Short Circuit Level 6

[SilverArc]

Hi Jghrist,
I could not find my answer so as per your suggestion, I am starting this new thread:

If I have a 10 MVA transformer with 7 % impedance at 69/13.8 KV...
The short circuit level at secondary is 5981 A and primary is 1196.2 A. When I know this is the possible primary short circuit in primary as well as seconday, then why do I need the short circuit MVA from Utility.

As you advised to have the utility impedance, Utility is an infinite source: I guess the main applications of the SC MVA is to decide the rating of the Primay protective device like a circuit switcher or a breaker and has nothing to do with the secondary rating:
But technically, if my primary short circuit utility short circuit is 40000A then secondary short circuit as per my trasnformer configuration should be 40000x(13800/69000)=8000 A, which is not possible as the transformer can only deliver 5981 A.

Do u think, if I am making sense ?
Thanks

 

[jghrist]

A short circuit on the primary side is restricted only by the system impedance. A short circuit on the secondary side is restricted by both the system impedance and the transformer impedance.

 

[DanDel]

When you say "The short circuit level at secondary is 5981 A and primary is 1196.2 A. " You appear to be using the transformer size and %Z to calulate both the secondary and primary fault currents. This method is only viable when trying to calculate the maximum possible secondary fault current.
Your calulated primary fault current would be correct if you were looking for the maximum contribution to the primary from the secondary side, and does not take into account the actual contribution from the line into the primary.
The Utility is not an infinite source; there is always an impedance associated with the Utility contribution.

 

[SilverArc]

So, Dandel,

Will that Contribution from Primary(including utility) will be transfered to secondary ?

As per the calculation, the max. secondary fault current can never be more than 5981 A.

So, if a fault occurs on secondary bushing of a transformer....The fault current will be still 5981 A ?

May be I just pushing my dilemna too far.
Thanks

 

[rbulsara]

Silverarc:
May be a simple exercise will help clarify the concept.
Assume all impedance as R and the utility source as a voltage source V.

Draw a simple electrical circuit loop with a voltage source (V), in series with a source impedance (R1), and a transformer impedance R2.

Now place a short between –V terminal and open end of R2. This will be equivalent to a short circuit on the secondary of the transformer. The current in the loop will be I=V/(R1+R2).

Now place a short from a point between the R1 and R2 and the –V terminal. R2 will be cut out of the circuit. The current will be I’=V/R1. This is similar to having a short circuit at the primary of the transformer.

The transformer ration only comes in picture when the fault is on the secondary side of the transformer.

What you need to learn further is how to account for the voltage ratios. This gets little complicated. For this freshen up on the per unit method . Refer to IEEE Buff book or the Red book. Or you need to convert %Z (or per unit) values to actual ohmic values and calculate. Hint: %Z = Per unit Z*100 and is based on the transformer rating and voltage as the base values.

If you assume the source impedance to be zero (infinite bus) the SCC on primary side will be “infinite”. In practice the source impedance is very small compared to the transformer impedance so it is convenient to assume it to be zero to get a conservative available SC current at the secondary of the transformer for a good starting point.

 

[DanDel]

SilverArc, for your system, the maximum secondary fault current contribution from the transformer primary can never be more than about 5981 A. You may also have more contributions from motors, another source or loop, etc., on the secondary side that will increase this value. But you can't calculate the primary fault current from the secondary contribution.

 

[SilverArc]

Thanks Everybody. I got it.

 

[SilverArc]

hi Everybody,
Sorry, I guess I was not done yet.

It is from Distribution guys working in utilities:
Normally in Northe America, After what distance normally we use a lighting arrester on poles.

Like after every 5 poles or some thing like that.

I just need a basic idea.
Thanks

 

[dpc]

Distance from what? Surge arrester should be as close as possible to whatever it is you are trying to protect.

 

[jghrist]

Arresters are not normally used just to protect overhead distribution lines in the USA. They are used to protect equipment or cable terminations connected to the line.

 

[davidbeach]

And even when used to protect equipment or cable terminations connected to the line, a lot will depend on where in the country you are. In a low lightning area like the Pacific Northwest the practices will be very different from high lightning areas like the front range in Colorado or in Florida.

 

[cuky2000]

The link below shows graphically the effect of the available SC on the secondary of a transformer as a function of utility short circuit level and the effect of dampening the SC current do to cable impedance.

The SC level at the utility side have small effect as shown in curves B and F for SC of 500,000kVA and 50,000kVA respectivelly. Cables on the other hand, have larger impact in the SC reduction, particularly at low voltage.

I hope this could help to clarify the questions.

http://cuky2000.250free.com/SC_Calc_Dist_Tranf_1.jpg

 

[DanDel]

Lightning Arrestors are typically used at the end of a line, i.e., at each transformer.

