Transfomer short circuit current?

[mbrooke]

Will there ever be a time when transformer L-G, L-L-G and LL short circuit currents exceed the typical 3 phase short circuit calculations? I am getting conflicting information saying that this is all you need to determine the AIC rating when selecting any piece of gear; where as in other cases I am being advised that I also need to calculate the L-G and LL values because they will exceed 3 phase values. Which is true?

 

[Phil Corso]

Will there ever be a time when transformer L-G, L-L-G and LL short circuit currents exceed the typical 3 phase short circuit calculations?

MBrooke...

Yes... in cases when the Xfmr's Zero-seq is less than the Pos-seq! Although, unlikely for the typical commercially-available 2-winding Xfmr, it may be possible when a 3-winding Xfmr is employed!

Regards, Phil Corso

 

[mbrooke]

MBrooke...

Yes... in cases when the Xfmr's Zero-seq is less than the Pos-seq! Although, unlikely for the typical commercially-available 2-winding Xfmr, it may be possible when a 3-winding Xfmr is employed!

Regards, Phil Corso

It most cases the zero sequence is at least equal to the positive sequence? The paper here is telling me to use 100%, so I can assume both are equal in theory?

I am a bit confused about the 50% value, Id assume 80%...

 

[Ingenieur]

Good primer on symmetrical components and fault analysis
https://cdn.selinc.com/assets/Liter...etrical-Pt2_AR_20140506.pdf?v=20170217-155658

if the pos, neg and zero seq Z's are equal and the fault Z = 0
the a 3 ph fault = single ph to gnd fault
these are not unreasonable assumptions when solving for the max

 

[Ingenieur]

But there is no 'one size fits all'
https://ocw.mit.edu/courses/electri...tems-spring-2011/readings/MIT6_061S11_ch4.pdf

this calcs
3 ph
1 line to gnd
2 line to gnd
line to line

with real world values
you can see what determines the magnitude of each

 

[mbrooke]

Good primer on symmetrical components and fault analysis
https://cdn.selinc.com/assets/Liter...etrical-Pt2_AR_20140506.pdf?v=20170217-155658

if the pos, neg and zero seq Z's are equal and the fault Z = 0
the a 3 ph fault = single ph to gnd fault
these are not unreasonable assumptions when solving for the max

That is a good paper, thank you

But there is no 'one size fits all'
https://ocw.mit.edu/courses/electri...tems-spring-2011/readings/MIT6_061S11_ch4.pdf

this calcs
3 ph
1 line to gnd
2 line to gnd
line to line

with real world values
you can see what determines the magnitude of each

On a standard pad-mount, how can I obtain the R and X (per unit) values?

 

[Ingenieur]

That is a good paper, thank you



On a standard pad-mount, how can I obtain the R and X (per unit) values?

Typically not given on the nameplate
the mfg can usually provide a detailed sheet
if not an estimate can be had from a graph like thisp

 

[mbrooke]

Typically not given on the nameplate
the mfg can usually provide a detailed sheet
if not an estimate can be had from a graph like thisp

:dunce:

Need more info on this graph :angel:

 

[Sahib]

:dunce:

Need more info on this graph :angel:

Perhaps the factors are not integers and decimal points missing in the graph?

 

[Sahib]

Will there ever be a time when transformer L-G, L-L-G and LL short circuit currents exceed the typical 3 phase short circuit calculations? I am getting conflicting information saying that this is all you need to determine the AIC rating when selecting any piece of gear; where as in other cases I am being advised that I also need to calculate the L-G and LL values because they will exceed 3 phase values. Which is true?

When a generator is operating in parallel as in distributed power generation, then L-G short circuit current may exceed 3 phase value.

 

[rian0201]

lets say three phase transformer ( with j0.1 source impedance equal +,-, and zero) transformer is delta wye, a SLG fault is greater that 3P fault at the secondary. this is very common in distribution utilities with solidly grounded system


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[Ingenieur]

phase fault
V = 1
2 x line = 1.732 x Zl
2 xfmr winding 1.732 x Zt
i fault ~ V / (1.732 x (Zl + Zt)) = (0.577 V) / (Zl + Zt)

phase-ground
V = 0.577
1 line Zl
1 xfmr winding Zt
ground path Zg (could be high if earth, could be low if Cu, depends on fault type also, arcing, bolted, etc.)
i fault ~ (0.557 V) / (Zl + Zt + Zg)

the ground fault will typically be a bit less due to the Zg

attached is a symmetrical component analysis that shows them to be equal (with assumptions: Z pos, neq, zero are equal, fault Z = 0)
note this is in PU

 

[ron]

I see it a lot where generators are the source, near the source distribution.

 

[rian0201]

is this for me? hihi

there are times SLG is greater just like in a generator.. low Z0.. one reason for neutral grounding..


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[mbrooke]

For those who know, and have experience, typically by how much does primary connection and core construction play a role in zero sequence currents?:


5 limb wye-wye

VS

3 limb delta-wye

 

[mbrooke]

phase fault
V = 1
2 x line = 1.732 x Zl
2 xfmr winding 1.732 x Zt
i fault ~ V / (1.732 x (Zl + Zt)) = (0.577 V) / (Zl + Zt)

phase-ground
V = 0.577
1 line Zl
1 xfmr winding Zt
ground path Zg (could be high if earth, could be low if Cu, depends on fault type also, arcing, bolted, etc.)
i fault ~ (0.557 V) / (Zl + Zt + Zg)

the ground fault will typically be a bit less due to the Zg

attached is a symmetrical component analysis that shows them to be equal (with assumptions: Z pos, neq, zero are equal, fault Z = 0)
note this is in PU

I guess I should also take into account the neutral between the padmount and service gear as that can change fault current significantly?

Also, brain freeze, in per unit to what? I've always thought of per unit as in relation to something. Ie, if the rated voltage is 480 volts, and the actual voltage is 480 volts, you have a per unit of 1. An actual reading of 500 volts would be a 1.05 per unit voltage.

 

[Ingenieur]

I guess I should also take into account the neutral between the padmount and service gear as that can change fault current significantly?

Also, brain freeze, in per unit to what? I've always thought of per unit as in relation to something. Ie, if the rated voltage is 480 volts, and the actual voltage is 480 volts, you have a per unit of 1. An actual reading of 500 volts would be a 1.05 per unit voltage.

I used pu = 1 for V to illustrate

the paper defines
v base, whatever you want, usually the fault location nominal
s is is usually the xfmr kva (or gen, doesn't matter)
z base = v^2 / s
i base = s base / (sqrt3 x v base)
very useful when slide rules ruled lol