grounding 13.8kV delta system

[odlanor]

We intend to ground a 13.8kV delta system with isolated cables, by zigzag transformer used for signal only, not for trip. Question is:
1- What current is convenient for alarm?
2- How many time of alarm? (5 minutes , One hour..)
3-What are recomendation for insulation level of cables and voltage.
4- is there any reference or guide for this application?

 

[davidbeach]

It really all depends. What would be the maximum ground fault current after you install the transformer?

 

[odlanor]

davidbeach,
I would say 15kA , if zig-zag neutral were solidly grounded.
But my intention is to install a high resistance in the neutral of zigzag.

 

[davidbeach]

Right, but if that resistor limits the current to 20A, then 10 or 15A might be a good alarm point, but if the current is limited to 5A then those settings wouldn't apply. Pick a maximum ground fault current, then some begin to fall out. Others are operational and rating issues - you wouldn't want to wait for 30 minutes to alarm on a resistor with a 10 minute rating, but if it has a continuous rating you can pick most any time you want. We can't make any of those decisions for you.

 

[odlanor]

I was not very clear:
My intention is to choose a ground fault current that can be supported 1 hour (for instance) after alarm. This time, is enough to remove the area of fault point by switching system ,keeping others parts of system in operation.

 

[davidbeach]

Then you want a resistor rated for continuous duty. You can pick pretty much any answer you want for any of the questions.

What current would you like? A hint - it should be higher than the capacitive charging current, but beyond that your choice.

Continuous duty resistor and it can be in alarm forever with no damage to the resistor. But that's a lot of heat. 5A is probably a minimum and you would need to dissipate around 40kW.

Everything on the system needs to be rated for full line-line voltage as the two unfaulted phase will be at line-line voltage to ground. This increases the risk of a second ground fault on a different phase. You really don't want that to happen.

 

[odlanor]

My justification is based on following;

BOOK:industrial power system - shoaib khan
Extract page 72 -4.3.3

4.3.3-example of a high resistance grounding calculation
The objective is to design a HR grounding for 4.16kV system. The system is connected to a 10 MVA transformer ...
..................
The faulted circuit should be removed quickly on a line-to-ground fault, and the tripping can be delayed to permit the operator to take corrective action. The following criteria must be met:
* the ground-fault current Ir + 3Ico <= 8.0 A
* the phase-to-ground insulation level for motors, switching equipment, and cables needs to be rated for phase-to-phase voltageif the faulted circuit is mainteained indefinitely, or 133% if the faulted circuit is removed within one hour.
.....

 

[davidbeach]

Well, there you go. I've never seen that 8A value before, but as long as Ir>3Ic0 you should be good to go. Pick something in that range. Hint: 5 and 10 amp resistors are probably available with less lead time than say a 6.5 amp resistor.

 

[potteryshard]

I'm not quite sure what was meant by 'isolated cables'...

If that implies unshielded cables, I would have to ask if such cables are available in the 25 kV class.

If it implies shielded cables grounded at the zig-zag, and isolated at the load (or vice versa) I would worry that any remote fault could create a large voltage difference between the shield(s) and the local ground potential. That could seem to eliminate the dead front rating of any equipment, and to risk shield-to-local-ground flashovers and personnel hazard.

 

[odlanor]

IEEE Std 142-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems:
page 26- 1.4.3 - resistance grounding
... In general the use of high-resistance grounding on systems where the line-to-ground fault current exceeds 10A should be avoided because of the damage potential of an arcing current larger than 10A in a confined space.
.....................
I would like to understand the meaning of CONFINED SPACE. In my project 13.8kV system with high-resistance grounding is a mesh of isolated cables inside ducts and cable-trays running 2 km of a hydro-powerplant complex.
This is a major river (madeira) in the Amazonian jungle climate with high humidity and high temperature.
Is this a confined space?

 

[dpc]

I think they are talking about the equipment, not the ground grid. Metal-clad switchgear, generator terminal enclosures etc. What they are trying to say is that you need to limit GF current to less than 10 A if you are doing HRG to avoid creating a lot of damage in the equipment. I've seen arcing faults burn holes in metal enclosed equipment. Also, current over 10 A increases the risk of the ground fault turning itself into a phase-phase fault.



David Castor

http://www.cvoes.com

 

[odlanor]

* the phase-to-ground insulation level for motors, switching equipment, and cables needs to be rated for phase-to-phase voltage if the faulted circuit is maintened indefinitely, or 133% if the faulted circuit is removed within one hour.

What does it mean 133%