 

[dgbpeakpwr]

I work in the transmission line and substation world (in Colorado and surrounding) and we typically place arrestors where the lines enter the station, high and low side of transformers and all feeder exits. I have not seen an instance where they were distribed on a line.

 

[RalphChristie]

First of all:

Please start each question in a new thread. It is easier to read and can be easier searched later if anyone have a similar question.

Both your questions are been solved, just one thing regarding arresters.

We use arresters (distribution level) on every tenth pole. This stems from old practices - used many years ago. Actually it was the norm in those days, someone a little lazy to do some studies and calculations just decided this is the norm, or copied it from somebody else. In my personal opinion it is a total overkill, but those old habits die not easily.

Do a study in your area, and based on that, do the decision where to put the arresters. Also, I am in total agreement with some of the other posters: put at least arresters near equipment or cables.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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Read the Site Policies at FAQ731-376

 

[SilverArc]

Thanks Ralph for your comments.
Next time, if I post a question , I will make sure I start a new thread.

But you will be surprised to know that unfortunately people who claim them selves to be sr. engg with more than 30 yrs know nothing about Electrical engineering and people like me have to turn to these forums for simple question, which I ask some times. I dont want to make views very apparent but beleive me, how some body is supposed to deal with situation where where one's sr. engineer at work has no clue what electrical engineering is.
Anyways, It was just some views about why peopl like me turn to guidance of smart people like you, davidbeach,waross, Jghrist etc.

Thanks

 

[RalphChristie]

Silverarc:

I am learning everyday something new on this site. Here are guys with a lot of experience.

Regarding seniors:
We can't expect the senior guys to know everything. The field is too big. Someone might be an expert on transformers while others may be experts on fault calculations. It all depends...
(I am feeling like a jack of all trades, master of none.....know just enough to be dangerous. Hehehehehe )
However, I must admit, I loves my job, it is a kind of a hobby. And that might be just the difference between me and normal Electrical guys. I am addicted to electricity....

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[cuky2000]

The Electric Power Research Institute (EPRI) performed a survey in the 1980's finding that is a common practice on the utility to have surge arrester every 5th pole.

Modern studies using transient analysis program, shown expected failure rate for different configurations as shown in the enclose link.

http://cuky2000.250free.com/Surge_Arrester_Line_Performance_2.jpg

Some surge arrester manufacturer claims that arrester may be a cost effective solution comparing with conventional overhead shield wire.

Recently there is available in the market line surge arrester. Placing intermediate arresters in addition to the end at the substation improve the overvoltage withstand of the line.

 

[SilverArc]

Thanks Cuky2000.

I guess I had a feeling that there is a thumb rule for it and every fifth pole is a thumb rule.Thanks again.

I have another confusion and pertains to utlity applications.
This question can be a possibly new thread but I am asking here. I hope you guys will pass me for this
1. In case of a pad mounted transformer application;

We have a fused cut out having a Expulsion type fuse and is technically for the protection of the feeder connected to the pad mounted transformer.

After that normally depending upon high or low value of fault current, we can have a partial range current limiting fuse and an expulsion type bay-onet fuse.
This is a normal trend. Normally the fuse cutout is an E Type fuse.

Here is my question(I hope what I wrote above is correct)

** Suppose I dont intend to use a partial current limiting fuse in the bushing and rather use a General purpose current limiting fuse in the fused cut out located overhead. Will this work ?

** What is the normal industry trend for fusing medium voltage switchgear. Is it Current limiting(partial range, General purpose or full range) or expulsion type fuses ?

I would appreciate a word.

Thanks

 

[apowerengr]

SilverArc - the under-oil partial-range current-limiting fuse is intended to clear primary winding faults in the transformer without taking out the E speed riser fuse; thus only the failed transformer is interrupted instead of all transformers on the loop. Remember the bayonet has a very limited fault-current interrupting rating (3500 amps or less, depends on voltage - think about how short the bayonet is compared to the length of a cutout door). The bayonet is only intended to clear secondary faults. Some utilities spend the additional $$ to protect the other transformers on the underground subloop. Using a current-limiting fuse at the riser cutout would limit the fault current for any primary fault on the loop (protects the feeder) however a primary winding failure in one transformer would take out the loop as the riser fuse would have to clear the fault. The use of current limiting fuses for switchgear is normally driven by the fault withstand of the gear vs. the available fault current on the system. Cutouts are typically rated to interrupt 7100 amps symmetrical with some distribution system able to deliver 30-40,000 amps. The E fuses (I assume S&C SMU-20) can interrupt 12-14,000 amps.