Insulation Coordination

[timm333]

Just trying to figure out how the insulation coordination should be done for a transformer being fed by a GIS (gas insulated substation) and the load on the transformer is also another GIS. Would the transformer be modeled as a capacitance which behaves as open circuit to switching transients. Thanks for help!

 

[Ingenieur]

I would think it would be modelled by a series L

 

[timm333]

Yes that is what I thought. But I heard that as the surge from the GIS breaker would be very fast (like 500 kV at a rise time of 12 ns) so the transformer would act as a capacitance which would be an open circuit. Also I heard that the underground cable between the breaker and transformer would reduce the surge-intensity very quickly, and so by the time the surge reaches the transformer, it will not be strong enough to damage the transformer. I just thought to ask, what you guys think about it?

 

[Ingenieur]

I pulled my text
there is a whole chapter on it
quite complex
depending on what is be studied
L, C or both

get your hands on this book
well worth it
Electrical Transients in Power Systems
Allan Greenwood

 

[mbrooke]

Not sure, but this might help:



http://www.ijsrp.org/research-paper-0313/ijsrp-p15121.pdf

https://www.google.com/url?sa=t&rct...872832&usg=AFQjCNF6aPHA7oY7XPhiLUg9Qiki3Pc7tg

 

[mbrooke]

I pulled my text
there is a whole chapter on it
quite complex
depending on what is be studied
L, C or both

get your hands on this book
well worth it
Electrical Transients in Power Systems
Allan Greenwood

I could be wrong, but the OP will probably need a very strong commuter program- at least this is how VFT GIS modeling is done around here.

 

[Ingenieur]

This may help

 

[timm333]

Thanks, I actually don't need a complete computer analysis, just need a rough idea by doing hand calculations. The first link above does not work, but the second link (https://www.google.com/url?sa=t&rct=...LUg9Qiki3Pc7tg) is helpful. On the first page of this link there is a graph, which indicates the factor for 400 kV is 0.85, which means 400*0.85=340 kV. So if the 340 kV is less than the BIL of the transformer, then will the transformer be safe?

Also the cable between the breaker and transformer would attenuate the surge-intensity very quickly. This reduction in surge intensity would depend on the C, L, and length values. Does anybody know the formula to calculate that how much would be the attenuation of the surge intensity per foot of 3-phase single conductor cable?

 

[Ingenieur]

Thanks, I actually don't need a complete computer analysis, just need a rough idea by doing hand calculations. The first link above does not work, but the second link (https://www.google.com/url?sa=t&rct=...LUg9Qiki3Pc7tg) is helpful. On the first page of this link there is a graph, which indicates the factor for 400 kV is 0.85, which means 400*0.85=340 kV. So if the 340 kV is less than the BIL of the transformer, then will the transformer be safe?

Also the cable between the breaker and transformer would attenuate the surge-intensity very quickly. This reduction in surge intensity would depend on the C, L, and length values. Does anybody know the formula to calculate that how much would be the attenuation of the surge intensity per foot of 3-phase single conductor cable?

It can be done with hand calcs
an equivilent ckt needs drawn
showing junctions: gis duct to cable, cable to xfmr prim, xfmr, xfmr to gis duct
the surge Z for each component needs calculated or estimated from tables
Surge Z = sqrt(L/C)

call the transient pulse the incident wave
for each junction the following needs calc'ed
coef reflection r = (z2-z1)/(z1+z2)
coef refraction/transmitted = 1+r
this will calc how much
incident x (1 + r) = passes thru
incident x r = reflected back on self (could add, could subtract)

for the times/distances we are concerned with dampening due to R is small and can be neglected
for the same reason a reflected wave may add to an incident increasing significantly the magnitude

with this the mag at each junction/component can be determined based on the initial pulse/transient
for switching use 2 x sqrt2 x Vll/sqrt3 for the incident magnitude

if you are doing this as a learning exercise go for it
if you are doing this for actual sizing/purchase consult with someone who is proficient in this
I am not, just a cub scout

 

[GoldDigger]

One thing that jumps to my mind after looking at the article from post #7 is that much if not all of the capacitive surge coupling from primary to secondary can be avoided by driving the primary from a source which is balanced to ground.
Not practical for distribution, but valuable for instruments and controls.

Sent from my XT1585 using Tapatalk

 

[Ingenieur]

One thing that jumps to my mind after looking at the article from post #7 is that much if not all of the capacitive surge coupling from primary to secondary can be avoided by driving the primary from a source which is balanced to ground.
Not practical for distribution, but valuable for instruments and controls.

Sent from my XT1585 using Tapatalk

Astute

the next page talks about mitigation
adding C on the sec drives the ratio towards 0
so little surge is refracted

 

[timm333]

I tried doing traveling waves calculation but it is complicated. Here is the exact situation: The GIS breaker is 132 kV 3-phase 60 Hz, and the transformer is 132/230 kV 3-phase 60 Hz. The cable which connects the breaker with transformer is 2x 3200 kCMIL (2 cables per phase of 3200 kCMIL), and the length of this cable is 1300 feet. Now if switching surge generated by GIS breaker is 600 kV with a rise time of 10 ns, how much will be the value of this switching surge (in kV) by the time it reaches the transformer? I know the answer would be somewhere around 150 kV, but am not able to find the formula to verify it. So I thought to ask you guys to see whether you can find this formula. Thanks for help.

