Line-Neutral Voltage on Low/High Resistance Grounded Systems

[ThePunisher]

Hi all,

I would like to obtain your technical opinion on the matter below. I thank you in advance for any clarification response I will get.

In high and low resistance grounded systems, in the event of a ground fault, the unfaulted phase conductors will be subjected a line-ground voltage equal to line-line voltage (especially if the NGR is open-circuited).

However, what about the neutral conductor (NGR to transformer X0? In my opinion, it will remain at system nominal L-N voltage and will not behave the same way as the unfaulted phase conductors.

Anyone agree with me? I only have difficulty justifying this understanding on the neutral conductor and would need some help. NEC 2017 section 250.187.(B).(2) -(Impedance grounded systems), which states that "The neutral conductor shall be insulated for the maximum neutral voltage"...by how much? is also not clear.

Regards,

 

[davidbeach]

The neutral of the transformer, X0 or equivalent, will be at phase-neutral voltage to ground. The whole voltage triangle shifts so that one corner is at ground and everything else moves together.

 

[ThePunisher]

Thanks David....your response still a bit vague to me as far as justifying that the X0-NGR voltage is still at system L-N even if there is a ground fault or if the NGR is open-circuited and ground fault occurs.

We have a 7.2 kV L-L system with the transformer secondary neutral connected to NGR. The L-N voltage is 4.16 kV. I used 5 kV with the perception that L-N voltage will remain at 4.16 KV regardless if a ground fault occurs withy NGR connected or open-circuited. Some folks are insisting to me to use 15 kv or 8 kV cable for the neutral connection from X0-NGR.

We have startco relay SE 330 with sensing resistor that will alarm and trip in the event of open-circuited NGR.

 

[waross]

To expand on David's answer:
Normally each phase is at phase to neutral voltage to ground.
There is negligible current across the grounding resistor and so there is negligible voltage drop across the grounding resistor.
When one phase goes to ground, the other two phases rise to phase to phase voltage to ground.
There will be full current through the grounding resistor as determined by the value of the grounding resistor.The voltage drop across the grounding resistor will be equal to the phase to neutral voltage.
One end of the resistor is connected to ground and the other end of the resistor is connected to the transformer neutral or X0 terminal.
Hence the system neutral rises to phase to neutral voltage above ground.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thanks Waross,

And by the statement "system neutral rises to phase to neutral voltage above ground, my neutral cable will then only be subjected to system L-N voltage only (in my case, 4.16 kV), right?

 

[ThePunisher]

See attached with the statement that confused me with regards to the neutral point voltage to ground highlighted.
I apologize for my so confused mind...which I thought won't hurt to ask.

 

[ThePunisher]

Folks in my turf is insisting to use 8 kV or 15 kV for the neutral cable from X0 terminal to NGR and I am the only one insisting for 5 kV as the L-N voltage at the NGR is 4160 V

 

[wroggent]

Depending on the distance you're working with and phase conductor size, it may be more practical or economical to buy a single type of cable even if it is larger or has a higher insulation rating than what you need.


There is an informational note below 250.187(B) in the 2017 NEC that says "The maximum neutral voltage in a three phase
wye system is 57.7 percent of the phase-to-phase voltage."


Regarding the highlighted section in the document, the additional voltage stress is on the two unfaulted phases (there will be a grounded shield on cables at this voltage level, i.e. the voltage across the insulation is phase-ground and when one phase faults the ground voltage is brought up to the phase voltage with respect to the other two phases).

 

[waross]

Another way of explaining it that:
The voltage from A phase to the neutral terminal is the line to neutral voltage.
If A phase goes to ground, the voltage from A phase to the neutral terminal is still line to neutral voltage and so the neutral is now at line to neutral voltage above ground.
I second the suggestion that if the cable lenth is short it may be more economical to use the same cable for all connections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Powerengineer_84]

A major benefit to the resistor grounded system is where there are motor or generators connected to the system, the resistor ground will limit the burning of iron. In systems with high short circuit availability, it will also help manage cable shield temperatures when the shield needs to carry ground current. A high resistance grounded system can be of value when a system must stay up to allow an orderly shutdown (allowing a ground fault to persist is of less detriment than a sudden trip of a process. Resistance grounding comes with its down sides as well, neutral shift as discussed above as well as needing an insulation system on the Y winding of the transformer that is not graded (full BIL all the way to the Y point) as well as arresters and surge caps that can deal with the overvoltages on unfaulted phases during an SLG fault. This will make the arresters some what less efficient in limiting surge voltages.