Grounding

[burntcoil]

While carrying out the calculation for Substation Grouding Grid as per IEEE Std. 80, I found that in most of the example Breaker Short Circuit withstand rating is used for single phase to ground fault current. For a substation where I have MV switchgear with Circuit Breaker rating of 31.5kA rated and LV Switchgear with 65kA rated circuit breaker, which short circuit rating value I will choose, 31.5kA or 65kA.

In the example I saw they are considering 31.5kA (or short circuit of MV Breakers). This makes me think that the MV network is normally three phase three wire and LV network is 3 phase 4 wire and the neutral is grounded. So why do we consider MV breaker short circuit withstand value.

Further to above, the single line to ground fault value will always be much less then the Circuit Breaker short time withstand, why we do not consider actual single line to ground fault current while following IEEE Std. 80-2000.

Thanks

 

[7anoter4]

According to IEEE-80/2000- Definitions:
"3.14 ground potential rise (GPR):The maximum electrical potential that a substation grounding grid may
attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage,
GPR,is equal to the maximum grid current times the grid resistance.
NOTE—Under normal conditions, the grounded electrical equipment operates at near zero ground
potential. That is, the potential of a grounded neutral conductor is nearly identical to the potential of remote
earth. During a ground fault the portion of fault current that is conducted by a substation grounding grid
into the earth causes the rise of the grid potential with respect to remote earth."
That means the fault current from remote source flowing through the surrounding earth and grounding grid
will rise the grid potential. If the transformer-the source of LV or MV current will be mounted on the substation grounding grid this does not produce GPR.
since the short-circuit current will flow through part of the grid but not penetrate the earth beneath the grid.
However, for conductor cross-section calculation-main circuit-you have to take it into consideration.

 

[7anoter4]

If we take a look on ch.15.11 Effect of future changes:
One could not be sure if the future change will not increase the present maximum fault grid current.
It is acceptable the maximum admissible current of the breaker will be close to the future value of the maximum ground fault maximum current.

 

[odlanor]

Zanorte4
I get lost in your answer. I could not link it to the question of the burntcoil!

 

[noreddine]

Dear all,
i've been working in a grounding system of wind power plants and i designed a grounding system according to IEEE STD 80. for your question aboit what to choose, LV single line to groud faul or MV one, it depends on the path of the short corcuit. this one depends on the neutral grounding. if you have LV neutral linked to ground than you should consider LV SLG faut. if it is MV wich is linked to ground then you should choose MV slg.

 

[odlanor]

I understood we have at the same grounding grid two grounded transformers estabilish two networks: the MV network is normally three phase three wire and LV network is 3 phase 4 wire and the neutral of each one is grounded.
Fault current ënters the ground near the fault and returns from ground to the neutral of transformer.

For MVslg all fault current returned is grid current.

For LVslg all fault current returned is divided between grid current AND neutral current. As neutral wire has impedance much minor then ground impedance,the greatest part of fault current go with neutral wire and the part of LVgrid current results less than MVgrid current.

 

[7anoter4]

There are 2 problems here:
1)GPR -only Medium Voltage current participates here.
2)Low voltage return current from the short-circuit point.
The fault current will split between grounding grid conductor and the grounding conductor -following the supply cable and close to it.
Since the reactance between faulted conductor and the grid is large[the distance
between live conductor and grid is large] and the reactance between live and grounding
conductor is small then most of current will flow through grounding conductor.
However, if you want to be sure, you may consider the grid conductor following the cable from transformer to the receiver[motor or else] as a return fault current conductor and calculate it accordingly.

 

[odlanor]

Zanorter4

GPR -only Medium Voltage current participates here if your transformer is MV/LV Ynyn.
WE use sometimes 2 transformers: MV/MV Ynd and MV/LV Dyn. In this case GPR will be present for Low Voltage slg.

 

[odlanor]

Zanorter4
CORRECTION!
GPR -only Medium Voltage current participates here if your transformer is MV/LV Ynyn.
WE use sometimes 2 transformers: MV/MV Ynd and MV/LV Dyn. In this case GPR will NOT be present for Low Voltage slg
because MV will see as 2 phase-fault.