 

[Ingenieur]

your pulse is way too large for a switching transient
for lightening OK

are you looking for SIL or BIL?
I ignored damping for both, travel time is 3.4 uSec
assume: v propagation = 0.4, cable surge Z = 30 Ohm and the transformer as an open circuit

SIL
pulse: 220 kv or 3 pu max (220/132/sqrt3) = 2.9 pu)
at CB 440 kV (the wave is 100% reflected and additive)

BIL
using your 600 kV (lightening or SIL)
assumed referenced to ground
1.2 MV

again
if for learning purposes, fine
if for sizing/procurement, seek professional consult

 

[Ingenieur]

this may help

 

[timm333]

I was talking about BIL. Yes 600 kV is high, but assumed it just to be on the safe side. Is 440 kV the maximum that the breaker can generate?
My question is that if it is 600 kV at breaker, then how to verify that only 150 kV would reach the transformer; if you could please provide a calculation for this, it would be very helpful. Thanks

 

[Ingenieur]

I was talking about BIL. Yes 600 kV is high, but assumed it just to be on the safe side. Is 440 kV the maximum that the breaker can generate?
My question is that if it is 600 kV at breaker, then how to verify that only 150 kV would reach the transformer; if you could please provide a calculation for this, it would be very helpful. Thanks

440 would be the switching transient
the xfmr could see more from a strike if it hit the cb load bushings

If you have a 600 kV pulse leaving the breaker it will be >> 150 kV at the transformer
If the xfmr is assumed an open ckt you will see 2 times whatever arrives
even if damped by 50% over that short distance (highly unlikely) you will see 300 + 300 = 600 at the cb

if we assume only 300 due to damping makes it to the xfmr and we have a xfmr surge impedance of 100
you will have 300 + (100-40)/(100 +40) x 300 = 430 at the cb
this assumes 50% damping and only 43% reflected!

I can't get to 150 unless absurd assumptions are made
dampen by 85% so only 90 kv makes it and 30 is transmitted leaving 150 (90+60) at the cb
that is not realistic

 

[timm333]

Thanks. A few questions:

When you used the surge impedance as 100 ohm, you used the other impedance (Zo) as 40 ohm. Did you just assume this 40 ohm value, or is it calculated?


When the transformer is assumed open circuit, the wave will be 100% reflected back through the underground cable. Will this double reflected voltage affect only the circuit breaker and underground cable, or will it also affect the transformer?


How did you calculate the travel time of 3.4 uSec? If the rate of rise of the switching surge from the GIS breaker is given as 10 nSec, is it possible to calculate the travel time from this? Thanks

 

[Ingenieur]

Thanks. A few questions:

When you used the surge impedance as 100 ohm, you used the other impedance (Zo) as 40 ohm. Did you just assume this 40 ohm value, or is it calculated?


When the transformer is assumed open circuit, the wave will be 100% reflected back through the underground cable. Will this double reflected voltage affect only the circuit breaker and underground cable, or will it also affect the transformer?


How did you calculate the travel time of 3.4 uSec? If the rate of rise of the switching surge from the GIS breaker is given as 10 nSec, is it possible to calculate the travel time from this? Thanks

The 40 is an avg of the range for cables
it can be calc'ed from cable data = sqrt(L/C)
We assumed the xfmr to be open/infinite Z
I chose 100 >> 40 to illustrate a concept

cb and xfmr prim plus associated bushings/insulators
surge arrestors should be based on it also

I used the average v for a cable of this class 0.4 x c
(c = 186000 mi/sec) iirc
you know the distance 1/4 mile : d = v x t
v can be calc'ed = 1/sqrt(L x C)
L and C MUST be per unit (the same as v) values, ie, Fd/mi, etc

since v and travel time >> rise time ignored it and we only are concerned with max magnitudes anyways

 

[GoldDigger]

The doubled surge voltage will appear at the terminals of the transformer itself. Not necessarily at other points along the line, depending on the length and whether the surge pulse has decayed before the reflection reaches that point. For short lines, double all the way.

 

[timm333]

Thanks. I tried to find the damping formula to calculate how much voltage would reach the receiving end, but could not find it.


The input side of this transformer has a BIL of 900 kV. So if the surge is 600 kV at the GIS breaker, we will see 1200 kV at the input terminals of the transformer (even though the transformer acts as open circuit.) 1200 kV exceeds the BIL of 900 kV.


However if the switching surge at the GIS breaker is less than 450 kV, then the doubled voltage at the input terminals of the transformer would be less than 900 kV, which would be Ok as it does not exceed the BIL.


I think 600 kV switching surge from 132 kV GIS breaker is a very high value, the actual value would be much less. Do you know how much would be the maximum possible switching surge (in kV) from a 132 kV GIS breaker? If it can be shown that the maximum possible surge from 132 kV GIS breaker is less than 450 kV, then the doubled voltage at input terminals of the transformer would be less than 900 kV (less than BIL). So it will be Ok and the complicated damping calculations would not be required.


Let me know what you think. I just thought to get the opinion that what you guys think, your information does not have to be accurate, of course.