Faults with Transformers

[electrofelon]

so you guys keep thinking that a xfmr is the best and primary means of fault attenuation

I must have missed the thread where someone said that.....

 

[mivey]

never seen a utility that wanted to burn $$$ by increasing system Z, you want to LOWER it, not RAISE it
system Z is not fault mitigation
careful selection/engineering of epd's is

why do they use <2% xfmrs?
they don't care about faults, their equipment can withstand them, and they have no liability to the customer

I worked with Tesla, been around awhile :lol:


can't spend too much time here, got 3 side projects
new gen swgr for a hi-rise sr home
develop an Act 537
EPC scada systems for 2 stp's

so you guys keep thinking that a xfmr is the best and primary means of fault attenuation :thumbsup:
don't quit your day jobs...what is brookes day job? it's a secret lol

No strawman please. Transformers are not necessarily the best and primary means and is not what I said. But there are times you increase the Z for that very reason.

Yes utilities care about fault current and no, the equipment can't always handle the fault current. Been there, done that, got the t-shirt. Been doing this a long time.

 

[Ingenieur]

No strawman please. Transformers are not necessarily the best and primary means and is not what I said. But there are times you increase the Z for that very reason.

Yes utilities care about fault current and no, the equipment can't always handle the fault current. Been there, done that, got the t-shirt. Been doing this a long time.

that is the premise of the thread

the only reason you would buy a xfmr with higher Z is that they are cheaper

 

[Besoeker]

w
xfmr's are not used for fault reduction

But they do.

 

[mbrooke]

never seen a utility that wanted to burn $$$ by increasing system Z, you want to LOWER it, not RAISE it
system Z is not fault mitigation
careful selection/engineering of epd's is

Sometimes you need to burn $$$$ to save on $$$$. The loop scheme thread is a perfect example for spending millions to save millions.

why do they use <2% xfmrs?
they don't care about faults, their equipment can withstand them, and they have no liability to the customer

I worked with Tesla, been around awhile :lol:

Less than 2% is for pole transformers, usually 167kva and below. Its to acheive good voltage regulation when the units are being driven at 300% overload. Outside of that power transformers usually have around 8-12.5% Z, sometimes ordered a lot higher to mitigate fault current.

can't spend too much time here, got 3 side projects
new gen swgr for a hi-rise sr home
develop an Act 537
EPC scada systems for 2 stp's

so you guys keep thinking that a xfmr is the best and primary means of fault attenuation :thumbsup:
don't quit your day jobs...what is brookes day job? it's a secret lol

I'm going to be honest- just honest- but I think you have tremendous experience in every aspect and industry involving electricity, minus POCOs. POCOs are a different ball game.

 

[mivey]

that is the premise of the thread

the only reason you would buy a xfmr with higher Z is that they are cheaper

OP asked about iso xfmrs. He knew they were used but not exactly how. For one, they are good for blocking zero sequence currents. I would not necessarily install one just to reduce fault current but might tweak the Z as an added measure if it were practical.

The thread wandered into a discussion about theory and practical fault reduction. That is where we find that if you are installing a transformer anyway or trying to use something readily available, a Z tweak/choice might prove practical in some cases.

The bottom line is that sometimes a higher transformer Z is chosen to limit fault current. I know its been done that way for at least near a hundred years or more (so well before we ever got involved).

 

[mivey]

that is the premise of the thread

the only reason you would buy a xfmr with higher Z is that they are cheaper

You are wrong and that is just they way it is. A higher Z is also used to reduce fault current. Just because you have no experience in that area does not mean it is not so. I have a lot of experience in that area and know it to be true.

But don't just take my word for it. I have many resources I could quote but don't feel like making a long list. Here are two:

From the RUS substation design guide:

Both regulation and efficiency are generally improved with lower impedance. However, these desirable results must be viewed along with higher through-fault currents permissible with a lower impedance.

Higher load side fault currents can be potentially damaging to the transformer and may also require higher current ratings of load side equipment at increased cost. Prudent compromises are thus often required in specifying transformer impedances.

From the J&P transformer book:

If the transformer is to be supplied from a system having a large fault capacity (and the interconnection of power networks tends towards this condition very frequently), the inherent impedance should be sufficient to protect the transformer against the mechanical forces which will be produced within the windings during an external short cirucit. Alternatively, values of impedance higher than normal may be specified in order to reduce the available short circuit MVA on the secondary side of the transformer and to permit the use of circuit breakers having rupturing capacities lower than would be otherwise possible.

 

[Ingenieur]

You are wrong and that is just they way it is. A higher Z is also used to reduce fault current. Just because you have no experience in that area does not mean it is not so. I have a lot of experience in that area and know it to be true.

But don't just take my word for it. I have many resources I could quote but don't feel like making a long list. Here are two:

From the RUS substation design guide:



From the J&P transformer book:

you are wrong
you do not understand the basics
you do not add line Z to reduce fault I, silly
I have more experience and education than you and 4 of your friends combined :lol:

compromise, not design for a higher Z
compromise, get properly rated CB's
patches, not good design
you design for lowest Z
might as well install a coil of wire to add Z lol, beter yet wrap it around an air core and call it a CLR

 

[mbrooke]

you are wrong
you do not understand the basics
you do not add line Z to reduce fault I, silly
I have more experience and education than you and 4 of your friends combined :lol:

compromise, not design for a higher Z
compromise, get properly rated CB's
patches, not good design
you design for lowest Z

Ummm... I may disagree with Mivey at times, I may challenge him for not dumbing down the blatantly obvious- but if there is one thing that can not be disputed is that Mivey has tremendous knowledge of utility power systems and the industry in general. He is either a professor at a university, or more likely a senior T&D engineer for some major utility or ISO. Less likely but still within possibility working for some company which manufactures utility equipment. Either way education is irreverent because his knowledge and his ability to apply it is like a super nova, you can spot it from far away.

 

[Ingenieur]

just met with a guy I trust
MSEE MIT, PE
30 yrs of mv/hv T&D design, over 250 substations

only a few times has he spec'ed higher Z, NOT for I fault reduction, but the utility required it order to minimize disruption on the primary
he usually wants lower, <5% for lower v drop during large motor starting

 

[jim dungar]

Now that this thread has fallen into comments on credentials, I find it is time to close it.