a magnitude that is relatively close to the magnitude of a three phase fault.
4. Often be found to be a less expensive application than use of grounding resistors if the desired current magnitude is several thousand amperes. These circumstances may arise in one of two possible instances:
a]. Large substation feeds a medium-voltage distribution system, and the total zero-sequence impedance of the step-down transformers in the station causes the SLG-fault current to greatly exceed the magnitude of a three-phase fault, and ground-fault limitation is desired to keep the total fault current within the reasonable limits. These conditions tend to occur most often in electric utility distribution practice.
b]. Serve single-line-to-neutral-connected load directly at the terminal voltage of generators.
See the enclosed neutral grounding info collected from various sources.